wed may 2, epi/norepi from AM

Epi/NE increase heart rate and inc contractility to increase cardiac output.

E/NE also increases vasoconstriction which increases peripheral resistance (blood vessels down stream of the arteries constrict, so their diameter gets smaller, so bloodflow exiting goes down, which increases bloodpressure).

If we put both of these together Blood Pressure (or mean arterior pressure MAP) (MAP is the product of cardiac output times peripheral resistance), so if we increase sympathetic NS activity, that increases cardiac output, increases perf resistance, which increases MAP.

Somebody suffering bloodloss, BP falls (MAP falls), to compensate for BP drop, increase in sympathetic NS stimulation, increases Epi/NE release, inc cardiac output, inc perf resist, inc in MAP.

You should have a good uncderstanding of the aqnate=amony of the symp neurons and the AM, draw the biosynth pathway including structure and enzyme names, what all the acronyms mean, and understand in preinciple what will happen to metabolic, and nervous systems when increase decrease of epi/norepi.

E/NE actions on the respritory system:
1) bind to beta-adrenergic receptors in the smooth muscle of bronchials (part of the tubing that carry air to the lungs). when it binds to the beta-adren. rec. it leads to smooth muscle relaxation, which increases the diameter of the bronchials, which decreases the resistance of air flow, and greatly increase air flow. albuterol is a beta-adrenergic binding agent that causes relaxation to open air flow into the lungs of asthma people.
2) E/NE raise arousal (you become more alert), regulate the secretion of a bunch of hormones such as renin secretion and inhibit insulin secretion.
3) E/NE increase metabolic rate and they increase thermogenesis. they increase the activity of outer-ring deiodinase in brown fat. that increases thyroid activity, by converting T4 into T3. Brown fat looks brown because it contains lots of mitochondria, and is the site of non-shivering thermogenesis. T3 stims synth or expression of the molecule thermogenin that is a therm0protein that uncouples from atp generation. they increase lypolysis, and triglyceride breakdown in fat, which increases the fatty acid concentration in fat, that increase opens the proton channels (the thermogenins). if you have an increase in the more active thyroid hormon, that sets it up to make more thermogenins, and they in themselves are closed. they can be regulated. if nor/epi are high, they are open and you have a rapid ability to gen more heat.

steroidogenesis.... You need to learn the boisynth path for these.... (on handout)you need to be able to draw all of the structures, and enzyme names, and patheways. you have to know everything on the sheet.... The parent of all of them is cholesterol. Part of it is chopped off and yo uare left with a ring structure with certain functional groups attached. If you count up thenumber of carbons in all of them, the first three are all 21 C steroids, the last 2 have 19, and then 18. Those are the natural versions, and they synthetic/man-made have different numbers. But you can count the number of carbons in the naturals to tell what the bioacti will be.

What is made hs to do with what enzymes are expressed in any particular cell, and no cells make every single steroid. If something goes wrong in the pathway, that influences other things being made in the pathway.

The way the handout works is it starts with cholesterol (you can look at another handout of cholsterol with a nomenclature of steroicogenic enzymes), and it shows the first structure of the pathway (prenenolone vs cholesterol, the side chanin is cleaved from pregnenolone to make cholesterol). You will want to mem prenenolone, and then memorize the numbering system fof the carbons. if you mem that scheme and then you learn the anems of the enzymes, you can figure out what the next structure cdown the line will look like. if you don't do that and you just go for mem them, you will probably drive yousrlf nuts. they loook too similar to each other.

the second part of the second sheet, gives all the enzymes with multiple names shown. We are going to use the 'trivial names' because they tell what it is actually doing. 17-alpha-hydroxylase; 17, 20 lyase: that tells exactly what it does... adds an OH at 17, and cleaves between 17 and 20.
There are 5 classes of steroid molecules:
Progestins
glucocorticoids
mineralocorticoids
androgens
estrogens

You want to look at the chart and id stuff you already know. At the lower left are mineralocorts ( lower left three)(aldosterone, corticosterone, and beta deoxycortosterone). next to that is glucocorticoid (2), the theird column (all ) is androgens. and the three to the far right are estrogens.

the way the enzymes work is tricky. the enzyme arrows point to what it catalyzes. If it doesn't go all the way across, then it doesn't make that item.

dezmolase is called 'side chain cleavage enzyme', 3 beta...(directly bleow it) goes by the name 3 beta hydroxy steroid dehydrogenase (3besta-hsd). it acts on carbon 3 to an OH, and something else.
and the lowest left enzyme is called aldosterone synthase (makes aldosterone).

The four major steroidogenic tissues testes, ovaries, adrenal cortex, and placenta.

You will run into 'peripheral conversion of steroids', means that step doesn't take place in a traditional steroidogenic tissue (like maybe body fat instead).

Let's talk about where they get cholesterol from, and what is the rate limiting step:
Cholesterol sources: some steroidogenic cells synth it from scratch from acetate. Most of the time, most of the cholesterol comes from the blood in the form of LDL (low density lypoproteins) they take them up, and use the cholesterol that is part of the LDL. uptaek is a place that steroidogenesis can be regulated. sites of steroid synth iwithin a cell... in either the mitochonodria, or in smooth ER. the cholesterol, and all of the subsequent molecules are shuttled between them. Smooth ER is called that because it doesn't lots of ribosomes attached. Ribosomes aren't needed for steroid synth. Steroids are not stored inside the cell. Cholesterol is processed through the pathway, it is highly lipid soluble, and goes right out of the cells. There are no vessicle storage of steroids. there is no way to regulate its release once it is made. If we look at the rate limiting step in the pathway, it is right at the beginning... it is the cleavage of the side-chain from cholesterol. It turns out that side-chain cleavage enzyme is in the inner mitochondrial membrane, and getting cholesterol to the inner mit mem is what makes it the rate limiting step. There is a protein repsonsible (StAR): steroidogenic acute regulatory protein, and it somehow fascilitates shuddling of cholesterol between the inner and outer mit mems. People with a mutation in StAR form little or no steroids. All of the regulatory pathways taht we are going to talk about, all of them act on StAR, as well as all the other enzymes in the pathway.

Last part of the general stuff: mech of steroid action... (handout in class)
Nuclear hormone receptor family... steroid receptors are part of that family. they jhave a hormoen binding domain and a DNA binding domain. thyroid hormones work differently than the steroids do. the thyroid response cell works in a dimer, binds, corepresors replaced by coacts. this is different. the basic concept is at the tope of the sheet.... read sheet. (fig 3-8 from je friffin and SR ojeda) availabel on eres) and fig 3-11 JE Griffin and SR Ojeda.

This is true for all of the molecules on the big pathway for steroids. there are distinct receptors for everything, but with some overlap between mineralocorts, and glucocorticoid receptors.

Adrenal cortex:
looks like a piramid with the inner part being the medulla. there are layers around the medulla (3 layers). the outer layer is called the zona-glomerulosa produces aldosterone; middle layer is called zona-fasiculata; innermost layer is zona-reticularis, both of which produce glucocortocoids and androgens. An easy way to remmber this as GFR from the outside in. (same acronym as from kidneys)

the ZGA layer is the only one that expresses aldosterone synthase. the products of the inner 2 are: androgens: androstenedioe, and DHEA (both are considered 'weak' androgens' which means they ahve relativly low affinity for receptors, not much testosterone or dihydortestosterone, and extermely little estrogens); and specific glucocorticoids: cortisol and corticosterone, these 2 are the most important in humans.

Glucocorticoids: (hypothalamic-pit-adreanl axis handout)...CRH: cortico releaseing hormone if there is no CRH then the pathway shuts down.

A stressor activiates this pathway.

Mon, April 30, adrenal receptor subtypes...

Today: Epi and Norepi.... we are basically one day behind, but we will get caught up by the end of the quarter.

Brief overview of sympathetic:
The hormones we are interested in are Ep and Norepi. if you are looking at a target cell somewhere in the body, have 2 sources that they can get Epi or Norepi from. 1) post-ganglionic neurons sympathetic neurons... realse Norepi as neurotransmitter 2) adrenal medulla (central part of the adrenal gland). AM releases both Epi and Norepi as hormones.

The sympathetic nervous system is part of the autonomic NS. The autonomic NS regulates endocrin and exocrin glands, contraction of smooth muscle, and cardiac muscle.
Autonomic neurons carry signals from the CNS out to the gland. There are 2 branches: parasympathic NS, and sympathetic NS. (you can disregard parasymp for this class). Sympathetic NS anatomy:
Post-ganglionic sympathetic fiber:

if we have a cell body of a neuron in the CNS, in the spinal cord (in this exp), an axon is sent out and is passed onto smooth or cardiac muscle, or endo/exo glands. The mid point between is a ganglion. it consists of lots of cell bodies and synapses. They are a grey whitish blob. It consists of thousands of axons synapsing on cell bodies. The one between CNS and the ganglion is called a pre0ganglionic fiber or neuron. between the ganglion and the endpoint is post-ganglionic fiber (neuron). At the axon terminal they release Norepi. And the NE reacts with the cells that are close to where it is released. It will either act right there, or go into the blood. This is one source of NE for these 4 targets.

the second source is the adrenal medulla:

The AM is really a highly modified post-symp ganglionic fiber. In the diagram for an AM:
starts with cell body in the spinal cord, and the pre-ganglionic fibers head towards the adrenal gland, and the pregang fiber has lots of axon terms, which make contact with cells, and these cells are called chromoffin cells, which are the source of the Epi/Norepi. Most of the Chromoffin cells release predominants Epi, and some release both Epi/Norepi. The cells release Epi/Norepi and the Epi/Norepi diffuse into the capillaries right next to the chromoffin cells. The part where the cells meet the caps, is all the adrenal medula. The AM is a 'sac' made of a group of cells, innervated by the pre-gang symp fibers. And when the PGSF fire they trigger the release of epi/ne.
IMportant points:
once the AM, when it rleases epi/ne, it is completely random where the epi/ne ends up.
when sympathetic neurons fire, THAT firing is highly directed.
The ratio of epi/norepi from AM, in humans, 80% epi, 20%ne. they also release some dopamine, but we aren't going to worry about that. Most of the hormone in the blood epi/ne, is epi. if it is being stim'd by epi comes from AM, norepi most likely it came from a post-gang symp fiber, even though a small amount comes from AM.

Now lets look at what the hormones look like. The diagram is on eres. tyrosine, dopa, dopamine, nor, epi.

this shows the biosynth pathways for these hormones. E, NE, and DA (dopamine) are all considered catecholamines. This refers to a certain structure of 2 hydroxyl groups off a aramatic ring, side by side (OH) then that is a catechol.

Tyrosine hydrosylase with tyrosine becaome dopa, and reacts with AA decarboxylase to become DA, and reacts with dopamine B-hydroxylase to make Nori, and cortisol triggers pheynylethanolamine-N-methyltranferase (PNMT) to make epi.

You really must know that.

The rate limiting step is the tyrosine hydroxylase step. Takes place in the chromoffin cells or in the post-gang.
Cortisol is produces in the cortex around the AM. Cortisol goes into the medula before it exits into the blood. And Cortisol stims the AM. If we look at the inputs that stimulate this pathway, regulation of Epi and NE secretion:
major regulator is the symp NS. it is always 'on' to a basal level. If the activity is activated beyond the basal level then it raises levels even more. Activation of symp NS is by flight or fight situations. other sits: during exercise, hypoglycemia, hypotension (low BP), and cold exposure. Some inputs trigger AM, and some trigger the PS/symp/NS. so leave yourselves open where it is not black or white. it is case dependant.

let's look at the receptors. 'adrenergic receptor subtypes' from eres:
Adrenergic receptors are the receptors for epi/norepil, do not call them epi-receptors.
The receptors are different in the G protein, and all have specific affinities. they have rougly similar afffinites for e/ne. since the receptors are all connected, but trigger different systems, so it is dependent upon which receptors are present in that area.

let's go on to look at the bioactivities...
the bio act. of e/ne have different catagories:
intermediary metabolism, cardiovascular system,, secretion of other hormones, arousal, metabolic rate.
Let's start with Int Met:
The effect of ep/norepi are mediated largely by beta-type adrenergic receptors. They increase the breakdown of fats and carbs into forms that can be used as fuel. The specific funtions are divided into different tissue types:
liver: (controls alot of int met): Epi/NE increase glycogenolosis (glsyogen breakdown), and increase gluconeogensis, and both of those act to increase blood glucose concentration.

muscle (skeletal): (a huge percentage of body weight): increase glycogenolosys, but the glucose that is freed up can't be released from the muscle because is it phos'd. that increases muscle glucose concetration to serve as fuel for the muscle, and enters glycolosys. it increases lactate and pyruvate production, which can leave the muscle and enter the circulation, and become substrates in the liver for making more glucose.

adapose tissue (fat):(less but still high majority):E/NE increase lipolysis (triglyceride is hydrolyzed to glycerol + fatty acids), and the glycerol is a substate for gluconeogenesis in the liver. the fatty acids can be used for fuel in cells outside the CNS, and alos be broken down into ketones to be used as fuel anyplace.

keypoints: actions on muscle and liver increase glucose concetration (and fat provides glycerol for gluconeogenesis), and fatty acids to allow for alternate fuels. (gluconeogenesis, only in liver or kidney).

Last part of this: in terms of protein met: epi/ne inhibit proteolysis. This is important later because it counteracts effects of cortisol. It is important to protect proteins to maintain structural integrity.

The big difffere3nce here between GH and epi are:
GH inhibits proteolysis, but GH stims aa uptake, but epi doesn't. epi just protects again proteolysis. epi does not act against glucose use in cells. it doesn't inhibit glucose uptake in cells, and doesn't counteract the uptake of insulin in cells like GH.

Let's move on to cardiovascular system:
effects on heart, blood vessels:
heart: mediated via beta 1 adrenergic receptors, what epi does: increases heart rate and contractility (force of contraction, larger means more blood pumped). the net result of both of these is they together increase cardiac output (volume of blood pumped by one ventricle per minute). Cardiac output is one of the things that determines how high blood pressure is.

Blood vessel adrenergic receptors:
-alpha Adrenergic recptors are in the smooth muscles that make upthe walls of blood vessels. Only alpha, then when they bind it causes the smooth muslces contract, and vessel diameter decreases (vasoconstriction)

-beta2 adr. recptors have the opposite effect: smooth muscle relaxes (vasodilation) and opens up blood vessels.

The effect on a blood vessel depends upon the concentrations of both of these. Most contain more alpha than beta adrenergic receptors. The response is vasoconstriction. the exceptions to this are: blood vessels in skeletal muscle, heart, liver, these contain more beta 2 than alpha, and so when epi/ne at high concetrations it causes vasodilation.

The interaction between going on from changing cardiac output vs changing blood vessel diameter. Heart to arteries to arterioles and returning capillaries, to veins and back to heart. (it is alphabetical) BP is measured in arteries. that pressure drives blood all the way through the cardiovascular system. it enters the arts and is at very high pressure, adn it flows down the pressure gradient. the only way for it to work properly is if arterial pressure is higher than veinous pressure, othewise it would go backwards. the body very closesly mointors and regs the pressure in arteries to keep it at the appropriate prssure. if it s too high then arteries in the brain will explode and you have a stroke and die. the two major things that determine blood pressure are: blood pressure entering arteries and blood pressure leaving the arteries. Blood in minus blood out, give you the blood pressure. you can increase the pressure in the arteries by: 1) increasing bloodflow into the arts,from the heart, (cardiac output) of 2) by decreasing the bloodflow out of the arts. what the sympathetic NS and epi/norepi do is to regulate cardiac output, and regulate bloodflow out of the arteries (decrease by vasoconstriction of blood flow out of arterioles. Vasoconstriction by raising E/NE binding to alpha-adrenergic cells.

wed, april 25

regulation of GH secreion: sim GH scc: GH affects metabolic pathways ... lobical that met factors ontreol GH
raised blood glucose stops secretion of GH and low blood gujlcoe raises SCC GH (short term/hours)
prolonged caloric deprivcation leads to raised GH (long term) probelmm that perons nom consuming sufficient protien rather than lower blood glucose levels (bf)
raised plasma AA (esp arginie) raises GH
acure stress raises GH (raised GH leads to raised glucose, and free fatty acid)
sleep leads to raised GH (GH has diuranl rhythm) inc sev hours after sleep

inhibition of GH:
low Th/hypotheyr.
raised corisol (classic strss hormoen) leads over time to raised cort low GH.
Emotional dep (bad emotional event)

Patterns of GH over life....
low from birth to childhood... goes up slightly in childhood and plattues until puberty.... spikes during puberty... and drops down to adulthood... and stays relatively stable after that....

GH no sig in prenatal growth (IGFs do
a+ at puberty, raises test and estrogens leads to GH secretion
estrgen acts directly on bone and everntually causes closeure of epiphyseal plate (cartilage)
in males, testo is converte to estrogen (closure)
test also acts dire on bone and dr stims growth (osteoblast activity)
estrogens alos stim bone growth
as people age greater than 60-70- GH declines and change in body comp lower muscle mass and raise percent fat.






disorders of GH secretion:
1) dwarfism - too little GH as a child

2) gigantism or giantism - too much
GH as a child

3) acromegaly - too much GH as an adult (pit tumor or hypoth. issue): increase in bone width, continued growth of cartilage, continued growth of soft tissues (liver, heart, tongue, kidneys), and tendency towards long standing hyperglycemia (diabetic over time)

Growth hormone mech of action...

this is the same for othes in the growth homrone family (prolactin)

one GH moelcue binds to two GH-R

JAK2 (just another kinase) is rebuited from the cytosol

JAK2 ginds to the GHR dimer and autophosphorylates on tyrosine residues (Y) (JAK2 is a tyrosine kinase)

JAK2 ph's the GHRs.

STATS (signal transducers and activators of transcription) bind to phos'd GHRs and are phos'd.

Two phos'd STATs are released into the cytoplasm and form a dimer and the dimer acts as a transcription factor to control gene expression.

(nothing to do with G protiens, no cAMP, etc)

GH circulates bound to a binding protein: extracellular doman of GH-R (aka GHR here )....
some GHR in liverare hydrolyzed and the extracellular domain leaves the plams membrane and is them GH-BP.

exam covers to here.....

Adrenal gland (medulla releases epi (E) and norepi (NE), is in the middle)(cortex surrounds medulla and releases steroid hormones)

E/NE are high in fight or flight sits. Cause heart rate to skyrocket during near miss.
they are secreted in response to stress.
but have other functions such as: regulate intermediary metabolism, regulate cardiovascular system, regulate secretion of other hormones.

april 23 how bones grow....

Bone: osteoid (collagen and other proteins)
hydorxyabpatite crystals (CaPo4) are the crystals found in bones.

If you look at the cross section of bone, the head part is the epiphysis, and the narrow part is the diaphysis, and is divided by the epiphsyeal plate (made of cartilage: collage and polysacarides) In order for bone to grow the cartilage plate expands (gets thicker), and as the epiphaseal plate thickens it pushes the head of the bone moves upward, and the cartilage is replaced by bone. Without the epiphyseal plate, you can't get growth. This method means that the bones get longer at each end. (only in long bones that have the epiphyseal plate) (bones in the skull grow by a different method)

The last thing you need to know before we look at this in more detail...
the cell types for making bone: osteoblasts, secrete osteoid (proteins) and enzymes that increase precipitation of calcium. (ca2+ to form CaPO4)
and osteoclasts are involved in bone RESORBTION, they produce enzymes that dissolve bone, and clean things up. They are developmentally related to macrophages.
and for making cartilage: chondrocytes, secrete collage, and polysacarides together make cart.


At the bottom part of the bone figr 18.16, you need to add a layer of cells that is below and immediately below the epiphysis is the progenitor chondocytes. They give rise to chondrocytes. They are stimulated to differentiate into chondrocytes. then the chondrocytes proliferate and each gets bigger and bigger. The secrete and lay down cartilage in this process. They organize themselves into columns in the direction of bone growth. Once th echondrocytes are mature, they stop growing and are totally surrounded by the cartilage, and the nutriants can't get through, and their lifespan is limited. When the chondrocytes start to die the osteoclasts clean up the mess and are replaced with osteoBLASTS, and they lay down new bone. This process continues in cycle so that the end of the bone gets pushed farther and farther from the diaphysis. each cartilage plate can only make a certain amount of chondrocytes, and then the cartilage is all replaced with bone, and growth stops. (regulated by sex hormones)

Calcified cartilage is not yet bone, but it is a step in the bone growth, by it being digested by the osteoCLASTS, and osteoBLASTS come in to replace with bone. The chondrocytes have to be in columns or growth won't happen efficiently.

GROWTH HORMONE:
stimulates differentiation of progenitor chondrocytes to chondrocytes and it stimulates proliferation of chondrocytes, and it stimulates proliferation and activity of osteoBLASTS. You need both hormones to do the job: IGF-1 raise ccchondrocyte prliferation and hypertrophy (enlarge). also raises osteoblast proliferation and activity. if a person is missing either, then long bone growth doesn't occur at the normal rate (greatly diminished). (IGF-1 is locally produced dependant upon the presence of GH (growth hormone) (IGF-1: insuline growth factor 1)

hypothyroid means that you don't grow, it has a direct effect on bone.
T3, T4: increase GH secretion, and increase GH receptor expression, and raise proliferation and hypertrophy of chondrocytes, and they are needed for normal columnar organization of the chondrocytes in the epiphyseal cartilage.

At puberty the epiphyseal plate disappears and growth stops. they think it is due the the activation of estrogens. (the epiphyseal plate 'closes', which means the cartilage is gone) It is due to loss of stem cells that give rise to chondrocytes and this process is accelerated by estrogens.

andgrogens in men are converted to estrogens and does the same thing. without andgrogen/estrogen receptors then the people get very tall, because it takes so much longer for the process to happen.

GH, IGF1, also increase bone width by increasing the activity of osteoblasts which lay down new bone around the sides of the bone... this continues after puberty.

The effects on intermediary metabolism. We won't worry about IGFs in terms of the intermediary tmetabolism. (sometimes they are opposite too, so it is super confusing, just forget about it in regards to int. met) just pay attention to the GH.

GH effect lipid/carbohydrate metabolism, protein metabolism, and ______ metabolism.
protein metabolism is relate to growth, by increasing protein synth, and aa uptake into cells, increases RNA synth, which goes along with protein synth, and inhibits proteolysis.

Lipid and carb metabolism:

the nervous sysstem requires glucose as its major energy. without it you go into coma. there are mechanisms to make sure it never gets to low. the approach that GH uses to control is that it regulates pathways and intermediate met that conserves protein. it provides through these actions, it provides fuels that can be used in place of AAs, because you don't want to process the AAs to make glucose. the major ways the body does that is by breaking down muscles through gluconeogenesis to get AAs into glucose. But that is bad. GH prevents this process. instead it raises blood glucose levels in a way that protects the proteins in the bodies, so that they don't get turned into glucose.

if we look first at lipid metabolism...GH increases lipolysis (triglycerides are hydrolized into fatty acids + glycerol). the fatty acids can be used as an energy source by cells outside the central nervous system. This spares glucose for use by the CNS. (muscles use the fatty acids INSTEAD of the glucose, so the glucose is left free for the CNS to use instead.
glycerol is a substrate for gluconeogenesis (glycerol can be turned into glucose).

carb metabolism: effects on carb met... GH decreases glucose uptake by cells outside of the CNS. decreases glucose use (decreases glycolosis). stimulates gluconeogenesis. all of these act to increase the glucose concentration of the blood. People discribe these effects as 'increasing insulin resistance'. you need to know that insulin does the exact opposite of what GH does. It acts to get glucose out of blood. (gluconeogenesis: making glucose from some other substrate... glycolosis backwards) If you have too much GH it takes FAR MORE insulin to lower the blood glucose levels.

You want proteins to be protected. They are inhibit from use of glucose with GH.

REGULATION OF GH SECRETION:
Look at handout for basic feedback loop.

There are to hypothalic:
GHRH and somatostatin.

GHRH is 44AAs.
somatostatin is 14 AAs.
They are both fairly short peptides. they are probably going to be secretes as proforms. they are made by different population in the hypoth. in terms of their mech of action. both types of recptors are serpatine. serpantine recpetors are coupled to G-proteins.
GHRH is coupled to G(sub S) (increases cAMP)
sinatistatin is coupled to G(sub i) (decrease cAMP)

as cAMP increase leads to calcium entry into the cell, and the increase in calcium(Ca2+) stimulates exocytosis of GH.

GHRH vs somatostatin are reciprocal. if you totally disconnect the pit from the hypth, the net action is a decrease in GH secretion.

GHRH is the more dominant/controlling factor. they both together regulate GH secretion.

At the bottom of the handout are other factors that regulate GH secretion.

Similar control as thyroid hormones.

Conditions that stim secretion:
hypglycemia, fasting; raise plasma AA (esp arg); prolonged caloric deprivation; acute stress; exercise; puberty (estrogens/androgens); sleep.

Conditions that inhibit secretion: hyperglycemia (raises fatty acids); hypthyroidism; aging; emotional deprivation; raised cortisol for extended period of time.

fri, april 20th.

hypothyroidism continued...
causes:
1) failure of thyroid gland to make enough TH (primary hypothyroidism is most common)
... in developing countries with insuff iodine hypothyroidism due to deficiency of iodine in diet. When iodine deficient you can't produce T3/T4, so loss of feedback control of TRH/TSH (negative feedback loop missing) which leads to increased secretion of TSH and thyroid gland grow to compensate. ( only sometimes do they get goiter). the thyroid gland is unusual in that it is able to store colloid. the thyroid evolved to protect individual where iodine is not plentiful. colloid can supply TH for weeks.
tx for hypothyroidism: thyroxine pills (T4) which is metabolically converted to T3, and over time would reinstate negative feedback look, and decrease TSH/TRH and hopefully decrease goiter. T4 suppliment because it has a much longer half life so it is easier to maintain steady levels with.

2) thyroid follicles are destroyed (hashimoto's disease): autoimmune disease .
the body can compensate by growing non-attacked follicles but eventually lose function.
tx-thyroxine

3)inactivation of TSH receptors: due to muation of inactiavtion by antibodies blocking receptors.
the thyroid gland is not stimulated to produce T3 and T4. Have raised TRH/TSH secretion but no enlargement of thyroid b/c receptor is inactive. only have growth of goiter if TSH receptoer is overactive.

second degree hypothyroidism: problem at hypothalamic or pit level.
1. decreased TRH secretion
and
2. decreased TSH secretion
both are much less common, and are potentially caused by tumor growth.

can determine if pit/hypoth cause of illness by measuring TSH levels in the blood if TSH is low that it is pit/hypoth NOT thyroid. if give bolus of TRH and TSH levels rise, then the issue is hypoth. if give bolus TRH and TSH still low, the its the pit.

Thyroid hormone resistant: TH made but cells not recognizing it.
1. loss of function mutation in 1+ TH receptor
2. loss of function mut in transporter in plasma membrane of target cells.

loss of TBG would decrease half life of TH but body would generally be able to compensate.

HYPERTHYROIDISM: cells recieve too much TH stimulation.
signs/symptoms: weight loss, feel warm/hot, jittery/nervous/anxious, rapid heart rate, muscle wasting (proteolysis>protein synth), weakness, fatigue, reproductive tract issue.
Causes:
1. tumors that secrete TRH/TSH/T3/T4 (rare)
2. injest thyroid supplements (sometimes for weight loss) (ie: exsessive use of kelp tabs)
3. Graves Disease (most common and tends in families): autoimmune disease that produces antibodies that activate the TSH receptors (antibodies = thyroid stimulating immunoglobulins TSIs)
TSH-R on thyr activated by TSI -> produces too much T3/T4 -> signals pit to reduce TSH production even though it is low already (negative feedback loop fails to mediate levels).

tx: initially attempt to limit T3/T4 production with TX, and next step is radioactive iodine which incorporates to colloid and helps to destroy thyroid gland. This makes the person hypothyroid, but that can be treated with TX (thyroxine).

The antibodies additionally tend to attack muscle tissue surrounding the eyes.

TSI can also cause goiter... goiter is not only a symptom of hypo/hyper thyroidism.

GROWTH HORMONE (see handout) (GH)
It is important because:
1) absolutely nec for post natal growth... no GH = stunted growth.
2) regulates a lot of metabolic pathways.
GHRH (Growth Hormone Releasing Hormone) + somatostatin control GH secretion.
... both act on ant.pit. (GHRH is positive control, and somatostatin is negative control) stimulates somatotropes.
1. GH binds target tissue and causes reaction
2. GH acts on liver and other tissues and stimulates them to produce insulin like growth factor (IGF1) .... binds target tissue and cause reaction.
>> both 1 and 2 needed together to cause the effect.
Approach: structure GH, structure IGF, biological activities, reglulation/secretion, GH-R, pathologies.

GROWTH HORMONE STRUCTURE:
-non-glycosylated single chain polypeptide (191aa, mw=20K)
-structurally, aa seq conserved between GH and prolactin
... structural similarity extends to placental lactogens as well.
......similarity makes them 'family':: GH-PRL-PL gene hormone family
-important of structural similarity>> get lot of info about mech action within family because mech of action is much the same.
-GH=large protein>> unusual binding (~40%) circulates bound to GH-BP in the blood
Half-life is roughly 30 mins (but that half life is increase by being bound to the BP.

INSULIN-LIKE-GROWTH-FACTORS
two types: IGF1 and IGF2 (or IGF-II)
IGF1 - major part of regulation via GH but not all regulation is by GH
IGF2 - regulated by lots of things
.. single chain polypeptide (mw ~ 7K)
.. circulates bound to BP
.. structure is similar to insulin
.. most IGF1 in the blood is from liver under regulation by GH, but IGF1 is also made in numerous other target tissues, and those are also regulated by GH, and others.
When IGF-1 is made in other tissues, they mostly act in situ, although some will get into circulation (autocrine/paracrine effect)

Somatomedan hypothesis: GH effects to raise IGF1 production by the liver and IGF1 then mediates ALL actions of GH (but that is a far oversimplification)

Current view (much more like handout): GH effects to raise IGF1 secretion in many tissues, and some systems require both GH & IGF1 to get biological effect.

In other systems GH can act independantly of IGF1 but you must look at specific systems to see that.

wed, april 18th: disorders of the thyroid hormone

Last time we talked about the mech of TRH action and TSH action, and both of those receptors are 7trans-serpantines that are coupled to G proteins. those 2 hormones when they bind to the specific receptor, they stim the associated the G protein and actiavte one of the pathways that we talked about. TSH activates primarily the Gf pathway and which stims the adenyl cyclase activity and results in an increase is cAMP in the cell, which increases the activity of protein kinase A which leads to various cellular effects like changes in the enzymes activity or rates of gene transcription. TRH receptor is coupled to Gq that acts when TRH bind it activates activity of phosopholypase C-beta which results in an increase in phosphoinostitol concentration which triggers an increase of Ca2+ in the cells which inc. diacyl glycerol levels and protien kinase C activigty which regulates enzymes and gene expression.

the last thing we talked about what the mech of acgtion of thyroid hormone, and a genomic mech of action. that refers to a thyroid hormone receptor acts as a transcription factor. it regulates the rate of gene expression, and that whole idea that a hormone enters a cell and interacts with a receptor that is bound to the regulatory region of the gene, and that complex regulates transcription is considered a 'genomic mech of action' for thyroid hormones and steroid hormones. this term you should know.

Some people refer to this as a classical mech of action, for steroids and thyroid hormones. (see handout) the hormone plus receptor acts as a transcription factor that regulates gene expression. This mech of action where there is an intracellular receptor that acts as a transcription factor. this is the first one discovered for steroid hormones and later realized that it was shared with thyroid hormones.

The other way that steroids and thyroid hormones work is taht they bind to specific receptors in the plasma membrane, and to distinguish that from the genomic reaction is called 'non-genomic' or non-classical mech of action. in this case the thyroid hormone receptors are in the plasma membrane, and are physically diffderent proteins as you would find associated with dna. there is some proof that the thyroid hormone receptors are G protein linked
. the major differnt in concept: if you ahve a hormone, it has to get into the cell, find the receptors, ditch corepressors, get coactivators, etc.... is slow like 30-60mins to see your transcription of a protein, acting as a transcription factor for antoher gene to be transcribed and thaty might be the end....

the non-genomic mech of action you see in just a few minutes.... all that has to happen is that they thyroid hormone binds to the receptor you activate the G protein, and then signal transduction cascade and that's it!

That last thing we are going to talk about then in terms of hormone mech of action are conceptual things....
Terms:

Agonist: a compound that binds to a hormone's receptor, and evokes a response that is both qualitatively, and quantitavely similar to that evoked by the actual hormone.

There are many drugs that act in this way (agonist) like a synthetic estrogen, that bind in the same way as the natural versoin.

Antagonist: the opposite. A compound that binds to the receptor, but then blocks activity, such as compettive inhibitors, or allosteric inhibitors. (allosteric: drug binds somewhere other than the binding site, that forces a change in the binding site)

Also partial-agonist or partial antagonist, bind, but the response is smaller, and so they block the full response from happening.

last page of handout?
the basic interaction is: hormone binds to the receptr yeilds the hormone receptor complex. it is reversable and noncovalent. we can define:
Kd: dissociation constant
Ka: association constant (they are inverses of each other)
You can see how the math works, it is the same as the equilibrium constant from biochem.

either of these terms refer to the affinity of the hormone receptor interaction. affinity is defined as a tendancy of the hormones to bind to the receptor. if you have a hormone receptor interaction has a high affinity then it moves to the right. High affinity: Kd is small, and Ka is large. for hormone receptor interactions to seem physiologically meaningful, it needs to be on the 10-8 molar or smaller. The equilibrium is strongly in favor of the formation of the hormone receptor complex. if it is lower than 10-8 (kd is greater, or ka is smaller) then you can demostrate mathmatcially that what is formed is so low that the physiological response is negligable.

The reason is that the lower the affinity, the greater the concetraion of hormone has to be in order to get a response.

In terms of what is important to the cell, you can demostrate that the biological response is directly proportional to the concentration of hormone receptor complex that forms in the cell. if you have lots of hormone bound to the cell , than if you have less bound. as you increase hormone concentration, you will drive the reaction towards making hormone receptor complex. If the amount increases to its max level, it tops out. the concentraiont of the hormone recptor complex is important in terms of the magnitude of the physiological response you will get. you can change the concentration by changing anything on the right hand side of the equation. if I increase hormone concentration that will drive it toward making more HRC. if I increase hormone receptors concentration, it increases the hromone receptor complex. if I increase the affinity of the hormone receptor interaction, that will also increase the HRC. But there is a plateau that can be reached.

When the affinity of a reaction become too low, as that number (Ka) gets smaller you will get less and less product. What people have determined experimentally, when the Ka gets below 10-8 molar yo udon't get product any more. so people need to know what the receptor is for a hormone, and if it is too small they will not consider it a relavent receptor. If H and R are both huge you can drive the way to the left, which allowws you to compensate for a low Ka. There is also a saturation effect, where they are finite, and one of the criteria is high affinity for the hormone, and that there are only a finite number of receptors. if you it were linear, then yo uwould know you aren't binding it to the cell.

These things change by altering, affinity of receptor, receptor concentration , and hormone concentration. lets look at changes in hormone concentration: peptide or protein, or hormones or amines. you can chagne the concentration of hormone in circulation by either increase/decrease the rate of synth, inc/dec rate of release of hormone, or inc/dec the clearance rate. if you change any of these things, then the concentration of hormone in circulation changes. in the case of the steroid hormones, all of these things apply except there is no regulation of hormone release because steroid hormones are not stored in the cell. If you are looking at the effects of TRH on thyrotropes (in an episodic or fulsityle manner by hyperthylamic neurons), so whenever a load of it hits the ant.pit there is an increase in hormone TRH concentration in the vicinity of the target thyrotrope which drives the reaction to making more TRH and TRH recptor complex, which triggers phosphyanostitol pathway inside the cells which leads to an increase in TSH secretion. the number of receptors in a cell can change, and this can be either up or down regulated. up regulated or down regulated is the language for the receptor concentration in cells. it can happen over the space of days... so one hormone can trigger the synthesis of receptors for another hormone... such as when we spoke of: thyroid hormones stim's that synth of beta-adanergic receptors in the heart. thyroid hormones up-regulate the concentration of beta-adanergic receptors in the heart. some hormones if present in constant amounts for hours or days down regulate expression of your own receptor, after a while the receptor concentration goes down because it was bombared by that hormone for a long time. This kind of thing keeps the cells from burning out. it is self-limiting mechanism if it is bombarded for too long. you can alos have short term changes in receptor concentration where a hormone will bind to a receptor and the complex will be internalized by endocytosis as to end the response. each target cell is different and each hormone is dfiferent.

The last part of this is changing Ka. It works at a few different levels. one of the things that can happen is if you have individuals whose receptor has ungone mutation, if that mutation is in the wrong part of the receptor, it can change the affinity of the hormone receptor interaction. Or a mutation in the hormone, can change the affinty too. other times you might see a change in the affinity of the receptor for the hormone, would be the hormone binds to the receptor which triggers the signal transduction pathway, and one of the enzymes triggered in the STP comes back and phos's the receptor. a phos'd receptor might have a lower affinity than a non-phos'd receptor. the final thing is that receptor affinity differs for different members of a class of hormones. an example of this is: there are 3 naturally occuring estrogens: 17beta- estrodiol, estrone, estriol. estrodiol is the most potent of the three. it is considered the most potent because the estrogen receptor has a much higher affinity than with the other two. If you compare estrone affinity is about 12 fold lower than for estrodiol. It is even lower for estriol. so here you have one class of estrogen receptors but the affinity of the receptor is different for which one you are talking about. ka*estrogen*est-rec -> ERC.... the bigger the Ka, the greater the ERC produced.

about Kd..... this is harder to conceptualize than Ka. unfortunately most literature is listed as Kd, not Ka. the reason for that is that Kd is the hormone concentration at which 50% of the receptors are filled/occupied by hormone. that is a convenient thing to know. (look at handout)

Usually if you are trying to charaterize a receptor, you have done all the work, then the receptors are considers physiologically relavent if the receptor Kd is of within the same order of magnitude as the concentration of the hormone. If the receptor is it 10^-9 and the hormone is at 10^-12 then by comparison the hormone is irrelivant, and conversely if the concentration of the hormone is higher by more than one magnitude, will always drive the reaction to fill the receptors, and there would be no regulation.

Thyroid hormones in disease....
In terms of definitions:
euthyroid: normal conditions.
hypothyroid: thyroid hormone concentration is too low
hyperthyroid: TH is too high
Goiter: enlarged thyroid gland regardless of cause

Let's start with hypothyroidism in adults: signs and symptoms: they will always be cold, and cold intolerant, typically they suffer from fatigue (hyper can too), because thyroid hormone has an effect on the central nervous system that are hypo, they think slow, their heartrate is lower than normal, there is an increase in polar molecules in extracellular space which attacts water, which leads to adema (general puffiness)(generalized myxadema: associated with a thyroid problem), they also have dry skin, and sometimes fowzy hair (dry, over-permed looking), and because they are cold, it drives up periferal vazoconstriction which drives up total periferal resistance, but the decrease in heartrate combats that, so mean arterial pressure doesn' t change that much. Also in hypothyroidism there is an increase in plasma cholesterol, because of a decrease in cholesterol clearance. this leads to cardiovascular disease.

hypothyroidism in children there is a profound mental, and growth retardation. this condition in children is called Cretinism. in utero they get it from the mom, but once born they would need to be supplimented with it. even if it did cross through breastmilk, it doesn't compensate enough for the low production. there is a lack of mylination, and dendrick sign formation, and decrease in formation of synapses, and neuronal migration patterns early in development. They test in the US to see if people can digest phenylalanine, and to make sure they are producing enough thyroid hormone. some people even believe that if you aren't making enough in the womb, that it isn't enough to suppliment after birth.

(endemic cretonism) refers to a lack of iodine in the diet, but creton means hypothyroid.

thyroid lowers production of polar molecules in extracellular space.

pictures: a goiter, someone who's thyroid is massively enlarged (can be either hyper/hypo)

mon, april 16th mech of hormone action G-protein cont.

G proteins continued....
back to the diagram from friday....
the receptors that use G proteins as part of the signal transduction mechanism is a serpantine (7 trans membrane domain receptor) a single polypeptide chain that weaves in and out 7 times. each receptor has a particular G protein associated with it. it is called a G protein because it binds GTP or GDP. It is a trimer with alpha, beta, gamma, and bound to GDP and is inactive,once the hormone binds to the extracellular change of the receptor, and that causes a conformation change in the signal transduction domain which triggers a conformational change in the G protein and so GDP dissociates and GTP binds to the alpha subunit splits from the beta and gamma subunits.... now alpha subunit with GTP (active and can regulate other enzyme activity or ion channels open/close in membrane) and the beta/gamma dimer (active too), and also regulate both as well, now. the specific actions of the different active subunits depends upon which G protein you are looking at. the rest of the process is summarized in the handout. (fig) In that case they show what the active alpha subunit can do or beta/gamma subunit can do on a different target protein. the active alpha subunit interacts with some target protein in the plasma membrane, such as an enzyme. the interaction between the active alpha subunit and the target protein either increases or decreases the activity of that target protein. this target protein remains activited or inactivated for as long as the alpha subunit GTP complex is bound to it. the process ends because the alpha subunit has inherent GTPase activity. when the alpha subunit binds to the target protein, it activates the GTPase actity, and it is hydrolyzed into GDP, and eventually recombines with the beta/gamma subunit and reforms the inactive trimer.

this is the general scheme. inthe specificity, there are different alpha, beta, and gamma subunits and there are differet target enzymes. specific serpantine hormone receptors are associated with specific G proteins. these G proteins function as part of the signal transduction pathway in lots of other systems besides just hormone systems. the only ones we are going to talk about are the ones that relate to what we are talking about in this class. there are at least 20 alpha subunits, and we are only interested in 3 of those: alpha f, alpha i, and alpha q. there are not as many different types of beta/gamma dimers. any G protein that contains an f, is GsubF, GsubQ, and GsubI, etc. thyroid stimulating hormone, and TRH both work through this mechanism. The alpha subunit of each G protein determines which signal transduction cascade in the cell is activated or inhibited. Gf and Gi regulate cAMP in the cells, and Gq regulates the phosphoanoxitide pathway.

We will start with Gf and Gi, both of these regulate Gf increases activity of adenylylcyclase activity and Gi decreases adenylylcylase activty. that basic pathway that couples adenylelcyclase to some end product or end event in the cell is summarized in the handout. Adenylylcyclase is a membrane associated enzyme that converts ATP into cAMP. cAMP is a very small molecule, that is considered a second messanger because it can freely difuse inthe cytoplasm of the cell. it is not associated with the membrane necessarily. the role it plays in the cell is that it activates the enzyme protein kinase A. (kinase is an enzyme that attaches phosphates to a protein, such as other enzymes that regulates them) Protein kinase A is a major regulator of other enzymes. those other enzymes regulated then can regulate many other processes in the cell, or a change in the expression of certain genes in the cell, or kinetics of ion opening/closing can change. the last part of the scheme shows that the enzyme phosphodiesterase, then is responsible for breaking down cAMP, and 5prime AMP and then is inactive. the beauty of this scheme is that this allows one hormone molecule to bind with one receptor and activate this pathway and the end result that a billion different reactions of one kind will happen in the cell. 'signal applification'. it is shown on the handout. if you use as an example: epi and it binds to liver cells and triggers breakdown of glycogen into glucose. if one epi binds to its receptor... Gf protein activated.... adenylelcyclase activated... protein kinase A is activated.... number of enzymes activated... and glycogen is broken down into glucose.... 10^8 glucose molecules created from a single epi molecule. in addition to applification this pathway is super rapid. less than a minute to get this going, because all that has to happen is enzymes being phos'd. But if you look at this handout to see that it can also control gene expression, you notice that that times a couple of hours by comparison.

I expect you to know the level of detail that is on the handout.

Gf activate the pathway, and Gi inhibit this pathway. the other pathway that is an issue, is the Gq pathway. if Gq is activated that results in an increase in phospholypase C-beta activity. And the activity what the function of phospholypase C-beta is an enzyme that is associated with the membrane, and within the membrane are phospholipids such as phosphoanostitol,4,5-disphosphate (on the left of handout), the part of this molecule that is slightly more interesting is the anostitol/phosphate group shown in blue and pink. phospholypase C-beta cleaves the phosphoanostitol group away from the rest of the phospholipid, and you are left with is diacylglycerol that is membrane bound (DAG) and inostitol 1,4,5 triphosphate (IP3) which is a mobile molecule in the cytoplasm, like cAMP, so it is considered a second messanger molecule too. DAG and IP3 are both regulatory molecules in the cell. the pathways: either activated either alphaQ subunit or the beta/gamma dimer, activates phospholypase C-beta and phosphoanostitol, 4,5-disphosphate is cleaved to DAG and IP3. IP3 is important because it interacts with the ER which stores calcium. so when it binds to sites on the ER, the calcium channels open and calcium difuses out of the ER into the cytosol. now it can do 2 things: bind to calmodulin which can then regulate other stuff, or calcium and diacylglycerol together convert inactive protein kinase C to active which is like protein kinase A that it regulates in that it regulates the activity of lots of differnt enzymes, and it can regulate gene expression by what it controls. The idea here is that one molecule binds to a recpetor and is bound to a G protein, it is possible for tons of stuff to happen in the cell due to a sincle molecule bound. But it is limited by which enzymes are present. IE: epi binding to a receptor that is linked to Gf in the liver, and the end result is glycogen breakdown to glucose, but if epi binds to pasemaker cells in the heart, and a G protein linked receptor in the heart an end result of destabilization of the heartrate. What effect a hormone has on a cell, depends upon what the signal transduction cascade is linked to. You don't need to know the structures of IP3 or PIP2, but you do need to know all of the pathways as words.

Now where the situation gets confusing, this information is presented so that hormones look like they activate specific things, and not others, but that isnt' true... there are tons of crossovers to other pathways. There are always other minor pathways that are also activated. When there is an increase in the cytosolic calcium concentration that can interact with other signal transduction pathways, so there can be crossover. if you do work in this area it can become a nightmare. so just leave youselves open that there is a lot of complexity.

G-protein linked pathways, or any signal transduction systems... what causes the signaling to end? probably the most obvious thing to happen is tht the hormoenn dissociates from its recptor. when that happens the stimulus is gone. Each hormone will bind to a given receptor only once or twice. a second way a pathway ends is if GTP is hydrolyzed. third is cAMP is hydrolyzed. fourth: calcium is removed from the cytosol. fifth: receptor is phos'd. both of these pathways activated protein kinase A or C. and either of those can turn around and phosphorylate the receptor. which reduces the affinity for the hormone, and reduces affinity for G-protein too. it also can activiate a pathway for endocytosis of the receptor. The most important thing depends upon the target cell. when people talk about receptors being down regulated, is reducing receptors on teh surface in some way.

In a real context TSH works by activating Gs, TRH binds to a receptor that is associated with Gp. The last thing to talk about with respect to hormone mech of action and the hypothylamic /thyroid/pit axis is the mech of action of T3 and T4.

Overview: T3 and T4 have 2 entirely different ways they work on cells. T3 and T4 acting inside a cell as transcriptional activators. Then we will talk about them binding to membrane bound receptors. To talk about their mech of action within the nucleus we have to do an overview of an entire family of hormone receptors. We will talk about the nucleur receptor super-family.

the nucleur rec superfamily. they all work as transcription factors. the hormone binds to a receptor inside the cell and the hormone receptor complex then acts as transcription factor that regulates gene expression. this nucleur super family contains close to 100 members. alot of the AAs seq of them are mRNA and don't know what the ligand is yet. for us whats important that they include receptors for thyroid hormone and for steriod hormones. there are distinct receptors for each steroid class and for thyroid and for vitD3 (also made from chol.). you have a thyroid hormone receptor, estrogen receptors, progestin receptors, glucocorticoid receptors, androgen receptors, and mineralcortacoid receptors. All these different receptors that belong to a super family of receptors because they have a basic similar structure. There is a DNA binding domain, and farther along is a hormone binding domain. the hormone binding domain are relatively specific for the class of hormones. there is an overlap between glucocort. receptors and mineralcort receptors though. the dna binding domain is the area of the molecule that interacts with DNA and so this particular part of the molecule binds to a nucleotide sequence in the dna called the hormone response element. The hormone response element is sometimes called an HRE, and if you want to talk about the specific horomone resposnse element for thyroid is TRE. the actual nucleotide seq to which the different steroids and thyroid hormone bind is very worked out. but you do not need to mem the nucleotide seq at all.

the way these work is that the dna binding domain binds to the hormone response element on the dna and that unit acts in the regulatory region of the gene. when that binding occurs the receptor acts as a transcription factor to regulate transcription of the gene. Now the thyroid method of action is slightly different in the binding details. and thyroids differ as well. See overhead.
General structure for thyroid/steriod hormone receptors... there are different subtypes of thyroid horm receptors and steroid hormone receptors... there are alpha1 THR, beta 1, and beta2 THR, all of these subtypes bind thyroid hormones but some are preferentiall expressed in only certain tissue. beta2 is usually only in the hypthal, and ant.pit. knowing this you can guess that they are the ones thought to mediate the feedback between TRH/TSH/T3/T4. someting in the liver won't be a part of that process.

Another figure from reading nubmer 2 on eres.

thyroid hormone receptors: thyroid hormone stims thermogenin or uncoupling protein in the mitochhondria, which uncouples the proton gradient from ATP. you give more thyroid turns on more thermogenin production (positive feedback). also T3 and T4 inhibit TRH, and TSH, so you add thyroid hormone and it inhibits gene expression, which is harder to visualise the negative feedback. (thermogenin is a protein that functions in the proton chanel in the inner mit. membrane and when it is open it fights the protons that are diffusing down their concentraion gradient and can bypass atpsynthase.)

Lets look at the mechanism that people think thyroid horomones turn on gene expression (see handout, 2 figures) Activating gene expression by thyroid hormones. in the resting state if there is no thyroid hormone present the THR is bound to the TH response element in the DNA, usually as a dimer of 2 THR, or a heterodimer of 1 THR and an RXR receptor (receptor for sys-retinormic acid) these 2 receptors bind the the HResp element and sit there. bound to them are molecules called c0-repressors. they respress transcription. and the corepressors inhibit transcription because they contain dipsome-deacetylase activity. they remove acetyl groups from histones that make up the chromatin. this leads to the hiding/blocking of the transcription start site. if you remember from bio20a: dna doesn't exisst in the nucleus as a naked dna. it is in histones, and is wrapped. the more tight it is the more difficult it is for the transcription factors to find the start sites and to begin. so tighter means less likely a gene will be transcribed. where the thyroid aren't bound to the receptor, the receptor is associated with the corepressors to keep the histones tightly bound around the gene of interest. when T3 is present it binds to the binding site on the thyroid hormone receptor to change the conformation of it and then the corepressor can no longer bind. and so the coactivators are now recruited and are able to bind (they act in the opposite fashion), they contain acetylase activity histones are acetylated and the chromatin opens up so now the transcription machinery can get to the dna and transcription starts. some of the coactivators (nucleur proteins) some of them are histone acetylase enzymes. they add acetyl groups to the histone proteins and the charge on the groups causes the histones to not be so tightly packed and allows easier access for the basal transcription machinery and rna polymerase to come in.

What the thyroid hormone does in this scenario where it increases gene expression is that when it binds to its receptor that causes a conformational change in the receptor, and it releases the corepressor, and leaves, and a coactivator binds instead, acts as a histone acetylase. As soon as you start transcription you can start getting mRHA from this gene.

This is the scheme that people accept as pretty much true. the difficulty comes when you have to use it to explain how T3 and T4 inhibit something. and the option is to turn it on. But in the case of TRH synthesis it is on and we need to turn it off. this is a very active area of research and they don't have it fully figured out. the current model is kind of the opposite of the above... for example, in a neuron producing TRH, where there is not much T3 present, you would have the thyroid hormone recptor diamer bound to DNA, and in that unbound, prerecpetor form, it is able to recruit activators. T3 shows up and binds and the coactivators are replaced with corepressors. the evidence is good that the receptor in both cases there has to be a receptor DNA interaction to get either effect.

fri, april 13th mech of hormone action G-protein

Look at regulation and thyroid hormone secretion. we talked about the pathway down that TRH acts on anterior pituitary with stimulates TSH secretion which acts on the thyroid gland which stimulates thyroid hormone secretion. YOu will notice at the top of the diagram is the word cold. one of the environmental influences that can increase thyroid hormone secretion in most mammals, is being cold, and the mech works in young humans. there is debate about how important it is in adults. as an adult if you went out in the cold it wouldn't ratchet up your thyroid instantly. but in eskimos that are long time cold, it is raised. increase in thyroid hormones increases metabolic rate, which makes more heat.

the last part of the diagram is the negative feedback loop between thyroid hormone concentration, and TRH and TSH production. imagine that for some reason TRH production increases a slight amount, that will stimulate the anterior pit to produce more TSH. the increase in TSH stims the thyroid gland to make T3 and T4. they circulate in the blood, and some will end up in the cells that make TRH, and some in the cells that make TSH. T3 and T4 will inhibit the production of TSH and TRH, when that happens the thyroid production goes down. the thyroid gland isn't stim'd as much. then thyroid concentration in the blood falls, and less and less thyroid hormone is acting on the hypothal and the pit gland so now TRH and TSH are not as inhibited, so they go up, and T3 T4 go up again, which again inhibit thyroid hormone. the levels oscillate around a set point. Classic endocrin feedback loop. T3 and T4 act on the cells that produce TRH and TSH: if we increase T3/T4 synthesis by the thyroid, that will increase the concentration of T3/T4 in the blood and they will act on the cells that secrete TRH and TSH which will decrease synthesis of TRH and TSH. The major effect is on thyrotropes (cells in the ant.pit.gland) what T3 and T4 do to thyrotropes is they increase expression of the TRH receptor. The net result of that is there is a decrease in the sensativity of the pit gland to TRH. In addition to that T3/T4 also directly inhibit TSH synthesis. T3 and T4 effect the expression of the TRH receptor? by reducing its synthesis or its expression into the plasma membrane of thyrotropes. ABSOLUTELY NOT COMPETITIVE INHIBITORS OF THE TRH OR THE RECEPTOR. Instead they prevent the receptor from being present. There is some kind of regulatory mechanism keeping them from being expressed.

You must really know what a negative feedback loop is.

If T3 are T4 are high, they inhibit production of TRH to TSH. It just turns it down gradually. It is not an extreme reaction. Since TRH/TSH go down, then T3 and T4 drop, and TRH/TSH are less inhibit, and rise slightly.

You need to know all of the cell types in the ant.pit gland and what they produce.

Play games like what if I remove something, what will help to every other things in the system? What will happen to TSH, or TRH, etc. If you give someone thyroid medication what will happen to each thing?

Negative feedback is a direct effect.

T3 and T4 circulate in the blood and they are not highly water sol molecules. They are bound to something like thyroid binding globulin or another large protein such as albumin. less than 1% is actually free T3, or T4. The rest is binding protein bound. It is generally expected that the bioactive hormone is the PREhormone. The dogma is that if it is with the binding protein that it is too large to pass a capillary wall to act on target cells, so it isn't bioactive unless it is the preform.

How does binding protein influence the negative feedback loop? If you have the ant.pit that produces TSH which acts on the thyroid which produces T3 and T4. Now the thyroid hormone binds to Thyroid hormone binding globulin, which is a noncovalent interaction. The concentration prethyroid hormone is regulated. the concentration bound to TBG is NOT regulated. Because it is not bioactive if it is bound. therefore total thyroid hormone concentration is not regulated. So freeT3 and freeT4 concentrations are regulated. The amount of concentration of Thyroid hormone binding globulin can change over time... if it is increased, it will force the freeTH to go down by binding more of it (mass action). when that happens it decreases the negative feedback on TSH and TRH secretion, which increase secretion of TRH and TSH, which then increase secretion of T3 and T4, and the final result will be that the concentration of pre T3 and T4 will return to the set point. the concentration of total T3 and T4 is increased. This causes issues in states such as pregancy. You need to make sure you are measuring the correct thing or you will be mislead.

neg vs pos feedback.... if it maintains the status quo, this it is negative feedback loop, but if it can escalate then it is a positive feedback loop (makes a thing that triggers more of that thing being made).

What hormones do in cells, they can have a number of different types of effects in cells. Mechanism of action.... one of the ways that hormones work in cells is they can regulate the transport of substances across the plasma membrane.... by regulating the activity of transporters and by regulating the regulating of opening or closing of ion channels. Such as in the case of the thyroid hormones which stimulate expression of sodium/pot ATPase, and sod/pot leak channels into the plasma membrane. Another basic thing is they can regulate enzymatic activity by either causing phosphorylation or de-phos of enzymes. In terms of terminology: Kinase refers to an enzyme that phos's proteins. a phosphitase is the opposite, it is an enzyme that dephos's. when it is phosphorylated that means that a phosphate is attached to a specific AA residue in a protein. Phosphated or not changes the 3d structure in vacinity of the phosphate group which alters the activity of that protein, if it is an enzyme. And there is no rule as to whether phosphorylation will turn somthing on or off. When they act in this way it is a very rapid action. It happens within minutes. the third thing is that they can regulate gene expression. they can inhibit or stimulate a certain gene, and it happens on the scale of minutes. in order for hormones to do any of these things they must bind to a receptor. A cell that does not have a recpetor for a hormone then it cannot respond to a hormone. receptors are basically hormone recognition molecules within a cell, and they a protein or a glycoprotein they can be located in different parts of the cells, and when the hormone binds to the receptors something happens like an increase in some activity, or an impossition of some activity.... it is not all or none. it is dose dependant... small dose is a small effect, and large dose makes a large effect. receptors for hormones have certain characteristics.

R is receptor....
one of the most important characteristics. The R contains a binding site for the hormone and this binding site is basically a part of the receptor molecule that has a structure that is highly complimentary to part of the structure of the hormone molecule. (like a lock and key) In order for it to be physiologically relavent the receptor binding sites have to be highly specific for the hormone (which is a ligand... a small molecule that binds to a larger molecule). when you say the receptor is highly specific for a hormone, it will only bind that one, and nothing else. Estrogen is one of the class of steroid hormones. there are 3 bioactive estrogens. estrogen receptors bind very tightly with high affinity for 2 estrogens, but not at all to glucocorticoids, because it is sufficiently different that it can not bind to the binding site. There is no way it can bind. The receptor has to be highly specific, or it would suck in everything that passed by and there would be no regulation. the interaction between recpetors and hormones in noncovalent. if it were covalent you could not undo it. even though it is noncovalent, it is of high affinity. It is bound very tightly. the final characteristic of hormone receptors is that hormone binding to the recptor triggers some event in the cell. including inhibiting something. The hormone causes something to happen in the cell. When someone talks about a mechansim of hormone action what they are really asking is: what is it exactly that goes between hormone binding to the recpetor and the end result. there are many steps in between, and you must know what all the steps are if you really want to understand this process.

lets look at recptor in terms of where they are in the cell....
receptors for peptides and proteins, and for epi/norepi and dopamine, are located in the plasma membrane. and this makes sense because most peptides/protein/hormones are water soluable, but not lipid soluable. it would be difficult for these hormone to get inside the cells easily. they would have to be taken up by endocytosis mechanism. highly water soluable which are also poorly lipid sol, act on receptors that are in the plasma membranes. (T3 and T4 are amines) our other choice are receptors for steroids and thyroid hormones are inside the cells, and in some cases in cytoplasm or nucleus, and there are also receptors in the plasma membrane. steroids are lipid sol because they are made from cholesterol, thyroid hormones are reasonablely lipid sol, and thyroid hormones do have carriers probably transports that transport them into the cell, but with both of these have recpetors in a) in the cell, and b) in the membrane. in detail: lets look at the receptors for TRH and TSH.... these are TRH is a tripeptide, and TSH is like a 30K mol weight protein, and neither can easily enter into cells, and the receptors for both are in the plasma membrane....the receptor has a serpantine structure, and it makes 7 passes through the plasma membrane, amino term is extracellular and the carboxy is intercellular. because of the structure, this type of receptor is given a differnt name: serpanting recpetor or a 7 pass trans membrane receptor, or 7 trans membrane domain receptor. in this kind of receptor structure, the hormone binding site has to be extracellular when the hormone binds to the receptor that causes a conformation change that triggers a change in the intracellular domain of the receptor, and triggers the signal transduction domain which activity the signal transduction cascade, and the end result is the effect you see on the cell.
again...
The hormone binds to the hormone binding domain which is extracellular, and that triggers a conformational change in signal transduction domain which is intracellular and then this leads to activation of a signal transduction cascade that culminates in the end result.

This is an example of cAMP being triggered. This general scheme of a hormone binding to a plasma membrane receptor and triggering a signal transduction cascade is a very general phenomenon that applies to really all of the hormones that bind to plasma membrane receptors. the details are differnt, but the general concept is the same. in the case of serpantine recptors the first step in activation of signal transduction that happens is activation of a G-protein(that is why they are called G-protein coupled receptors.). G proteins (it is on the handout on eres) are a specific family that get their name because they bind GTP or GDP. That is why they are called G. They have 3 sub units, alpha, beta, gamma. alpha is the GTP/GDP binder. when the trimer is intact and GDP is bound then the G protein is 'inactive'. when the trimer disassociates into an alpha sub and bound to GTP, and then into beta/gamma dimer then G protein is active and regulates the activity of various enzymes in the cell. In addition to regulating enzyme activity it can also regulate ion channel opening/closing. What happens here is shown in the fig on the handout. the signal molecule is the hormone which binds to the serpantine receptor and each receptor associates with a G protein that is very close by to it in the plasma membrane. when a hormone binds, the receptors signal transduction domain changes conformation, which causes the conformation of the G protein to change, which causes two things to happen: GDP dissociates and is replaced by GTP, for that to happen the alpha subunit separates from the beta/gamma subunit, which creates an active alpha subunit is created, and an active beta/gamma dimer subunit, and either/both actives can get close to the plasma membrane and interact with differnt enzymes and regulate their actitivey or interact with ion channels in the membrane to make them open or close. Eventually the GTP will be hydrolyzed to GDP + P, and start the cycle over again.

wed april 11, thyroid hormones continued

overhead shows different T3 and T4, and products that are not made in the thyroid....

that is true depending upon waht you mean by made in the thyroid gland.

RT3 is never incorporated into the thyroglobulin. the only ones you will find attached to the thyroglobulin are MIT, VIT, T3 and T4. RT3 is not made as part of the coupling reaction. once the thyroglobulin is hydrolyzed, by endocytosis, and combines with lysosomes, and permiases hydrolyzed the thryoglobulin then it is possible for T4 to be acted on by an inner ring diiodinase and so within the ep cell then it is possible to make RT3. (RT3 is bio-inactive)

Let's talk about the bio activity of thyroid hormones, and then regulation and secretion of thyroid hormone action. there are 2 thyroid hormones T3, and T4, T3 the thyroid hormone receptor has a much higher affinity for T3 than T4. T3 is there for the biologically the more potent hormone. T4 is a precursor that T3 is made. But if T4 does happen to bind, it does the same action as T3, but it is FAR less likely to bind. the general list of activities of thyroid hormones is sumarized at the bottom of the handout. It should be right at the front.

thyroid activities in mammals:
one of the key biological activites of TH (thyroid hormones) they increase the metabolic activity of most cells. the exceptions here are: not neurons, testes or spleen. when bound metabolic rates increase in the cell because this happens it then increases the metabolic rate in the entire animal. what basal metobolic rate is: minimum amount of activity required to sustain life in terms of metobolic processes. As soon as you eat something your metabolic rate goes up, as with any movement too. in practical terms it is very expensive to shut someone up to find their basal rate, so instead they check their exhale breath for o2 consumption, and they can figure it that way.

if you give someone thyroid hormone it increases their metabolic rate, and it increases for a number of reasons. increase in the number of mitochondria from increased TH. and increases all the levels of enzymes in the mitochondria. also increase the nubmer of sodium/potasium atpases in the plasma membrane of those cells. plus there is an increase in sodium/pot leak channels in the plasma membrane. increased leakage of pot out, and sod in to the cell, with everything trying to maintain the status quo and works that much harder to keep the balance. which increases o2 consumption which by definition is an increase in metabolism.

plasma membrane always contain sod/pot channels that are called 'leak channels' the electo-chemical gradiant drives it. the atpase pumps the sodium back out, and the pot back in. increasing the leak channels and atpase you are just speeding it all up and running it more times for higher metabolism.

another thing TH do, is they have a large effect on intermediary metabolism. there are numerous effects and what makes it confusing is the effects are dose-dependant. what happen is that the TH effect most if not all of the major metabolic pathways. TH increase lipogenesis, and lipolosis (break down), and they increase protein synth but they can also increase proteolosis depending upon the dose. if you look at those together, usually what happens is that if the TH is too high then there is a net catabolic effect. you tear down more than you make. in the case of carb metabolism TH increase and stimulate gluconeogenesis, glycogenolysis, and depending upon the dose, stimulate glycogen synthesis. if it is too high the net effect is an increase in glucose concentration. if you look at this, TH stimulate anabolic and canabolic pathways, which means if you increase the TH you make things faster and you tear them down faster, you increase more you run it even faster. if it is really high they cycles become futile cycles, you make it and tear it down. net effect: there can be an increase in futile cycles as TH concentration increases.

one of the reasons this is important is that it leads us to the next bio activity of TH which is to increase heat production in the body. the more chemical reactions taking place, means more heat. also due to an increase in the synth called uncoupling protein (aka thermogenin?) (final step of atp synth, adp phosphorylation to atp occurs in the mitochondria. because of the activity of the e transport chain there ends up being a high concentration of protons in the intermembrane space. the proton gradiant is important in atp production because it drives the activity of the enzyme that converts adp to atp. enzyme is called atpsynthase. because of the concentration between space and the inner mit space, the protons move through the atp synthase and that allows adp to be phos'd into atp. without the protons moving through and interacting with the atpsynthase the atp doesn't' get made. what the uncoupling is basically a proton channel that is present in the inner mit membrane, and when it is open, protons don't necessarily interact with the atpsynthase. they can go thru the wide open channel, and atp isn't produced. the cell went through a great deal of trouble to make a proton gradiant, and heat, but is wasted because it bypasses the gradient and doesn't make atp. With more TH, you make more heat, but nothing to show for it. TH make the creation of more H channels. The net effect is that if you increase TH, you increase number of futile cycles, and rate, to generate heat, and thermogenin (uncoupling protein) which produces a lot more heat. TH goes up, and you feel warm. this effect is an increase in heat production due to TH is called the thermogenic effect of THs.

these two effects (increase in met rate, and heat production) are the universal effects of TH in mammals. they also regulate a number of other pathways too. TH has a very long half-life in the circulation that takes several days to get a peak response. other effects of TH we can put in the developmental catagory, and are extremely important. such as effects on the CNS. during development THs do several things within the nervous system. theyincrease milanation of axon, and increase nerve growth, and increase dendridic branching, and regulate neuronal migration.

milan sheet wraps around axons and acts as an insulator, and keeps ionic current from leaking out. the net effect is that a milanated axon conducts aaction potentials much more rapidly than an non-milinated axon. babies are not coordinated because their axons are not milinated. adults are fully milinated and so are super quick. dendritic branches, the input part of the neuron consists of the cell body and the dendrite. dendrite is where most synapses in the nervous system occure. branches occur so that you have lots of places to connect.

neuronal migration is important because they move through the nervous system, and without the movement you don't develop normally. without T3 and T4 at birth, within 2-3 weeks from birth, there is profound and permanent mental retardation happens. they check blood if babies are born in hospitals, to make sure your thyroid is good. it isn't correctable if it isn't caught right away. the mother gives some TH, but still there could be developmental issues because it isn't enough.

the other time that THs are important is that they increase alertness and wakefulness. If it is too low in adults, the net effect is lethargy, and a mental slowness. another important effect is it is needed for normal body growth. and this is true because TH increase GH secretion and TH increase GH receptor expression. GH is required for post-natal growth. in addition TH stimulate bone maturation.

also needed for normal reproductive production. and if you look at symptoms of hypo/hyper thyroid there are problems with the mentstral cycle. there is no clear explanantion. but pretty much everythign seems to be effected by the TH, and it triggers cascades that mess up reproductive function.

the TH increase the sensativeity to epi and nor-epi. it does this partly by increasing expression of ?beta-adanergic? receptors. the net results is that you get an increase in heart rate and cardiac output. This is a permissive effect. Epi and nor-epi are adrenalin and nor-adrenalin, which are major regulators of the heart. if you raise those concetration it causes heart rate to go up, and cardiac output to go up (volume of blood pumped every minute). Ordinarily they increase heartrate and cardiac output. TH increase expression of epi and nor-epi receptors in the heart. For them to act on the heart there has to be a receptor for it to bind to. the adranergic receptors are what bind the epi and nor-epi. the more recpetors there are the more the effect is on the heart. if there were no adranergic receptors in the heart, the epi and nor-epi could not effect the heart at all, no matter the concentrations. this is called a permissive effect because with the TH itself, you aren't aware it is going on, by stimulating the increase of adranergic receptors, 'gives permission' for epi and nor-epi to act on the heart. hormones work in the background to regulate expression for another hormone, and allow it to do its thing. besides increasing expression of these receptors, there is also a direct effect on the heart that increase heartrate and cardiac output directly from the TH itself.

REGULATION AND THYROID HORMONE SECRETION
look at the pathway on the handout.
the hypothalamus secretes thyrotropin releasing hormone into a capallary bed in the median eminans, blood carries it through portal vessels, down to the pit gland where it exits a capillary bed and if it binds to a thyrotrope it stimulates secretion of TSH and then is released into the circulation. if it gets back the the thyroid gland it binds to the phelicular epithelial cells stimulating T3 and T4 from the thyroid folicles.

in terms of TRH stucture, it is a tripeptide. it is very simple. the book tells you the aas there are, but you don't need to know that. expect TRH to be a pro-hormone because it is so small. pro-TRH is hydrolyzed to yeild 6 molecules. TSH belongs to a family of hormones called the glycoprotein family. that means all the members of the family are very similar. members of the glycoprotein family there are 4 members: TSH, FSH, LH, (pit products), Human colionic gonadatropin HCG (produced only during pregancy in the embrio). these molecules are glycoproteins (they are glycosolated) and they consist of 2 subunits (alpha and beta). they combine to mol weight of 30K. so each subunit is a single polypeptide chain, plus glycosolation (plus sugar). the 2 subunits are non-covalently associated. if you look at the alpha TSH/LH/FSH/HCG they have identical AA sequences. alpha is always the same in that family. there are some differences in glycosolation. the beta subunits are totally unique to each hormone. the beta subunit confer receptor specificity. whatever beta subunit there is, it will only bind to its appropriate receptor. LH to LH receptor, etc. the alpha needs to be there for necesary for signal transduction.

the glycosolation is important because it increases the half-life of the hormone in circulation. (30mins to an hour or so). glycosolation protects against endomatic degradation, but does not keep it from being excreted in the urine (that is how a preg test works). TRH can also stimulate prolactin creation. in the pit gland which cell it binds to dictates what happens next. the TRH molecule doesn't have a choice. It is the minor prolactin regulator. thyrotropes: TSH secretion, or lactotrope then prolactin secretion. It is totally random as to which cell it binds to.

TRH is the major regulator for TSH secretion. in terms of TSH inthe thyroid gland, it does alot of different things. it increases iodine uptake from the extracellular fluid, by increasing expression of iodine transporters. it increases thyroglobulin synthesis, and increases iodination of tyrosine, and increases coupling of iodinated tyrosines, and stimulates endocytosis of choline, stimulates cytoglobulin proteonlosis, stimulates activity of thyroid peroxidase, and it them overall stimulates folicular cell hypertrophy, and hyperplasia. TSH stimulates every part of the pathway making thyroid hormones, except of moving iodine from inside of the epithelial cells into the coloid. that will happen without TSH. TSH upregulates every other part of the TH making process. in addition to that it stimulates hypertrophy, and hyperplasia of the cells that make up the folicles. hypertrophy means one cell gets bigger, hyperplasia means an increase in the total number of cells. just by making more TH producing cells means you get more TH.

Monday April 9th thyroid

Monday... last week was pituitary, and now we move on to thyroid....

Thyroid hormones, hypothylamic thyroid access.

Tonight some proactice prbomes will be posted to the website. And the first midterm from last quarter will also be listed. Don't assume that this is what you will be tested on from before. There will be different questions. The questions will be totally different and on diffferent topics. But it will show you more or less what the length of the exam is, and what style. Some of the topics wno't be covered in time for our first midterm.

Also, there will be a list of topics you will be responsible for.

first image: hypthalamic-pituitary-thyroid axis....

this is a specific exampe of the heirarcy of control we spoke of on friday. Hypothalaumus makes TRH with stimuclates Ant Pit to make TSH which is released in the systemic blood, and goes into the thyroid glad (front of throat), and triggers growth in the thyroid aand to make thyosine, and tri-iodothyronine (t4 and t3).

All cells in the body respond to the thyroid hormones and we are starting with them rather than with the other pituitary hormones. we are starting with tis particuluar group of hormoneos because it is reasonably straight forward. It illistrates the basic relationships tha tyou will see in the endocrin systems througout the body. The order we will do this in is:
The first thing we will talk about today is the structure of T3and T4, then thri synthesis, then their transport in the blood, and then threir bilological activities, and then their regulation and secretion, and then take a break to talk about hormone mechanism of action, part of that discussion will be directly about the thyroid hormones, but part will be more general. Then we will finish the thyroid hormones by looking at disorders about T3 and T4 secretion.

the first thing to look at is the structure of the molecules. (see page with tyrosine and what it makes in, from eres).

Start with tyrosine. the first step in the biosynthetic pathsway is attaching an iodine to the 3 carbon of the phenol ring. The first carb to accept an Iodine is always called carbon 3. with a 3,5 iodine it is called 3,5 diiodotyrosine (DIT), then with 2 DITs you get T4 (4 means 4 iodines in the structure). If you have a single I, and then attach it to one with 2 I on it (MIT+DIT) you make T3 (3 iodine). Both T4 and T3 are biologically active. Inner ring is the one in the middle, and the outer ring is the one fartherst from the peptide bond. The biologically active molecule with 3 iodines is always an inner ring with 2I, and an outer with 1. If you have a 'reversed' T3, where the inner ring has 1 I, and the outer has 2, is NOT biologically active. T2, and T1 are both NOT biologically active either. (MIT and DIT are NOT biologically active)

Rev T3, is NOT a product made within the thyroid gland, but it is made peripherally.

Now you need to understand how and where it takes place. The thyroid glad consistems mainly of thyroid folicals. A folical is just a sphere of cells. (fig 76-1) The ball is filled with colloid. both the ep cells and the colloid produce thyroid hormones. the colloid contains thyroglobulin (mol wt ~660Kilodaltons) (dimer) It is secreted into the ep cells into the follicle. They thryoglobulin contains about a 100 tyrosine residues. some of which are turned into T3and T4. If you look at fig 7-5 from eres, it shows what has to happen. thyroglobulin is a long chain with tyrosine is part of the chain. they are iodinated (i9o0dine is added) to become DIT molecules. (aka organification) The tyrosine then are in close proximity of each other. the iodinated phenol is hydrolized and is bonede to another DIT. the inner ring is always a DIT, and the outer ring can be either a DIT or an MIT. this is the basic coupling reaction. two things have to happen for the tyrosine to become a T3 or T4. it has to be iodinated, and then coupled. the product is still attached to thyroglobulin molecule. so a peptide bond has to be hydrolized for it to become T3 or T4. (thyroglobulin is the colloid)

fig 76-2 there is an outline that goes with this picture (overview of thyroid hormone biosynthesis)thyroid peroxidase iodinates the tyrosine on thyroglobulin, and it catalyzes the coupling reaciton. if you have read the book, or bgo back, the books talks in one little part talks about an iodinate which is the same, and has both activities.. TG (thyroglobulin) contains MIT, DIT, T3, and T4 (but not RT3 is not normally occuring, that is a typo). the TG just sits there all the peptide bonds are intact, until there is a signal for T3 and T4 to be released, when TSH is present, and reaches out formes it into a vessicle inslide the ep cell (colloid droplet is a membrane bound structure). It them colleses with the lysosomes with contain proteases which hydrolyse the colloid. You have ffree amino accides and T3, T4, MIT and DIT. T3, and T4 are secreted into capilaries. the MIT and DIT are acted on by Diodinaces that remove the iodine and the iodine is recycled into the colloid. thyroglobulin is taken up back into the cell only if TSH is present.

quantitatively:
fewer than 20 tyrosines are iodinated per thryoglobuliln. this is not an efficent process. about 90% of the product that is released from TG is T4, and about 10% is T3, but different books give different numbers. But remember the vast majority is T4, and small amount is T3.

Because the way the process works. the thyroid gland can store colloid for a very long time. if you thyroid were normal, but had a hypphectomy, but the thryoid would holdonto the colloid for several weeks. If tehre isn't a stimulous to be taken up, it just sits there.

so 10% is T3/90% is T4, does that really matter? do they have similar bioactivities, or do they have different roles? they have different roles. if you look at them, most people feel that T3 is physiological releveant thyroid hormone. the reason is because affinity of the thyroid hormone receptor is about 10% higher for T3 than T4. We'll talk next week about a formal def of hormone recptors affinity and their percentage. We make more T4, but the receptor preferes T3. T4 has a different role in the body. aFter T4 is released from thyroid it can have 2 different fates: 1) action of outer ring deiodinase emzyme to make T3, or 2) inner ring deiodinase to produce RT3 (no bioact), or 3) stay T4. which happens dictates whether T3 goes up, or down. T4 acts as a pro-hormone from which T3 is made outside of the thyroid gland. A prohormone is a long peptide string that has protective junk around it, but it is also used as a term to mean it gives rise to something else. where these T4 to T3 changes happen. ther are 2 types of outer ring deiodinases. type 1 in liver, kidneys, and thryoid is responsible for generating T3, that ends up in the blood from T4 that passes through. type 2 deiodinase: taget tissues (thrydoid taget hormone cells) eg pituitary gladn, CNS, placenta, the T4 leaves the blood, enters the cell in the CNS and is converted into T3 within the cell itself. the cell controls when/how much is made.

One more quantitative thing... if you measure the concentration of T3 and T4 in the blood. the total T4 concentration is about 50xs greater than the total T3 concentration. in target cell cytoplasm the estimate is T4 and T3 are relatively equal (inside the target cells). they are both more lypid soluble, not water soluble.

transport in the blood (T3, T4):
this will introduce a topic for thyroid, and steroid hormones. these molecules are very small. maybe one would think that because of their size that they are water sol, but they aren't. that has created during the course of evolution. molecules that are important, but not water soluble makes it difficult to get much of them into the circulation. so they circulate bound to a binding protein. roughly 99.98% of T4, and greater than 99.5% of T3 circulates in the plasma bound to a binding protein. (BP) they are large molecular weight proteins with timpically made by the liver (60-90Kilodaltons) made by the liver and are released into the circulation and present in the plasma. each hormone has their own BP. For T3 and T4 (70%) is called thyroid binding globulin (TBG). and in addition to that some T3 and T4 circulate bound to albumin instead (30%), or a few other proteins. Binding is very specific for TBG, and is non=covalent. nothing else can use it. T3+ TBG=T3-TBG but free T3 is called 'free T3'. Total T3 is free T3 PLUS the stuff bound to BPs. TBG is only in plasma. the Dogma is that only T3 and T4 that are free have biological activity. While it is bound, it is NOT biologically active. the reason for that is, the BP are too large to leave the capillaries, and so it is stuck in the plasma. Only the unbound T3/T4 can make it through the wall to get out of the circ and into the target cell. the binding protein serves the purpose of:
1) raise Hormone solubility in the blood which acts as a reservoir of hormones,
2) increase the half-life/decrease the clearance of hormones from the blood. t1/2 is the shorthand for half-life, which is the time it atkes for 1/2 a compound to be cleared/degraded. (two ways to do this:
a) decrease filtration into urine in kidney (binding proteins can't be filtered because it makes them too big which decreases their excretion).
b)and protect hormones againt attack by degradative enzymes.
both of these mechs work so well in relation of thyrsoid hormones that the t1/2 of T4 is about 7 days, and T3 is about 1 day. Compared with epinephrine (also from tyrosine) estimated t1/2 is 1 min, and others are 20-30 mins! So 1-7 days is super long. the importance of a long half-life is that clinically, you can't instantly change the levels.