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.