13.2 Messengers and Targets

Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.

I · Three chemistries, one bloodstream

A hormone is one of three things. A peptide or protein — a chain of amino acids, from ADH’s nine to the glycoproteinsProteins with carbohydrate groups attached, involved in cell signaling. TSH, FSH and LH — made on ribosomesSmall structures responsible for protein synthesis, either free-floating or attached to the rough ER, stored in vesicles, released on demand. A monoamine from a single amino acidThe building blocks of proteins, consisting of an amino group, carboxyl group, and side chain.: tyrosine gives the catecholamines and thyroid hormone (TH): Includes T3 and T4, regulating metabolism., tryptophan gives melatoninRegulates sleep-wake cycles.. A steroid cut from cholesterolA lipid molecule that is a key component of cell membranes and a precursor for bile acids and steroi — cortisol, aldosteroneA hormone that increases sodium and water reabsorption in the kidneys, helping regulate blood pressu, calcitriol, the sex steroids — made the moment it is needed, because a lipid cannot be stored in a vesicle. How a hormone is built decides how it works. Peptides and catecholamines are polar: they travel free in plasmaThe liquid component of blood., cannot cross a membrane, and knock at a surface receptorA structure that detects stimuli.. Steroids are nonpolar, and so is thyroid hormone, the amine that behaves like a steroid (the lecture’s exception): they ride albumin, thyroxine-binding globulin, transthyretin or transcortin, walk through the membrane to a receptor in the cytosol or nucleusThe control center of the cell that contains DNA and directs cellular activities., and change which genes are transcribed; thyroid hormone’s receptor is already sitting on the DNA. More than 99% of T4 is bound, about 90% of cortisol; the bound fraction is a reservoir, the free fraction is the signal, and most labs report the total.

See it move: Scene 1 · One Hallway, Many Messages · Scene 2 · Who Needs a Ride

II · Knock or walk in, and five things a hormone can do

When a polar hormone knocks, the receptor hands the message to a G proteinA signaling protein activated by neurotransmitter receptors that triggers internal cellular response, which switches on adenylate cyclase; ATPThe energy currency of cells used for muscle contraction. becomes cyclic AMP; cAMP(Cyclic AMP): Second messenger in many hormone signaling pathways. wakes a protein kinase; the kinase phosphorylates enzymesProteins that speed up chemical reactions in the body. down a cascade — calcitoninA hormone from the thyroid that lowers blood calcium levels by inhibiting osteoclasts., glucagonIncreases blood sugar by promoting glycogen breakdown., TSH, ACTH and epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla. all use it. Each step multiplies: one hormone molecule, thousands of products — amplification — which is why hormones work at nanomolar concentrations. Phosphodiesterase breaks cAMP down; caffeine blocks it, which is why coffee feels like epinephrine. The second door: phospholipase C splits PIP₂ into DAG and IP₃, IP₃ releases calcium from the endoplasmic reticulum, calcium binds calmodulinProtein that binds calcium to activate cellular processes. — angiotensin IIA powerful vasoconstrictor that increases blood pressure and stimulates aldosterone release. and GHRH use it. Somatostatin’s receptor lowers cAMP. Whichever door, a hormone can do five things: change membrane permeability or potential, build or switch on enzymes, change secretion, trigger mitosis, or — the steroid route — change transcription. And the cell decides how loudly it listens: a hormone kept high for weeks makes it pull receptors in (down-regulation); kept low, put more out (up-regulation). Hormones share targets three ways — permissive (thyroid hormone lets epinephrine receptors be made), synergistic (glucagon and epinephrine on liver glycogenA storage form of glucose found in animals.), antagonistic (insulin against glucagon; PTH against calcitonin).

See it move: Scene 3 · Knock or Walk In · Scene 4 · Lock and Key, and How Many Locks

III · Balance in the blood, and the thirty-day lesson

Think of a bathtub. Secretion is the tap. Carrier binding is a side reservoir — the lecture’s parking lot, full of cars not driving. Clearance is the drain: liver and kidney remove hormone at a metabolic clearance rate, and the half-lifeTime for hormone levels to reduce by half. is how long the level takes to halve — a minute for epinephrine, an hour for cortisol, a week for T4. Receptors and carriers saturate: every seat taken, more hormone adds nothing. The thirty-day lesson: give prednisone, a cortisol look-alike, for a month and the pituitary stops making ACTH because the loop reads “enough”; the cortex shrinks; stop suddenly and the tap cannot reopen for weeks — so steroids are tapered. Now read her chart. Total calcium is bound plus free, and only the free half fires a parathyroid cell. Total cortisol is mostly transcortin’s reservoir, and one 8 am number says nothing about the rhythm. 25-hydroxyvitamin D is the storage form; the hormone is made in the kidney and was never measured. And every target tissue keeps its own receptors, and inflammatory cytokines are among the things that change how many a cell keeps. A hormone can be made in the right amount and misheard — the question moves from the gland to the listener.

See it move: Scene 5 · Balance in the Blood

5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: Present in a normal amount, possibly misread, Neither on its own, or A gland making too little or too much. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.