GI Hormones & Body-Mass Homeostasis

BCH 120 β€” Metabolic & Endocrine Biochemistry Β· Dr. Radi

build Jul 17 Β· 11:10 Β· CC BY-NC-SA 4.0 Β· owned figures (RDKit / matplotlib / PyMOL)
Dr. Radi

By the end of this unit, you can…

  • Describe how the classical GI hormones act both locally in the gut and on the brain, and the roles of gastrin (acid), secretin (bicarbonate), and CCK (bile + pancreatic enzymes) with their source cells, triggers, and regulation
  • Explain leptin's control of appetite (adipose β†’ hypothalamus; ob/ob vs db/db mice; NPY vs Ξ±-MSH; JAK-STAT) and its sympathetic thermogenesis via UCP1
  • Integrate the appetite hormones β€” insulin, ghrelin, PYY₃₋₃₆ β€” with leptin, and explain leptin resistance in obesity
Dr. Radi

Today's route πŸ—ΊοΈ

  1. GI Hormones β€” The Classical Three
  2. Gastrin, Secretin & CCK in Detail
  3. Leptin & the Control of Appetite
  4. Burning Fuel, Ghrelin, PYY & Leptin Resistance
Dr. Radi

1 Β· GI Hormones β€” The Classical Three

"Your gut is the body's largest endocrine organ. It runs digestion with its own hormones β€” and, surprisingly, the same hormones talk to your brain about hunger. Meet the three classics: gastrin, secretin, and CCK."

Dr. Radi

GI hormones lead a double life

Your GI system has three core jobs β€” digest food, absorb nutrients, and eliminate waste β€” and it coordinates them with its own hormones. Here's what makes those hormones unusual: they're made by the gut itself, and though they ride the blood, their main targets are right next door β€” other parts of the gut (the local job). But they have a second audience: most also have receptors in the hypothalamus, where they shape hunger and satiety. So the gut isn't just digesting β€” it's reporting to the brain about the meal.

Dr. Radi

The three classical hormones

Learn these three and their one-line jobs; the rest of the lecture just adds detail. Gastrin, from G cells in the stomach, drives gastric acid (and pepsin). Secretin, from S cells in the duodenum, triggers pancreatic bicarbonate to neutralize that acid. CCK, from I cells in the duodenum, releases pancreatic enzymes and squeezes the gallbladder for bile. One runs the stomach; two run the duodenum and pancreas β€” together, they orchestrate a meal.

Dr. Radi

2 Β· Gastrin, Secretin & CCK in Detail

"Walk a meal through your gut and these three hormones fire in sequence β€” acid, then neutralize, then digest fat. Each one has a source cell, a trigger, and a clinical story worth knowing."

Dr. Radi

Gastrin β€” the acid hormone

Food hits the stomach and gastrin fires. Made by G cells (stomach and duodenum) in response to a meal β€” especially protein β€” plus neural signals, it drives gastric acid so hard the stomach can hit pH 1. That acid activates pepsin, kills pathogens, and gastrin also triggers histamine and intrinsic factor (needed to absorb vitamin B₁₂). Two clinical hooks: a gastrin-secreting tumor causes Zollinger-Ellison (runaway acid, ulcers); losing intrinsic factor causes pernicious anemia. Gastrin shuts off when the stomach empties.

Dr. Radi

Secretin β€” the acid-neutralizer

As that acidic stomach content pours into the duodenum, the pH drops β€” and secretin answers. Released by S cells whenever duodenal contents turn acidic (pH < 4.5), secretin tells the pancreas to pump out bicarbonate and water, neutralizing the acid so the pancreatic enzymes can work at their basic optimum. It also inhibits gastric acid, easing off the stomach. Historical note: secretin was the very first hormone ever discovered (Bayliss & Starling, 1902) β€” the experiment that coined the word "hormone."

Dr. Radi

CCK β€” the fat & protein hormone

Now the fat arrives, and cholecystokinin (CCK) takes over. Released by I cells in response to fat and protein in the gut, CCK does two things its name and old name spell out: it makes the gallbladder contract (chole-cysto-kinin β†’ bile out to emulsify fat) and it triggers pancreatic enzyme secretion (its "pancreozymin" activity). CCK is also made by neurons in the brain, where it's a satiety signal β€” one more gut hormone telling you you're full.

Dr. Radi

3 Β· Leptin & the Control of Appetite

"How does your body know how much fat it's carrying? A hormone called leptin, sent from fat to brain. Two famous obese mice β€” one missing leptin, one missing its receptor β€” cracked the whole system open."

Dr. Radi

Leptin: fat talks to the brain

Leptin is the body's fuel gauge. It's made by adipose tissue in proportion to how much fat you carry, travels to the hypothalamus, and says: eat less. We know this from two obese mice. ob/ob mice can't make leptin (OB gene) β€” they eat nonstop and balloon, but leptin injections reverse it. db/db mice can't hear leptin β€” the DB gene encodes its hypothalamic receptor, so they're obese and diabetic and leptin can't help them. Hormone missing vs receptor missing β€” same result.

Dr. Radi

Two opposing appetite signals

In the hypothalamus, appetite is a tug-of-war between two neurohormones. NPY (neuropeptide Y) is orexigenic β€” it screams "eat!", and it rises in starvation. Ξ±-MSH is anorexigenic β€” it says "stop!" Leptin wins the war for stop: it silences NPY and turns up Ξ±-MSH, pushing both levers toward eating less. Insulin does the same. So a well-fed body quiets the "eat" neuron and boosts the "stop" neuron.

Dr. Radi

Leptin signals through JAK-STAT

How does leptin turn up Ξ±-MSH? Through a pathway you already know β€” JAK-STAT (from unit 1). Leptin binds and its receptor dimerizes; a soluble JAK kinase phosphorylates the receptor; STAT proteins dock, get phosphorylated, dimerize, and go to the nucleus to raise transcription of the Ξ±-MSH precursor. The leptin receptor has no built-in kinase, so it borrows JAK β€” exactly the erythropoietin trick. Same machinery, new job: telling you you've eaten enough.

Dr. Radi

4 Β· Burning Fuel, Ghrelin, PYY & Leptin Resistance

"Appetite isn't one hormone β€” it's a whole committee. Leptin also burns fat as heat, insulin and PYY say 'stop,' ghrelin says 'eat now,' and the sad twist: in obesity, the leptin signal stops working."

Dr. Radi

Leptin also burns fuel as heat

Leptin doesn't just curb eating β€” it spends energy too. From the hypothalamus it fires the sympathetic nervous system, releasing norepinephrine onto fat cells' β₃-adrenergic receptors, which crank up transcription of UCP1 (thermogenin). Remember UCP1 from bioenergetics? It lets protons leak back into the mitochondrion without making ATP β€” so the energy comes out as heat, and fat is burned to stay warm. Leptin thus works both ends: eat less and burn more.

Dr. Radi

The appetite hormone board

Zoom out and appetite is a committee vote. On the "stop eating" side: leptin (from fat, long-term), insulin (from the pancreas after a meal), and PYY₃₋₃₆ (from the intestine and colon after you eat). On the "eat now" side: ghrelin β€” the hunger hormone from the stomach that rises between meals β€” which switches on the hypothalamic NPY neurons. Your appetite at any moment is the sum of these signals.

Dr. Radi

Why leptin wasn't the obesity cure

Here's the twist that broke a lot of hearts (and a few stock prices). In ob/ob mice, giving leptin melts the weight off β€” it works. So the drug companies raced to give leptin to obese people β€” and it did almost nothing. Why? Most obese people already have high leptin; the problem is downstream β€” the signal arrives but isn't heard. It's called leptin resistance, and it's a big reason obesity is so hard to treat with a single hormone.

Dr. Radi

Can you…?

  • ☐ describe how the classical GI hormones act both locally in the gut and on the brain, and the roles of gastrin (acid), secretin (bicarbonate), and CCK (bile + pancreatic enzymes) with their source cells, triggers, and regulation?
  • ☐ explain leptin's control of appetite (adipose β†’ hypothalamus; ob/ob vs db/db mice; NPY vs Ξ±-MSH; JAK-STAT) and its sympathetic thermogenesis via UCP1?
  • ☐ integrate the appetite hormones β€” insulin, ghrelin, PYY₃₋₃₆ β€” with leptin, and explain leptin resistance in obesity?

If any box stays empty, the practice site has a drill for it. πŸ§ͺ

Dr. Radi