When the pH of the stomach lumen falls below 3, the antrum of the stomach releases a peptide that acts locally to inhibit gastrin release. This peptide is:
First, I remember that gastrin is a hormone that stimulates gastric acid secretion. It's released from G cells in the stomach antrum. But when there's already a lot of acid (low pH), the body needs to reduce gastrin to prevent overproduction. So there's a feedback mechanism here.
The peptide that inhibits gastrin in this scenario is probably somatostatin. Wait, but somatostatin is produced by D cells in the stomach. Alternatively, maybe it's another hormone like secretin or CCK, but those act more on pancreatic enzymes or bile. Wait, no. Secretin does inhibit gastrin, but it's released from the duodenum when pH is low, not the stomach antrum. Hmm.
Wait, the question says the antrum releases the peptide. So if it's the antrum itself, maybe it's a local regulator. D cells in the antrum produce somatostatin, which inhibits gastrin release from G cells. So when pH is low, D cells in the antrum release somatostatin to inhibit gastrin. That makes sense. So the answer should be somatostatin.
But wait, some sources say that gastric inhibitory polypeptide (GIP) also plays a role. No, GIP is more related to inhibiting gastric acid secretion, not directly gastrin. Secretin is released from the duodenum, so that's not from the antrum. So the correct answer must be somatostatin, produced by D cells in the antrum.
So the options would be A. Secretin B. CCK C. Somatostatin D. GIP. Then the correct answer is C. Somatostatin.
**Core Concept:** Gastrin secretion is regulated by a negative feedback loop involving pH-sensitive mechanisms in the stomach. When gastric lumen pH drops below 3, the antral D cells release somatostatin to inhibit gastrin release from G cells, preventing excessive acid secretion.
**Why the Correct Answer is Right:** Somatostatin, a peptide hormone produced by D cells in the gastric antrum, acts locally via G-protein-coupled receptors to inhibit gastrin release. It suppresses paracrine signaling from G cells when luminal acidity is high, maintaining homeostasis. This autocrine/paracrine feedback is critical to avoid acid hypersecretion and mucosal damage.
**Why Each Wrong Option is Incorrect:**
**Option A: Secretin**—Secretin is released by duodenal S cells in response to low pH, inhibiting gastric acid secretion via systemic effects but not directly suppressing gastrin.
**Option B: Cholecystokinin (CCK)**—CCK, released by I cells in the duodenum, stimulates gallbladder contraction and pancreatic enzyme secretion, unrelated to gastrin inhibition.
**Option D: Gastric Inhibitory Polypeptide (GIP)**—GIP inhibits gastric acid secretion indirectly via the vagus nerve but does not target gastrin release directly.
**Clinical Pearl / High-Y