A 52-year-old man with a 10-year history of GERD presents with epigastric pain. Endoscopy reveals a duodenal ulcer. Which of the following best explains the mechanism of increased gastric acid secretion in this patient?

Explanation

## Mechanism of Gastric Acid Secretion **Key Point:** Gastric acid secretion is regulated by three main pathways: acetylcholine (ACh), gastrin, and histamine. All three converge on the parietal cell to increase H⁺ secretion. ### The Three Pathways: 1. **Acetylcholine (Vagal/Enteric):** Acts via M3 muscarinic receptors → ↑ intracellular Ca²⁺ → H⁺ secretion 2. **Gastrin (Hormonal):** Released by G cells in response to amino acids, peptides, and gastric distension → acts on CCK-B receptors on parietal cells AND stimulates ECL cells to release histamine 3. **Histamine (Paracrine):** Released by ECL cells → acts via H2 receptors on parietal cells → ↑ cAMP → H⁺ secretion ### Why Option 2 (Increased histamine via gastrin) is Correct: In duodenal ulcer disease, there is often increased gastrin secretion (either from G cells or ectopic sources). Gastrin stimulates ECL cells to release histamine, which is the **final common pathway** for acid secretion. Histamine acts on H2 receptors, increasing cAMP and activating the proton pump. This is why H2 blockers and PPIs are effective in ulcer treatment. **Clinical Pearl:** The three pathways (ACh, gastrin, histamine) are interdependent. Blocking any one pathway reduces overall acid secretion, but histamine is often the most potent final mediator. **High-Yield:** Gastrin → ECL cells → Histamine → H2 receptors → cAMP → Proton pump activation → H⁺ secretion