## Mechanisms of Gastric Acid Regulation ### Histamine's H2 Receptor Pathway: **Key Point:** Histamine is released by ECL cells and acts on H2 receptors on parietal cells via a cAMP-dependent mechanism. ### The H2 Receptor Signaling Cascade: 1. Histamine binds to H2 receptors on parietal cell membrane 2. G-protein coupling → adenylyl cyclase activation 3. ↑ cAMP → activates protein kinase A (PKA) 4. PKA phosphorylates and activates the H⁺/K⁺-ATPase (proton pump) 5. Result: H⁺ secretion into gastric lumen **Clinical Pearl:** This is why H2 blockers (ranitidine, famotidine) and PPIs (omeprazole) are effective — they interrupt this pathway at different points. ### Comparison of Three Main Pathways: | Substance | Source | Receptor | Mechanism | Final Effect | |-----------|--------|----------|-----------|---------------| | **Acetylcholine** | Enteric neurons | M3 muscarinic | ↑ Ca²⁺ | H⁺ secretion | | **Gastrin** | G cells | CCK-B | ↑ Ca²⁺ + ECL stimulation | H⁺ secretion | | **Histamine** | ECL cells | H2 | ↑ cAMP | H⁺ secretion | **Mnemonic:** **"ACH-CAM"** — ACetylcholine and Calcium; Histamine and cAMP
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