Which muscarinic receptor subtype mediates the pirenzepine-sensitive inhibition of gastric acid secretion, and what is its primary G-protein coupling?

• A M1 receptors coupled to Gq, leading to increased IP3 and intracellular calcium ✓
• B M2 receptors coupled to Gi, leading to decreased cAMP
• C M3 receptors coupled to Gq, causing direct parietal cell stimulation
• D M4 receptors coupled to Gs, increasing adenylyl cyclase activity

Explanation

Pirenzepine selectively blocks M1 muscarinic receptors on ECL cells and ganglionic neurons in the gastric plexus, reducing histamine-mediated acid secretion. M1 receptors couple to Gq protein, activating phospholipase C with consequent IP3 generation and intracellular calcium rise. M2 (cardiac) couples to Gi; M3 on parietal cells and smooth muscle also couples to Gq but is less sensitive to pirenzepine; M4 receptors are largely CNS-based and Gi-coupled.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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