Which of the following muscarinic receptor subtypes is predominantly responsible for the pirenzepine-sensitive inhibition of gastric acid secretion?

• A M2 on parietal cell surface inhibiting adenylyl cyclase
• B M3 directly on parietal cells activating phospholipase C
• C M1 on gastric ECL cells and enteric ganglia ✓
• D M4 on D cells stimulating somatostatin release

Explanation

Pirenzepine is a selective M1 antagonist. M1 receptors on gastric enterochromaffin-like (ECL) cells and enteric ganglia facilitate histamine release and cholinergic input to parietal cells; blocking M1 reduces acid secretion indirectly. M3 receptors on parietal cells directly stimulate HCl secretion via PLC-IP3 pathway, but pirenzepine has low affinity for M3. M2 receptors are predominantly cardiac. M4 receptors are not the primary site for acid regulation via pirenzepine.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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