A 62-year-old man under general anaesthesia receives neostigmine to reverse residual non-depolarizing neuromuscular blockade. Shortly after administration, his heart rate drops to 42 beats per minute. Which muscarinic receptor subtype primarily mediates this bradycardia?

• A M1
• B M4
• C M3
• D M2 ✓

Explanation

M2 receptors are the predominant muscarinic subtype in the heart, mediating negative chronotropy (slowed SA node firing) and dromotropy (slowed AV nodal conduction). Neostigmine increases acetylcholine at cardiac muscarinic receptors, producing bradycardia via M2 activation. M3 receptors mediate smooth muscle contraction and glandular secretion but not cardiac slowing.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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