A patient takes a drug that blocks muscarinic M2 receptors in the heart. The expected physiological effect would be:

• A Bradycardia and increased PR interval
• B Tachycardia and reduced PR interval (increased AV conduction) ✓
• C Negative inotropy with unchanged heart rate
• D Coronary vasodilation and reduced myocardial oxygen demand

Explanation

M2 muscarinic receptors mediate parasympathetic (vagal) effects on the heart through Gi protein coupling. Activation of M2 opens GIRK channels (IKACh), hyperpolarizing SA node cells (bradycardia) and slowing AV conduction (PR prolongation). Blocking M2 therefore removes vagal tone, resulting in tachycardia and enhanced AV conduction (shortened PR interval). This is the mechanism of atropine in treating bradycardia and heart block.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.