A patient presents with miosis, bradycardia, and excessive secretions after agricultural exposure. Atropine is given in large doses. Which muscarinic receptor subtype on the heart mediates the bradycardia being treated?

• A M2 (Gi-coupled, decreased cAMP and increased K+ conductance) ✓
• B M1 (Gq-coupled, IP3/DAG pathway)
• C M3 (Gq-coupled, vascular smooth muscle contraction)
• D M4 (Gi-coupled, CNS inhibitory neuromodulation)

Explanation

Cardiac muscarinic receptors are predominantly M2 subtype, coupled to Gi protein. M2 activation decreases cAMP, opens GIRK (G-protein-coupled inwardly rectifying potassium) channels, hyperpolarizes the SA node, and slows conduction through the AV node causing bradycardia. Atropine competitively blocks M2 receptors, reversing organophosphate-induced bradycardia. M1 receptors are mainly on CNS neurons; M3 receptors on smooth muscle and glands; M4 on CNS.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.