A 30-year-old woman undergoes elective surgery. During reversal of neuromuscular blockade, neostigmine is administered but she develops severe bradycardia requiring atropine. Which receptor subtype mediates the cardiac slowing produced by neostigmine in this scenario?
- A M2 receptors on the sinoatrial node ✓
- B M1 receptors on the sinoatrial node
- C N1 receptors at the autonomic ganglia
- D M3 receptors on cardiac conducting tissue
Correct answer: A. M2 receptors on the sinoatrial node
Explanation
Cardiac slowing via vagal tone is mediated through M2 muscarinic receptors (Gi-coupled) on the sinoatrial node, which reduce heart rate by decreasing the slope of the pacemaker potential. Neostigmine amplifies acetylcholine accumulation at all cholinergic synapses including cardiac, causing M2-mediated bradycardia. M1 receptors are predominantly in the CNS and ganglia; M3 receptors are on smooth muscle and glands; N1 receptors at ganglia are nicotinic.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.