A young woman with intermittent wheeze undergoes a bronchial provocation test. After inhaling stepwise concentrations of a choline ester, her FEV1 falls by more than 20 percent, confirming airway hyperresponsiveness. Which receptor mediates the bronchoconstriction produced by this agent?

• A Nicotinic receptors at pulmonary sensory nerve endings
• B Beta-2 adrenergic receptors on bronchial epithelium
• C M3 muscarinic receptors on bronchial smooth muscle ✓
• D M2 muscarinic receptors on postganglionic parasympathetic neurons

Explanation

Methacholine is a muscarinic agonist used in bronchoprovocation testing. Bronchoconstriction results from activation of M3 receptors on airway smooth muscle, triggering Gq-coupled phospholipase A signalling and calcium-mediated contraction. M2 receptors are presynaptic autoreceptors that inhibit further acetylcholine release, so their stimulation would reduce rather than increase bronchoconstriction. Beta-2 activation relaxes airway smooth muscle, and nicotinic receptors are not located on bronchial smooth muscle.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.