A 28-year-old man is brought to the emergency department with acute asthma exacerbation. Nebulized ipratropium bromide is added to salbutamol. The bronchodilatory action of ipratropium is mediated by blockade of which receptor in the airways?
- A Muscarinic M1 receptors on parasympathetic ganglia
- B Muscarinic M2 receptors on vagal nerve endings
- C Muscarinic M3 receptors on airway smooth muscle and submucosal glands ✓
- D Nicotinic receptors on airway sensory nerve terminals
Correct answer: C. Muscarinic M3 receptors on airway smooth muscle and submucosal glands
Explanation
Ipratropium is an anticholinergic bronchodilator that blocks muscarinic M3 receptors on airway smooth muscle and submucosal glands, preventing acetylcholine-mediated bronchoconstriction and mucus secretion. Vagal parasympathetic tone is the dominant neural bronchoconstrictor pathway. M1 receptors facilitate ganglionic transmission but are not the primary target for bronchodilation. M2 autoreceptors, when blocked, would increase acetylcholine release and worsen bronchospasm, but ipratropium's clinical benefit is from M3 blockade.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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