A 24-year-old asthmatic using an over-the-counter oral cold remedy containing a non-selective beta-blocker notices her salbutamol inhaler no longer relieves wheeze. Which receptor-level interaction explains this treatment failure?

• A Competitive antagonism at beta-2 receptors by the beta-blocker ✓
• B Downregulation of beta-2 receptors caused by chronic salbutamol use
• C Inhibition of adenylate cyclase downstream of the muscarinic receptor
• D Potentiation of acetylcholine release at airway ganglia

Explanation

Salbutamol is a selective beta-2 agonist acting at bronchial beta-2 receptors coupled to Gs and adenylate cyclase. A non-selective beta-blocker such as propranolol occupies these same receptors competitively, shifting the agonist dose-response curve rightward and blunting bronchodilation. Receptor downregulation takes weeks of heavy agonist exposure, and the other mechanisms are unrelated to competitive beta-receptor blockade.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.