In severe acute asthma (status asthmaticus) not responding to inhaled beta-2 agonists, IV magnesium sulphate is given. The mechanism by which it produces bronchodilation is:

• A Competitive antagonism of histamine H1 receptors on bronchial smooth muscle
• B Direct activation of beta-2 adrenoceptors on bronchial smooth muscle
• C Inhibition of mast cell degranulation by stabilising the mast cell membrane
• D Blockade of voltage-gated calcium channels in bronchial smooth muscle, reducing intracellular Ca2+ and causing relaxation ✓

Explanation

Magnesium is a physiological calcium channel antagonist; Mg2+ ions block voltage-gated calcium channels, reducing Ca2+ influx into bronchial smooth muscle cells and causing relaxation (bronchodilation). It may also reduce acetylcholine release from cholinergic nerve terminals and stabilise mast cells. IV MgSO4 (1.2–2 g over 20 min) is recommended in severe acute asthma as a third-line bronchodilator when maximal inhaled beta-2 agonists and ipratropium and systemic corticosteroids have not achieved adequate response. It does not act on beta-2 receptors or H1 receptors.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.