A 30-year-old woman with paroxysmal supraventricular tachycardia is given adenosine IV. The drug terminates the arrhythmia by which mechanism?

• A Blocking L-type calcium channels in the AV node
• B Blocking beta-1 receptors in the sinoatrial node
• C Blocking sodium channels, slowing Phase 0 depolarization
• D Activating A1 adenosine receptors, increasing IKAdo (potassium) conductance in the AV node ✓

Explanation

Adenosine acts on A1 receptors (Gi-coupled) in the AV node, which opens inwardly rectifying potassium channels (IKAdo), hyperpolarising the nodal cells and transiently blocking AV conduction—terminating re-entry SVT. Its effect is extremely short (half-life ~10 seconds) due to rapid cellular uptake. Calcium channel blockers (verapamil, diltiazem) also block the AV node but via an entirely different mechanism, and beta-blockers and sodium-channel blockers are not the mechanism for adenosine.

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

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