What is the mechanism of action of glucagon in beta-blocker overdose?

• A. Increases cAMP independently of beta receptors
• B. Decreases cAMP
• C. Blocks calcium channels
• D. Inhibits beta receptors

Answer: (A)

Glucagon provides a separate way to boost heart function when beta receptors are blocked. It binds to glucagon receptors on cardiac cells, which are Gs-coupled. This directly activates adenylate cyclase, increasing cAMP levels inside the cell. The rise in cAMP activates protein kinase A, which enhances calcium entry and handling in the heart, leading to higher heart rate (chronotropy) and stronger contractions (inotropy) even though beta-adrenergic receptors are blocked. This bypasses the need for beta receptor stimulation, which is why glucagon is useful in beta-blocker overdose.