Which agent is most likely to shorten ERP in AV nodal tissue?

• A. Sympathetic activation
• B. Vagal activation
• C. Ischemia
• D. Exercise

Answer: (A)

Autonomic tone modulates the AV node’s refractoriness. Sympathetic activation speeds the heart’s conduction machinery by raising cAMP and activating protein kinase A, which boosts L-type calcium current and accelerates repolarization in the AV nodal cells. This makes the tissue recover excitability sooner after each beat, shortening the effective refractory period. In contrast, vagal (parasympathetic) activation slows calcium entry and enhances conductance changes that prolong the refractory period, increasing AV nodal delay. Ischemia and exercise influence conduction differently: ischemia can disrupt ionic balance and often prolong refractoriness or cause blocks, while exercise mainly acts as a surge of sympathetic tone and would likewise tend to shorten ERP, though the question points to a specific agent, and the direct one here is sympathetic activation.