Calcium channel blockade by certain drugs would primarily shorten which phase of the ventricular action potential?

• A. Phase 0 (rapid depolarization)
• B. Phase 2 (plateau)
• C. Phase 1 (brief initial repolarization)
• D. Phase 3 (repolarization)

Answer: (B)

The plateau phase relies on a sustained inward calcium current through L-type calcium channels to counterbalance outward potassium currents. Blocking these calcium channels reduces the inward Ca2+ current, so the membrane can’t stay depolarized as long and the plateau shortens. The rapid depolarization relies on sodium, the brief initial repolarization on transient outward K+, and repolarization mainly on K+ efflux, so calcium channel blockade chiefly shortens the plateau.