Which phase of the non-pacemaker action potential is repolarization with high K+ currents?

• A. Phase 3 repolarization
• B. Phase 2 plateau
• C. Phase 1 repolarization
• D. Phase 4 resting potential

Answer: (A)

Repolarization driven by strong outward potassium currents occurs during the final repolarization phase. In non-pacemaker (ventricular) action potentials, this is phase 3, when delayed rectifier K+ channels (I_Kr, I_Ks) open and push positive charge out of the cell to return toward the resting potential.

Early repolarization (phase 1) is mainly due to a transient outward K+ current (not as sustained or large), and the plateau (phase 2) is balanced by calcium influx with some outward K+ as well. The resting phase (phase 4) is maintained mainly by inward rectifier K+ current. Therefore, the phase with repolarization powered by high K+ currents is phase 3.