Transient outward channels contribute to which phase of non-pacemaker APs?

Transient outward channels contribute to which phase of non-pacemaker APs?

• A. Phase 1
• B. Phase 2
• C. Phase 3
• D. Phase 0

Answer: (A)

Transient outward potassium currents shape the early repolarization of the non-pacemaker ventricular action potential. After the rapid upstroke driven by Na+ entry (Phase 0), these Ito channels open quickly and then inactivate, producing a brief outward K+ current that creates the notch right at the start of repolarization. This brief outward current defines Phase 1. The plateau phase (Phase 2) follows, governed by Ca2+ influx balanced by K+ efflux, and then Phase 3 is final repolarization dominated by other K+ currents. In non-pacemaker cells, the transient outward current is the key contributor to the Phase 1 notch.

Transient outward potassium currents shape the early repolarization of the non-pacemaker ventricular action potential. After the rapid upstroke driven by Na+ entry (Phase 0), these Ito channels open quickly and then inactivate, producing a brief outward K+ current that creates the notch right at the start of repolarization. This brief outward current defines Phase 1. The plateau phase (Phase 2) follows, governed by Ca2+ influx balanced by K+ efflux, and then Phase 3 is final repolarization dominated by other K+ currents. In non-pacemaker cells, the transient outward current is the key contributor to the Phase 1 notch.