The voltage-gated potassium channels (Kv) are activated by depolarization. The currents produced by these channels include the transient out potassium current Ito1. This current has two components. Both components activate rapidly, but Ito,fast inactivates more rapidly than Ito, slow. These currents contribute to the early repolarization phase (phase 1) of the action potential.
Phase 3 is indicative of final repolarization and during it, the membrane potential returns to resting level. This phase in ventricular cells corresponds to T wave on the electrocardiogram (ECG) (Fig. 1C). Different K+ currents are responsible for phase 3 such as rapid and slow delayed rectifier K+ currents (IKr and IKs), inwardly rectifier K+ current (IK1) and transient outward K+ current (Ito). From the onset of the action potential until part of phase 3, the cell does not respond to new excitation, which is called the effective (or absolute) refractory period (ERP or ARP).
The ionic events underlying a cardiac action potential are illustrated in (C) and include the depolarizing inward Na+ (INa) and calcium (ICa)currents, and the repolarizing transient outward current (Ito), and three outward potassium currents (IKr, IKs, IK1).
