Phase 3 - rapid repolarisation

In phase 3 (the "rapid repolarisation" phase), the L-type Ca2+ channels close, while the slow delayed rectifier (IKs) K+ channels remain open as more K+ leak channels open. This ensures a net outward positive current, corresponding to negative change in membrane potential, thus allowing more types of K+ channels to open. These are primarily the rapid delayed rectifier K+ channels (IKr) and the inwardly rectifying K+ current, IK1 (Kir). This net outward, positive current (equal to loss of positive charge from the cell) causes the cell to repolarize. The primary delayed rectifier K+ currents (IKs and IKr) are generated by K+ efflux mediated by potassium voltage-gated channel subfamily KQT member 1 (KCNQ1 aka Kv7.1) and potassium voltage-gated channel subfamily H member 2 (KCNH2 aka HERG) channels respectively. Specific to the atria, an ultra-rapidly activating delayed rectifier outward K+ current (IKur) generated primarily by potassium voltage-gated channel subfamily A member 5 (KCNA5) helps to repolarize atrial cells.

Summary from Reactome (R-HSA-5576890), read 2026-09-23.