Phase 1 of Myocardium AP is mainly due to:

Phase 0 (depolarization), fast sodium channels open. Phase 1 (initial repolarization), fast sodium channels close.  Phase 2 (plateau), calcium channels open and fast potassium channels close. Phase 3 (rapid repolarization), calcium channels close and slow potassium channels open. Phase 4 (resting membrane potential) averages about −90 millivolts. Key Concept: Phase 1 of myocardium AP is due to K+ opening >> inactivation of Na+ channel.   Phase 2 of myocardium AP starts with Ca++ entry. Long duration of myocardium AP is due to phase 2. ​