Initial rapid repolarization in cardiac muscle is mediated through:

Opening  of  K+ channel >> Closing of Na channel  The  main  factor  is  opening of K+ channel.  Although  Ganong has  maintained  both closure  of  Na+ channels  and  opening of one  type  of  K+ channel and closure of Na channel only, The recent concept is K+ channel opening is the main reason of phase 1.  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.