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What is Initial rapid repolarization in cardiac muscle is mediated through: and explain in detail?
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.