<prompt> Initial rapid repolarization in cardiac muscle is mediated through:Opening of K channelOpening of Ca channelOpening of Na channelClosing of Na channel  <response>: 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. 