Plateau phase of ventricular muscle is due to opening of:

Phase 2 (plateau), calcium channels open and fast potassium channels close. A brief initial repolarization occurs and the action potential then plateaus as a result of: (1)  Increased calcium ion permeability. (2) Decreased potassium ion permeability. The voltage-gated calcium ion channels open slowly during phases 1 and 0, and calcium enters the cell. Potassium channels then close, and the combination of decreased potassium ion efflux and increased calcium ion influx causes the action potential to plateau. Key Concept: Ca++and K+ channels remain open during plateau phase (phase 2) of AP. But Ca2+ initiates the plateau phase, as K+ channels are already open from phase 1. This Ca++ channel in phase 2 is also known as calcium-sodium channel.