Which ion current is primarily responsible for the plateau phase (phase 2) of the ventricular action potential, and why is calcium entry during this phase critical?

• A Persistent sodium current (INaL) sustaining depolarization; critical for action potential propagation between cells
• B L-type Ca²⁺ current (ICaL) balanced by outward K⁺ current (IKr, IKs); critical because it triggers Ca²⁺-induced Ca²⁺ release from SR (CICR), the mechanism coupling electrical excitation to mechanical contraction ✓
• C Funny current (If) maintaining the plateau; critical for spontaneous pacemaker activity
• D Inward rectifier K⁺ current (IK1) sustaining the plateau by preventing repolarization

Explanation

Phase 2 (plateau) of the ventricular AP is maintained by a balance between slow inward L-type Ca2+ current (ICaL) and slow outward K+ currents (IKr — rapid delayed rectifier, and IKs — slow delayed rectifier). ICaL entry into the cell triggers a much larger Ca2+ release from the SR via ryanodine receptors (RyR2) — calcium-induced calcium release (CICR) — amplifying the trigger 10-fold to produce the cytosolic Ca2+ transient needed for actomyosin cross-bridge cycling and contraction. IK1 (inward rectifier) is actually responsible for maintaining phase 4 resting potential and is inhibited during the plateau. The funny current (If, HCN channel) is the pacemaker current in SA node.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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