During phase 2 (plateau) of the cardiac ventricular action potential, which ionic currents are most responsible for maintaining the prolonged depolarization?

• A Persistent inward Na+ current (INaL) and inward Ca2+ current (ICaL) balanced by slow outward K+ current (IKr, IKs) ✓
• B Rapid Na+ influx through fast Na+ channels maintaining depolarization
• C Inward Cl- current combined with outward K+ current
• D Na+/Ca2+ exchanger (NCX) operating in reverse mode as the sole inward current

Explanation

The plateau phase (phase 2) of the ventricular action potential — unique to cardiac muscle and enabling the refractory period that prevents tetany — is maintained by a balance between sustained inward L-type Ca2+ current (ICaL, the principal plateau current) and a late/persistent Na+ current (INaL), opposed by slowly activating outward K+ currents (IKr — rapid delayed rectifier and IKs — slow delayed rectifier). ICaL also triggers Ca2+-induced Ca2+ release from the SR. Drugs that block ICaL (e.g., verapamil, diltiazem) shorten the plateau and slow conduction.

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

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