During phase 2 (plateau) of the cardiac ventricular action potential, the net inward current is primarily carried by which channel, and its prolonged opening is responsible for which clinically important property?

• A T-type Ca²⁺ channels (ICaT); responsible for pacemaker automaticity
• B L-type Ca²⁺ channels (ICaL); responsible for the long refractory period preventing tetanic contraction ✓
• C Funny current (If) channels; responsible for the plateau duration in working myocardium
• D Rapid inward Na⁺ current (INa); responsible for the absolute refractory period

Explanation

The plateau of the ventricular action potential is maintained by L-type (long-lasting, dihydropyridine-sensitive) Ca²⁺ channels whose prolonged opening balances outward K⁺ currents. This extended plateau creates the long absolute refractory period (~250 ms), which prevents the summation of contractions (tetanus) — a critical safety feature of cardiac muscle. T-type channels and If contribute to pacemaker activity in nodal cells, not to the plateau of working myocardium.

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

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