The rapid upstroke (phase 0) of the ventricular action potential is generated by which ion channel, and what is its approximate reversal potential?

• A L-type Ca²⁺ channels (ICaL); approximately +120 mV
• B IKr channels; approximately -90 mV
• C If (funny current) channels; approximately -40 mV
• D Fast Na⁺ channels (INa); approximately +40 mV ✓

Explanation

Phase 0 of the ventricular action potential is mediated by voltage-gated fast Na⁺ channels (INa) that open rapidly when membrane potential reaches threshold (approximately -65 to -70 mV). The equilibrium (Nernst) potential for Na⁺ is approximately +60 mV, and the actual peak during phase 0 reaches about +20 to +40 mV due to simultaneous outward currents. L-type Ca²⁺ channels contribute mainly to phase 2 (plateau), IKr to phase 3 repolarization, and If to spontaneous depolarization in pacemaker cells.

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

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