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

• A L-type Ca2+ channels (ICaL); reversal potential ~+120 mV
• B Ih (funny current, HCN channels); reversal potential ~ -30 mV
• C Voltage-gated Na+ channels (INa); reversal potential ~+50 mV ✓
• D IKr channels (rapid delayed rectifier); reversal potential ~ -90 mV

Explanation

Phase 0 of the ventricular action potential is produced by rapid opening of voltage-gated Na+ channels (INa), which have an equilibrium (reversal) potential near +50 mV (ENa). This creates the steep, fast upstroke. L-type Ca2+ channels are responsible for phase 2 (plateau), not phase 0. Funny current (Ih) drives spontaneous depolarisation in the SA node. IKr contributes to phase 3 repolarisation.

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

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