Which ionic mechanism best distinguishes the plateau phase (Phase 2) of the ventricular action potential from the rapid repolarization phase (Phase 3)?

Explanation

## Ionic Basis of Cardiac Action Potential Phases ### Phase 2 (Plateau Phase) — Ventricular Myocyte **Key Point:** Phase 2 is characterized by a **balance between inward (depolarizing) and outward (repolarizing) currents**, resulting in a relatively flat membrane potential. - **Inward current:** L-type (dihydropyridine-sensitive) calcium channels remain open, allowing sustained Ca²⁺ influx - **Outward current:** Delayed rectifier K⁺ channels (I_K) begin to open but are initially overwhelmed by Ca²⁺ entry - **Net result:** Slow depolarization or near-zero slope — the "plateau" - **Duration:** 200–300 ms in ventricular tissue; shorter in atrial tissue ### Phase 3 (Rapid Repolarization) — Ventricular Myocyte **Key Point:** Phase 3 is dominated by **potassium efflux exceeding any inward current**. - **Inward current:** L-type Ca²⁺ channels inactivate; Na⁺/Ca²⁺ exchanger may contribute inward current - **Outward current:** Delayed rectifier K⁺ channels (I_K and I_Ks) fully activate; inward rectifier K⁺ channels (I_K1) activate - **Net result:** Steep negative slope as K⁺ efflux dominates - **Membrane potential:** Returns from +20 mV toward −90 mV ### Comparison Table | Feature | Phase 2 (Plateau) | Phase 3 (Repolarization) | |---------|-------------------|------------------------| | **Dominant inward current** | L-type Ca²⁺ channels | Minimal (Ca²⁺ channels inactivated) | | **Dominant outward current** | Early I_K activation (weak) | Full I_K, I_Ks, I_K1 activation (strong) | | **Net current** | Balanced (near zero) | Outward (K⁺ efflux >> inward) | | **Membrane potential slope** | Flat or slightly positive | Steeply negative | | **Membrane potential range** | −10 to +20 mV | +20 mV to −90 mV | | **Duration** | 200–300 ms | 100–200 ms | **High-Yield:** The **plateau phase is unique to ventricular and atrial myocytes** — it is absent in sinoatrial (SA) nodal cells, which lack L-type Ca²⁺ channels and have no plateau. **Clinical Pearl:** Drugs that block L-type Ca²⁺ channels (e.g., verapamil, diltiazem) shorten Phase 2 and reduce the action potential duration (APD). Drugs that block K⁺ channels (e.g., amiodarone, sotalol) **prolong** Phase 2 and APD. **Mnemonic:** **"PLAT = Positive, Long, Ample, Two-current"** — Phase 2 is positive (depolarized), long (200–300 ms), ample (sustained), and balanced between two currents (Ca²⁺ in, K⁺ out).