Repolarization in Excitable Cells is Caused by Potassium Efflux
The repolarization phase in excitable cells is caused by potassium (K) efflux (option 3). During phase 3 of the action potential, the transmembrane potential repolarizes from its plateau voltage of approximately +10 mV back to its resting level of approximately -85 mV through the selective movement of potassium ions out of the cell 1.
Mechanism of Repolarization
Voltage-gated potassium channels open in response to depolarization and facilitate selective efflux of potassium ions across the plasma membrane, making them the major determinants of cellular repolarization in excitable cells 2.
The Repolarization Process
- The rapid repolarization phase (phase 3) is driven by potassium currents that activate following membrane depolarization 1
- Multiple distinct K+ channels contribute to repolarization, with some driving early repolarization and others responsible for late repolarization 3
- In ventricular myocytes, two kinetically distinct voltage-dependent K+ currents participate: a rapidly activating/inactivating transient outward current (Ito) and a slowly activating/inactivating delayed rectifier (IK) 4
Ionic Basis
- Potassium efflux (K+ moving out of the cell) returns the membrane potential from positive values back toward the negative resting potential 1
- This process creates the interventricular and intraventricular voltage gradients that generate the T wave on the body surface ECG 1
- The repolarization sequence generally proceeds from epicardium to endocardium, opposite to the direction of depolarization 1, 5
Why Other Options Are Incorrect
- Sodium influx (option 2) causes depolarization (phase 0), not repolarization
- Sodium efflux (option 1) does not play a significant role in the repolarization phase
- Potassium influx (option 4) would further depolarize rather than repolarize the cell
Clinical Relevance
- Abnormalities in potassium channel function can cause primary repolarization abnormalities manifested as ST-segment and T-wave changes on ECG 1
- These abnormalities may be caused by electrolyte disturbances (particularly potassium and calcium), drugs, toxins, ischemia, or inherited channelopathies 1
- Understanding that repolarization depends on K+ efflux explains why hypokalemia prolongs repolarization and hyperkalemia shortens it 1