Inward rectifier channels help maintain which aspects of the cardiac action potential?

• A. Late phase 3 repolarization and phase 4 negativity
• B. Upstroke phase 0
• C. Plateau phase 2
• D. Early repolarization

Answer: (A)

Inward rectifier channels mainly set and stabilize the resting membrane potential, and they contribute to the final part of repolarization. These channels conduct K+ most effectively when the cell is hyperpolarized, pulling the membrane toward the potassium equilibrium potential and keeping the resting voltage negative during phase 4. As the action potential finishes and the membrane repolarizes toward Ek, these channels help complete late phase 3 repolarization by providing a stabilizing K+ conductance as the membrane returns to its negative resting level. This combination—strong stabilization of diastolic negativity and assistance in ending repolarization—is why they’re key for the aspects described.

They don’t drive the rapid upstroke, which is Na+ influx; they don’t primarily shape the plateau, which is balanced Ca2+ influx and K+ efflux during phase 2; and they aren’t the main players in early repolarization, which is dominated by other K+ currents like Ito.