A contractile cardiac cell exposed to a calcium channel blocker would have which of the following changes to its action potential?

• A. Increased amplitude
• B. Shortened duration
• C. Width or duration reduced
• D. No change

Answer: (C)

In contractile cardiac cells, the plateau phase of the action potential (phase 2) is sustained by a calcium current through L-type calcium channels. When a calcium channel blocker is present, this inward Ca2+ current is reduced. With less depolarizing current during the plateau, the repolarizing potassium currents dominate sooner, so the plateau ends earlier and the action potential duration is shortened. The initial upstroke is driven mainly by sodium channels, so the peak amplitude isn’t increased by blocking calcium entry; the main change is a shorter plateau.