Ace the CVS Pharmacy Exam 2025 - Your Prescription for Success!

The upstroke of the pacemaker action potential in the heart is primarily mediated by L-type calcium channels. During this phase, a significant influx of calcium ions occurs, which is critical for the depolarization that characterizes the action potential in pacemaker cells of the sinoatrial (SA) node and other parts of the conducting system of the heart.

L-type calcium channels open in response to membrane depolarization and allow calcium ions (Ca++) to flow into the cells. This influx of calcium is essential for initiating the action potential and is a key factor in cardiac muscle contraction. In pacemaker cells, the action potential doesn't rely predominantly on sodium channels like in other excitable tissues; instead, the L-type calcium channels are crucial for the characteristic rhythm and pacing of the heart.

Though T-type calcium channels do contribute to the early phases of depolarization in some types of cardiac cells, they are not the primary mediators during the upstroke of the pacemaker action potential. Sodium channels are mainly active in faster depolarizing tissues, and potassium channels are involved in repolarization and not in mediating the upstroke. This makes the L-type calcium channels the correct choice for this question.