Which current predominates during phase 0 in pacemaker cells?

Which current predominates during phase 0 in pacemaker cells?

• A. Fast Na+ current
• B. L-type calcium current
• C. T-type calcium current
• D. Potassium outward current

Answer: (B)

In pacemaker cells, the rapid upstroke of the action potential (phase 0) is driven mainly by calcium entry through L-type voltage-gated calcium channels. Unlike ventricular and atrial myocytes, which rely on a fast sodium current for phase 0, pacemaker cells have little fast INa and rely on ICa,L to depolarize to threshold quickly. The T-type calcium current can contribute to earlier or slower depolarization in some pacemaker contexts, but it is not the primary driver of phase 0. The outward potassium current, on the other hand, helps repolarize (phase 3) rather than initiate depolarization. So the predominant current during phase 0 in pacemaker cells is the L-type calcium current.

In pacemaker cells, the rapid upstroke of the action potential (phase 0) is driven mainly by calcium entry through L-type voltage-gated calcium channels. Unlike ventricular and atrial myocytes, which rely on a fast sodium current for phase 0, pacemaker cells have little fast INa and rely on ICa,L to depolarize to threshold quickly. The T-type calcium current can contribute to earlier or slower depolarization in some pacemaker contexts, but it is not the primary driver of phase 0. The outward potassium current, on the other hand, helps repolarize (phase 3) rather than initiate depolarization. So the predominant current during phase 0 in pacemaker cells is the L-type calcium current.