Initial rapid repolarization in cardiac muscle is mediated through:
**Core Concept**
The initial rapid repolarization in cardiac muscle is a crucial phase of the cardiac action potential, which helps to restore the resting membrane potential. This process involves the closure of voltage-gated sodium channels, the opening of voltage-gated potassium channels, and the activation of various repolarizing potassium currents.
**Why the Correct Answer is Right**
The initial rapid repolarization is primarily mediated through the rapid activation of the delayed rectifier potassium current (IKr) and the transient outward potassium current (Ito). These potassium currents help to rapidly decrease the membrane potential, leading to repolarization. The IKr current is carried by the hERG channel, which is sensitive to certain medications like sotalol and dofetilide. The Ito current is carried by the Kv4.3 channel, which is involved in the early phase of repolarization.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the initial rapid repolarization is not primarily mediated through the L-type calcium current (ICa,L). While ICa,L does contribute to the plateau phase of the cardiac action potential, it is not involved in the initial rapid repolarization.
**Option B:** This is incorrect because the initial rapid repolarization is not primarily mediated through the sodium-potassium pump (Na+/K+-ATPase). The Na+/K+-ATPase is involved in maintaining the resting membrane potential, but it is not a major player in the initial rapid repolarization.
**Option C:** This is incorrect because the initial rapid repolarization is not primarily mediated through the inward rectifier potassium current (IK1). While IK1 does contribute to the resting membrane potential, it is not involved in the initial rapid repolarization.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the hERG channel, which carries the IKr current, is sensitive to certain medications like sotalol and dofetilide, which can prolong the QT interval and increase the risk of torsades de pointes.
**Correct Answer:** C. The transient outward potassium current (Ito) and the rapid activation of the delayed rectifier potassium current (IKr).