Verapamil acts by –
**Core Concept**
Verapamil is a calcium channel blocker that belongs to the phenylalkylamine class. It primarily affects the L-type calcium channels in cardiac and smooth muscle cells, leading to a decrease in intracellular calcium levels.
**Why the Correct Answer is Right**
Verapamil acts by binding to the α1 subunit of the L-type calcium channels, which blocks the inward flow of calcium ions. This results in a decrease in the contractility of the heart and a subsequent decrease in blood pressure. Additionally, verapamil has a negative inotropic effect on the heart, which means it decreases the force of contraction of the heart muscle. This is particularly useful in treating conditions such as hypertension, angina pectoris, and certain types of arrhythmias.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because verapamil does not primarily act through potassium channels. While it may have some effect on potassium channels, its primary mechanism of action is through the blockade of L-type calcium channels.
**Option B:** This option is incorrect because verapamil is not a beta-blocker. Beta-blockers act by blocking the effects of epinephrine on the heart, whereas verapamil blocks the influx of calcium ions.
**Option C:** This option is incorrect because verapamil does not primarily act through sodium channels. While it may have some effect on sodium channels, its primary mechanism of action is through the blockade of L-type calcium channels.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that calcium channel blockers like verapamil can cause a reflex increase in sympathetic tone, leading to an increase in heart rate and blood pressure. This is known as a "reflex tachycardia" and can be a side effect of these medications.
**Correct Answer:** D.