Calcium channel blockers
Calcium channel blockers are commonly used to prevent anginathat doesn’t respond to drugs in either of the other antianginal classes. They’re the drug of choice to treat Prinzmetal’s angina. As mentioned earlier, several of the calcium channel blockers are also used as antiarrhythmics and to treat hypertension. Calcium channel blockers used to treat angina include:
When administered orally, calcium channel blockers are absorbed quickly and almost completely. Because of the first-pass effect, however, the bioavailability of these drugs is much lower. The cal-cium channel blockers are highly bound to plasma proteins.
All calcium channel blockers are metabolized rapidly and almost completely in the liver.
Calcium channel blockers prevent the passage of calcium ions across the myocardial cell membrane and vascular smooth-muscle cells. This causes dilation of the coronary and peripheral arteries, which decreases the force of the heart’s contractions and reduces the workload of the heart.
Also, by preventing arterioles from constricting, calcium channel blockers reduce afterload. Decreasing afterload further decreases the oxygen demands of the heart. (See How calcium channelblockers work)
Some calcium channel blockers (diltiazem and verapamil) also re-duce the heart rate by slowing conduction through the SA and AV nodes. A slower heart rate reduces the heart’s need for additional oxygen.
Calcium channel blockers are used for long-term prevention of angina only, not short-term relief of chest pain. Calcium channel blockers are particularly effective for preventing Prinzmetal’s angina.
· Calcium salts and vitamin D reduce the effectiveness of calcium channel blockers.
· Nondepolarizing blocking drugs may have an enhanced muscle-relaxant effect when taken with calcium channel blockers.
· Verapamil and diltiazem increase the risk of digoxin toxicity, en-hance the action of carbamazepine, and may cause myocardial de-pression. (See Adverse reactions to calcium channel blockers.)