Calcium-channel blockers inhibit the inward ﬂow of calcium through the specialised slow channels of cardiac and arterial smooth-muscle cells. By thus relaxing the smooth muscle, they have important applications in the treatment of HYPERTENSION and ANGINA PECTORIS. Various types of calcium-channel blockers are available in the United Kingdom; these diﬀer in their sites of action, leading to notable diﬀerences in their therapeutic eﬀects. All the drugs are rapidly and completely absorbed, but extensive ﬁrst-pass metabolism in the liver reduces bioavailability to around one-ﬁfth. Their hypotensive eﬀect is additive with that of beta blockers (see BETA-ADRENOCEPTOR-BLOCKING DRUGS); the two should, therefore, be used together with great caution – if at all. Calcium-channel blockers are particularly useful when beta blockers are contraindicated, for example in asthmatics. However, they should be prescribed for hypertension only when THIAZIDES and beta blockers have failed, are contraindicated or not tolerated.
Verapamil, the longest-available, is used to treat angina and hypertension. It is the only calcium-channel blocker eﬀective against cardiac ARRHYTHMIA and it is the drug of choice in terminating supraventricular tachycardia. It may precipitate heart failure, and cause HYPOTENSION at high doses. Nifedipine and diltiazem act more on the vessels and less on the myocardium than verapamil; they have no antiarrhythmic activity. They are used in the prophylaxis and treatment of angina, and in hypertension. Nicardipine and similar drugs act mainly on the vessels, but are valuable in the treatment of hypertension and angina. Important diﬀerences exist between diﬀerent calcium-channel blockers so their use must be carefully assessed. They should not be stopped suddenty, as this may precipitate angina. (See also HEART, DISEASES OF.)