Calcium-channel blocking agents.

The role of calcium in the cardiovascular system, and the pharmacology, pharmacokinetics, and studies evaluating the clinical use of three calcium-channel blocking agents--verapamil hydrochloride, nifedipine, and diltiazem hydrochloride--are reviewed. Inhibition of calcium conductance and alteration of calcium availability cause profound changes in: slow inward current of the cardiac action potential, myocardial contractility and metabolism, blood pressure regulation, and smooth-muscle activity. Calcium-channel blocking agents affect the movement of calcium through these channels in smooth and cardiac muscle; the specific agents in this class differ markedly in their inhibitory effects. Verapamil hydrochloride is useful intravenously for treating supraventricular rhythm disturbances. It is absorbed well when taken orally, but there is an extensive first-pass effect, so that about 20% enters the systemic circulation. The incidence of side effects in patients receiving verapamil is 9-10%; about 1% require discontinuation of therapy. Verapamil is contraindicated in patients with sinus-node disease, unstable atrioventricular block, and shock. Nifedipine has proven useful for hypertension, coronary-artery spasm, and exertional angina; it has little negative inotropic effect. Approximately 90% of an oral dose is absorbed, and 65-70% reaches the systemic circulation after first-pass metabolism. Protein binding of nifedipine ranges from 92 to 98%. Side effects of nifedipine, usually associated with the peripheral vasodilatory action, occur in approximately 15% of patients, requiring discontinuance in 2-5%. Diltiazem hydrochloride has been shown effective in the treatment of coronary-artery spasm; limited studies indicate it may be useful in treating exertional angina, hypertension, and possibly arrhythmias. Diltiazem's oral bioavailability is good (90% reaches systemic circulation), but there is significant interindividual variability between administered dose and resulting plasma concentration. Geriatric patients have delayed absorption and reduced clearance of diltiazem given in sustained-release tablets. Studies of diltiazem are limited at this time. The exact role of calcium-channel blocking agents has not yet been elucidated. However, their ability to influence the calcium channel greatly expands the therapeutic armamentarium for cardiovascular disease and other disorders.