Cellular actions and pharmacology of calcium-channel blockers.

The calcium-channel blockers represent a diverse group of chemical structures that block calcium-selective channels in the plasma membrane of a variety of excitable cells. As the calcium fluxes carried by these channels allow ionic calcium (Ca2+) to gain access to the cell interior, where Ca2+ serves as an activator--messenger, calcium-channel blockers generally act to inhibit cell function. By reducing the depolarizing currents caused by the entry of positively charged Ca2+ into the negatively charged interior of resting cells, the calcium-channel blockers also inhibit excitatory processes that depend on calcium entry across the plasma membrane. These principles account for most of the effects of calcium-channel blockers on the cardiovascular system. In vivo, the calcium-channel blockers inhibit contractile function in the heart and vascular smooth muscle and, because the initial depolarizing currents in the sinoatrial and atrioventricular nodes are carried by calcium channels, slow the heart rate and prolong atrioventricular conduction. However, in vivo in human studies, there are differences among the calcium-channel blockers. The vasodilatory effects of the calcium channel blockers, which can reduce systemic blood pressure, offer a primary basis for their potential use in the treatment of hypertension. The tissue specificity exhibited by some of the calcium-channel blockers may enhance their therapeutic value in selected hypertensive patients. Of the three calcium-channel blockers now available for use in the United States (diltiazem, nifedipine, and verapamil), diltiazem and verapamil are approximately equipotent in inhibiting calcium-channel function in cardiac and vascular smooth muscle, whereas nifedipine is more potent in vascular smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)