Excitation-Contraction Coupling in Rat Myocardium: Modulation with Verapamil and Calcium
Verapamil (Isoptin, Knoll) (1), a non-nitrate derivative of papaverine is a vasodilator that produces a short-lasting increase in coronary blood flow and decrease in blood pressure (2) and a long- lasting depression of contractile force (3–5). The uncoupling of excitation from contraction in heart muscle by verapamil is thought to result from its Ca-antagonistic properties (3). Data from both electrophysiological (6–9) and radioactive Ca experiments (4,10) indicate that these drugs reduce transmembrane Ca-influx, a process which is an integral step of excitation-contraction coupling in cardiac muscle (11). Inhibition of calcium influx through the sarcolemma has been postulated for the negative inotropic effect of the drug (3,12,13). This electrogenic influx of calcium occurs during depolarization and has been described by Reuter (11) as a slow inward flow of current and is usually thought to be associated with the plateau of the transmembrane action potential.
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