Inhibitory effects of cocaine on Ca2+ transients and contraction in single cardiomyocytes

The effects of cocaine on the Ca2+ transient responsible for excitation-contraction coupling were studied in single rat heart cells loaded with the fluorescent Ca2+ indicator fura 2. A high-speed imaging technique using a charge-coupled device as detector and transient image store [O'Rourke et al., Am. J. Physiol. 259 (Heart Circ. Physiol. 28): H230-H242, 1990] was used to measure cytosolic free Ca2+ concentration ([Ca2+]i) and contraction simultaneously from the images of fluorescence. Cardiomyocytes maintained a basal [Ca2+]i of approximately 140 nM in presence or absence of cocaine. After electrical field stimulation, [Ca2+]i increased to a peak of 498 +/- 25 nM under control conditions. This was reduced to a peak [Ca2+]i of 389 +/- 25 nM after treatment with 50 microM cocaine. Cocaine also reduced the rate of rise of [Ca2+]i but did not affect the time to peak or the half time for resequestration of the released Ca2+. The rate and extent of cell shortening was reduced by cocaine in parallel with the inhibition of the [Ca2+]i transient. Cocaine had no effect on the half time for relaxation. Cocaine did not modify the relationship between contraction and the elevation of [Ca2+]i over a range of extracellular Ca2+ concentrations. The intracellular pool of Ca2+ releasable by caffeine was also unaffected by cocaine. In the presence of the beta-adrenergic agonist isoproterenol, which caused a large enhancement of peak [Ca2+]i and contraction, cocaine still inhibited both parameters. However, cocaine did not reverse the ability of isoproterenol to enhance the rate of Ca2+ reuptake and cell relaxation. Whole cell voltage-clamp studies showed that 50 microM cocaine reduced both the Na+ current (50%) and the Ca2+ current (30%). These data suggest that sarcolemmal ion channels are the primary site that, in cardiac muscle, mediate the negative inotropic effects and the suppression of [Ca2+]i transients by cocaine.