T- and L-Type Ca2+-Channel Antagonists Reduce Contractility in Guinea Pig Cardiac Myocytes

Summary: The aim of this study was to investigate the influence of L- and T-type Ca2+-channel blockade on myocardial contractility in guinea pig cardiomyocytes. Left ventricular myocardium from guinea pig contains both L- and T-type Ca2+ channels. The T-type Ca2+ influx was inhibited with mibefradil (1-100 μM), a novel compound with a threefold higher affinity for T- compared with L-type Ca2+ channels. In comparison, L-type Ca2+ influx was reduced by the benzodiazepine diltiazem (1-100 μM). The effect of mibefradil and diltiazem on electrically driven (0.5 Hz) isolated cardiomyocytes (n = 12) was studied in a concentration-dependent manner. The change of the contraction amplitude (percentage of cell shortening) was continuously recorded with an one-dimensional high-speed camera. Both mibefradil and diltiazem concentration-dependently reduced (p < 0.05 vs. control) the contraction amplitude in isolated myocytes from guinea pig. The concentration at which the contraction amplitude of guinea pig cardiomyocytes was reduced by 50% (EC50) was 31.6 μM for diltiazem and 6.3 μM for mibefradil, indicating that the T-type Ca2+-channel blocker mibefradil is more potent in reducing contractility in guinea pig cardiac myocytes in comparison with the L-type Ca2+-channel antagonist diltiazem. Mean values for cell shortening in percentage ± SEM for mibefradil (0, 1, 10, 100 μM) were 100%, 78 ± 9.2%, 36 ± 5.4%, and 24 ± 3.6%. The corresponding values for diltiazem were 100%, 92 ± 12.5%, 79 ± 8.9%, and 35 ± 2.6%. In contrast, the increase of the extracellular Ca2+ concentration (2-7.5 mM) resulted in a significant increase of the contraction amplitude (+213 ± 14%). Therefore, blockade of the Ca2+ influx through voltage-dependent T- or L-type Ca2+ channels decreases contraction in isolated cardiac myocytes from guinea pigs containing L- and T-type Ca2+ channels.