Effects of mibefradil, a selective T-type Ca2+ channel antagonist, on sino-atrial node and ventricular myocardia

The effects of mibefradil, a non-dihydropyridine Ca2+ channel antagonist, o n the action potential configuration of isolated rabbit sino-atrial node pr eparations, membrane currents of guinea-pig ventricular myocytes and the co ntractile force of isolated ventricular papillary muscles were examined. In sino-atrial node preparations, 10 mu M mibefradil decreased the slope of t he pacemaker depolarization (phase 4 depolarization) and maximum rate of ri se, and shifted the threshold potential to the positive direction with no e ffect on action potential duration. In ventricular myocytes, 1 mu M mibefra dil inhibited the T-type Ca2+ current by about 40% while it had no effect o n the T-type Ca2+ current. At 10 mu M, mibefradil inhibited the L-type and T-type Ca2+ currents by about 40% and 90%, respectively. Mibefradil had no effect on contractile force at concentrations up to 1 mu M. Thus, mibefradi l was shown to produce potent prolongation of the pacemaker depolarization, mainly through inhibition of the T-type Ca2+ current. It is suggested that the T-type Ca2+ current may not be involved in ventricular contraction.