Abstract

The effects of chlorpromazine on calcium channel currents were studied in cultured mouse neuroblastoma cells (N1E-115) using the whole-cell variation of the patch-clamp technique. Two types of calcium channel currents (type I and type II) as carried by barium ions were recorded separately on the basis of their different voltage- and time-dependent kinetics. Chlorpromazine reversibly reduced the amplitude of both types of calcium channel currents. The concentration dependence of the calcium channel block indicated a one-to-one binding stoichiometry for both type I and type II currents, with an apparent dissociation constant of 15 microM in both cases. The block of calcium channels was dependent on the holding potential for both type I and type II currents, being enhanced by depolarization. This voltage dependence of the block was due to a higher affinity of chlorpromazine for the inactivated state of the calcium channels than the resting state, as demonstrated by a hyperpolarizing shift of the steady-state inactivation curve. The activation kinetics were not affected by chlorpromazine in either type I or type II current. The time course of inactivation of the type I current was not changed by chlorpromazine, whereas that of the type II current was accelerated, suggesting an involvement of an open channel block. Chlorpromazine block of type I current was independent of the level of test depolarizing pulse, whereas that of type II current was augmented with an increase in depolarization. It was concluded that chlorpromazine binds preferentially to the inactivated form of both type I and type II calcium channels, without affecting the gating kinetics of channel activation.