Determinants of voltage-dependent inactivation affect Mibefradil block of calcium channels

The voltage gated calcium channel family is a major target for a range of therapeutic drugs. Mibefradil (Ro 40-5967) belongs to a new chemical class of these molecules which differs from other Ca2+ antagonists by its ability to potently block T-type Ca2+ channels. However, this molecule has also been shown to inhibit other Ca2+ channel subtypes. To further analyze the mechanism governing the Ca2+ channel-Mibefradil interaction, we examined the effect of Mibefradil on various recombinant Ca2+ channels nov expressed in mammalian cells from their cloned cDNAs, using Ca2+ as the permeant ion at physiological concentration. Expression of a(1A) a(1C) and a(1E) in tsA 201 cells resulted in Ca2+ currents with functional characteristics closely related to those of their native counterparts. Mibefradil blocked alpha(1A) and ct,, with a Kd comparable to that reported for T-type channels, but had a lower affinity (similar to 30-fold) for alpha(1C). For each channel, inhibition by Mibefradil was consistent with high-affinity binding to the inactivated state. Modulation of the voltage-dependent inactivation properties by the nature of the coexpressed beta subunit or the al splice variant altered block at the Mibefradil receptor site. Therefore, we conclude that the tissue and sub-cellular localization of calcium channel subunits as well as their specific associations are essential parameters to understand the in vivo effects of Mibefradil. (C) 1999 Elsevier Science Ltd. All rights reserved.