Isradipine interacts with the open state of the L-type calcium channel at high concentrations.

Triple mutation of Tyr1485, Met1486 and Ile1493 in the IVS6 segment of alpha 1C-b subunit of the L-type calcium channel results in a loss of the high affinity inhibition by isradipine. The mutant channel (Ch30) yet exhibits a concentration-dependent inhibition by isradipine with a 110-fold lower affinity. The mechanisms underlying the remaining low affinity block were investigated. Isradipine accelerated the current decay in Ch30 but not in wild type channel in a concentration dependent manner. Dependence of the current amplitude inhibition on holding potential was parallel in Ch30 and in wild type channels, while the acceleration of current decay in Ch30 was independent of the membrane potential. The recovery from voltage-dependent inactivation was biphasic in both channels and was slowed down by isradipine in the wild type but not in the Ch30 channel. The change of the charge carrier (Ba2+ or Ca2+) and calcium chelator (EGTA or BAPTA) did not affect the acceleration of current decay indicating that isradipine did not interact with the Ca(2+)-inactivated state of the channel. These results demonstrate that the mutations of Ch30 affect selectively the high affinity inhibition of an inactivated channel and unmask a low affinity interaction of isradipine with an open state of the channel.