Mechanism of voltage- and use-dependent block of class A Ca²⁺channels by mibefradil

1 The action of mibefradil was studied on wild type class A calcium (Ca2+) channels and various class A/L-type channel chimaeras expressed in Xenopus oocytes. The mechanism of Ca2+ channel block by mibefradil was evaluated with two microelectrode voltage clamp. 2 Resting-state dependent block (or initial block) of barium currents (I-Ba) through class A Ca2+ channels was concentration dependent with an IC50 value of 208+/-23 mu M. 3 Mibefradil (50 mu M) did not significantly affect the midpoint voltage of the steady-state inactivation curve suggesting that inactivation does not promote Ca2+ channel block. Chimaeric class A/L-type Ca2+ channels inactivating with faster or slower kinetics than wild type class A channels were equally well inhibited by mibefradil as wild type class A channels. 4 Frequent Ca2+ channel activation facilitated I-Ba inhibition by mibefradil (use-dependent block). Recovery from use-dependent block was voltage-dependent. being slower at depolarized membrane potentials (tau = 75 +/- 15 s at -70 mV, (n = 6) vs tau = 20 +/- 2 s at - 100 mV, (n = 6), P<0.05). 5 We suggest that use-dependent block of class A Ca2+ channels by mibefradil occurs because of slow recovery from open channel block (SROB) and not because of drug binding to inactivated channels. 6 Voltage-dependent slow recovery from open state-dependent block provides a molecular basis for understanding the cardiovascular profile of mibefradil such as selectivity for vasculature and relative lack of negative inotropic effects.