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

Abstract

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 μM. 3. Mibefradil (50 μM) 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 (τ = 75 ± 15 s at -70 mV, (n = 6) vs τ = 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.