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.

Full Text

The Full Text of this article is available as a PDF (339.9 KB).