Mibefradil inhibition of T-type calcium channels in cerebellar Purkinje neurons

The antihypertensive agent mibefradil completely and reversibly inhibited T -type calcium channels in freshly isolated rat cerebellar Purkinje neurons. The potency of mibefradil was increased at less hyperpolarized holding pot entials, and the apparent affinity was correlated with the degree of channe l inactivation. At 35 degrees, the apparent dissociation constant K-app was 1 mu M at a holding voltage of -110 mV (corresponding to noninactivated ch annels) and 83 nM at a holding voltage of -70 mV (corresponding to 65% inac tivation). The increased affinity was attributable mainly to a decreased of f-rate. Mibefradil also inhibited P-type calcium channels in Purkinje neuro ns, but inhibition was much less potent. At a holding potential of -70 mV, the K-app for mibefradil inhibition of P-type channels was similar to 200-f old higher than that for inhibition of T-type channels. Mibefradil should b e a useful compound for distinguishing T-type channels from high voltage-ac tivated calcium channels in neurons studied in vitro.