Mibefradil block of cloned T-type calcium channels

Mibefradil is a tetralol derivative chemically distinct from other calcium channel antagonists. It is a very effective antihypertensive agent that is thought to achieve its action via a higher affinity block for low-voltage-activated (T) than for high-voltage-activated (L) calcium channels. Estimates of affinity using Ba(2+) as the charge carrier have predicted a 10- to 15-fold preference of mibefradil for T channels over L channels. However, T channel IC(50) values are reported to be approximately 1 microM, which is much higher than expected for clinical efficacy because relevant blood levels of this drug are approximately 50 nM. We compared the affinity for mibefradil of the newly cloned T channel isoforms, alpha1G, alpha1H, and alpha1I with an L channel, alpha1C. In 10 mM Ba(2+), mibefradil blocked in the micromolar range and with 12- to 13-fold greater affinity for T channels than for L channels (approximately 1 microM versus 13 microM). When 2 mM Ca(2+) was used as the charge carrier, the drug was more efficacious; the IC(50) for alpha1G shifted to 270 nM and for alpha1H shifted to 140 nM, 4.5- and 9-fold higher affinity than in 10 mM Ba. The data are consistent with the idea that mibefradil competes for its binding site on the channel with the permeant species and that Ba(2+) is a more effective competitor than Ca(2+). Raising temperature to 35 degrees C reduced affinity (IC(50) 792 nM). Reducing channel availability to half increased affinity ( approximately 70 nM). This profile of mibefradil affinity makes these channels good candidates for the physiological target of this antihypertensive agent.