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-a ctivated (T) than for high-voltage-activated (L) calcium channels. Estimate s of affinity using Ba2+ as the charge carrier have predicted a 10- to 15-f old preference of mibefradil for T channels over L channels. However, T cha nnel IC50 values are reported to be similar to 1 mu M, which is much higher than expected for clinical efficacy because relevant blood levels of this drug are similar to 50 nM. We compared the affinity for mibefradil of the n ewly cloned T channel isoforms, alpha 1G, alpha 1H, and alpha 1I with an L channel, alpha 1C. In 10 mM Ba2+, mibefradil blocked in the micromolar rang e and with 12- to 13-fold greater affinity for T channels than for L channe ls (similar to 1 mu M versus 13 mu M). When 2 mM Ca2+ was used as the charg e carrier, the drug was more efficacious; the IC50 for alpha 1G shifted to 270 nM and for alpha 1H 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 com petes for its binding site on the channel with the permeant species and tha t Ba2+ is a more effective competitor than Ca2+. Raising temperature to 35 degrees C reduced affinity (IC50 792 nM). Reducing channel availability to half increased affinity (similar to 70 nM). This profile of mibefradil affi nity makes these channels good candidates for the physiological target of t his antihypertensive agent.