Mibefradil, a T-type and L-type calcium channel blocker, limits infarct size through a glibenclamide-sensitive mechanism.

Mibefradil is a novel calcium channel blocker with activity at both L-type and T-type calcium channels. There are data suggesting that this compound can protect the ischemic/reperfused myocardium in spite of the fact that there is a very low abundance of T-type calcium channels within ventricular tissue. The aims of this study were two-fold. First, we wished to study the protective effect of mibefradil on ischemia/reperfusion injury in the isolated rat heart using infarct size as the endpoint of injury. In this respect, we compared mibefradil with amlodipine, a well-known and potent L-type calcium channel blocker, and with ischemic preconditioning, an intervention known to reduce infarct size consistently. Secondly, we investigated the possible mechanisms through which protection was achieved. For this second purpose, we examined the effects on protection of glibenclamide (an ATP-dependent K+ channel blocker) and chelerythrine (a protein kinase C inhibitor). Isolated rat hearts were perfused in the Langendorff mode at constant pressure. Control, mibefradil-treated (0.3 microM), mibefradil plus glibenclamide (50 microM), and mibefradil plus chelerythrine (10 microM) treated hearts underwent 35 minutes regional ischemia followed by 120 minutes reperfusion. At the end of the experiments, infarct size was determined with triphenyltetrazolium chloride and was expressed as a percentage of the ischemic risk zone (I/R%). A significant reduction in infarct size with mibefradil treatment was observed (I/R 11.1 +/- 2.1% vs. 35.5 +/- 3.1% in controls). This was comparable with the infarct reduction seen with two 5-minute cycles of ischemic preconditioning (17.7 +/- 2.5%). Amlodipine 0.1 microM, a concentration that caused equivalent coronary vasodilatation as that produced by mibefradil treatment, had no significant effect on infarct size (I/R 29.7 +/- 3.5%). The protective effect of mibefradil was not significantly modified by the presence of the PKC inhibitor chelerythrine 10 microM (I/R 19.1 +/- 4.9%) but was abolished when glibenclamide 50 microM was coadministered with mibefradil prior to ischemia (I/R 28.1 +/- 4.7%). Neither chlelerythrine nor glibenclamide alone had any influence on infarct size. We conclude from these data that mibefradil, unlike amlodipine, markedly reduces infarct size in the rat isolated heart. This protection is sensitive to inhibition by glibenclamide, suggesting that KATP channel opening may be an important additional and novel mechanism of mibefradil's action.