Characterization of mibefradil block of the human heart delayed rectifier hKv1.5

The goal of this study was to analyze the effects of mibefradil on a human cardiac K+ channel (hKv1.5) stably expressed in Chinese hamster ovary cells using the whole-cell configuration of the patch-clamp technique. Mibefradi l inhibited in a concentration-dependent manner the hKv1.5 current with a K -D value of 0.78 +/- 0.05 muM and a Hill coefficient of 0.97 +/- 0.06. Bloc k induced by mibefradil was voltage dependent, consistent with a value of e lectrical distance of 0.13. The apparent association (k) and dissociation ( I) rate constants measured at +50 mV were found to be 7.3 +/- 0.5 X 10(6) M -1 . s(-1) and 4.3 +/- 0.1 s(-1), respectively. Block increased rapidly bet ween -20 and +10 mV, coincident with channel opening and suggested an open channel block mechanism, which was confirmed by a slower deactivation time course resulting in a "crossover" phenomenon when tail currents recorded un der control conditions and in the presence of mibefradil were superimposed. Shifts toward negative potentials of the maximum conductance and the activ ation curve were observed, confirming the voltage dependence of block. Mibe fradil induced a significant use-dependent block when trains of depolarizat ion at frequencies between 0.02 and 2 Hz were applied. In the presence of m ibefradil, recovery of inactivation was faster than under control condition s, suggesting that mibefradil might compete with the inactivation gate of h Kv1.5. These results indicate that mibefradil blocks hKv1.5 channels in a c oncentration-, voltage-, time- and use-dependent manner and the concentrati ons needed to observe these effects are in the therapeutic range.