State-dependent block of rabbit vascular smooth muscle delayed rectifier and Kv1.5 channels by inhibitors of cytochrome P450-dependent enzymes.

ABSTRACT

The effects of the cytochrome P450 inhibitors clotrimazole, ketoconazole, and 1-aminobenzotriazole (1-ABT) on native delayed rectifier (K(DR)) and cloned Kv1.5 (RPV Kv1.5) K+ channels of rabbit portal vein (RPV) myocytes were determined using whole-cell and single channel patch-clamp analysis. Clotrimazole reduced K(DR) and RPV Kv1.5 whole-cell current with respective Kd values of 1.15 +/- 0.39 and 1.99 +/- 0.6 microM. Clotrimazole acted via an open state blocking mechanism based on the following 1) the early time course of K(DR) current activation was not affected, but inhibition developed with time during depolarizing steps and increased the rate of decay in current amplitude; 2) the inhibition was voltage-dependent, increasing steeply over the voltage range of K(DR) activation; and 3) mean open time of RPV Kv1.5 channels in inside-out patches was decreased significantly. Ketoconazole reduced K(DR) current amplitude with a Kd value of 38 +/- 3.2 microM. However, ketoconazole acted via a closed (resting) state blocking mechanism 1) K(DR) amplitude was reduced throughout the duration of depolarizing steps and the rate of decay of current was unaffected, 2) there was no voltage dependence to the block by ketoconazole over the K(DR) activation range, and 3) ketoconazole did not affect mean open time of RPV Kv1.5 channels in inside-out membrane patches. 1-ABT between 0.5 and 3 mM did not affect native K(DR) or RPV Kv1.5 current of rabbit portal vein myocytes. Clotrimazole and ketoconazole, but not 1-ABT, suppress vascular K(DR) channels by direct, state-dependent block mechanisms not involving the modulation of cytochrome P450 enzyme activity.