IONIC MECHANISM OF MINOXIDIL SULFATE-INDUCED SHORTENING OF ACTION-POTENTIAL DURATIONS IN GUINEA-PIG VENTRICULAR MYOCYTES

Patch-clamp techniques were used to study pharmacological effects of minoxidil sulfate (MNXS) on the membrane currents of enzymatically isolated guinea pig ventricular myocytes. In the whole-cell current-clamp mode, MNXS (100 muM) shortened the action potential duration without affecting the resting membrane potential. This action was antagonized in part by 1 muM glibenclamide, a specific blocker of ATP-sensitive K+ channel. Under the whole-cell voltage-clamp condition, MNXS increased the time-independent outward current, in a dose-dependent manner, at voltages more positive to -73.5 mV. This MNXS-induced outward current was inhibited completely by 1 muM glibenclamide. In inside-out patch membranes, MNXS (100 muM) applied to the cytosolic side produced a reversible activation of ATP-sensitive K+ channels. This MNXS-dependent increase in the single-channel activity was abolished by increasing the ATP concentration to 3 mM or by adding 1 muM glibenclamide. Even after complete rundown of the channel activity in inside-out patches, MNXS could reactivate in part the channel in 22 of 35 patches. In addition, MNXS was found to suppress whole-cell L-type Ca++ channel currents in a dose-dependent manner. This MNXS effect on Ca++ currents was not antagonized by 1-3 muM glibenclamide. We conclude that MNXS shortens the cardiac action potential duration by both increasing ATP-sensitive K+ channel currents and decreasing L-type Ca++ channel currents.