In vivo electrophysiological effects of a selective slow delayed-rectifier potassium channel blocker in anesthetized dogs: potential insights into class III actions.
There is a high incidence of diuretic use among patients who develop exaggerated QT prolongation and polymorphic ventricular tachycardia (torsade de pointes) during treatment with action potential-prolonging agents. Diuretic-induced hypokalemia is thought to be the usual mechanism, but a direct effect of diuretic drugs on repolarizing currents is an additional possibility. Therefore, in this study, we examined the effects of the diuretic agents chlorthalidone and indapamide on the cardiac delayed rectifier current. In guinea pig ventricular myocytes, this current is made up of two components: IKr, a rapidly activating, inwardly rectifying current blocked by most action potential-prolonging antiarrhythmics, and IKs, a slowly activating component. In this preparation, indapamide blocked outward current in a time-, voltage- and concentration-dependent fashion, whereas chlorthalidone (1 mmol/L) was without effect. The following features of the effect of indapamide strongly suggest selective block of IKs: (1) Indapamide block was significantly greater with 5000-millisecond activating pulses (-43 +/- 5% at +50 mV [100 mumol/L indapamide]) than with 225-millisecond ones (-20 +/- 5%; n = 5, P < .01), and the signature of the indapamide-sensitive current was a slowly activating delayed rectifier current. (2) The voltage dependence of indapamide block (EC50, 101 mumol/L at +50 mV and 196 mumol/L at +10 mV) was consistent with preferential block of IKs relative to IKr. (3) In the presence of indapamide, an envelope-of-tails test for IKr was satisfied. The drug-insensitive current had rectifying properties similar to those described for IKr in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)