Block of IKs, the Slow Component of the Delayed Rectifier K+ Current, by the Diuretic Agent Indapamide in Guinea Pig Myocytes

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 componentsIKr, 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 μmol/L indapamide]) than with 225-millisecond ones (−20±5%n=5, P