Clozapine, a commonly used antipsychotic drug, can induce QT prolongation, which may lead to torsades de pointes and sudden death. To investigate the arrhythmogenic side effects of clozapine, we studied the impact of clozapine on human ether-a-go-go-related gene (HERG) channels expressed in Xenopus oocytes and HEK293 cells, and on the delayed rectifier K(+) currents of guinea-pig cardiomyocytes. Clozapine dose-dependently decreased the amplitudes of the currents at the end of voltage steps, and the tail currents of HERG. The IC(50) for the clozapine blockade of HERG currents in Xenopus oocytes progressively decreased relative to depolarization (39.9 microM at -40 mV, 28.3 microM at 0 mV and 22.9 microM at +40 mV), whereas the IC(50) for the clozapine-induced blockade of HERG currents in HEK293 cells at 36 degrees C was 2.5 microM at +20 mV. The clozapine-induced blockade of HERG currents was time dependent: the fractional current was 0.903 of the control at the beginning of the pulse, but declined to 0.412 after 4 s at a test potential of 0 mV. The clozapine-induced blockade of HERG currents was use-dependent, exhibiting more rapid onset and greater steady state blockade at higher frequencies of activation, with a partial relief of blockade observed when the frequency of activation was decreased. In guinea-pig ventricular myocytes held at 36 degrees C, treatment with 1 and 5 microM clozapine blocked the rapidly activating delayed rectifier K(+) current (I(Kr)) by 24.7 and 79.6%, respectively, but did not significantly block the slowly activating delayed rectifier K(+) current (I(Ks)). Our findings collectively suggest that blockade of HERG currents and I(Kr), but not I(Ks), may contribute to the arrhythmogenic side effects of clozapine.