Protriptyline, a tricyclic antidepressant for psychiatric disorders, can induce prolonged QT, torsades de pointes, and sudden death. We studied the effects of protriptyline on human ether-à-go-go-related gene (HERG) channels expressed in Xenopus oocytes and HEK293 cells. Protriptyline induced a concentration-dependent decrease in current amplitudes at the end of the voltage steps and HERG tail currents. The IC(50) for protriptyline block of HERG current in Xenopus oocytes progressively decreased relative to the degree of depolarization, from 142.0 microM at -40 mV to 91.7 microM at 0 mV to 52.9 microM at +40 mV. The voltage dependence of the block could be fit with a monoexponential function, and the fractional electrical distance was estimated to be delta=0.93. The IC(50) for the protriptyline-induced blockade of HERG currents in HEK293 cells at 36 degrees C was 1.18 microM at +20 mV. Protriptyline affected channels in the activated and inactivated states, but not in the closed states. HERG blockade by protriptyline was use-dependent, exhibiting a more rapid onset and a greater steady-state block at higher frequencies of activation. Our findings suggest that inhibition of HERG currents may contribute to the arrhythmogenic side effects of protriptyline.