Fig. 4. Halothane changed HERG  current activation and deactivation rates. (A ) Left, sample traces of currents at −10 mV elicited following the I-V protocol shown in figure 1A, inset. Right, sample traces of currents at −120 mV after a test pulse to 20 mV obtained following the protocol shown in the inset. Control and 1.5% halothane records are shown superimposed; the halothane trace was scaled to match the control current value at 20 mV. Dashed lines represent the zero current level. (B ) Activation and deactivation time constants as a function of voltage. Activation time course was fit to a single exponential relaxation. Double exponential relaxation was used to fit deactivation time courses. HERG  currents after 1.5% halothane treatment had slower activation (control vs.  1.5% halothane: 459 ± 52 ms vs.  595 ± 105 ms at −10 mV, and 76 ± 11 ms vs.  147 ± 38 ms at 10 mV, n = 5), and accelerated deactivation (control vs.  halothane: 49.4 ± 1.4 ms vs.  39.4 ± 1.7 ms at −120 mV, and 91.8 ± 4.0 ms vs.  69.8 ± 3.9 ms at −100 mV, n = 5). Differences were significant between −10 mV and 40 mV for activation and in the range between −150 mV and −50 mV for deactivation.