Anomalous R666G tail currents do not support a change in POPEN to explain the current saturation at hyperpolarized voltages. The I-V relationship of the steady-state currents in a symmetrical K+ gradient (from Fig. 3 A) was transformed to a G-V relationship in A by assuming a reversal potential of 0 mV and therefore a linear open-state current through the origin. These conductance values were then normalized to the peak conductance (filled black circles). Normalized peak tail current amplitudes (after leak correction) are plotted against the voltage of the preceding pulse at which the normalized conductance values were obtained (filled green circles, right axis). No change in the amplitudes of these tail currents (all measured at −100 mV) was observed at voltages <−80 mV in a region where the normalized conductance declines. (B) A comparison of individual raw current traces extracted from Fig. 1 A confirms that the tail currents exhibit an anomalous increase in amplitude after more depolarizing voltage pulses during which the gating pore is occluded (red trace) and are absent after hyperpolarized pulses during which the gating pore remains open (black trace). The slow activation kinetics of the tail currents are shown C, in which tail currents were recorded at −100 mV after depolarizing pulse to 0 mV of different durations (see inset at top of figure). Current traces are arranged along the abscissa in proportion to conditioning pulse duration, and the peak current is fit with a single exponential function (dashed blue line), revealing an activation time constant of 1.2 s at 0 mV.
