The recombinant Kv3.4 channel expressed in Xenopus oocytes undergoes strict OSI. (A) Family of whole oocyte outward currents evoked by a series of voltage steps (inset). (B) Normalized G_P-V and steady-state inactivation curves (closed and open circles, respectively). The steady-state inactivation protocol used 10-s conditioning pulses and a test pulse to +70 mV. Red solid lines are the best-fit fourth-order Boltzmann (G_P-V curve) and a Boltzmann (steady-state inactivation) function (best-fit parameters and descriptive statistics are summarized in Table 1). (C) Voltage dependence of the time constant of inactivation. Time constants or weighted time constants were determined from the best-fit exponential or sum of two exponentials that describes the decay phase of the currents, respectively. For comparison, the red symbol in C indicates the time constant determined from the double-pulse protocol. (D) Outward currents evoked by a double-pulse protocol with a 5-ms resetting gap between the two pulses (inset). The conditioning pulse activated 12% of the peak conductance. (E) Kinetics of double-pulse inactivation. The normalized peak current evoked by the test pulse of the protocol in D plotted against the duration of the conditioning pulse (closed circles). The solid red line represents the following empirical best-fit sum of exponential terms: I/I₀ (t) = 0.52 − 0.38(1 − e^−t/88)^1.3 + 0.48e^−t/280. (F) Overlay of the average outward current (mean ± SEM; black and gray, respectively) evoked by a single pulse (−100 to 0 mV) and the rate of inactivation (red) at 0 mV. The rate of inactivation is the scaled negative first derivative of the red solid line in E. The blue solid line is the exponential function that best describes the decay phase of the current trace (black). The scaled negative first derivative of the observed time course of double-pulse inactivation is also plotted (open black circles) for comparison.