Voltage dependence of Tz1-induced gating modification. (A; top) Protocol used to measure the voltage-dependent preactivation of NaV1.4 channels in the presence of Tz1. (Bottom) Current traces at −70 mV in response to conditioning pulses (here to −40 mV) are shown in gray and are offset in time by the conditioning durations. The current trace corresponding to a conditioning time of 64 ms is highlighted (black). The continuous line is a single-exponential fit to the peak current responses after conditioning, Ia. (B; top) Protocol used to measure the voltage-dependent channel deactivation in the presence of Tz1. (Bottom) Current traces at −70 mV in response to conditioning pulses (here to −160 mV) are shown in gray and are offset in time by the conditioning durations. The current trace corresponding to a conditioning time of 32.768 s is highlighted. The continuous line is a single-exponential fit to the peak current responses after conditioning (Id) and describes the deactivation of NaV1.4 channels in the presence of toxin. (C) Normalized Ia as a function of the conditioning time, shown for the indicated conditioning voltages with superimposed single-exponential data fits. (D) Normalized Id as a function of conditioning time, shown for the indicated conditioning voltages with superimposed single-exponential data fits. (E) Time constants of Tz1-induced gating modification in the activation (open symbols) and deactivation (closed symbols) direction as a function of voltage. The continuous line is a data fit according to a transition-state model (Eq. 6) assuming the transfer of q = 2.4 ± 0.1 e0 with a symmetry factor of δ = 0.21 ± 0.05 (n = 3–5). Tz1 concentration was 25 µM in all panels.
