Relaxation rates as function of CTX concentration. (A) Family of TEVC traces recorded at +60 mV from the same oocyte in the absence and presence of increasing CTX concentrations in the bath. These measurements were carried out in Hi-Na⁺ solution. (B) Point-by-point current ratios between the traces at different CTX concentrations, divided by the control trace (black), as described in Materials and methods. The trace colors match the CTX concentrations indicated in A. The blue line on top of each trace is a single-exponential fit to the relaxation, and the corresponding τ values are shown to the right of each trace. (C) Plot of 1/τ versus [CTX] for several oocytes and voltage pulses applied at different potentials. The continuous lines are linear fits, where the slope corresponds to k_on and the intercept to k_off. Each measurement is the average ± SD for three to five experiments. (D) Linear fit parameters from the data in C, plotted as a function of voltage. The solid lines are fits of Eq. 4 with the following parameters: k_ono = 0.045 ± 0.02 nM⁻¹ s⁻¹, zδ = 0.15 ± 0.09 and k_offo = 0.21 ± 0.052 s⁻¹, zδ = 0.78 ± 0.09. Error bars represent the SEM. See also Data S1 for underlying data.