Figure 1.

Outward TTX-R Na⁺ currents inhibited by external Cd²⁺. (A) The cell was held at −130 mV and stepped every 4 s to the test pulse (0 mV) for 60 ms. With 150 mM Na⁺ internal solution and 150 mM Cs⁺ external solution, outward TTX-R Na⁺ currents were elicited by depolarization to 0 mV and were inhibited by 300 and 1,000 μM Cd²⁺. The two control sweeps were obtained before and after Cd²⁺ inhibition, demonstrating rapid reversibility of the inhibition. The dotted line indicates the zero current level. (B) Inhibition of outward TTX-R Na⁺ currents by different concentrations (30–3,000 μM, as indicated beside each series of symbols) of Cd²⁺ at different test pulse voltages. The experimental conditions and pulse protocols were generally similar to that described in A, except that the test pulse was varied from −20 to 40 mV in 20-mV steps (the horizontal axis). The relative current (the vertical axis) at each test pulse potential is defined by normalization of the peak currents in the presence of Cd²⁺ to the peak current in the control (Cd²⁺-free) solution (n = 3–9). The inhibition is clearly Cd²⁺ concentration dependent, yet shows only minimal voltage dependence. (C) The mean relative current in B is plotted against [Cd²⁺] (the concentration of Cd²⁺) in semilogarithmic scale. The lines are best fits for each set of data points of the form: relative current = 1/{1 + ([Cd²⁺]/K _app,o)}, where K _app,o stands for the apparent dissociation constant of Cd²⁺ in such an experimental configuration (150 mM Cs⁺ outside and outward Na⁺ current). The K _app,o from the fits are given in the parentheses in the figure. (D) The K _app,o obtained in C are plotted against test pulse voltage in semilogarithmic scale. The line is the best fit to the data points of the form: K _app,o = 1480 μM * exp(V/230), where V stands for the test pulse voltage in mV.