Single sodium channels from canine ventricular myocytes: voltage dependence and relative rates of activation and inactivation.

Single sodium channel currents were recorded from canine ventricular myocytes in cell-attached patches. The relative rates of single-channel activation vs. inactivation as well as the voltage dependence of the rate of open-channel inactivation were studied. 2. Ensemble-averaged sodium currents showed relatively normal activation and inactivation kinetics, although the mid-point of the steady-state inactivation (h infinity) curve was shifted by 20-30 mV in the hyperpolarizing direction. This shift was due to the bath solution, which contained isotonic KCl to depolarize the cell to 0 mV. 3. Steady-state activation showed less of a voltage shift. The threshold for eliciting channel opening was around -70 mV and the mid-point of activation occurred near -50 mV. 4. The decline of the ensemble-averaged sodium current during a maintained depolarization was fitted by a single exponential function characterizing the apparent time constant of inactivation (tau h). The apparent rate of inactivation was voltage dependent, with tau h decreasing e-fold for a 15.4 mV depolarization. 5. The relative contributions of the rates of single-channel activation and inactivation in determining the time course of current decay (tau h) were examined using the approach of Aldrich, Corey & Stevens (1983). Mean channel open time (tau o) showed significant voltage dependence, increasing from 0.5 ms at -70 mV to around 0.8 ms at -40 mV. At -70 mV tau h was much greater than tau o, while at -40 mV the two time constants were similar. 6. The degree to which the kinetics of single-channel activation contribute to tau h was studied using the first latency distribution. The first latency function was fitted by two exponentials. The slow component was voltage dependent, decreasing from 19 ms at -70 mV to 0.5 ms at -40 mV. The fast component (0.1-0.5 ms) was not well resolved. 7. Comparing the first latency distribution with the time course of the ensemble-averaged sodium current at -40 mV showed that activation is nearly complete by the time of peak inward sodium current. However, at -70 mV, activation overlaps significantly with the apparent time course of inactivation of the ensemble-averaged current. 8. Using the methods of Aldrich et al. (1983) we also measured the apparent rate of open-channel closing (a) and open-channel inactivation (b). Both rates were voltage dependent, with a showing an e-fold decrease for an 11 mV depolarization and b showing an e-fold increase for a 30 mV depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)