Characterization of the Inactivation Process of the Sodium Channel in Frog Ventricular Cells

The voltage clamp experiments were conducted on single frog ventricular cells with the oil gap method for characterization of the sodium current (INa). The results obtained can be summarized as follows:

1. The falling phase of INa could be fitted by a single-exponential function when the series resistance was small (0.81-2.25 MO) and compensated.

2. Normalized steady-state inactivation (h∞) curve could be fitted by the equation h∞ = 1/(1 + exp(V-Vh)/k), where the half-inactivation voltage (Vh) was -59.8 mV and the slope factor (k) was 5.64 m V. No shift of this curve was observed throughout the experiment.

3. There was a delay of onset of inactivation development.

4. The time course for recovery of INa from inactivation exhibited a single time constant.

5. Almost all properties of the inactivation process of Na channel can be described by the original Hodgkin-Huxley's (H-H) kinetic model, except the presence of the delayed onset of inactivation. In order to incorporate this discrepancy, modification of H-H model is required, that the transition rate constant from the open state to the inactivated state may well be larger than that from the closed state to the inactivated state.