KINETIC CHARACTERIZATION OF RAT-BRAIN TYPE IIA SODIUM-CHANNEL ALPHA-SUBUNIT STABLY EXPRESSED IN A SOMATIC-CELL LINE

1. The rat brain type IIA Na+ channel alpha-subunit was stably express ed in Chinese hamster ovary (CHO) cells. Current through the expressed Na+ channels was studied using the whole-cell configuration of the pa tch clamp technique. The transient Na+ current was sensitive to TTX an d showed a bell-shaped peak current vs. membrane potential relation. 2 . Na+ current inactivation was better described by the sum of two expo nentials in the potential range -30 to +40 mV, with. a dominating fast component and a small slower component. 3. The steady-state inactivat ion, h(infinity), was related to potential by a Boltzmann distribution , underlying thr ee states of the inactivation gate. 4. Recovery of th e channels from inactivation at different potentials in the range -70 to -120 mV were characterized by al? initial delay which decreased wit h hyperpolarization. The time course was well fitted by the sum of two exponentials. In this case the slower exponential was the major compo nent, and both time constants decreased with hyperpolarization. 5. For a working description of the Na+ channel inactivation in this prepara tion, with a minimal deviation from the Hodgkin-Huxley model, a three- state scheme of the form O reversible arrow I-1 reversible arrow I-2 w as proposed, replacing the original two-state scheme of the Hodgkin-Hu xley model, and the rate constants are reported. 6. The instantaneous current-voltage relationship showed marked deviation from linearity an d was satisfactorily fitted by the constant-field equation. 7. The tim e course of activation was described by an m(x) model. However, the be st-fitted value of x varied with the membrane potential and had a mean value of 2. 8. Effective gating charge was determined to be 4.7e from the slope of the activation plot, plotted on a logarithmic scale. 9. The rate constants of activation, alpha(m) and beta(m), were determine d. Their functional dependence on the membrane potential was investiga ted.