Voltage dependencies of the fast and slow gating modes of RIIA sodium channels

Rat brain IIA sodium channel alpha-subunits were expressed in Xenopus oocytes, and the sodium currents were measured by intracellular voltage clamping with large agarose-tipped electrodes and by excised membrane patch-clamp recording to separate and characterize the properties of the fast and slow channel gating modes. The currents showed biexponential inactivation properties with fast and slow phases that could be isolated as distinct gating modes through differences in their inactivation properties. At holding potentials more negative than -55 mV, fast mode currents inactivated within a few milliseconds of depolarization, and could be distinguished by their rapid recovery from inactivation. Single sodium channels in the fast mode opened early after depolarization and rarely showed re-openings. At holding potentials positive to -55 mV, fast mode currents were inactivated, revealing slow mode currents which had slower activation and inactivation kinetics and showed sustained single channel activity during depolarizing pulses. The steady-state voltage dependencies of fast and slow mode activation were very similar. In contrast, slow mode inactivation occurred at potentials 27 mV more positive than fast mode inactivation. The slow mode appears to be due to destabilization of a voltage-insensitive conformation of the channel. The fast gating process dominated at high current levels, perhaps due to alpha-subunit interactions.