VOLTAGE DEPENDENCIES OF THE FAST AND SLOW GATING MODES OF RIIA SODIUM-CHANNELS

Rat brain IIA sodium channel a-subunits were expressed in Xenopus oocy tes, and the sodium currents were measured by intracellular voltage cl amping with large agarose-tipped electrodes and by excised membrane pa tch-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 isola ted as distinct gating modes through differences in their inactivation properties. At holding potentials more negative than -55 mV, fast mod e currents inactivated within a few milliseconds of depolarization, an d could be distinguished by their rapid recovery from inactivation. Si ngle sodium channels in the fast mode opened early after depolarizatio n 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 sust ained single channel activity during depolarizing pulses. The steady-s tate voltage dependencies of fast and slow mode activation were very s imilar. 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 th e channel. The fast gating process dominated at high current levels, p erhaps due to alpha-subunit interactions.