SODIUM-CHANNEL INACTIVATION KINETICS OF RAT SENSORY AND MOTOR-NERVE FIBERS AND THEIR MODULATION BY GLUTATHIONE

Na+ channel currents of rat motor and sensory nerve Fibres were studie d with the patch-clamp technique on enzymatically demyelinated axons. Differences between motor and sensory fibres in multi-channel inactiva tion kinetics and the gating of late single-channel currents were inve stigated. In the axon-attached mode, inactivation of multi-channel Na currents in sensory axons was best fitted with a single time constant while for motor axons two time constants were needed. Late single-cha nnel currents in sensory axons were characterized by short openings wh ereas motor axons exhibited additional long single-channel openings. I n contrast, in excised, inside-out membrane patches, no differences be tween motor and sensory fibres were found: in both types of fibre inac tivation of multi-channel Na+ currents proceeded with two time constan ts and late single-channel currents showed short and long openings. Af ter application of the reducing agent glutathione to the cytoplasmic s ide of excised inside-out patches, inactivation of Na+ currents in bot h motor and sensory fibres proceeded with a single, fast exponential t ime constant and late currents appeared with short openings only. Thes e data indicate that the axonal metabolism may contribute to the diffe rent inactivation kinetics of Na+ currents in motor and sensory nerve fibres.