Sodium channel inactivation kinetics of rat sensory and motor nerve fibres and their modulation by glutathione

Na+ channel currents of rat motor and sensory nerve fibres were
studied with the patch-clamp technique on enzymatically
demyelinated axons. Differences between motor and sensory fibres in
multi-channel inactivation kinetics and the gating of late
single-channel currents were investigated. 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-channel currents in
sensory axons were characterized by short openings whereas motor
axons exhibited additional long single-channel openings. In
contrast, in excised, inside-out membrane patches, no differences
between motor and sensory fibres were found: in both types of fibre
inactivation of multi-channel Na+ currents proceeded with two time
constants and late single-channel currents showed short and long
openings. After application of the reducing agent glutathione to
the cytoplasmic side of excised inside-out patches, inactivation of
Na+ currents in both motor and sensory fibres proceeded with a
single, fast exponential time constant and late currents appeared
with short openings only. These data indicate that the axonal
metabolism may contribute to the different inactivation kinetics of
Na+ currents in motor and sensory nerve fibres.