Journal of Neuroscience, Vol 5, 2559-2564, Copyright © 1985 by Society for Neuroscience
Voltage clamp analysis of tetrodotoxin-sensitive and -insensitive sodium channels in rat muscle cells developing in vitro
T Gonoi, SJ Sherman and WA Catterall
Sodium currents in cultured rat muscle cells converted to myoballs by
treatment with colchicine were recorded using a giga-ohm seal voltage clamp
procedure in the whole cell configuration. The mean peak Na+ conductance of
the myoballs was 90 pS/microns2 of surface membrane. Half-maximal
activation of Na+ currents was observed for test pulses to -31 mV and
half-maximal inactivation was observed for prepulses to -74 mV. Titration
of the inhibition of Na+ currents by tetrodotoxin (TTX) yielded a biphasic
inhibition curve consistent with the presence of two classes of Na+
channels differing in affinity for TTX. The TTX- sensitive channels carried
28% of the Na+ current and had an apparent KD for TTX of 13 nM at 20
degrees C. The TTX-insensitive Na+ channels had an apparent KD for TTX of
3.2 microns. Inhibition of TTX- insensitive Na+ channels by TTX was
enhanced by repetitive stimulation of the myoballs at 2 Hz, whereas the
inhibition of TTX-sensitive Na+ channels by TTX was not frequency
dependent. We conclude that rat muscle cells developing in vitro synthesize
physiologically functional, TTX-sensitive Na+ channels in the absence of
innervation. These channels, which are characteristic of adult skeletal
muscle, function in parallel with TTX-insensitive Na+ channels that are
present in embryonic muscle.
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