EFFECTS OF MEMBRANE-POTENTIAL ON JUST DETECTABLE MOVEMENT IN RAT SKELETAL-MUSCLE - EFFECTS OF DENERVATION

The potential, Vt, at which a brief test depolarization first elicited movement was determined using two-microelectrode point voltage clamp. We expected that inactivation of excitation-contraction coupling at c onditioning potentials between -60 and 0 mV would shift Vt to more pos itive potentials, and that fibers would become inactivatable with less conditioning depolarization in EDL than soleus. The curve relating Vt to conditioning potential had a negative slope (which was insensitive to addition of 1 mM cobalt or replacement of calcium with 20 mM CaEGT A) between -60 and -35 mV and a steep positive slope with further depo larization. Unexpectedly, fibers became inactivatable with less condit ioning depolarization in soleus than in EDL when Vt was measured with 50 msec test pulses. However, the positive shift in Vt became less ste ep as test pulse duration lengthened in soleus fibers. When Vt obtaine d with test pulses approaching rheobase (10 msec in EDL and 500 msec i n soleus) was compared, EDL fibers became inactive with less condition ing depolarization than soleus fibers. The increase in Vt became steep er with 1 mM cobalt or 20 mM CaEGTA and was shifted to more positive p otentials by denervation in soleus fibers. We conclude that inactivati on (i) does not strongly influence threshold contractions at condition ing potentials between -60 and -40 mV and (ii) influences Vt between - 40 and 0 mV in a manner that depends on test pulse duration.