Membrane charge movement in contracting and non-contracting skeletal muscle fibres

1. The single gap voltage clamp technique (Kovács & Schneider, 1978) was used to monitor membrane charge movement in tendon-terminated short segments of cut frog skeletal muscle fibres.2. Experiments were performed both on fibres able to contract and on fibres in which contraction was eliminated by exposing the open end to a solution containing 20 mm-EGTA. In both cases ionic conductances were minimized by using a predominantly caesium glutamate solution at the open end and a predominantly tetraethyl-ammonium sulphate solution with tetrodotoxin at the closed end.3. Modifications of previously used charge movement analysis procedures included synthesis of a ;mean linear' ON and OFF capacitative transient from the OFFs of several different hyperpolarizing pulses and use of only the first 35 msec of the ;mean linear' transient so that base lines could be fitted to unaltered latter parts of ON and OFF currents for depolarizing pulses.4. Simultaneous two-micro-electrode and gap current recording from gap-clamped fibres with blocked contraction established the validity of gap-recorded charge movement currents.5. For pulses to below about 0 mV in non-contracting fibres the charges Q(ON) and Q(OFF) moved by the non-linear transient currents at pulse ON and OFF were approximately equal. For pulses to between about 0 and +50 mV Q(OFF) exceeded Q(ON), with the charge inequality increasing with both pulse amplitude and pulse duration.6. Use of 20 mm-cobalt in the solution at the closed end eliminated the ON:OFF charge inequality for large depolarizations by decreasing Q(OFF).7. The charge inequality and cobalt effect indicate that, in the absence of cobalt, ionic conductance was being slowly activated during depolarizations to between 0 and +50 mV and that inward calcium current tails were contributing to the measured Q(OFF) values. The small and slowly developing ionic current during large depolarizations was probably removed with the straight sloping base line so that Q(ON) was minimally affected by conductance activation.8. Average Q vs. V results for pulses to at most 0 mV in eighteen non-contracting fibres were well fitted by the two-state Boltzmann model where Q = Q(max)/[1+exp-(V - V)/k] with Q(max) = 26.7+/-0.6 nC/muF, k = 16.7+/-0.6 mV and V = -32.9+/-1.0 mV (least-squares values+/-s.d. obtained from fit).9. In contracting fibres the only apparent artifact produced by contraction in the I(Q) records for pulses to at most 0 mV was a ;bowing' of the OFF base lines for the larger pulses. The ON records appeared to be unaffected by contraction artifacts.10. The average Q vs. V relationship for pulses to at most 0 mV in contracting fibres was virtually identical to the one obtained from fibres in which contraction was blocked.11. The ON portions of I(Q) records for pulses to between about -50 and -25 mV exhibited prolonged tails, plateaux or secondary rising phases whereas the OFF portions decayed smoothly. I(Q) time courses were not noticeably different with or without blockage of contraction by internal EGTA.