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

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Abstract
The single gap voltage clamp technique (Kovacs et Schneider, 1978) was used to monitor membrane charge movement in tendon-terminated short segments of cut frog skeletal muscle fibers. Experiments were performed on fibers able to contract and on those in which contraction was eliminated by exposing the open end to a solution containing 20 mM-EGTA [ethylene glycol bis (.beta.-aminoethyl ether) tetraacetate]. In both cases ionic conductances were minimized by using a predominantly cesium glutamate solution at the open end and a predominantly tetraethylammonium sulfate solution with tetrodotoxin at the closed end. Modifications of previously used charge movement analysis procedures included synthesis of a mean linear ON and OFF capacitative transient from the OFF of several different hyperpolarizing pulses and use of only the first 35 ms of the mean linear transient so that base lines could be fitted to unaltered latter parts of ON and OFF currents for depolarizing pulses. Simultaneous 2 micro-electrode and gap current recording from gap-clamped fibers with blocked contraction established the validity of gap-recorded charge movement currents. For pulses to below about 0 mV in non-contracting fibers the charges QON and QOFF moved by the non-linear transient currents at pulse ON and OFF were approximately equal. For pulses to between 0 and +50 mV QOFF exceeded QON, with the charge inequality increasing with both pulse amplitude and pulse duration. Use of 20 mM-Co in the solution at the closed end eliminated the ON:OFF charge inequality for large depolarizations by decreasing QOFF. The charge inequality and Co effect indicate that in the absence of Co, ionic-conductance was being slowly activated during depolarizations to between 0 and +50 mV and that inward Ca current tails were contributing to the measured QOFF values. The small and slowly developing ionic current during large depolarizations was proably removed with the straight sloping base line so that QON was minimally affected by conductance activation. Average Q vs. V results for pulses to at most 0 mV in 18 non-contracting fibers were well fitted by the 2-state Boltzmann model where Q = Qmax/[1 + exp-(V - .hivin.V)/k] with Qmax = 26.7 .+-. 0.6 nC/.mu.F, k = 16.7 .+-. 0.6 mV and .hivin.V = -32.9 .+-. 1.0 mV (least-squares values .+-. SD obtained from fit). In contracting fibers, the only apparent artifact produced by contraction in the IQ records for pulses to at most 0 mV was a bowing of the OFF base lines for the larger pulses. The ON records appeared unaffected by contraction artifacts. The average Q vs. V relationship for pulses to at most 0 mV in contracting fibers was virtually identical to that obtained from fibers in which contrastion was blocked. The ON portions of IQ records for pulses to between about -50 and -25 mV exhibited prolonged tails plateaux or secondary rising phases whereas the OFF portions decayed smoothly. IQ time courses were not noticeably different with or without blockage of contraction by internal EGTA.