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Department of Physiology, Rush University, School of Medicine, Chicago, IL 60612.

1. Membrane currents were measured in cut skeletal muscle fibres voltage-clamped in a double Vaseline gap in solutions that had impermeant ions substituted for Na+, K+ and Cl-. The fibres were maintained at a holding potential of 0 mV. Pulses to positive voltages elicited outward currents that were proportional to voltage at all times; these were used to estimate linear capacitive currents, which in turn were used in the construction of non-linear current transients. 2. Large negative-going pulses elicited proportionally larger inward currents that decayed during the pulse with voltage-dependent kinetics. A portion of the non-linear current could be eliminated by solutions containing EGTA, as well as by large negative conditioning pulses of 200 ms or more. This portion was probably an inward Ca2+ current. 3. The non-linear current remaining in EGTA-containing solutions had characteristics of intramembrane charge movement ('charge 2'). This charge depended on voltage according to a two-state Boltzmann function of average parameters Qmax = 47.7 nC/microF, V = -115 mV, K = 21.5 mV (seven fibres). 4. The charge movement current transients were single-exponential decays (after a short rising phase) with time constants (tau) that depended on voltage (V). A single-barrier Eyring rate model described well the dependence of time constant on voltage. This fit permitted an independent estimate of a transition voltage, V, and a slope parameter K related to apparent valence of the mobile particle. The values of V and K that best fitted the kinetic data were close to the corresponding values estimated from the charge vs. voltage distribution. 5. Effective capacitance was measured by the transfer of capacitive charge by a small pulse superimposed on a variable pre-pulse. The capacitance was found to depend on pre-pulse voltage. The voltage dependence of the capacitance was as expected from the properties of charge 2 measured independently in the same fibres. 6. The presence of charge 2, defined as charge that moves in a very negative voltage range, was compared on the same fibres in a depolarized and a normally polarized (holding potential = -100 mV) situation. All fibres had less charge 2 at a holding potential of -100 mV (14 nC/microF average reduction). In these fibres charge 1, explored with pulses from -70 mV to 0 mV, was greater at a holding potential of -100 mV (18 nC/microF average increase).(ABSTRACT TRUNCATED AT 400 WORDS)

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