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Physiological Laboratory, Cambridge.

1. Charge movements were compared in normally polarized and depolarized intact frog muscle fibres under voltage clamp. 2. The membrane capacitance was linear through positive control steps made consistently from a holding voltage of -10 mV, in agreement with earlier reports from cut fibres. 3. A shift in holding voltage from -90 to -10 mV reduced both the absolute amount and the voltage dependence of charge movement elicited by voltage steps imposed from a fixed conditioning voltage of -180 mV. The charge transferred by steps from -180 to -20 mV was 43.8 +/- 1.14 nC/microF in fully polarized fibres and 21.7 +/- 1.49 nC/microF in the same depolarized fibres (means +/- S.E. of the mean; four fibres). 4. Charge movement in response to steps from -90 to -20 mV increased from 10.4 +/- 1.60 nC/microF to 28.4 +/- 2.42 nC/microF (five fibres) within 30s of changing the holding voltage from -10 to -90 mV. 5. The same fibres also showed significant charge movement between voltages of -180 and -90 mV. However, shifts in holding voltage did not significantly alter the maximum value of this charge, around 10-11 nC/microF. 6. Membrane capacitance as measured by small steps to a voltage of -90 mV remained constant despite holding potential changes, or lidocaine (10 mM) treatment. 7. The same results were obtained whether the above procedures were applied to fibres exposed to normal extracellular [Ca2+], or in Ca(2+)-free media. In both cases tubular cable corrections did not affect the results. 8. These findings suggest independent charge I and charge II systems in which inactivation of charge I is not associated with its interconversion into charge II.