CHARGE INACTIVATION IN THE MEMBRANE OF INTACT FROG STRIATED-MUSCLE FIBERS

1. Charge movements were compared in normally polarized and depolarize d intact frog muscle fibres under voltage clamp. 2. The membrane capac itance was linear through positive control steps made consistently fro m 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 b oth 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/muF in fully polarized fibres and 21.7 +/- 1.49 nC/muF 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/muF to 28.4 +/- 2.42 nC/muF (five fibres) withi n 30 s of changing the holding voltage from -10 to -90 mV. 5. The same fibres also showed significant charge movement between voltages of -1 80 and -90 mV. However, shifts in holding voltage did not significantl y alter the maximum value of this charge, around 10-11 nC/muF. 6. Memb rane capacitance as measured by small steps to a voltage of -90 mV rem ained constant despite holding potential changes, or lidocaine (10 mm) treatment. 7. The same results were obtained whether the above proced ures were applied to fibres exposed to normal extracellular [Ca2+], or in Ca2+-free media. In both cases tubular cable corrections did not a ffect 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.