Initial charge distribution and capacity transients in frog skeletal muscle.

The effect of prior charge transfers on intramembrane charge movements was investigated in voltage-clamped frog skeletal muscle fibres. 10 mV steps between fixed prepulse and test voltages were imposed at varying intervals after a 5 mV prepulse step made from a previous conditioning level. The amount of charge transfer prior to the test step was varied by altering the extent to which the test step intercepted slow (q gamma) currents elicited in the prepulse. The greater the prior charge transfer, the more rapid the transients in the subsequent test step. In contrast, where the prepulse had not reached the threshold of slow currents, or at larger depolarizations where the then faster charging currents were not intercepted, charge movements induced by the test step had a constant size and form. Charging kinetics thus varied with preceding charge transfer, consistent with higher-order cooperative (Huang, 1984), or sequential (Almers, 1978) schemes for the nonlinear charge.