'Off' tails of intramembrane charge movements in frog skeletal muscle in perchlorate-containing solutions.

1. Charge movements in response to hyperpolarizing ('off') voltage-clamp steps were examined in frog skeletal muscle fibres in the presence of 8 mM-perchlorate. 2. The appearance of prolonged 'off' decays, of duration 50-100 ms coincided with those of slow 'q gamma' charge transfers in preceding 'on' transients. 3. Charge was conserved in the presence of perchlorate whether testing or pre-pulse voltages were varied. Additionally, steady-state charge as a function of voltage was independent of the direction from which the voltage was reached. 4. 'Off' recoveries were most prolonged at voltages around -90 to -100 mV and became less marked with depolarization. 5. Charging currents in response to hyperpolarizing steps that intercepted prolonged ('q gamma') 'on' decays showed reduced slow 'off' tails but intact 'off' decays at early times. 6. Transients elicited by depolarizing steps that intercepted 'off' tail currents showed decreased slow 'on' components. By varying the time at which 'off' responses were so intercepted, it was shown that 'off' tails can require well over 100 ms to attain a steady state in perchlorate. 7. Prolonged depolarization to a holding potential of -30 mV inactivated delayed ('q gamma') components in both 'on' and 'off' responses but left the more rapid ('q beta') decays intact. 8. These observations are easier to reconcile with parallel systems responsible for independent early ('q beta') and delayed ('q gamma') transients in both 'on' and 'off' steps than with the existence of 'q beta' and 'q gamma' transitions in a causal sequence.