1. Calcium currents and intramembrane charge movements were measured in cut twitch muscle fibres of the frog and the time course of activation of the current was studied using various conditioning pulse protocols. 2. When a conditioning activation was produced by a depolarizing pulse which ended before inactivation occurred, a subsequent depolarization led to a faster onset of activation, indicating that the system had not completely returned to the initial state during the interval between the two pulses. 3. The interval between conditioning and test pulse was varied at different subthreshold potentials to study the time course of restoring the steady-state conditions. Complete restoration required a waiting period of about 1 min at the holding potential of -80 mV due to a very slow process but partial recovery was reached within 100 ms. This initial recovery process was strongly voltage dependent and became considerably slower when the interval potential approached the threshold for current activation. 4. Stepping to a roughly 10 mV subthreshold potential without applying a conditioning activation caused no change in the time course of the current produced by a subsequent test depolarization. Depolarizing just to the current threshold caused a slowly progressing acceleration of test current activation. 5. The peak current-voltage relation in the fast gating regime caused by a conditioning activation coincided with the current-voltage relation measured under steady-state conditions, indicating not that a new channel population had become activated but that the same channels showed a different gating behaviour. 6. Intramembrane charge movements measured in 2 mM-Cd2+ and tested at potentials between -40 and +40 mV showed negligible changes when preceded by a strong depolarization. 7. We discuss several possible models which can explain the fact that the current is speeded up by a conditioning activation while the charge movements remain unchanged. It is possible that the fast voltage-dependent transition which becomes visible after conditioning pulses reflects a rapid conformational change of the Ca2+ channel molecule which also occurs during its normal gating mode but remains undetectable in terms of conductance. In view of the hypothesis that the Ca2+ channel molecule forms a voltage sensor for excitation-contraction coupling this fast transition could be coupled to the control of Ca2+ release from the sarcoplasmic reticulum.
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