1. Currents of intramembranous charge movement were recorded, together with intracellular [Ca2+], in single muscle fibres subjected to voltage-clamp depolarization and 'pulses' of extracellular solution with a Ca2+ release-inducing concentration of caffeine (10 mM). 2. When caffeine was present prior to and during the voltage pulses, the charge transferred by pulses to between -60 and -40 mV increased by about 40%. 3. In fibres depleted of Ca2+ in the sarcoplasmic reticulum (SR), caffeine had no effect on charge transfer or kinetics. 4. Whenever the prior exposure to caffeine resulted in a large elevation in [Ca2+]i at the start of the depolarizing pulse, there was an increase in Iβ, the monotonically decaying component of charge movement. When the presence of caffeine enhanced Ca2+ release induced by the pulse, there was increase in Iγ, the hump-like component. 5. The charge transferred during a pulse to -50 mV increased with time of exposure to caffeine. Ca2+ release induced by the voltage pulse grew during the first second of caffeine exposure, then decreased with longer exposure time. The enhancement of charge transfer by caffeine was therefore not due to the increase in Ca2+ release caused by the drug. 6. The increase in charge transfer was a uniform, monotonically increasing function of the [Ca2+]i attained at the end of the voltage pulse. 7. Charge transfer, as a function of [Ca2+]i, pulse voltage and time, was simulated with a model, used previously, in which Ca2+ binds to intracellular sites and increases the electrical potential near the voltage sensors. Two sites were needed to fit the observations, with dissociation constants of 60 nM and 2 to 10 μM. 8. In the presence of caffeine, the voltage-driven movement of a given amount of intramembranous charge resulted in greater activation of release permeability. 9. All effects of caffeine observed in this and the preceding paper could be explained assuming a single action: caffeine increases the tendency of the release channels to open. This results in opening of closed channels and an increase in their susceptibility to activation by the voltage sensors.
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