ACTIVATION OF CA2-MUSCLE( RELEASE BY CAFFEINE AND VOLTAGE IN FROG SKELETAL)

1. Using a fast flow computer-controlled, two-Vaseline-gap chamber, si ngle muscle fibres were subjected to 'pulses' of caffeine at Ca2+ rele asing concentrations, combined with voltage-clamp depolarizations, whi le monitoring intracellular [Ca2+]. 2. Ca2+ release flux elicited by c affeine reached 2.5 mM s(-1), or less, after 3 s of exposure, then dec ayed to zero. The caffeine-releasable pool of sarcoplasmic reticulum ( SR) Ca2+ was 2.9 +/- 0.4 mM (mean +/- S.E.M., n = 10). 3. In parallel with release induced by caffeine, release induced by voltage pulses ap plied during a caffeine exposure increased in the first second of expo sure, then decreased, to abolition after 5 s. 4. The amount of Ca2+ re leasable by depolarizing pulses was always equal to the amount of Ca2 in the caffeine-releasable pool. Therefore, there is a single releasa ble Ca2+ pool. This pool is well stirred - it takes much more time to lose its Ca2+ by release than to diffusionally homogenize its [Ca2+]. Its depletion explains quantitatively the decay of release induced by caffeine or voltage during an exposure to caffeine. 5. A 1.5 s pulse t o 10 mV, applied during exposure to caffeine, resulted in large Ca2+ r elease and, upon repolarization, termination of the caffeine-induced r elease. This is similar to repolarization-induced stop of caffeine con tracture (RISC) in embryonic murine myoballs. The permeability elicite d by caffeine (ratio of flux to calcium in the releasable pool) was no t affected by depolarizing pulses. Therefore, the mechanism of the RIS C-like effect was Ca2+ depletion. 6. Caffeine-induced release did not depend on the holding potential. 7. Whether caffeine was present or no t, release activated by voltage remained always under voltage control, ending rapidly upon repolarization. A depolarizing pulse induced a re lease permeability with an early peak, followed by decay to a steady l evel. Caffeine (10 mM) shifted the mid-activation voltage of both peak and steady components by -15 mV and increased the steepness of their voltage dependence by 15%. The maximum permeability increased by 30% f or the peak and 25% for the steady component (n = 5). These results ne ither support nor disprove the hypothesis that the peak of Ca2+ releas e is activated by Ca2+. 8. The similar potentiation by caffeine of bot h components of release, the continued ability of voltage to control r elease in the presence of caffeine, and its failure to alter caffeine- induced permeability indicate that caffeine and the voltage sensor enh ance independently the channel's tendency to open.