Isometric force and 45Ca loss from fiber to bath were measured simultaneously in skinned fibers from frog muscle at 19 degrees C. In unstimulated fibers, 45Ca efflux from the sarcoplasmic reticulum (SR) was very slow, with little or no dependence on EGTA (0.1-5 mM) or Mg++ (20 micrometer-1.3 mM). Stimulation by high [Cl] at 0.11 mM Mg++ caused rapid force transients (duration approximately 10 s) and 45Ca release. This response was followed for 55 s, with 5 mM EGTA added to chelate myofilament space (MFS) Ca either (a) after relaxation, (b) near the peak of the force spike, or (c) before or with the stimulus. When EGTA was present during Cl application, stimulation of 45Ca release was undetectable. Analysis of the time-course of tracer loss during the three protocols showed that when EGTA was absent, 16% of the fiber tracer was released from the SR within approximately 3 s, and 70% of the tracer still in the MFS near the peak of the force spike was subsequently reaccumulated. The results suggest that (a) the Cl response is highly Ca-dependent; (b) stimulation increases 45Ca efflux from the SR at least 100-200-fold; and (c) the rate of reaccumulation is much slower than the influx predicted from published data on resting fibers, raising the possibility that depolarization inhibits active Ca transport by the SR.

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