Properties of chloride-stimulated 45Ca flux in skinned muscle fibers.

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Abstract
Isometric force and 45Ca loss from fiber to bath were measured simultaneously in skinned fibers from frog muscle at 19.degree. C. In unstimulated fibers, 45Ca efflux from the sarcoplasmic reticulum (SR) was very slow, with little or no dependence on EGTA [ethylene glycol bis (.beta.-aminoethyl ether) N,N,N'',N''-tetraacetic acid] (0.1-5 mM or Mg2+ (20 .mu.M-1.3 mM). Stimulation by high [Cl] at 0.11 mM Mg2+ caused rapid force transients (duration .apprx. 10 s) and 45Ca release. This response was followed for 55 s, with 5 mM EGTA added to chelate myofilament space (MFS) Ca either after relaxation, near the peak of the force spike, or 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 3 protocols showed that when EGTA was absent, 16% of the fiber tracer was released from the SR within .apprx. 3 s and 70% of the tracer still in the MFS near the peak of the force spike was subsequently reaccumulated. The Cl response apparently was highly Ca-dependent; stimulation increased 45Ca efflux from the SR at least 100- to 200-fold; and the rate of reaccumulation was much slower than the influx previously predicted on resting fibers, raising the possibility that depolarization inhibits active Ca transport by the SR.