Characteristics of phosphate-induced Ca(2+) efflux from the SR in mechanically skinned rat skeletal muscle fibers.

The effects of P(i) on sarcoplasmic reticulum (SR) Ca(2+) regulation were studied in mechanically skinned rat skeletal muscle fibers. Brief application of caffeine was used to assess the SR Ca(2+) content, and changes in concentration of Ca(2+) ([Ca(2+)]) within the cytosol were detected with fura 2 fluorescence. Introduction of P(i) (1-40 mM) induced a concentration-dependent Ca(2+) efflux from the SR. In solutions lacking creatine phosphate (CP), the amplitude of the P(i)-induced Ca(2+) transient approximately doubled. A similar potentiation of P(i)-induced Ca(2+) release occurred after inhibition of creatine kinase (CK) with 2,4-dinitrofluorobenzene. In the presence of ruthenium red or ryanodine, caffeine-induced Ca(2+) release was almost abolished, whereas P(i)-induced Ca(2+) release was unaffected. However, introduction of the SR Ca(2+) ATPase inhibitor cyclopiazonic acid effectively abolished P(i)-induced Ca(2+) release. These data suggest that P(i) induces Ca(2+) release from the SR by reversal of the SR Ca(2+) pump but not via the SR Ca(2+) channel under these conditions. If this occurs in intact skeletal muscle during fatigue, activation of a Ca(2+) efflux pathway by P(i) may contribute to the reported decrease in net Ca(2+) uptake and increase in resting [Ca(2+)].