Characteristics of phosphate-induced Ca2+ efflux from the SR in mechanically skinned rat skeletal muscle fibers

The effects of Pi on sarcoplasmic reticulum (SR) Ca2+ regulation were studied in mechanically skinned rat skeletal muscle fibers. Brief application of caffeine was used to assess the SR Ca2+ content, and changes in concentration of Ca2+([Ca2+]) within the cytosol were detected with fura 2 fluorescence. Introduction of Pi (1–40 mM) induced a concentration-dependent Ca2+ efflux from the SR. In solutions lacking creatine phosphate (CP), the amplitude of the Pi-induced Ca2+ transient approximately doubled. A similar potentiation of Pi-induced Ca2+ release occurred after inhibition of creatine kinase (CK) with 2,4-dinitrofluorobenzene. In the presence of ruthenium red or ryanodine, caffeine-induced Ca2+ release was almost abolished, whereas Pi-induced Ca2+ release was unaffected. However, introduction of the SR Ca2+ ATPase inhibitor cyclopiazonic acid effectively abolished Pi-induced Ca2+ release. These data suggest that Pi induces Ca2+ release from the SR by reversal of the SR Ca2+ pump but not via the SR Ca2+ channel under these conditions. If this occurs in intact skeletal muscle during fatigue, activation of a Ca2+efflux pathway by Pi may contribute to the reported decrease in net Ca2+ uptake and increase in resting [Ca2+].