Mechanisms of reduced SR Ca2+ release induced by inorganic phosphate in rat skeletal muscle fibers

The effects of inorganic phosphate (P-i) on Ca2+ release from the sarcoplas mic reticulum (SR) were studied in mechanically skinned rat skeletal muscle fibers. Application of caffeine or T-tubule depolarization was used to ind uce Ca2+ release from the SR, which was detected using fura 2 fluorescence. Addition of P-i (1-40 mM) caused a reversible and concentration-dependent reduction in the caffeine-induced Ca2+ transient. This effect was apparent at low P-i concentration (<5 mM), which did not result in detectable precip itation of calcium phosphate within the SR. The inhibitory effect of P-i ex hibited a marked dependence on free Mg2+ concentration. At 0.5 mM free Mg2, 5 mM P-i reduced the caffeine-induced transient by 25.1 <plus/minus> 4.1% (n = 13). However, at 1.5 mM free Mg2+, 5 mM P-i reduced the amplitude of caffeine-induced Ca2+ transients by 68.9 +/- 3.1% (n = 10). Depolarization- induced SR Ca2+ release was similarly affected. These effects of P-i may be important in skeletal muscle fatigue, if an inhibitory action of P-i on SR Ca2+ release is augmented by the rise in cytosolic Mg2+ concentration, whi ch accompanies ATP breakdown.