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

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Abstract
The effects of inorganic phosphate (Pi) on Ca2+ release from the sarcoplasmic reticulum (SR) were studied in mechanically skinned rat skeletal muscle fibers. Application of caffeine or T-tubule depolarization was used to induce Ca2+ release from the SR, which was detected using fura 2 fluorescence. Addition of Pi (1–40 mM) caused a reversible and concentration-dependent reduction in the caffeine-induced Ca2+ transient. This effect was apparent at low Pi concentration (i exhibited a marked dependence on free Mg2+ concentration. At 0.5 mM free Mg2+, 5 mM Pi reduced the caffeine-induced transient by 25.1 ± 4.1% ( n = 13). However, at 1.5 mM free Mg2+, 5 mM Pi 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 Pi may be important in skeletal muscle fatigue, if an inhibitory action of Pi on SR Ca2+ release is augmented by the rise in cytosolic Mg2+ concentration, which accompanies ATP breakdown.