Mechanisms of Pi regulation of the skeletal muscle SR Ca2+ release channel

Inorganic phosphate (P-i) accumulates in the fibers of actively working muscle where it acts at various sites to modulate contraction. To characterize the role of P-i as a regulator of the sarcoplasmic reticulum (SR) calcium (Ca2+) release channel, we examined the action of P-i on purified SR Ca2+ release channels, isolated SR vesicles, and skinned skeletal muscle fibers. In single channel studies, addition of P-i to the cis chamber increased single channel open probability (P-o; 0.079 +/- 0.020 in 0 P-i, 0.157 +/- 0.034 in 20 mM P-i) by decreasing mean channel closed time; mean channel open times were unaffected. In contrast, the ATP analog, beta,gamma-methyleneadenosine 5'-triphosphate (AMP-PCP), enhanced P-o by increasing single channel open time and decreasing channel closed time. P-i stimulation of [H-3]ryanodine binding by SR vesicles was similar at all concentrations of AMP-PCP, suggesting P-i and adenine nucleotides act via independent sites. In skinned muscle fibers, 40 mM P-i enhanced Ca2+-induced Ca2+ release, suggesting an in situ stimulation of the release channel by high concentrations of P-i. Our results support the hypothesis that P-i may be an important endogenous modulator of the skeletal muscle SR Ca2+ release channel under fatiguing conditions in vivo, acting via a mechanism distinct from adenine nucleotides.