Mechanisms of Pi regulation of the skeletal muscle SR Ca21 release channel

Balog, Edward M., Bradley R. Fruen, Patricia K. Kane, and Charles F. Louis. Mechanisms of Pi regulation of the skeletal muscle SR Ca21 release channel. Am. J. Physiol. Cell Physiol. 278: C601–C611, 2000.—Inorganic phosphate (Pi) accumulates in the fibers of actively working muscle where it acts at various sites to modulate contraction. To characterize the role of Pi as a regulator of the sarcoplasmic reticulum (SR) calcium (Ca21) release channel, we examined the action of Pi on purified SR Ca21 release channels, isolated SR vesicles, and skinned skeletal muscle fibers. In single channel studies, addition of Pi to the cis chamber increased single channel open probability (Po; 0.079 6 0.020 in 0 Pi, 0.157 6 0.034 in 20 mM Pi) by decreasing mean channel closed time; mean channel open times were unaffected. In contrast, the ATP analog, b,g-methyleneadenosine 58-triphosphate (AMP-PCP), enhanced Po by increasing single channel open time and decreasing channel closed time. Pi stimulation of [3H]ryanodine binding by SR vesicles was similar at all concentrations of AMP-PCP, suggesting Pi and adenine nucleotides act via independent sites. In skinned muscle fibers, 40 mM Pi enhanced Ca21-induced Ca21 release, suggesting an in situ stimulation of the release channel by high concentrations of Pi. Our results support the hypothesis that Pi may be an important endogenous modulator of the skeletal muscle SR Ca21 release channel under fatiguing conditions in vivo, acting via a mechanism distinct from adenine nucleotides.