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Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, St. Paul, Minnesota 55455
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 (Ca2+) release channel, we examined the action
of Pi on purified SR Ca2+ 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 ± 0.020 in 0 Pi, 0.157 ± 0.034 in 20 mM
Pi) by decreasing mean channel closed time; mean channel
open times were unaffected. In contrast, the ATP analog,
,
-methyleneadenosine 5'-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 Ca2+-induced
Ca2+ 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 Ca2+ release
channel under fatiguing conditions in vivo, acting via a mechanism
distinct from adenine nucleotides.
ryanodine receptor; sarcoplasmic reticulum; excitation-contraction coupling; muscle fatigue; inorganic phosphate; calcium-induced calcium release
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