Am. Duke et Ds. Steele, Characteristics of phosphate-induced Ca2+ efflux from the SR in mechanically skinned rat skeletal muscle fibers, AM J P-CELL, 278(1), 2000, pp. C126-C135
The effects of P-i on sarcoplasmic reticulum (SR) Ca2+ regulation were stud
ied in mechanically skinned rat skeletal muscle fibers. Brief application o
f caffeine was used to assess the SR Ca2+ content, and changes in concentra
tion of Ca2+ ([Ca2+]) within the cytosol were detected with fura 2 fluoresc
ence. Introduction of P-i (1-40 mM) induced a concentration-dependent Ca2efflux from the SR. In solutions lacking creatine phosphate (CP), the ampli
tude of the P-i-induced Ca2+ transient approximately doubled. A similar pot
entiation of P-i-induced Ca2+ release occurred after inhibition of creatine
kinase (CK) with 2,4-dinitrofluorobenzene. In the presence of ruthenium re
d or ryanodine, caffeine-induced Ca2+ release was almost abolished, whereas
P-i-induced Ca2+ release was unaffected. However, introduction of the SR C
a2+ ATPase inhibitor cyclopiazonic acid effectively abolished P-i-induced C
a2+ release. These data suggest that P-i induces Ca2+ release from the SR b
y reversal of the SR Ca2+ pump but not via the SR Ca2+ channel under these
conditions. If this occurs in intact skeletal muscle during fatigue, activa
tion of a Ca2+ efflux pathway by P-i may contribute to the reported decreas
e in net Ca2+ uptake and increase in resting [Ca2+].