Comparison of the effects of inorganic phosphate on caffeine-induced Ca²⁺ release in fast- and slow-twitch mammalian skeletal muscle

Abstract

We compared the effects of 50 mM P i on caffeine-induced Ca 2+ release in mechanically skinned fast-twitch (FT) and slow-twitch (ST) skeletal muscle fibers of the rat. The time integral (area) of the caffeine response was reduced by ∼57% (FT) and ∼27% (ST) after 30 s of exposure to 50 mM P i in either the presence or absence of creatine phosphate (to buffer ADP). Differences in the sarcoplasmic reticulum (SR) Ca 2+ content between FT and ST fibers [∼40% vs. 100% SR Ca 2+ content (pCa 6.7), respectively] did not contribute to the different effects of P i observed; underloading the SR of ST fibers so that the SR Ca 2+ content approximated that of FT fibers resulted in an even smaller (∼21%), but not significant, reduction in caffeine-induced Ca 2+ release by P i . These observed differences between FT and ST fibers could arise from fiber-type differences in the ability of the SR to accumulate Ca 2+ -P i precipitate. To test this, fibers were Ca 2+ loaded in the presence of 50 mM P i . In FT fibers, the maximum SR Ca 2+ content (pCa 6.7) was subsequently increased by up to 13 times of that achieved when loading for 2 min in the absence of P i . In ST fibers, the SR Ca 2+ content was only doubled. These data show that Ca 2+ release in ST fibers was less affected by P i than FT fibers, and this may be due to a reduced capacity of ST SR to accumulate Ca 2+ -P i precipitate. This may account, in part, for the fatigue-resistant nature of ST fibers.