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 Pion caffeine-induced Ca2+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 Piin either the presence or absence of creatine phosphate (to buffer ADP). Differences in the sarcoplasmic reticulum (SR) Ca2+content between FT and ST fibers [∼40% vs. 100% SR Ca2+content (pCa 6.7), respectively] did not contribute to the different effects of Piobserved; underloading the SR of ST fibers so that the SR Ca2+content approximated that of FT fibers resulted in an even smaller (∼21%), but not significant, reduction in caffeine-induced Ca2+release by Pi. These observed differences between FT and ST fibers could arise from fiber-type differences in the ability of the SR to accumulate Ca2+-Piprecipitate. To test this, fibers were Ca2+loaded in the presence of 50 mM Pi. In FT fibers, the maximum SR Ca2+content (pCa 6.7) was subsequently increased by up to 13 times of that achieved when loading for 2 min in the absence of Pi. In ST fibers, the SR Ca2+content was only doubled. These data show that Ca2+release in ST fibers was less affected by Pithan FT fibers, and this may be due to a reduced capacity of ST SR to accumulate Ca2+-Piprecipitate. This may account, in part, for the fatigue-resistant nature of ST fibers.