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

Posterino GS, Dunn SL. Comparison of the effects of inorganic phosphate on caffeine-induced Ca release in fastand slowtwitch mammalian skeletal muscle. Am J Physiol Cell Physiol 294: C97–C105, 2008. First published October 24, 2007; doi:10.1152/ajpcell.00155.2007.—We compared the effects of 50 mM Pi on caffeine-induced Ca 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 Pi in either the presence or absence of creatine phosphate (to buffer ADP). Differences in the sarcoplasmic reticulum (SR) Ca content between FT and ST fibers [ 40% vs. 100% SR Ca content (pCa 6.7), respectively] did not contribute to the different effects of Pi observed; underloading the SR of ST fibers so that the SR Ca content approximated that of FT fibers resulted in an even smaller ( 21%), but not significant, reduction in caffeine-induced Ca 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 Ca -Pi precipitate. To test this, fibers were Ca loaded in the presence of 50 mM Pi. In FT fibers, the maximum SR Ca 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 Ca content was only doubled. These data show that Ca release in ST fibers was less affected by Pi than FT fibers, and this may be due to a reduced capacity of ST SR to accumulate Ca -Pi precipitate. This may account, in part, for the fatigue-resistant nature of ST fibers.