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MUSCLE CELL BIOLOGY AND CELL MOTILITY

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

Discipline of Physiology, School of Molecular and Biomedical Sciences, The University of Adelaide, Adelaide, South Australia, Australia

Submitted 12 April 2007 ; accepted in final form 22 October 2007

We compared the effects of 50 mM P_i on 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 P_i in 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 P_i observed; 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 P_i. These observed differences between FT and ST fibers could arise from fiber-type differences in the ability of the SR to accumulate Ca²⁺-P_i precipitate. To test this, fibers were Ca²⁺ loaded in the presence of 50 mM P_i. 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 P_i. In ST fibers, the SR Ca²⁺ content was only doubled. These data show that Ca²⁺ 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 Ca²⁺-P_i precipitate. This may account, in part, for the fatigue-resistant nature of ST fibers.

fatigue; Ca²⁺ precipitation; excitation-contraction coupling