Effects of fatigue on sarcoplasmic reticulum and myofibrillar properties of rat single muscle fibers

Force decline during fatigue in skeletal muscle is attributed mainly to pro gressive alterations of the intracellular milieu.:Metabolite changes and th e decline in free myoplasmic calcium influence the activation and contracti le processes. This study was aimed at evaluating whether fatigue also cause s persistent modifications of key myofibrillar and sarcoplasmic reticulum ( SR) proteins that contribute to tension reduction. The presence of such mod ifications was investigated in chemically skinned fibers, a procedure that replaces the fatigued cytoplasm from the muscle fiber with a normal medium. Myofibrillar Ca2+ sensitivity was reduced in slow-twitch muscle (for examp le, the pCa value corresponding to 50% of maximum tension was 6.23 +/- 0.03 vs. 5.99 +/- 0.05, P < 0.01, in rested and fatigued fibers) and not modifi ed in fast-twitch muscle. Phosphorylation of the regulatory myosin light ch ain isoform increased in fast-twitch muscle. The rate of SR Ca2+ uptake was increased in slow-twitch muscle fibers (14.2 +/- 1.0 vs. 19.6 +/- 2.5 nmol .min(-1) mg fiber protein(-1), P < 0.05) and not altered in fast-twitch fib ers. No persistent modifications of SR Ca2+ release properties were found. These results indicate that persistent modifications of myofibrillar and SR properties contribute to fatigue-induced muscle force decline only in slow fibers. These alterations may be either enhanced or counteracted, in vivo, by the metabolic changes that normally occur during fatigue development.