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

1 Dipartimento di Anatomia e Fisiologia Umana, and 2 Consiglio Nazionale delle Ricerche, Centro di Studio per la Biologia e la Fisiopatologia Muscolare, I-35131 Padova, Italy Force decline during fatigue in skeletal muscle is attributed mainly to progressive alterations of the intracellular milieu. Metabolite changes and the decline in free myoplasmic calcium influence the activation and contractile processes. This study was aimed at evaluating whether fatigue also causes persistent modifications of key myofibrillar and sarcoplasmic reticulum (SR) proteins that contribute to tension reduction. The presence of such modifications was investigated in chemically skinned fibers, a procedure that replaces the fatigued cytoplasm from the muscle fiber with a normal medium. Myofibrillar Ca 2+ sensitivity was reduced in slow-twitch muscle (for example, the pCa value corresponding to 50% of maximum tension was 6.23 ± 0.03 vs. 5.99 + 0.05, P