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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
Ca2+ 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 < 0.01, in
rested and fatigued fibers) and not modified in fast-twitch muscle.
Phosphorylation of the regulatory myosin light chain 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
protein1, P < 0.05) and not
altered in fast-twitch fibers. 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.
myofibrillar calcium sensitivity; chemically skinned fibers
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