Danieli-Betto D, Germinario E, Esposito A, Biral D, Betto R
Dipartimento di Anatomia e Fisiologia Umana, Centro di Studio per la Biologia e la Fisiopatologia Muscolare, I-35131 Padova, Italy.
J Appl Physiol (1985). 2000 Sep;89(3):891-8. doi: 10.1152/jappl.2000.89.3.891.
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 < 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 Ca(2+) 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 fibers. No persistent modifications of SR Ca(2+) 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.
骨骼肌疲劳时力量下降主要归因于细胞内环境的渐进性改变。代谢物变化和游离肌浆钙的下降会影响激活和收缩过程。本研究旨在评估疲劳是否还会导致关键肌原纤维和肌浆网（SR）蛋白的持续性改变，这些改变会导致张力降低。在化学去膜纤维中研究了此类改变的存在情况，化学去膜是一种用正常培养基替代肌肉纤维中疲劳细胞质的方法。慢肌纤维中肌原纤维Ca(2+)敏感性降低（例如，对应于最大张力50%的pCa值在静息和疲劳纤维中分别为6.23±0.03和5.99 + 0.05，P < 0.01），而快肌纤维中未改变。快肌纤维中调节性肌球蛋白轻链同工型的磷酸化增加。慢肌纤维中SR Ca(2+)摄取速率增加（14.2±1.0对19.6±2.5 nmol·min(-1)·mg纤维蛋白(-1)，P < 0.05），快肌纤维中未改变。未发现SR Ca(2+)释放特性有持续性改变。这些结果表明，肌原纤维和SR特性的持续性改变仅在慢肌纤维中导致疲劳诱导的肌肉力量下降。在体内，这些改变可能会被疲劳发展过程中正常发生的代谢变化增强或抵消。
