Department of Life Sciences, Manchester Metropolitan University, Research Centre for Musculoskeletal Sciences & Sport Medicine, Manchester M1 5GD, UK.
Institute of Sport Science and Innovations, Lithuanian Sports University, LT-44221 Kaunas, Lithuania.
Medicina (Kaunas). 2020 May 20;56(5):249. doi: 10.3390/medicina56050249.
: Muscle fatigue is characterised by (1) loss of force, (2) decreased maximal shortening velocity and (3) a greater resistance to stretch that could be due to reduced intracellular Ca and increased Pi, which alter cross bridge kinetics. Materials and Methods: To investigate this, we used (1) 2,3-butanedione monoxime (BDM), believed to increase the proportion of attached but non-force-generating cross bridges; (2) Pi that increases the proportion of attached cross bridges, but with Pi still attached; and (3) reduced activating Ca. We used permeabilised rat soleus fibres, activated with pCa 4.5 at 15 °C. The addition of 1 mM BDM or 15 mM Pi, or the lowering of the Ca to pCa 5.5, all reduced the isometric force by around 50%. Stiffness decreased in proportion to isometric force when the fibres were activated at pCa 5.5, but was well maintained in the presence of Pi and BDM. Force enhancement after a stretch increased with the length of stretch and Pi, suggesting a role for titin. Maximum shortening velocity was reduced by about 50% in the presence of BDM and pCa 5.5, but was slightly increased by Pi. Neither decreasing Ca nor increasing Pi alone mimicked the effects of fatigue on muscle contractile characteristics entirely. Only BDM elicited a decrease of force and slowing with maintained stiffness, similar to the situation in fatigued muscle. This suggests that in fatigue, there is an accumulation of attached but low-force cross bridges that cannot be the result of the combined action of reduced Ca or increased Pi alone, but is probably due to a combination of factors that change during fatigue.
肌肉疲劳的特征是（1）力量损失，（2）最大缩短速度降低，（3）拉伸阻力增加，这可能是由于细胞内 Ca 减少和 Pi 增加，改变了横桥动力学。
为了研究这一点，我们使用了（1）2,3-丁二酮单肟（BDM），据信它会增加附着但不产生力的横桥的比例；（2）增加附着横桥比例的 Pi，但仍附着 Pi；（3）减少激活 Ca。我们使用透化的大鼠比目鱼肌纤维，在 15°C 时用 pCa 4.5 激活。添加 1 mM BDM 或 15 mM Pi，或降低 Ca 至 pCa 5.5，都会使等长力降低约 50%。当纤维在 pCa 5.5 下激活时，刚性与等长力成比例下降，但在 Pi 和 BDM 存在下保持良好。在拉伸后力增强与拉伸长度和 Pi 呈正相关，表明 titin 起作用。在 BDM 和 pCa 5.5 存在下，最大缩短速度降低约 50%，但 Pi 略有增加。单独降低 Ca 或增加 Pi 都不能完全模拟疲劳对肌肉收缩特性的影响。只有 BDM 会引起力和刚度的降低，类似于疲劳肌肉的情况。这表明，在疲劳中，存在附着但低力的横桥的积累，这不能仅仅是由于减少 Ca 或增加 Pi 的联合作用的结果，而是可能由于在疲劳过程中发生变化的组合因素引起的。
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