Lynch G S, Stephenson D G, Williams D A
Department of Physiology, University of Melbourne, Parkville, Victoria, Australia.
J Muscle Res Cell Motil. 1995 Feb;16(1):65-78. doi: 10.1007/BF00125311.
To understand how the coexistence of fast and slow contractile and regulatory systems within single skeletal muscle fibres might affect contractile behaviour, fibre segments from the fast-twitch extensor digitorum longus and predominantly slow-twitch soleus muscle of the adult rat were tied together, either in parallel or in series, and then activated in Ca(2+)- and Sr(2+)-buffered solutions. Experimental force-pCa and force-pSr relations were compared with theoretical force-pCa and force-pSr curves predicted by a model for composite fibres, which accounted for the coexistence of fast and slow myosin within the contractile unit and enabled an estimate to be made of the relative contribution of fast- and slow-twitch elements within the tied-fibre combinations. The contractile behaviour of a fast-twitch and a slow-twitch muscle fibre tied either in series or in parallel, were compared with the force-pCa and force-pSr data predicted from the composite fibre model. Interestingly, the resultant force-pCa(-pSr) curves of the parallel-tied fibre combinations were well fitted with those predicted by the composite model. However, the experimental force-pCa(-pSr) curves of the series-tied fibres were not well fitted by a composite curve based on the known proportion of fast- and slow-twitch fibre components. A total force-length diagram was devised to take into account changes in the length of the fibre segments tied in series during activation, as well as possible differences in fibre diameter. Using this diagram it was possible to explain accurately the Ca2+ and Sr2+ activation curves of known fast- and slow-twitch segments tied in series. The results from this study are important for the interpretation of contractile data obtained from single muscle fibres exhibiting mixed fast- and slow-twitch contractile characteristics. Such muscle fibres have previously been identified in animals affected by muscular diseases (e.g. dystrophy), in mammalian extraocular muscles and in animals subjected to long-term exercise training.
为了理解单根骨骼肌纤维内快速和慢速收缩及调节系统的共存如何影响收缩行为，将成年大鼠快肌趾长伸肌和主要为慢肌比目鱼肌的纤维段以平行或串联方式绑在一起，然后在钙（Ca²⁺）和锶（Sr²⁺）缓冲溶液中激活。将实验得到的力-pCa和力-pSr关系与复合纤维模型预测的理论力-pCa和力-pSr曲线进行比较，该模型考虑了收缩单元内快肌球蛋白和慢肌球蛋白的共存，并能够估计绑在一起的纤维组合中快肌和慢肌成分的相对贡献。将串联或并联的快肌和慢肌纤维的收缩行为与复合纤维模型预测的力-pCa和力-pSr数据进行比较。有趣的是，并联纤维组合的合力-pCa（-pSr）曲线与复合模型预测的曲线拟合良好。然而，串联纤维的实验力-pCa（-pSr）曲线不能很好地用基于已知快肌和慢肌纤维成分比例的复合曲线拟合。设计了一个总力-长度图，以考虑激活过程中串联绑在一起的纤维段长度的变化以及纤维直径可能存在的差异。利用该图可以准确解释已知串联的快肌和慢肌段的Ca²⁺和Sr²⁺激活曲线。这项研究的结果对于解释从表现出快肌和慢肌混合收缩特征的单根肌纤维获得的收缩数据很重要。此前已在患有肌肉疾病（如营养不良）的动物、哺乳动物眼外肌以及接受长期运动训练的动物中发现了这种肌纤维。
