Parry D J, DiCori S
Department of Physiology, University of Ottawa, Ont., Canada.
Can J Physiol Pharmacol. 1990 Jan;68(1):51-6. doi: 10.1139/y90-007.
Post-tetanic potentiation was measured in motor units, isolated functionally by ventral root splitting, of soleus and extensor digitorum longus muscles of mouse. All motor units from the extensor digitorum longus had times to peak twitch tension less than 13 ms; there was a linear relationship between time to peak tension and post-tetanic potentiation, with the faster units exhibiting greater potentiation. When soleus motor units were similarly analyzed, it appeared that there may be two distinct populations of units. Those units with times to peak tension less than 13 ms were virtually indistinguishable from those of extensor digitorum longus. On the other hand, the slope of the relationship between post-tetanic potentiation and time to peak tension was significantly lower for soleus units with times to peak tension of 13 ms or more. Approximately three-quarters of the soleus units were of the latter slow type, whereas only one-half of the muscle fibres could be classified as type I by means of immunohistochemistry, suggesting that the myosin heavy chain may not be the major determinant of post-tetanic potentiation. Single, chemically skinned fibres of soleus were analyzed for myosin heavy and light chain components by polyacrylamide gel electrophoresis. All fibres with type I heavy chain contained only the two slow light chains. On the other hand, almost all of the fibres with type IIA myosin heavy chain contained both fast and slow light chains. It is suggested that the discrepancy between the proportions of physiologically "fast" motor units and histochemical type IIA fibres may be the consequence of variable amounts of slow light chain associated with the fast IIA myosin heavy chain.
在小鼠比目鱼肌和趾长伸肌中，通过腹根分支功能分离运动单位，测量强直后增强。趾长伸肌的所有运动单位达到峰值抽搐张力的时间均小于13毫秒；达到峰值张力的时间与强直后增强之间存在线性关系，速度较快的单位表现出更大的增强。当对比目鱼肌运动单位进行类似分析时，似乎可能存在两个不同的单位群体。那些达到峰值张力时间小于13毫秒的单位与趾长伸肌的单位几乎没有区别。另一方面，对于达到峰值张力时间为13毫秒或更长的比目鱼肌单位，强直后增强与达到峰值张力时间之间关系的斜率明显较低。大约四分之三的比目鱼肌单位属于后一种慢类型，而通过免疫组织化学方法只有一半的肌纤维可归类为I型，这表明肌球蛋白重链可能不是强直后增强的主要决定因素。通过聚丙烯酰胺凝胶电泳分析比目鱼肌单根化学去皮纤维的肌球蛋白重链和轻链成分。所有含有I型重链的纤维仅含有两条慢轻链。另一方面，几乎所有含有IIA型肌球蛋白重链的纤维都同时含有快轻链和慢轻链。有人提出，生理上“快”的运动单位比例与组织化学IIA型纤维比例之间的差异可能是与快IIA型肌球蛋白重链相关的慢轻链数量不同的结果。
