Program in Physical Therapy, and Departments of Neurology, Biomedical Engineering and Orthopaedic Surgery, Washington University School of Medicine, St Louis, MO 63110, USA.
Shirley Ryan AbilityLab and Departments of Physical Medicine and Rehabilitation, Physiology and Biomedical Engineering, Northwestern University, Chicago, IL 60611, USA
J Exp Biol. 2018 Nov 16;221(Pt 22):jeb182089. doi: 10.1242/jeb.182089.
Differences in passive muscle mechanical properties between amphibians and mammals have led to differing hypotheses on the functional role of titin in skeletal muscle. Early studies of frog muscle clearly demonstrated intracellular load bearing by titin, but more recent structural and biological studies in mice have shown that titin may serve other functions. Here, we present biomechanical studies of isolated frog and mouse fibers, and fiber bundles to compare the relative importance of intracellular versus extracellular load bearing in these species. Mouse bundles exhibited increased modulus compared with fibers on the descending limb of the length-tension curve, reaching a 2.4-fold elevation at the longest sarcomere lengths. By contrast, frog fibers and bundles had approximately the same modulus at all sarcomere lengths tested. These findings suggest that in the mouse, both muscle fibers and the ECM are involved in bearing whole muscle passive tension, which is distinct from the load bearing process in frog muscle, where titin bears the majority of whole muscle passive tension.
由于两栖动物和哺乳动物的被动肌肉力学特性存在差异，导致人们对肌联蛋白在骨骼肌中的功能作用提出了不同的假设。早期对青蛙肌肉的研究清楚地表明了肌联蛋白具有细胞内的承载能力，但最近在老鼠中的结构和生物学研究表明，肌联蛋白可能具有其他功能。在这里，我们对分离的青蛙和老鼠纤维以及纤维束进行了生物力学研究，以比较这些物种中细胞内与细胞外承载相对重要性。与长度-张力曲线下降支上的纤维相比，老鼠束在最长肌节长度时表现出增加的模量，达到 2.4 倍的升高。相比之下，青蛙纤维和束在所有测试的肌节长度下具有大致相同的模量。这些发现表明，在老鼠中，肌肉纤维和细胞外基质都参与了承载整个肌肉的被动张力，这与青蛙肌肉中的承载过程不同，在青蛙肌肉中，肌联蛋白承载了整个肌肉的大部分被动张力。
