Mehta D, Wu M F, Gunst S J
Department of Physiology and Biophysics, Indiana University School of Medicine, Indianapolis 46202, USA.
Am J Physiol. 1996 Jan;270(1 Pt 1):C243-52. doi: 10.1152/ajpcell.1996.270.1.C243.
The active isometric force developed by a muscle decreases at muscle lengths below an optimal length (Lo). However, when the length of an actively contracting muscle is abruptly decreased, a lower level of isometric force is reached during force redevelopment than when the contraction is initiated at the shorter length. This has been attributed to a deactivation of contractile proteins caused by shortening. In this study, intracellular Ca2+ and myosin light chain (MLC) phosphorylation were measured to assess the mechanisms for the modulation of isometric force caused by changing smooth muscle length before or during isometric contraction. The decline in isometric force between Lo and 0.5Lo was associated with decreases in MLC phosphorylation and intracellular Ca2+ during contractions elicited by acetylcholine or 60 mM KCl. Quick release of the muscle during contraction depressed force redevelopment at the shorter length but not MLC phosphorylation. We conclude that decreases in Ca(2+)-calmodulin-dependent MLC phosphorylation contribute significantly to the decline in isometric force at lengths below Lo, but the depression of contractility associated with the quick release of actively contracted smooth muscle is not caused by a shortening-induced deactivation of contractile proteins.
肌肉产生的主动等长力在肌肉长度低于最佳长度（Lo）时会下降。然而，当主动收缩的肌肉长度突然缩短时，在力量重新发展过程中达到的等长力水平低于从较短长度开始收缩时的水平。这被归因于缩短引起的收缩蛋白失活。在本研究中，测量了细胞内Ca2+和肌球蛋白轻链（MLC）磷酸化，以评估在等长收缩之前或期间改变平滑肌长度引起的等长力调节机制。在乙酰胆碱或60 mM KCl引发的收缩过程中，Lo与0.5Lo之间等长力的下降与MLC磷酸化和细胞内Ca2+的减少有关。收缩过程中肌肉的快速释放抑制了较短长度下的力量重新发展，但不影响MLC磷酸化。我们得出结论，Ca（2+）-钙调蛋白依赖性MLC磷酸化的降低显著导致了长度低于Lo时等长力的下降，但主动收缩的平滑肌快速释放相关的收缩力降低不是由缩短诱导的收缩蛋白失活引起的。
