Szeto B, Hai C M
Department of Physiology, Brown University, Providence, Rhode Island 02912, USA.
Arch Biochem Biophys. 1996 May 15;329(2):241-8. doi: 10.1006/abbi.1996.0215.
Phosphorylation of the 20,000 dalton myosin light chain (LC) is a central regulatory mechanism of smooth muscle contraction. Our previous findings on airway smooth muscle have led us to hypothesize that length-dependent and length-independent modulation of myosin phosphorylation coexist in smooth muscle. In this study, we tested the general applicability of this hypothesis by investigating the length dependences of myosin phosphorylation and contractile force in bovine coronary arterial smooth muscle. Comparison of the time courses of myosin phosphorylation at optimal (Lo) and preshortened lengths indicated that the initial peak myosin phosphorylation induced by K(+)-depolarization, histamine, and endothelin were all length dependent. Additional experiments focusing on the length dependence of steady-state myosin phosphorylation revealed that the length dependence of K(+)-depolarization-induced steady-state myosin phosphorylation (0.13 mol P(i)/mol LC/Lo) was significant, but smaller than that found in airway smooth muscle. In contrast, the length dependence of endothelin-induced steady-state myosin phosphorylation (0.03 mol P(i)/mol LC/Lo) was insignificant. The different length dependences of depolarization- and endothelin-induced myosin phosphorylation were found to correlate with different length-force relations. The length-force relationships in K+-depolarized and endothelin-activated tissues remained different even when the force was normalized by the maximum value at Lo. Variable length-force relations have been reported but the biochemical basis is not understood. Results from this study suggest that length-dependent modulation of myosin phosphorylation may be an important determinant of length-force relationship in smooth muscle.
20,000道尔顿肌球蛋白轻链（LC）的磷酸化是平滑肌收缩的核心调节机制。我们之前关于气道平滑肌的研究结果使我们推测，肌球蛋白磷酸化的长度依赖性和非长度依赖性调节在平滑肌中共存。在本研究中，我们通过研究牛冠状动脉平滑肌中肌球蛋白磷酸化和收缩力的长度依赖性，来检验这一假设的普遍适用性。比较在最佳长度（Lo）和预缩短长度下肌球蛋白磷酸化的时间进程表明，由钾离子去极化、组胺和内皮素诱导的初始峰值肌球蛋白磷酸化均具有长度依赖性。聚焦于稳态肌球蛋白磷酸化长度依赖性的额外实验表明，钾离子去极化诱导的稳态肌球蛋白磷酸化的长度依赖性（0.13摩尔无机磷/摩尔LC/Lo）显著，但小于在气道平滑肌中发现的长度依赖性。相比之下，内皮素诱导的稳态肌球蛋白磷酸化的长度依赖性（0.03摩尔无机磷/摩尔LC/Lo）不显著。发现去极化和内皮素诱导的肌球蛋白磷酸化的不同长度依赖性与不同的长度-力关系相关。即使将力用Lo时的最大值进行归一化，钾离子去极化和内皮素激活组织中的长度-力关系仍然不同。已有报道称存在可变的长度-力关系，但其生化基础尚不清楚。本研究结果表明，肌球蛋白磷酸化的长度依赖性调节可能是平滑肌中长度-力关系的重要决定因素。
