Ratz P H, Hai C M, Murphy R A
Department of Physiology, School of Medicine, University of Virginia, Charlottesville 22908.
Am J Physiol. 1989 Jan;256(1 Pt 1):C96-100. doi: 10.1152/ajpcell.1989.256.1.C96.
Cross-bridge phosphorylation associated with agonist-stimulated contraction of vascular smooth muscle is often transiently elevated. Such observations led to the concept that phosphorylation of the 20-kDa myosin regulatory light chain (Mp) was required for initial activation and cross-bridge cycling but might not be necessary for steady-state maintenance of stress in the latch state. The possibility that stress maintenance is not regulated by phosphorylation has received some experimental support in contractions induced by phorbol esters and the calcium channel activator BAY K 8644 in which significant increases in Mp were not detected. Our aim was to test the hypothesis that phosphorylation is both necessary and sufficient for activation and for maintenance of steady-state stress. Activation of swine carotid media using agents that bypass receptor activation and elevate Ca2+ influx without mobilizing intracellular Ca2+ stores (BAY K 8644 and ionomycin) produced monotonic increases in both stress and Mp. Transient initial peaks in Mp were absent. Steady-state stress induced by both receptor- and nonreceptor-mediated activation was dependent on small increases in Mp. Increases in Mp greater than 0.3 mol Pi/mol myosin light chain had small effects on stress but produced large increases in the maximum rate of cross-bridge cycling at zero load (Vo). The experimentally determined dependence of stress on Mp was quantitatively predicted by our working hypothesis. This model proposes that Ca2+-stimulated cross-bridge phosphorylation is obligatory for cross-bridge attachment. However, dephosphorylation of attached cross bridges to form noncycling "latch bridges" allows stress maintenance with reduced Mp and cycling.
与激动剂刺激的血管平滑肌收缩相关的横桥磷酸化常常会短暂升高。这些观察结果引出了这样一个概念，即20 kDa肌球蛋白调节轻链（Mp）的磷酸化是初始激活和横桥循环所必需的，但对于维持闩锁状态下的稳态张力可能并非必要。在佛波酯和钙通道激活剂BAY K 8644诱导的收缩中，未检测到Mp显著增加，这为张力维持不受磷酸化调节的可能性提供了一些实验支持。我们的目的是检验磷酸化对于激活和维持稳态张力既必要又充分这一假设。使用绕过受体激活并增加Ca2+内流而不动员细胞内Ca2+储存的试剂（BAY K 8644和离子霉素）激活猪颈动脉中膜，会使张力和Mp单调增加。Mp没有出现短暂的初始峰值。受体介导和非受体介导的激活所诱导的稳态张力均依赖于Mp的小幅增加。Mp增加超过0.3摩尔磷酸根/摩尔肌球蛋白轻链时，对张力的影响较小，但在零负荷下横桥循环的最大速率（Vo）会大幅增加。我们的工作假设定量预测了实验确定的张力对Mp的依赖性。该模型提出，Ca2+刺激的横桥磷酸化对于横桥附着是必不可少的。然而，附着的横桥去磷酸化以形成非循环的“闩锁桥”，可以在Mp降低和循环减少的情况下维持张力。
