Campbell Kenneth S
Department of Physiology, University of Kentucky, Lexington, Kentucky, USA.
Biophys J. 2006 Feb 15;90(4):1288-94. doi: 10.1529/biophysj.105.067504. Epub 2005 Nov 18.
Permeabilized rat soleus muscle fibers were subjected to rapid shortening/restretch protocols (20% muscle length, 20 ms duration) in solutions with pCa values ranging from 6.5 to 4.5. Force redeveloped after each restretch but temporarily exceeded the steady-state isometric tension reaching a maximum value approximately 2.5 s after relengthening. The relative size of the overshoot was <5% in pCa 6.5 and pCa 4.5 solutions but equaled 17% +/- 4% at pCa 6.0 (approximately half-maximal Ca2+ activation). Muscle stiffness was estimated during pCa 6.0 activations by imposing length steps at different time intervals after repeated shortening/restretch perturbations. Relative stiffness and relative tension were correlated (p < 0.001) during recovery, suggesting that tension overshoots reflect a temporary increase in the number of attached cross-bridges. Rates of tension recovery (k(tr)) correlated (p < 0.001) with the relative residual force prevailing immediately after restretch. Force also recovered to the isometric value more quickly at 5.7 < or = pCa < or = 5.9 than at pCa 4.5 (ANOVA, p < 0.05). These results show that k(tr) measurements underestimate the rate of isometric force development during submaximal Ca2+ activations and suggest that the rate of tension recovery is limited primarily by the availability of actin binding sites.
将大鼠比目鱼肌纤维通透化处理后，在钙离子浓度（pCa）值范围为6.5至4.5的溶液中，使其经历快速缩短/拉伸实验方案（肌肉长度缩短20%，持续时间20毫秒）。每次拉伸后力量都会重新恢复，但会暂时超过稳态等长张力，在重新拉长后约2.5秒达到最大值。在pCa 6.5和pCa 4.5的溶液中，超调量的相对大小小于5%，但在pCa 6.0（约为最大钙离子激活量的一半）时等于17%±4%。在重复进行缩短/拉伸扰动后的不同时间间隔施加长度步长，以此来估计pCa 6.0激活过程中的肌肉刚度。在恢复过程中，相对刚度和相对张力具有相关性（p<0.001），这表明张力超调反映了附着的横桥数量的暂时增加。张力恢复速率（k(tr)）与拉伸后立即出现的相对残余力具有相关性（p<0.001）。在5.7≤pCa≤5.9时，力量恢复到等长值的速度也比在pCa 4.5时更快（方差分析，p<0.05）。这些结果表明，k(tr)测量值低估了亚最大钙离子激活过程中等长力量发展的速率，并表明张力恢复速率主要受肌动蛋白结合位点可用性的限制。
