Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, John Dalton Building, Oxford Road, Manchester, UK.
Exp Physiol. 2011 Feb;96(2):171-8. doi: 10.1113/expphysiol.2010.054239. Epub 2010 Oct 1.
Reducing the activating calcium concentration with skinned fibres is known to decrease isometric force and maximal shortening velocity, both of which will reduce the peak power. However, power is also a function of the curvature of the force-velocity relationship, and there is limited information on how changes in activating calcium affect this important property of muscle fibres. Force-velocity relationships of permeabilized single type I fibres from rat soleus muscle were determined using isotonic contractions at 15°C with both maximal (pCa 4.5) and submaximal activation (pCa 5.6). The rate of tension redevelopment (k(tr)), which provides a measure of sum of the apparent rate constants for cross-bridge attachment and detachment (f(app) + g(app)) following a rapid release and restretch, was also measured. Compared with pCa 4.5, specific tension (P(o)) at pCa 5.6 declined by 22 ± 8% (mean ± s.d.) and the maximal velocity of shortening (V(max)) fell by 44 ± 7%, but curvature of the force-velocity relationship (a/P(o)) rose by 47 ± 31%, indicating a less concave relationship. The value of k(tr) declined by 23 ± 7%. The change in a/P(o) reduced the impact of changes in P(o) and V(max) on peak power by approximately 25%. Fitting the data to Huxley's model of cross-bridge action suggests that lower activating calcium decreased both the rate constant for cross-bridge attachment (f) and that for detachment of negatively strained cross-bridges (g(2)). The fact that V(max) (and thus g(2)) changed to a greater extent than k(tr) (f(app) + g(app)) is the reason that reduced activation results in a reduction in curvature of the force-velocity relationship.
用去皮纤维降低激活钙浓度已知会降低等长力量和最大缩短速度，这两者都会降低峰值功率。然而，功率也是力-速度关系曲率的函数，关于激活钙的变化如何影响肌肉纤维的这个重要特性，信息有限。使用 15°C 的等张收缩，用最大（pCa 4.5）和亚最大激活（pCa 5.6），测定了来自大鼠比目鱼肌的通透性单型 I 纤维的力-速度关系。张力再发展速率（k(tr)）也进行了测量，该速率提供了快速释放和再拉伸后交叉桥附着和脱离的表观速率常数（f(app) + g(app)）之和的度量。与 pCa 4.5 相比，pCa 5.6 时的特定张力（P(o)）下降了 22 ± 8%（均值 ± s.d.），最大缩短速度（V(max)）下降了 44 ± 7%，但力-速度关系的曲率（a/P(o)）上升了 47 ± 31%，表明关系不太凹。k(tr)的值下降了 23 ± 7%。a/P(o)的变化使 P(o)和 V(max)变化对峰值功率的影响降低了约 25%。将数据拟合到 Huxley 的横桥作用模型表明，较低的激活钙降低了横桥附着的速率常数（f）和负应变横桥的脱离速率常数（g(2)）。V(max)（因此 g(2)）变化程度大于 k(tr)（f(app) + g(app)）是降低激活导致力-速度关系曲率降低的原因。
