Brenner B, Chalovich J M, Greene L E, Eisenberg E, Schoenberg M
Biophys J. 1986 Oct;50(4):685-91. doi: 10.1016/S0006-3495(86)83509-3.
The stiffness of single skinned rabbit psoas fibers was measured during rapid length changes applied to one end of the fibers. Apparent fiber stiffness was taken as the initial slope when force was plotted vs. change in sarcomere length. In the presence of MgATP, apparent fiber stiffness increased with increasing speed of stretch. With the fastest possible stretches, the stiffness of relaxed fibers at an ionic strength of 20 mM reached more than 50% of the stiffness measured in rigor. However, it was not clear whether apparent fiber stiffness had reached a maximum, speed independent value. The same behavior was seen at several ionic strengths, with increasing ionic strength leading to a decrease in the apparent fiber stiffness measured at any speed of stretch. A speed dependence of apparent fiber stiffness was demonstrated even more clearly when stiffness was measured in the presence of 4 mM MgPPi. In this case, stiffness varied with speed of stretch over about four decades. This speed dependence of apparent fiber stiffness is likely due to cross-bridges detaching and reattaching during the stiffness measurement (Schoenberg, 1985. Biophys. J. 48:467). This means that obtaining an estimate of the maximum number of cross-bridges attached to actin in relaxed fibers at various ionic strengths is not straightforward. However, the data we have obtained are consistent with other estimates of cross-bridge affinity for actin in fibers (Brenner et al., 1986. Biophys. J. In press.) which suggest that ~60-90% of the cross-bridges attached in rigor are attached in relaxed fibers at an ionic strength of 20 mM and ~2-10% of this number of cross-bridges are attached in a relaxed fiber at an ionic strength of 170 mM.
在对单根去表皮兔腰大肌纤维一端施加快速长度变化的过程中，测量了纤维的刚度。当绘制力与肌节长度变化的关系图时，表观纤维刚度被视为初始斜率。在存在MgATP的情况下，表观纤维刚度随拉伸速度的增加而增加。在尽可能快的拉伸速度下，离子强度为20 mM时松弛纤维的刚度达到强直收缩时测量刚度的50%以上。然而，尚不清楚表观纤维刚度是否已达到与速度无关的最大值。在几种离子强度下都观察到了相同的行为，离子强度增加导致在任何拉伸速度下测量的表观纤维刚度降低。当在4 mM MgPPi存在的情况下测量刚度时，表观纤维刚度对速度的依赖性表现得更为明显。在这种情况下，刚度随拉伸速度在大约四个数量级上变化。表观纤维刚度对速度的这种依赖性可能是由于在刚度测量过程中横桥的分离和重新附着（Schoenberg，1985.《生物物理杂志》48:467）。这意味着要获得在不同离子强度下松弛纤维中附着于肌动蛋白的横桥最大数量的估计并非易事。然而，我们获得的数据与纤维中横桥对肌动蛋白亲和力的其他估计结果一致（Brenner等人，1986.《生物物理杂志》即将发表），这些估计表明，在离子强度为20 mM时，强直收缩时附着的横桥中有60 - 90%在松弛纤维中附着，而在离子强度为170 mM时，松弛纤维中附着的横桥数量约为此数量的2 - 10%。
