Metzger J M
Department of Physiology, University of Michigan School of Medicine, Ann Arbor 48109, USA.
Biophys J. 1996 Jan;70(1):409-17. doi: 10.1016/S0006-3495(96)79584-X.
The effects of added phosphate and MgADP on unloaded shortening velocity during maximal and submaximal Ca2+ activation of the thin filament were examined in skinned single skeletal fibers from rabbit psoas muscle. During maximal Ca2+ activation, added phosphate (10-30 mM) had no effect on unloaded shortening velocity as determined by the slack-test technique. In fibers activated at submaximal concentrations of Ca2+ in the absence of added phosphate, plots of slack length versus duration of unloaded shortening were biphasic, consisting of an initial high velocity phase of shortening and a subsequent low velocity phase of shortening. Interestingly, in the presence of added phosphate, biphasic slack-test plots were no longer apparent. This result was obtained in control fibers over a range of submaximal Ca2+ concentrations and in maximally Ca2+ activated fibers, which were first treated to partially extract troponin C. Thus, under conditions that favor the appearance of biphasic shortening (i.e., low [Ca2+], troponin C extraction), added phosphate eliminated the low velocity component. In contrast, in fibers activated in the presence of 5 mM added MgADP, biphasic slack-test plots were apparent even during maximal Ca2+ activation. The basis of biphasic shortening is not known but it may be due to the formation of axially compressed cross-bridges that become strained to bear a tension that opposes the relative sliding of the myofilaments. The present findings could be explained if added phosphate and MgADP bind to cross-bridges in a strain-dependent manner. In this case, the results suggest that phosphate inhibits the formation of cross-bridges that bear a compressive strain. Added MgADP, on the other hand, may be expected to detain cross-bridges in strong binding states, thus promoting an increase in the population of cross-bridges bearing a compressive strain. Alterations in the population of strained cross-bridges by added phosphate and MgADP would alter the internal load within the fiber and thus affect the speed of fiber shortening.
在兔腰大肌的去皮肤单根骨骼肌纤维中，研究了添加磷酸盐和MgADP对细肌丝在最大和次最大Ca2+激活期间的无负荷缩短速度的影响。在最大Ca2+激活期间，通过松弛测试技术测定，添加的磷酸盐（10 - 30 mM）对无负荷缩短速度没有影响。在没有添加磷酸盐的情况下，以次最大浓度的Ca2+激活的纤维中，松弛长度与无负荷缩短持续时间的关系图呈双相，包括初始的高速度缩短阶段和随后的低速度缩短阶段。有趣的是，在添加磷酸盐的情况下，双相松弛测试图不再明显。在一系列次最大Ca2+浓度的对照纤维以及首先进行部分肌钙蛋白C提取的最大Ca2+激活纤维中都得到了这一结果。因此，在有利于双相缩短出现的条件下（即低[Ca2+]、肌钙蛋白C提取），添加的磷酸盐消除了低速成分。相反，在添加5 mM MgADP的情况下激活的纤维中，即使在最大Ca2+激活期间，双相松弛测试图也很明显。双相缩短的基础尚不清楚，但可能是由于形成了轴向压缩的横桥，这些横桥变得紧张以承受与肌丝相对滑动相反方向的张力。如果添加的磷酸盐和MgADP以应变依赖的方式与横桥结合，那么本研究结果就可以得到解释。在这种情况下，结果表明磷酸盐抑制了承受压缩应变的横桥的形成。另一方面，添加的MgADP可能会使横桥保持在强结合状态，从而促进承受压缩应变的横桥数量增加。添加的磷酸盐和MgADP对应变横桥数量的改变会改变纤维内部的负荷，从而影响纤维缩短的速度。
