Schiereck P, van Heijst B G, Jansen P M, Schiereck J, van der Leun M, Bras W, de Beer E L
Department of Medical Physiology and Sports Medicine, University of Utrecht, The Netherlands.
Eur Biophys J. 1998;27(6):575-81. doi: 10.1007/s002490050168.
Skinned muscle fibres from the gracilis muscle of the rabbit were used to record small angle X-ray diffraction spectra under various contractile conditions. The intracellular calcium concentration, expressed as pCa, was varied between 8.0 and 5.74. Equatorial diffraction spectra were fitted by a function consisting of five Gaussian curves and a hyperbola to separate the (1.0), (1.1), (2.0), (2.1) and Z-line diffraction peaks. The hyperbola was used to correct for residual scattering in the preparation. The ratio between the intensities of the (1.1) and (1.0) peaks was defined as the relative transfer of mass between myosin and actin, due to crossbridge formation after activation by calcium. The relation between the ratio and the relative force of the fibre (normalized to the force at pCa 5.74 and sarcomere length 2.0 microns) was linear. At high pCa (from pCa 6.34 to 8.0) no active force was observed, while the ratio still decreased. Sarcomere length was recorded by laser diffraction. The laser diffraction patterns did not show changes in sarcomere length due to activation in the high pCa range (between 8.0 and 6.34). From these results the conclusion is drawn that crossbridge movement occurs even at subthreshold calcium concentrations in the cell, when no active force is exerted. Since no force is generated this movement may be related to crossbridges in the weakly bound state.
利用兔股薄肌的去皮肌纤维，在各种收缩条件下记录小角X射线衍射光谱。以pCa表示的细胞内钙浓度在8.0至5.74之间变化。赤道衍射光谱由五条高斯曲线和一条双曲线组成的函数拟合，以分离(1.0)、(1.1)、(2.0)、(2.1)和Z线衍射峰。双曲线用于校正制剂中的残余散射。(1.1)和(1.0)峰强度之比定义为钙激活后由于横桥形成导致的肌球蛋白和肌动蛋白之间的相对质量转移。该比值与纤维的相对力（相对于pCa 5.74和肌节长度2.0微米时的力进行归一化）之间的关系是线性的。在高pCa（从pCa 6.34到8.0）时未观察到主动力，而该比值仍下降。通过激光衍射记录肌节长度。激光衍射图案未显示在高pCa范围（8.0至6.34之间）激活导致的肌节长度变化。从这些结果得出结论，即使在细胞内钙浓度低于阈值且未施加主动力时，横桥运动也会发生。由于未产生力，这种运动可能与弱结合状态的横桥有关。
