Department of Biological, Chemical and Physical Sciences, Illinois Institute of Technology, Chicago, Illinois, USA.
Biophys J. 2011 Mar 16;100(6):1499-508. doi: 10.1016/j.bpj.2011.01.059.
We studied the effect of titin-based passive tension on sarcomere structure by simultaneously measuring passive tension and low-angle x-ray diffraction patterns on passive fiber bundles from rabbit skinned psoas muscle. We used a stretch-hold-release protocol with measurement of x-ray diffraction patterns at various passive tension levels during the hold phase before and after passive stress relaxation. Measurements were performed in relaxing solution without and with dextran T-500 to compress the lattice toward physiological levels. The myofilament lattice spacing was measured in the A-band (d(1,0)) and Z-disk (d(Z)) regions of the sarcomere. The axial spacing of the thick-filament backbone was determined from the sixth myosin meridional reflection (M6) and the equilibrium positions of myosin heads from the fourth myosin layer line peak position and the I(1,1)/I(1,0) intensity ratio. Total passive tension was measured during the x-ray experiments, and a differential extraction technique was used to determine the relations between collagen- and titin-based passive tension and sarcomere length. Within the employed range of sarcomere lengths (∼2.2-3.4 μm), titin accounted for >80% of passive tension. X-ray results indicate that titin compresses both the A-band and Z-disk lattice spacing with viscoelastic behavior when fibers are swollen after skinning, and elastic behavior when the lattice is reduced with dextran. Titin also increases the axial thick-filament spacing, M6, in an elastic manner in both the presence and absence of dextran. No changes were detected in either I(1,1)/I(1,0) or the position of peaks on the fourth myosin layer line during passive stress relaxation. Passive tension and M6 measurements were converted to thick-filament compliance, yielding a value of ∼85 m/N, which is several-fold larger than the thick-filament compliance determined by others during the tetanic tension plateau of activated intact muscle. This difference can be explained by the fact that thick filaments are more compliant at low tension (passive muscle) than at high tension (tetanic tension). The implications of our findings are discussed.
我们通过同时测量来自去表皮兔腰大肌被动纤维束的被动张力和低角度 X 射线衍射图案，研究了肌联蛋白的被动张力对肌节结构的影响。我们使用了一种拉伸-保持-释放方案，在保持阶段之前和之后，在被动张力松弛前后，在不同的被动张力水平下测量 X 射线衍射图案。测量是在无和有葡聚糖 T-500 的松弛溶液中进行的，葡聚糖 T-500 将晶格压缩到生理水平。肌节的 A 带（d(1,0)）和 Z 盘（d(Z)）区域测量了肌丝晶格间距。从第六肌球蛋白子午线反射（M6）确定了厚丝骨架的轴向间距，并从第四肌球蛋白层线峰值位置和 I(1,1)/I(1,0) 强度比确定了肌球蛋白头部的平衡位置。在 X 射线实验过程中测量了总被动张力，并使用差分提取技术确定了胶原和肌联蛋白的被动张力与肌节长度之间的关系。在所采用的肌节长度范围内（2.2-3.4 μm），肌联蛋白占被动张力的>80%。X 射线结果表明，当纤维去表皮后肿胀时，肌联蛋白以粘弹性方式压缩 A 带和 Z 盘晶格间距，而当晶格被葡聚糖减少时，肌联蛋白以弹性方式压缩。肌联蛋白还以弹性方式增加了轴向厚丝间距 M6，无论是否存在葡聚糖。在被动张力松弛过程中，I(1,1)/I(1,0) 或第四肌球蛋白层线的峰值位置都没有检测到变化。被动张力和 M6 的测量值被转换为厚丝顺应性，得到的数值为85 m/N，这比其他人在激活完整肌肉的强直张力平台期间确定的厚丝顺应性大几个数量级。这种差异可以通过这样一个事实来解释，即在低张力（被动肌肉）下，厚丝比在高张力（强直张力）下更具顺应性。我们的发现的意义将被讨论。
