Magid A, Ting-Beall H P, Carvell M, Kontis T, Lucaveche C
Adv Exp Med Biol. 1984;170:307-28. doi: 10.1007/978-1-4684-4703-3_26.
This report concerns structural forces in resting muscle and proposes three additions to the sliding filament model to account for these mechanical properties. The proposal includes: connecting filaments (C-filaments) which connect the ends of each thick filament to the neighboring Z-lines, core filaments which support the myosin of the thick filament and which attach to the C-filaments, and side-struts which bind the thick filaments together along their length and restrict their radial movement. C-filaments would act as the parallel elastic element and transmit the passive tension to the thick filaments. Isolated myofibrils (mechanically-skinned and detergent-treated frog semitendinosus fibers) when stretched progressively showed exponentially-increasing passive tension which did not disappear when filament overlap was exceeded, but continued to rise. SL was monitored with a HeNe laser. Passive tension phasically exceeded 3 X 10(5) N/M2. Electron microscopy (thin-sectioned and freeze-fracture/deep-etch specimens) of non-overlap fibers showed orderly fibril structure with clear separation of A- and I-bands. In the gap between them could be seen filaments, 40-50 A in diameter, connected to the thick filament ends. Unlike actin, these filaments did not become decorated by myosin S-1. Equatorial X-ray measurements showed that stretching relaxed skinned muscles squeezed the thick filaments closer; this radial compression continued beyond filament overlap. Extreme stretch of fibers caused the thick filaments to strain several-fold. Treatment of non-overlap fibers with a high ionic strength pyrophosphate myosin solvent caused a large drop in passive tension and stiffness, but no change in SL was detected nor was myofibril continuity detectably affected. Non-overlap fibrils, when treated with elastase, released A-segments which retain three-dimensional coherency . Deep-etch EM's of non-overlap fibers disclosed abundant structures (about 75 A) wide attaching adjacent thick filaments.
本报告涉及静息肌肉中的结构力，并提出对肌丝滑动模型进行三项补充以解释这些力学特性。提议包括：连接丝（C丝），其将每条粗肌丝的末端连接到相邻的Z线；核心丝，其支撑粗肌丝的肌球蛋白并附着于C丝；以及侧支柱，其沿粗肌丝的长度将它们结合在一起并限制其径向运动。C丝将充当平行弹性元件，并将被动张力传递给粗肌丝。逐渐拉伸分离的肌原纤维（经机械去皮和去污剂处理的青蛙半腱肌纤维）时，被动张力呈指数增加，当超过肌丝重叠时不会消失，而是继续上升。用氦氖激光监测肌节长度（SL）。被动张力阶段性地超过3×10⁵ N/m²。非重叠纤维的电子显微镜检查（超薄切片和冷冻断裂/深度蚀刻标本）显示出有序的纤维结构，A带和I带清晰分离。在它们之间的间隙中可以看到直径为40 - 50埃的细丝，连接到粗肌丝末端。与肌动蛋白不同，这些细丝不会被肌球蛋白S - 1装饰。赤道X射线测量表明，拉伸松弛的去皮肌肉会使粗肌丝挤压得更近；这种径向压缩在超过肌丝重叠后仍会继续。纤维的极度拉伸会使粗肌丝应变几倍。用高离子强度的焦磷酸肌球蛋白溶剂处理非重叠纤维会导致被动张力和刚度大幅下降，但未检测到肌节长度变化，也未检测到肌原纤维连续性受到明显影响。用弹性蛋白酶处理非重叠纤维时，会释放出保持三维连贯性的A段。非重叠纤维的深度蚀刻电子显微镜显示有大量宽约75埃的结构连接相邻的粗肌丝。
