Nonuniform elasticity of titin in cardiac myocytes: A study using immunoelectron microscopy and cellular mechanics
Nonuniform elasticity of titin in cardiac myocytes: A study using immunoelectron microscopy and cellular mechanics
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DOI：
10.1016/s0006-3495(96)79586-3
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发表时间：
1996-01-01
影响因子：
3.4
通讯作者：
Trombitas, K
中科院分区：
文献类型：
--
作者：
Granzier, H;Helmes, M;Trombitas, K
肌连接蛋白（又称连接蛋白）是一种在肌节内发现的肌肉特异性巨蛋白，从z线延伸到m线。titin的i带片段被认为是一个分子弹簧，当肌节被拉伸时产生张力（被动张力）。最近对骨骼肌的研究表明，并不是titin的整个i带段表现为弹簧；为了更好地了解心脏被动张力的发展机制，其中被动张力在泵送功能中起着至关重要的作用，我们利用结构和机械技术研究了心肌细胞中titin的弹性段，将单个心肌细胞拉伸不同程度，然后在拉伸状态下进行免疫标记和电镜处理。我们使用识别不同的titin表位的单克隆抗体，研究了titin表位在肌瘤中的位置与肌瘤长度的关系。我们发现，titin的I-band片段只有一小部分区域是弹性的；其轮廓长度估计约为75 nm，仅占titin i带段总长度的40%。被动张力测量表明，心脏产生多少被动张力的基本决定因素是titin弹性段的应变。此外,我们发现,在观察松弛(长度,类似于1.85μm)弹性肌段高度折叠本身之上,基于数据,我们提出一个两阶段机制的被动张力心脏,发展,肌节长度之间的类似于1.85μm,类似于2.0μm,肌的弹性部分整理了一下,在长度超过2.0μm,类似的分子构成肌的弹性域段解开,肌节缩短到松弛以下的长度（类似于1.85 μ m）也会导致弹性肌凝素段变直，从而产生一种反对缩短的力，这种力往往会使肌节恢复到松弛长度。
Titin (also known as connectin) is a muscle-specific giant protein found inside the sarcomere, spanning from the Z-line to the M-line. The I-band segment of titin is considered to function as a molecular spring that develops tension when sarcomeres are stretched (passive tension). Recent studies on skeletal muscle indicate that it is not the entire I-band segment of titin that behaves as a spring; some sections are inelastic and do not take part in the development of passive tension, To better understand the mechanism of passive tension development in the heart, where passive tension plays an essential role in the pumping function, we investigated titin's elastic segment in cardiac myocytes using structural and mechanical techniques, Single cardiac myocytes were stretched by various amounts and then immunolabeled and processed for electron microscopy in the stretched state. Monoclonal antibodies that recognize different titin epitopes were used, and the locations of the titin epitopes in the sarcomere were studied as a function of sarcomere length, We found that only a small region of the I-band segment of titin is elastic; its contour length is estimated at similar to 75 nm, which is only similar to 40% of the total I-band segment of titin, Passive tension measurements indicated that the fundamental determinant of how much passive tension the heart develops is the strain of titin's elastic segment. Furthermore, we found evidence that in sarcomeres that are slack (length, similar to 1.85 mu m) the elastic titin segment is highly folded on top of itself, Based on the data, we propose a two-stage mechanism of passive tension development in the heart, in which, between sarcomere lengths of similar to 1.85 mu m and similar to 2.0 mu m, titin's elastic segment straightens and, at lengths longer than similar to 2.0 mu m, the molecular domains that make up titin's elastic segment unravel, Sarcomere shortening to lengths below slack (similar to 1.85 mu m) also results in straightening of the elastic titin segment, giving rise to a force that opposes shortening and that tends to bring sarcomeres back to their slack length.