Li Yong, Lang Patrick, Linke Wolfgang A
Department of Cardiovascular Physiology, Ruhr University Bochum, Germany.
Sci Rep. 2016 Apr 15;6:24492. doi: 10.1038/srep24492.
The contractile units of striated muscle, the sarcomeres, comprise the thick (myosin) and thin (actin) filaments mediating active contraction and the titin filaments determining "passive" elasticity. We hypothesized that titin may be more active in muscle contraction by directly modulating thick-filament properties. We used single-myofibril mechanical measurements and atomic force microscopy of individual sarcomeres to quantify the effects of sarcomere strain and titin spring length on both the inter-filament lattice spacing and the lateral stiffness of the actin-myosin overlap zone (A-band). We found that strain reduced the lattice spacing similarly in sarcomeres with stiff (rabbit psoas) or compliant titin (rabbit diaphragm), but increased A-band lateral stiffness much more in psoas than in diaphragm. The strain-induced alterations in A-band stiffness that occur independently of lattice spacing effects may be due to titin stiffness-sensing by A-band proteins. This mechanosensitivity could play a role in the physiologically important phenomenon of length-dependent activation of striated muscle.
横纹肌的收缩单位——肌节，由介导主动收缩的粗肌丝（肌球蛋白）和细肌丝（肌动蛋白）以及决定“被动”弹性的肌联蛋白丝组成。我们推测，肌联蛋白可能通过直接调节粗肌丝特性在肌肉收缩中发挥更积极的作用。我们使用单肌原纤维力学测量和单个肌节的原子力显微镜来量化肌节应变和肌联蛋白弹簧长度对肌丝间晶格间距以及肌动蛋白-肌球蛋白重叠区（A带）横向刚度的影响。我们发现，在具有刚性（兔腰大肌）或柔顺性肌联蛋白（兔膈肌）的肌节中，应变同样会减小晶格间距，但腰大肌中A带横向刚度的增加幅度比膈肌大得多。独立于晶格间距效应而发生应变诱导的A带刚度变化可能是由于A带蛋白对肌联蛋白刚度的感知。这种机械敏感性可能在横纹肌长度依赖性激活这一重要生理现象中发挥作用。
