Smith D A, Mijailovich S M
Department of Physiology, Monash University, Clayton, VIC 3800, Australia.
Ann Biomed Eng. 2008 Aug;36(8):1353-71. doi: 10.1007/s10439-008-9514-z. Epub 2008 May 28.
The contractile behavior of a single half-sarcomere has been calculated from the lattice model with dimeric myosin and extensible filaments, using the model cycle with two working strokes, explicit Pi-release transitions and faster binding for the second head of the dimer. The mean-field approximation is used to generate independent state probabilities for myosin heads, assuming that the positional symmetry of actin filaments in the half-sarcomere is preserved. This model predicts absolute values of the active tension, stiffness and ATPase of fast fibers and their variation with shortening velocity, the phase-2 tension response to a length-release step and the transient tension rise during ramp stretching, in reasonable agreement with experimental data for frog muscle. It accounts for three observations beyond the reach of traditional models: (i) with elastically stiff myosin, a two-stroke model explains the rate of rapid tension recovery as a function of step size, (ii) slow Pi release from A.M.ADP.Pi after the first stroke generates the flat tension response observed after rapid recovery from a small release step, (iii) a discrete lattice model generates undamped oscillations in the isotonic length response to a force step, as observed when the sarcomeres are highly ordered. The discrete lattice also generates length-dependent oscillations in the tension-length curve and the tension response to ramp shortening, which may be smoothed out if lattice symmetry is broken.
利用具有两个工作冲程、明确的磷酸根释放转变以及二聚体第二个头部更快结合的模型循环，从包含二聚体肌球蛋白和可伸展细丝的晶格模型计算出单个半肌节的收缩行为。假设半肌节中肌动蛋白丝的位置对称性得以保留，采用平均场近似来生成肌球蛋白头部的独立状态概率。该模型预测了快肌纤维的主动张力、刚度和ATP酶的绝对值及其随缩短速度的变化、对长度释放步骤的第二阶段张力响应以及斜坡拉伸过程中的瞬时张力上升，与青蛙肌肉的实验数据合理相符。它解释了传统模型无法解释的三个现象：（i）对于具有弹性刚性的肌球蛋白，两冲程模型解释了快速张力恢复速率与步长的函数关系；（ii）在第一个冲程后，从A.M.ADP.Pi缓慢释放磷酸根产生了从小释放步骤快速恢复后观察到的平坦张力响应；（iii）离散晶格模型在等张长度对力阶跃的响应中产生无阻尼振荡，这在肌节高度有序时可以观察到。离散晶格还在张力-长度曲线以及对斜坡缩短的张力响应中产生长度依赖性振荡，如果晶格对称性被破坏，这些振荡可能会被平滑掉。
