Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT.
Department of Biological Sciences and Biomedical Engineering Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, Troy, NY.
J Gen Physiol. 2020 Apr 6;152(4). doi: 10.1085/jgp.201912484.
The force response of cardiac muscle undergoing a quick stretch is conventionally interpreted to represent stretching of attached myosin crossbridges (phase 1) and detachment of these stretched crossbridges at an exponential rate (phase 2), followed by crossbridges reattaching in increased numbers due to an enhanced activation of the thin filament (phases 3 and 4). We propose that, at least in mammalian cardiac muscle, phase 2 instead represents an enhanced detachment rate of myosin crossbridges due to stretch, phase 3 represents the reattachment of those same crossbridges, and phase 4 is a passive-like viscoelastic response with power-law relaxation. To test this idea, we developed a two-state model of crossbridge attachment and detachment. Unitary force was assigned when a crossbridge was attached, and an elastic force was generated when an attached crossbridge was displaced. Attachment rate, f(x), was spatially distributed with a total magnitude f0. Detachment rate was modeled as g(x) = g0+ g1x, where g0 is a constant and g1 indicates sensitivity to displacement. The analytical solution suggested that the exponential decay rate of phase 2 represents (f0 + g0) and the exponential rise rate of phase 3 represents g0. The depth of the nadir between phases 2 and 3 is proportional to g1. We prepared skinned mouse myocardium and applied a 1% stretch under varying concentrations of inorganic phosphate (Pi). The resulting force responses fitted the analytical solution well. The interpretations of phases 2 and 3 were consistent with lower f0 and higher g0 with increasing Pi. This novel scheme of interpreting the force response to a quick stretch does not require enhanced thin-filament activation and suggests that the myosin detachment rate is sensitive to stretch. Furthermore, the enhanced detachment rate is likely not due to the typical detachment mechanism following MgATP binding, but rather before MgADP release, and may involve reversal of the myosin power stroke.
心肌快速拉伸时的力响应通常被解释为代表附着的肌球蛋白横桥的拉伸（相 1）和这些拉伸横桥以指数速率的脱离（相 2），随后由于薄丝的激活增强，横桥以增加的数量重新附着（相 3 和 4）。我们提出，至少在哺乳动物心肌中，相 2 代表由于拉伸导致的肌球蛋白横桥的增强脱离速率，相 3 代表相同横桥的重新附着，相 4 是具有幂律松弛的被动类似粘弹性响应。为了验证这一想法，我们开发了一种横桥附着和脱离的两态模型。当横桥附着时赋予单位力，并且当附着的横桥移位时产生弹性力。附着速率 f(x) 以总幅度 f0 进行空间分布。脱离速率被建模为 g(x) = g0+ g1x，其中 g0 是常数，g1 表示对位移的敏感性。解析解表明，相 2 的指数衰减率表示（f0 + g0），相 3 的指数上升率表示 g0。相 2 和相 3 之间的低谷深度与 g1 成正比。我们制备了去皮的小鼠心肌，并在不同无机磷酸盐（Pi）浓度下施加 1%的拉伸。产生的力响应很好地符合解析解。随着 Pi 的增加，相 2 和相 3 的解释与较低的 f0 和较高的 g0 一致。这种快速拉伸的力响应的解释方案不需要增强的薄丝激活，并表明肌球蛋白脱离速率对拉伸敏感。此外，增强的脱离速率可能不是由于 MgATP 结合后的典型脱离机制，而是在 MgADP 释放之前，并且可能涉及肌球蛋白动力冲程的反转。
