Bachouchi N, Morel J E
Département de Biologie, CEN Saclay, Gif-sur-Yvette Cédex, France.
J Theor Biol. 1989 Nov 21;141(2):143-57. doi: 10.1016/s0022-5193(89)80014-1.
The maximum chord of the myosin heads is comparable to the closest surface-to-surface spacing between the myofilaments in a muscle at the slack length. Therefore, when the sarcomere length increases or when the fibre is compressed, the surface-to-surface myofilament spacing becomes lower than the head long axis. We conclude that, in stretched or compressed fibres, some crossbridges cannot attach, owing to steric hindrance. When the amount of compression is limited, this hindrance may be overcome by a tilting of the heads in the plane perpendicular to the filament axes; in this case, there is no consequence as concerns the crossbridge properties. In highly compressed fibres, the crossbridges become progressively hindered and all the crossbridges are hindered for an axis-to-axis spacing representing about 60% of the spacing observed under zero external osmotic pressure. In this case, both the isometric tension and the ATPase activity of the fibre are zero. In fibres stretched up to 3.77 microns (sarcomere length corresponding to the disappearance of the overlap between the thick and the thin filaments), the ratio of hindered crossbridges over the functional crossbridges may be estimated at about 55%. In stretched fibres, a noticeable proportion of crossbridges are sterically hindered and the crossbridges performance (e.g. constants of attachment and detachment) depends on filament spacing, i.e. on sarcomere length. Therefore, we think it is probably impossible to consider the crossbridges as independent force converters, since this idea requires that the crossbridge properties are independent of sarcomere length. In this connection, all the experiments performed on osmotically compressed fibres are of major importance for the understanding of the true mechanisms of muscle contraction.
肌球蛋白头部的最大弦长与肌肉在松弛长度时肌丝间最接近的表面到表面间距相当。因此，当肌节长度增加或纤维被压缩时，肌丝间的表面到表面间距会低于头部的长轴。我们得出结论，在拉伸或压缩的纤维中，由于空间位阻，一些横桥无法附着。当压缩量有限时，这种阻碍可能会通过头部在垂直于细丝轴的平面内倾斜而被克服；在这种情况下，横桥特性不会受到影响。在高度压缩的纤维中，横桥逐渐受到阻碍，对于轴到轴间距约为零外部渗透压下观察到的间距的60%时，所有横桥都会受到阻碍。在这种情况下，纤维的等长张力和ATP酶活性均为零。在拉伸至3.77微米（肌节长度对应于粗、细肌丝重叠消失）的纤维中，受阻横桥与功能性横桥的比例估计约为55%。在拉伸的纤维中，相当一部分横桥受到空间位阻，并且横桥的性能（例如附着和脱离常数）取决于细丝间距，即取决于肌节长度。因此，我们认为将横桥视为独立的力转换器可能是不可能的，因为这个想法要求横桥特性与肌节长度无关。在这方面，所有关于渗透压压缩纤维的实验对于理解肌肉收缩的真正机制都非常重要。
