X-ray diffraction studies on striated and smooth muscles

Abstract

It is appropriate to start this contribution with a tribute to the pioneer small-angle X-ray studies of H. E. Huxley (1953) on living, resting, striated muscle, reported at another discussion meeting of this Society. These studies led to the prediction of many features of the sliding-filament model, and were ahead of their time in technique, as is evident, since it was nearly ten years before any other similar papers were published. 1. The filament lattice of striated muscle The work of Elliott, Lowy & Worthington (1963) on the filament lattice of striated muscle has shown that for both living, resting, muscle and glycerol- extracted muscle the relative intensity of the equatorial reflexions of the hexagonal lattice depends in the same way on sarcomere length. For long sarcomeres the intensity of the (1, 0) reflexion is greater than that of the (1, 1) reflexion, for short sarcomeres the reverse is true. The explanation offered was that the actin filaments contribute to the equatorial pattern only when stabilized by inter-molecular forces within the hexagonal myosin lattice and that they (the actin filaments) are comparatively disordered in the I -band. The actin contribution is in phase for the (1, 1) reflexion and out of phase for the (1, 0), so that, as the actin filaments slide in and the actin contribution increases, the (1, 0) becomes progressively weaker, and the (1, 1) progressively stronger. Calculations on this basis agreed well with the observed effect. It was found that the sarcomere length at which the two reflexions had equal intensity was greater for mammalian than for frog muscle. From this fact it was predicted that the actin filaments in mammalian muscle should be about 0.3 μm longer than in frog muscle; Miss Sally Page has now observed such a difference directly (Page & Huxley 1963).