The structure of the myosin elements in vertebrate smooth muscles

Abstract

Low-angle X -ray diffraction experiments with living relaxed and contracting guinea-pig taenia coli muscles have led to the discovery of a m eridional reflexion at about 14.3 nm whose presence establishes the existence of a regular assembly of myosin molecules in the form of filamentous elements (Lowy, Poulsen & V ibert 1970). From measurements of the shape of the 14.3 nm reflexion it is possible to deduce that in the axial direction the diffracting elements must be at least 500 nm long, while in a direction perpendicular to the fibre axis these elements must diffract coherently over a lateral distance of at least 60 nm. The latter feature can be straightforwardly interpreted in terms of the lateral register of the cross bridges across the faces of the ribbon-like myosin elements seen in the electron microscope by Lowy & Small (1970), Small, Lowy & Squire (1971) and Small & Squire (1972). The intensity of the 14.3 nm reflexion varies in patterns from muscles in various relaxed and contracted states. Nevertheless, we have detected no significant changes in the shape of this reflexion when the muscle passes from a resting to a contracted state, when it is loaded with weights from 1 to 10 g, or when it is incubated at different temperatures in Ringer solutions m ade hypertonic to various extents by the addition of sucrose. These observations suggest that the structure of the myosin elements responsible for the 14.3 nm reflexion persists unchanged regardless of the states of the muscle investigated. In our view this can be interpreted to indicate that the ribbons are present in relaxed muscles and interact with actin during contraction.