Low-angle X-ray diffraction studies of living striated muscle during contraction

Abstract

The spacings and intensities of the small-angle equatorial X-ray reflexions from living toad sartorius muscle were studied in the resting and contracted state. On contraction, there is a small and nearly constant decrease of 6 to 12 Å in the lattice spacing over the whole range of muscle lengths studied and the 1,1 reflexion becomes slightly more intense relative to the 1,0 reflexion. In both resting and contracting muscle, the centre-to-centre distance between actin and myosin filaments depends on sarcomere length, being about 80 Å larger in the short muscles (sarcomere length 2.1 μ) than in the long ones (sarcomere length 3.6 μ). In another set of experiments with the toad sartorius muscle, it was found that on contraction no change occurred in the spacings of the meridional X-ray reflexions from either the actin or the myosin filaments. We have also observed a decrease in intensity of the myosin layer lines (given by the projections on the myosin filaments) as first reported by Huxley, Brown & Holmes (1965). By indicating that there is no change in length in the actin or myosin filaments during contraction, these results provide general support for the sliding filament hypothesis.