EVALUATION OF FREEZE-SUBSTITUTION IN RABBIT SKELETAL-MUSCLE - COMPARISON OF ELECTRON-MICROSCOPY TO X-RAY-DIFFRACTION

Rabbit psoas muscle fibres, relaxed and in rigor, have been freeze sub stituted for electron microscopy. Fourier transforms and average densi ty maps of micrographs of transverse sections have been obtained and c ompared to X-ray diffraction data. The Fourier amplitudes from rigor a nd relaxed muscle are comparable to equatorial data from X-ray diffrac tion of muscle if there is more disorder in the electron micrographs w hich can be described by a 'temperature' factor. The phases of reflect ions out to the 3,2 have been determined; those reflections at the sam e radius and therefore not separable in the X-ray patterns, such as th e 2,1 and the 1,2, are separated in the transforms of sections through the A band. In transforms from both rigor and relaxed muscle they hav e the same phase. In rigor muscle they have different amplitudes. All the phases are positive or negative showing that the lattice is centro symmetric at the resolution obtained. The phases obtained generally su pport those suggested by model building studies using X-ray diffractio n data. In rigor muscle, areas where the cross-bridges are regularly a ttached are clearly seen in thin transverse sections. A handedness to this structure is indicated by a lack of mirror symmetry, in both the Fourier transform of thick sections, and in the averaged density map. This correlates well with the arrangement where the myosin head is bou nd as in the acto-S1 structure but only to actin monomers within a lim ited azimuthal range.