Structural studies of muscle during force development in various states.

Structural studies concerned with force generating mechanisms in striated muscle fibers in different states are described. The first study deals with fibers in the "rigor" state, where ATP is absent and all the myosin heads form cross-bridges with the actin-containing filaments. In this state large axial forces are developed when ionic strength is reduced to very low levels. At the same time, the fiber expands radially, as indicated by both X-ray diffraction and light microscopy. Comparison of the latter two measurements indicates that force is developed in part because of differences in the lateral expandability of different parts of the sarcomere. Thus, under these conditions, force appears to be modulated by factors that operate at the filament rather than the cross-bridge level. The second study deals with the location of the myosin heads in the relaxed state, and the mass movement that takes place when the fiber is physiologically activated. By using the intense X-ray source at the Stanford Synchrotron Radiation Laboratory, five equatorial reflections were recorded for both the relaxed and the activated state, and a spatial resolution of 100 A was obtained. Analysis of the data indicates that (1) in the resting state, myosin heads protrude out from the thick filaments and extend toward the thin filaments, and (2) upon activation, significant loss of mass occurs only in the region peripheral to the thick filament backbone through a movement that has a pronounced azimuthal component. The latter movement can be taken as the overall shift in myosin subfragment 1 during the cross-bridge cycle.