Time-resolved X-ray diffraction studies on stretch-activated insect flight muscle.

The specific feature of stretch activation of the indirect flight muscle of the tropical waterbug Lethocerus was used to correlate mechanical and structural aspects of muscle contraction. The time courses of the changes in intensities of the strongest equatorial reflections, the (10) and (20) and of the first meridional reflection at 14.5 nm-1 were monitored using synchrotron radiation as a high intensity X-ray source. The ratio of the intensities of the equatorial reflections, (I20/I10), which reflects the mass distribution within the filament lattice array, increases by about 10% relative to the Ca(2+)-activated level when a rapid stretch is imposed, compared with a 200% change seen when fibres change from the relaxed to the rigor state, while the spacing of the lattice planes decreases by about 1%. The intensity of the first meridional reflection at 14.5 nm-1 decreases by about 35% during stretch activation with a slightly faster time course than the delayed tension increase. The results suggest that the average structure of cycling crossbridges is different from that present in the rigor state.