In Vivo Time Resolved X-Ray Diffraction Reveals Radial Motions of Myofilaments in Insect Flight Muscle

Abstract

The constraint of constant volume for a contracting muscle cell implies a radial expansion that occurs during axial shortening will lead to increases in the radial spacing of the lattice of myofilaments. That change in filament spacing, in turn, can have a profound effect on the attachment rate of crossbridges (Williams et al. (2010) PLoS Comput Biol 6:e1001018.). At the same time, cross-bridges attaching to thin filaments and generating axial (longitudinal) forces and may simultaneously restrict the extent to which the filament lattice can expand or contract in the radial direction as muscle shortens. Here we examined radial changes in the lattice of the flight muscle of the hawk-moth Manduca sexta using high speed time resolved X-ray diffraction to directly measure the time course of changes in filament spacing as a function both the length of muscle and the timing of activation. Interestingly, the measured lattice spacing (1) strongly reflected activation timing and (2) varied considerably during the cycle of shortening and lengthening and (3) did not follow the pattern predicted by a constant lattice volume. Three key issues arise from these data. (1) Large changes in lattice spacing suggest that models of cross-bridge force generation should consider radial separation of thick and thin filaments; (2) cross-bridges may restrict the expansion of the filament lattice and may experience considerable radial force; and (3) the mismatch between measured and predicted radial motions of myofilaments indicates that there is fluid movement between subcellular compartments that has not been considered in the mechanics and energetics of force generation by muscle. Finally, radial tensions may play a key role in elastic energy storage for insect flight. Supported by NSF IOS-1022058 and NIH P41 GM103622-17.