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

INTRODUCTION 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 [1]. 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. This leads to an unexplored interaction between force generation, muscle shortening, and radial changes in the geometry of the contractile machinery. While the current evidence suggests lattice spacing changes are small in intact muscle fibers, data were derived from very small strains in the flight muscles of Drosophila[2]. In other animals, much larger muscle strains accompany movement. As such we examined radial changes in the lattice of the flight muscle of the hawk-moth Manduca sexta. We used 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.