Meeting Abstract
The energetic and functional versatility of muscle at the macroscopic level depends on the collective action of myosin motors in the contractile lattice. For example, myosin heads on thick filaments are out of register with actin binding sites, which limits crossbridge binding. However, due to compliance in myofilaments, filament strain change alters actin-myosin kinetics facilitating crossbridge cooperativity. Recent work in isometric active and passive muscle shows that thick filament strain at the sarcomere level has a nonlinear relationship with whole muscle force. However we do not know if this relationship holds under dynamic conditions. If not, then the dynamics of strain in the filaments could help shape work production in an intact muscle. To see how force and thick filament strain are related dynamically, we performed work loops at different phases of activation on isolated Manduca sexta flight (DLM) muscle with simultaneous time-resolved x-ray diffraction. Consistent with earlier results, we found that the thick filament underwent strain changes of .2+/-.1% and the overall elastic response was similar to that of vertebrate muscle. However the relationship between thick filament strain and force during the course of the work loop was hysteretic, with the difference in thick filament strain at the same force at different points in the work loop cycle being between 30 to 60% of the total amplitude. Taking into account potential contributions of non-filament based passive forces (e.g. extracellular matrix) could not account for the hysteresis. Changing the phase of activation modulates the hysteresis with in vivo conditions produce only half that of peak negative power conditions. Taken together these results mean that there is not a one-to-one relationship between myofilament strain and muscle force.
