Using White Noise to Probe Actomyosin Cycling Kinetics During Shortening and Lengthening in Drosophila Flight Muscle Fibers

Abstract

Our laboratory routinely measures demembranated muscle fiber mechanics using small amplitude sinusoidal length perturbation analysis and estimates kinetics of actomyosin cycling and myosin attachment time. We have developed a complementary measurement technique using small, random changes in muscle length (white-noise length stimuli) that simultaneously cover a broad frequency spectrum, comparable with the sinusoidal length perturbations. We find the white-noise technique provides a description of the viscoelastic properties that is consistent with sinusoidal analysis measurements, rapidly capturing much of the physiological behavior associated with contracting muscle fibers. The white-noise technique samples a vast range of system behavior in a fraction of the time required to complete sinusoidal analysis, and does not require the linear response underlying sinusoidal analysis methods. Thus, we combined the white-noise stimuli with a linear shortening and lengthening transients to probe cross-bridge cycling behavior during these periods of varied load. Preliminary measurements using demembranated dorsal longitudinal flight muscle fibers from Drosophila melanogaster indicate faster cross-bridge cycling kinetics during lengthening and slower cycling during shortening, compared to isometric contraction. During isometric contraction we estimate a myosin attachment time of 5.0 ms, and 4.6 versus 5.5 ms during the lengthening and shortening transients, respectively. These initial applications of the white-noise system analysis technique show promise for future studies probing molecular processes that underlie complicated length transients associated with normal muscle contraction.