Frequency of Maximal Power Output at in vivo Myofilament Lattice Spacing Matches Drosophila Wing Beat Frequency

Abstract

In striated muscle, actomyosin cross-bridge behavior is dependent on the distance between thick and thin filaments. Demembranated (skinned) fibers are often used to probe cross-bridge cycling kinetics, where the filament lattice is typically swollen from in vivo due to plasma membrane removal. To investigate the functional consequences of returning lattice spacing to in vivo values in skinned, indirect flight muscle (IFM) fibers we added a large, neutral, long-chain polysaccharide (4% w/v Dextran T-500, 500 kDa) to the bathing solution. X-ray diffraction measurements of living Drosophila melanogaster and skinned IFM fibers allowed us to measure in vivo and control for in vitro lattice spacing values. Small amplitude sinusoidal length perturbation analysis measured frequencies of maximal oscillatory work (112±3 Hz) and power (145±5 Hz) at in vivo lattice spacing (15°C). This suggests that in vivo cross-bridge kinetics are tuned for power output, as the wing beat frequency of fruit flies is ∼150 Hz at 15° C. To confirm these shifts in cross-bridge kinetics arise from changes in lattice spacing rather than osmotic pressure, we matched the osmotic pressure of 4% T-500 using 0.34% Dextran T-10 (10 kD), a shorter polysaccharide that minimally affects filament lattice spacing at this concentration. Work and power were unchanged at 0.34% T-10, compared to measurements without dextran. Importantly, the frequencies of maximal work and power output were 143±6 and 202±4 Hz in a swollen lattice without dextran, demonstrating a significant difference from in vivo spacing. These results signify that in vivo lattice spacing optimizes cross-bridge cycling kinetics for power output, not work, during flight.