The Contribution of Calcium and Stretch-Activated Tension to Power Generation by Drosophila Indirect Flight Muscle

Abstract

Recent studies suggest small insects might regulate muscle power output by operating their stretch activated indirect flight muscles (IFM) at less than saturating calcium levels during flight instead of varying the number of muscle fibers recruited. Calcium levels have also been found to correlate with wing beat frequency, suggesting calcium concentration might influence aerodynamic power. To test the effects of calcium concentration on muscle power generation, we used the work loop technique to measure Drosophila indirect flight muscle (IFM) oscillatory power generation under optimal length change parameters. Maximum power was 2.2±0.4 nW/mm3 at pCa = 4.7±0.09. The threshold for muscle power generation occurs at ∼ pCa 6.1, with pCa50 = 5.8±0.05 and the Hill coefficient = 7.2±3.7. Over the steepest portion of the curve, and above the estimated minimum power required for flight, about pCa 6.0 to 5.5, muscle power increases 2.5-fold. We compared the contributions to muscle power from calcium activated isometric tension to the contribution from the tension generated by stretch-activation during flight by imposing a 2.5% ML, 1.5 ms stretch. The pCa50 for calcium activated tension was 6.1±0.05 and Hill coefficient was 4.6±1.3. The pCa50 for stretch activated tension was 5.8±0.07 and Hill coefficient was 3.8±1.3. Following a stretch at pCa 4.5, total isometric tension increased from 2.5±0.3 mN/mm2 to 16.4±1.2 mN/mm2 of which 11.4±1.1 mN/mm2 was contributed by passive elastic elements. As calcium is increased from pCa 6.0 to 5.5, stretch-activated tension contributes 1.0±0.3 mN/mm2 of additional tension while calcium tension contributes 0.4±0.2 mN/mm2. We conclude that if calcium levels vary between pCa 6.0 to 5.5 during flight, then the contribution of stretch tension to additional power is about twice that of calcium tension.