Abstract
Most flying insect species use "asynchronous" indirect flight muscles (A-IFMs) that are specialized to generate high mechanical power at fast contraction frequencies. Unlike individual contractions of "synchronous" muscles, those of A-IFMs are not activated and deactivated in concert with neurogenically controlled cycling of myoplasmic [Ca(2+)] but rather are driven myogenically by oscillatory changes in length. The motor neurons of the A-IFMs, which fire at a rate much slower than contraction frequency, are thought to play the limited role of maintaining myoplasmic [Ca(2+)] above the critical threshold that maintains the muscle in a stretch-activatable state. Despite this asynchronous form of excitation-contraction coupling, animals can actively regulate power output as required for different flight behaviors, although the neurobiological and biophysical basis of this regulation is unknown. While presenting tethered flying fruit flies, Drosophila melanogaster, with visual stimuli, we recorded membrane potential spikes in identified A-IFM fibers. We show that mechanical power output rises and falls in concert with the firing frequency of all A-IFM fibers and cannot be explained by differential recruitment of separately innervated motor units. To explore the hypothesis that myoplasmic [Ca(2+)] might similarly rise and fall in concert with firing frequency, we genetically engineered Drosophila to express the FRET-based Ca(2+) indicator cameleon selectively within A-IFMs. The results show that Ca(2+) levels increase in proportion to muscle firing rate, both during spontaneous flight and when muscle spikes are elicited electrically. Collectively, these experiments on intact animals support an active role for [Ca(2+)] in regulating power output of stretch-activated A-IFM.
