Force Enhancement in the Drosophila Jump Muscle

Abstract

Force Enhancement (FE), a history-dependent phenomenon observed in skeletal muscle, is characterized by an elevated steady-state force after an active stretch compared to that of an isometric contraction at the corresponding length. FE has been observed for nearly a century, and demonstrated in whole muscle and single fiber preparations. Although the underlying mechanism(s) are yet to be elucidated, FE has been well characterized on both the ascending and descending limb of the force-length relationship, demonstrating a positive correlation with stretching amplitude and no clear relation to stretching rate. Furthermore, evidence has suggested that the mechanism for FE is multi-fold, encompassing both a passive structural element (titin) and kinetic mechanism. The limitations of investigating FE arise from the ability to manipulate molecular structure of the sarcomere in skeletal muscle. For that reason, it is advantageous to study alternative models. We have recently demonstrated the Drosophila's Tergal Depressor of the Trochantor (TDT), or jump muscle, to be an analogous to that of skeletal muscle. Furthermore, our ability to genetically manipulate the structure and subsequent myosin kinetics of the TDT muscle through transgene expression allows unprecedented insight into an analogous muscle model. Therefore, the TDT offers unique opportunities to investigate the underlying mechanism(s) associated with FE. The purpose of this investigation was to confirm the presence of FE in a wild-type (WT) Drosophila TDTs. TDT muscles were dissected, prepared, and mechanically evaluated on a custom-built, microscope-based mechanics rig as previously established. Preliminary results not only suggest that FE exists in WT TDTs, but that this phenomenon is also characteristically similar to that observed in mammalian skeletal muscle.