Comparison of the Regulation of Contraction in Insect Flight Muscle and Vertebrate Cardiac Muscle

The indirect flight muscle of insects (IFM), such as Drosophila and Lethocerus, is activated by periodic stretches at a priming concentration of calcium. The alternate activation of opposing muscles produces the rapid wing movements needed for flight. Cardiac muscle develops more force at longer sarcomere lengths (length-dependent activation), and force is enhanced by a rapid stretch at each beat. Troponin bridges between thick and thin filaments have been identified in Lethocerus IFM (Perz-Edwards et al, 2014) and these may be stretch sensors. Cardiac TnC and the TnC-F2 isoform in IFM both bind one regulatory calcium in the N-lobe. The force-pCa curve for IFM with only the TnC-F2 isoform (pCa50 5.8 and co-operativity nH 3.2) is similar to that of cardiac trabeculae, suggesting the co-operative behaviour is produced in the same way. TnC-F1, the IFM isoform needed for stretch activation, binds one calcium in the C-lobe, which is not regulatory. The force-pCa curve for IFM fibres, in which TnC-F1 is the main isoform, shows greater calcium-sensitivity and less co-operativity (pCa50 6.2, nH 1.3). The force-pCa curve of IFM fibres is similar to that of cardiac trabeculae with bound C1-C2 fragment of MyBP-C, in which calcium-sensitivity is increased and co-operativity greatly reduced. In this case, cardiac and IFM fibres achieve the same mechanical properties by different mechanisms. In order to determine if troponin forms part of a link between thick and thin filaments in IFM, we have tested binding between a tropomyosin-troponin complex and isolated thick filaments or myosin. The subunits of tropomyosin-troponin and the conditions needed for binding will be described.