Mechanical properties of insect fibrillar muscle at large amplitudes of oscillation

Abstract Glycerol-extracted fibres from Lethocerus flight muscle were subjected to large-amplitude sinusoidal oscillation in a solution containing Mg-ATP and Ca2+ at room temperature, under conditions designed to eliminate the effects of diffusion of nucleotides into and out of the fibre. They produced work up to 4 to 8 % peak-to-peak oscillation amplitude at 1.5 to 2.5 Hz and up to lower amplitudes at higher frequencies; freshly extracted fibres produced work at higher amplitudes than aged fibres. The mean tension in the muscle was shown to be critical for work production. Provided this parameter was maintained constant, the mean fibre length, Ca2+ concentration and ADP concentration could be varied considerably without loss of oscillatory work; these factors are therefore not in themselves critical for oscillation. The tension/length relationship of relaxed fibres is non-linear at high amplitudes of oscillation. Oscillatory tension/length loops are not elliptical at high amplitude and change shape when mean tension and frequency are changed. Linear control theory cannot be used to analyse performance at high amplitudes. At the upper limit of mechanical performance the oscillation amplitude and work-per-cycle performed by the freshly extracted fibres were similar to those of live flight muscle, and the active tension generated was similar to that from other maximally activated striated muscles. Calculation suggests that most or all of the myosin in the muscle was activated in each cycle of oscillation. The limits of performance are therefore probably present in life and are characteristics of the entire contractile material; their possible significance is discussed.