The effects of fibre length and calcium ion concentration on the dynamic response of glycerol extracted insect fibrillar muscle.

1. The property of insect fibrillar muscle which enables it to oscillate continuously when it is connected to a resonant load is the delayed activation by stretch of contractile activity. The dynamic response has been measured at different fibre lengths and at different calcium ion concentrations, to see what effects these conditions have on the magnitude and rate constant of the delayed tension.2. Bundles of ten fibres of the dorsal longitudinal muscle of the water bug, Lethocerus cordofanus, which had been in glycerol for less than 5 days were used. Graded activation was obtained with buffered calcium ion concentrations between 10(-7) and 10(-5)M.3. A computer-controlled apparatus was used to measure the dynamic mechanical properties of the muscle fibres. This allowed many measurements to be made on the same preparation. Further computer programs analysed the Nyquist plots produced by the experiment.4. When the mean length of a fibre bundle was increased, the magnitude of the delayed tension was increased (for a constant amplitude of sinusoidal length change), the rate constant was unaltered, and the stiffness of the fibres was increased. When the calcium ion concentration was raised, the magnitude of the delayed tension was increased, the rate constant increased, and the stiffness of the fibres fell. Calcium activation and stretch activation are thus clearly separable in their effects, and so the mechanisms must be separate.5. The various different effects of calcium cannot be explained by any simple model of activation, for example, an on-off switch mechanism controlling the number of bridges in action.6. The stretch-induced activity is proportional to a power of the length of two or greater and this non-linearity aids the efficient operation of the oscillatory mechanism.