The relationship of adenosine triphosphatase activity to tension and power output of insect flight muscle.

1. On a simple model of actomyosin interaction, the tension cost (ATP hydrolysed/unit tension) and the frequency of low amplitude oscillation optimum for work production are both determined by the rate of detachment from the actin filament of the myosin crossbridge. To test this model, the two parameters were measured under different conditions using glycerol-extracted Lethocerus cordofanus dorsal longitudinal flight muscle fibres. 2. The ATPase activity of the static muscle rose by an amount approximately proportional to the rise in tension as the muscle was stretched. 3. When the muscle fibres were sinusoidally oscillated at 5-10 Hz by 2% of their resting length they produced a large amount of mechanical power and hydrolysed approximately twice as much ATP per unit mean tension as they did when static. The ATPase activity was linearly related to the mean tension during oscillation. 4. The experiments were repeated at temperatures between 12 and 30 degrees C and the tension cost and the optimal frequency of oscillation of the fibres were found to rise with temperature. 5. Removal of phosphate from the incubating medium reduced both the tension cost and the optimal working frequency. Addition of pyrophosphate or sulphate reduced both parameters still further. 6. From these results the tension cost of static muscle was shown to be proportional to its optimal working frequency. 7. ATPase activity rose monotonically with power production at work-producing frequencies and at moderate degrees of stretch. A high absolute efficiency was found under a wide range of conditions. 8. The proportionality between tension cost and optimal frequency is evidence for the proposed model of actomyosin interaction.