ATP hydrolysis by muscle and related topics

Insect flight muscle is an advantageous preparation for energetic studies as it permits unambiguous measuring of the rate of ATP hydrolysis accompanying power output and tension production under different mechanical and chemical conditions.
The ATPase activity and mean tension of a single fibre incubated in activating solution were compared at different extensions under isometric and oscillatory conditions. Analysis of individual experiments showed that the relationship between ATPase activity and tension is linear and so the regression of ATPase activity upon tension gives a measure of the cost of tension production. This was found to be much higher when the fibres were oscillated at 2% amplitude than when they were stretched isometrically.
Power output at 2% amplitude was varied by changing extension and frequency of oscillation. ATPase activity rises approximately linearly with power output at moderate extensions and over a range of frequencies around that optimal for power production. The slope of the regression of ATPase activity upon power was taken as a measure of the efficiency of energy conversion by the muscle.
The monotonic relationship between energy usage and energy production breaks down when degree of extension or oscillatory frequency are too high. The fibres can be stretched beyond an optimum tension so that power output declines but ATPase activity and tension continue to rise. At frequencies too fast for work production, ATPase activity and tension may be high. Thus the rate of ATP hydrolysis is not inflexibly linked to the power output of the muscle and is more closely coupled with tension production.
From measurements of the mid-length tension difference over a wide range of frequencies the optimum frequency for work production at 2% and 0.2% amplitude was obtained.
The experimental conditions were varied to examine the effect of changes in temperature, pH, ionic strength and anion composition of incubating medium on five indices of the muscles' performance: tension cost under conditions of static stretch; tension cost under conditions of a 2% oscillatory length change; efficiency of power output; optimum frequency for work output at 0.2% amplitude, fmax1; and at 2% amplitude, fmax2.
The efficiency was found not to vary under a wide range of conditions and, despite a variation of one decade in the optimum frequency for work, a mean usage of 40% of the free energy available from ATP hydrolysis was observed with a range of ±andnbsp;20%.
The cost of static and dynamic tension production varied proportionally as conditions changed. Static tension cost was half the dynamic tension cost.
Optimum frequency for work production at 2% and 0.2% also varied in proportion, the optimum frequency at 0.2% being 1.5 times that at 2%.
Tension cost and fmax under different conditions were plotted against each other and found to be proportional. On a model of muscular contraction based on a crossbridge cycle of attachment and detachment and involving hydrolysis of one ATP molecule per cycle, both tension cost and fmax depend on the duration of attachment. This is determined by the rate constant of the detachment step, and a proportionality between tension cost and optimum frequency would be predicted. As this was observed such a model can be accepted.
The change of tension cost when the muscle is oscillated will occur if the mean force of the bridge falls when interfilament movement occurs. The simplest model giving such a prediction is that of a stretched spring.
On this model, allowing a 10andnbsp;nm movement of the crossbridge spring, the tension cost should double with 2% amplitude of oscillation, as is observed. The measurements of efficiency and tension cost at 0.2% amplitude are also as predicted by the model.
From the physical characteristics of a stretched spring, the force exerted by a crossbridge in isometric contraction is calculated as 12andnbsp;pN. The rate constant, and hence time of attachment, varies with the external conditions; in phosphate buffered activating solution at 30anddeg;C, the conditions tested most resembling life, the rate constant was approximately 100andnbsp;secandminus;1 and the duration of attachment 10andnbsp;msec. The percentage of bridges attached during oscillatory work production was approximately 15%.

Advisor: Tregear, R