Abstract
We were interested to estimate the maximum mechanical efficiency with which chemical energy derived from ATP hydrolysis is converted into mechanical work by individual cross-bridges when they perform their powerstroke synchronously. Glycerinated rabbit psoas muscle fibres, containing ATP molecules almost equal in number to the cross-bridges within the fibre, were activated to shorten under various afterloads by laser-flash photolysis of caged Ca(2+). In these conditions, almost all the cross-bridges are in the state where the ATP is hydrolyzed but the products have not yet been released from the cross-bridge (M-ADP-P(i)) immediately before activation, and can hydrolyze only one ATP molecule during the flash-induced mechanical response. Power output records of the fibres following activation indicated that the cross-bridges actually started their powerstroke almost synchronously. The amount of ATP utilized at 1 s after activation was estimated from the amount of isometric force developed after interruption of fibre shortening, while the amount of work done was calculated by multiplying the amount of afterload by the distance of fibre shortening. A conservative estimation of the maximum mechanical efficiency at a load of 0.5-0.6 P(o) was 0.7, suggesting that the actual maximum mechanical efficiency of cross-bridge powerstrokes may be close to unity.
Highlights
Muscle contraction is caused by attachment–detachment cycle between the cross-bridges on the thick filament and the thin filament coupled with ATP hydrolysis
The mechanical efficiency with which chemical energy derived from ATP hydrolysis is converted into mechanical work in demembranated muscle fibres has been estimated recently by measuring the amount of ATP utilized for work production, using fluorescence of a phosphate-binding protein (He et al, 1997, 1999) or NADH (Reggiani et al, 1997; Sun et al, 2001)
The higher the initial peak, the larger the area under the power output trace, i.e. the amount of work done by fibre shortening
Summary
Muscle contraction is caused by attachment–detachment cycle between the cross-bridges on the thick filament and the thin filament coupled with ATP hydrolysis The cross-bridges attach to the thin filament to perform their powerstrokeproducing positive forces, and produce negative forces before being detached from the thin filament F. Huxley, 1957) On this basis, the overall mechanical efficiency of muscle fibres may be much smaller than that of individual cross-bridges during their powerstroke, since positive forces are always opposed by negative forces, due to asynchronous cross-bridge activity. To accurately estimate the mechanical efficiency of individual cross-bridges when they perform their powerstroke-producing positive force, it is necessary to perform experiments under conditions in which the crossbridges start their powerstroke synchronously
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