High mechanical efficiency of the cross-bridge powerstroke in skeletal muscle

Abstract

SUMMARY 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 Ca2+. 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-Pi) 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 Po was 0.7, suggesting that the actual maximum mechanical efficiency of cross-bridge powerstrokes may be close to unity.

The Sliding Filament Theory Since Andrew Huxley: Multiscale and Multidisciplinary Muscle Research

“…Documenting the efficiency of individual myosin motors in the lattice is challenging since, by operating asynchronously, they perform work on negatively strained neighbors (83). To estimate myosin motor efficiency, Sugi et al (182) recorded the power generated by glycerinated fibers when cross-bridge powerstrokes were synchronized by limiting the amount of ATP in the system to approximately one per myosin motor and using the laser-induced release of caged calcium for activation. Their study showed that the efficiency of individual myosin motors within the filament lattice is, conservatively, 70% (182), which is much greater than previous estimates in vertebrate muscle.…”