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The Journal of Experimental Biology 206, 1201-1206 (2003)
doi: 10.1242/jeb.00232

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

Haruo Sugi^*, Hiroyuki Iwamoto, Tsuyoshi Akimoto and Hirohiko Kishi

Department of Physiology, School of Medicine, Teikyo University, Itabashi-ku, Tokyo 173-8605, Japan

^* Author for correspondence (e-mail: sugi{at}med.teikyo-u.ac.jp)

Accepted 13 January 2003

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-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.

Key words: mechanical efficiency, cross-bridge, skeletal muscle, caged Ca²⁺, isotonic shortening, muscle work, ATP utilization, rabbit

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