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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

View larger version (23K): [in a new window]   Fig. 1. Laser flash-induced mechanical response. Typical fibre length as a % of L0 (A) and force (B) changes of the preparation, contracting isometrically (a) or shortening isotonically (b–f) under five different afterloads. Length recordings a, b, c, d, e and f correspond to force recordings a', b', c', d', e' and f', respectively. The loads were Po (isometric contraction, a, a'), 0.78 Po (b, b'), 0.53 Po (c, c'), 0.35 Po (d, d'), 0.09 Po (e, e') and 0 (unloaded shortening, f, f'), respectively. The initial (relaxed) length L0 and the cross-sectional area of the fibre were 2.8 mm and 6.8x10–5 cm2, respectively.

View larger version (20K): [in a new window]   Fig. 2. Power output during flash-induced fibre shortening. (A) Power output recordings under four different afterloads. (B) Power output records normalized relative to the peak values attained. The load was 0.09 Po (a, a'), 0.35 Po (b, b'), 0.53 Po (c, c') and 0.78 Po (d). The recordings were obtained from the experiment shown in Fig. 1.

View larger version (31K): [in a new window]   Fig. 3. Fibre length (A) and force (B) changes of a preparation that was first made to shorten isotonically under five different afterloads, and then subjected to quick releases at 1 s after activation to drop the force to zero. After each release, the preparation redeveloped isometric force at the decreased fibre length. Length recordings a–g correspond to force recordings a'–g', respectively. The load was Po (isometric condition; b, b'), 0.63 Po (c, c'), 0.41 Po (d, d'), 0.20 Po (e, e'), 0.09 Po (f, f'), 0 Po (unloaded condition; g, g'). Recordings a, a' were obtained during isometric contraction without quick release.

View larger version (14K): [in a new window]   Fig. 4. Dependence of the amount of ATP utilized for mechanical response (Pu, filled circles) and the amount of work done (W, open circles) on the isotonic load (P) at 1 s after flash activation. Values are means ± S.E.M. obtained from eight different data sets.

View larger version (19K): [in a new window]   Fig. 7. Dependence of the mechanical efficiency of individual cross-bridges (E) on the amount of isotonic load (P) obtained from the results shown in Figs 4 and 5. Values are scaled to adjust the value at 1.0 P/Po to that in Fig. 4. The amount of work done (W), ATP utilized for whole mechanical response (Pu), ATP utilized for preceding isometric force development (Pi), and ATP utilized for isotonic shortening (Ps) are also shown as functions of load.

View larger version (9K): [in a new window]   Fig. 5. Relationship between the amount of ATP utilized (Pi) and the isometric force developed (P). Values are means ± S.E.M. obtained from five different data sets.

View larger version (10K): [in a new window]   Fig. 6. Dependence of the amount of ATP utilized (Ps) on the distance of fibre shortening. Values are means ± S.E.M. obtained from eight different data sets.