Non-Linear Force-Length Relationship in the ADP-Induced Contraction of Skeletal Myofibrils
Yuta Shimamoto 1, Fumiaki Kono 1, Madoka Suzuki 2 and Shin'ichi Ishiwata 1*
1 Department of Physics, Faculty of Science and Engineering, Waseda University
2 Consolidated Research Institute for Advanced Science and Medical Care, Waseda University
* To whom correspondence should be addressed. E-mail: ishiwata{at}waseda.jp.
Submitted on April 12, 2007
Revised on May 30, 2007
Accepted on 24 July 2007
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Abstract |
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The regulatory mechanism of sarcomeric activity has not been fully clarified yet because of its complex and cooperative nature, which involves both Ca2+ and cross-bridge binding to the thin filament. To reveal the mechanism of regulation mediated by the cross-bridges, separately from the effect of Ca2+, we investigated the force-sarcomere length (SL) relationship in rabbit skeletal myofibrils (a single myofibril or a thin bundle) at SL>2.2 µm in the absence of Ca2+ at various levels of activation by exogenous MgADP (4-20 mM) in the presence of 1 mM MgATP. The individual SLs were measured by phase-contrast microscopy to confirm the homogeneity of the striation pattern of sarcomeres during activation. We found that at partial activation with 4-8 mM MgADP, the developed force non-linearly depended on the length of overlap between the thick and the thin filaments, that is, contrary to the maximal activation, the maximal active force was generated at shorter overlap. Besides, the active force became larger, while this non-linearity tended to weaken, with either an increase in [MgADP] or the lateral osmotic compression of the myofilament lattice induced by the addition of a macromolecular compound, dextran T-500. The model analysis, which takes into account the [MgADP]- and the lattice spacing-dependent probability of cross-bridge formation, was successfully applied to account for the force-SL relationship observed at partial activation. These results strongly suggest that the cross-bridge works as a cooperative activator, the function of which is highly sensitive to as little as 
1 nm changes in the lattice spacing.
Key Words:
ADP-bound cross-bridges, cooperativity, dextran, lattice spacing, length-dependent activation, sarcomere
