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Rigor-Force Producing Cross-Bridges in Skeletal Muscle Fibers Activated by a Substoichiometric Amount of ATP

Takenori Yamada ^*, Yasunori Takezawa , Hiroyuki Iwamoto ^*, Suechika Suzuki ^* and Katsuzo Wakabayashi 

^* Department of Physiology, School of Medicine, Teikyo University, Tokyo 173-8605, Japan; and Division of Biophysical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

Correspondence: Address reprint requests to Dr. Takenori Yamada, Dept. of Physics (Biophysics Section), Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. Tel.: 81-3-5228-8228; Fax: 81-3-5261-1023; E-mail: yamada{at}rs.kagu.tus.ac.jp.

Isometric skinned muscle fibers were activated by the photogeneration of a substoichiometric amount of ATP and their cross-bridge configurations examined during the development of the rigor force by x-ray diffraction and electron microscopy. By the photogeneration of 100 µM ATP, 2/3 of the concentration of the myosin heads in a muscle fiber, muscle fibers originally in the rigor state showed a transient drop of the force and then produced a long-lasting rigor force (50% of the maximal active force), which gradually recovered to the original force level with a time constant of 4 s. Associated with the photoactivation, muscle fibers revealed small but distinct changes in the equatorial x-ray diffraction that run ahead of the development of force. After reaching a plateau of force, long-lasting intensity changes in the x-ray diffraction pattern developed in parallel with the force decline. Two-dimensional x-ray diffraction patterns and electron micrographs of the sectioned muscle fibers taken during the period of 1–1.9 s after the photoactivation were basically similar to those from rigor preparations but also contained features characteristic of fully activated fibers. In photoactivated muscle fibers, some cross-bridges bound photogenerated ATP and underwent an ATP hydrolysis cycle whereas a significant population of the cross-bridges remained attached to the thin actin filaments with no available ATP to bind. Analysis of the results obtained indicates that, during the ATP hydrolysis reaction, the cross-bridges detached from actin filaments and reattached either to the same original actin monomers or to neighboring actin monomers. The latter cross-bridges contribute to produce the rigor force by interacting with the actin filaments, first producing the active force and then being locked in a noncycling state(s), transforming their configuration on the actin filaments to stably sustain the produced force as a passive rigor force.

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