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An X-Ray Diffraction Study on Early Structural Changes in Skeletal Muscle Contraction

Japan Synchrotron Radiation Research Institute, SPring-8, Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan

Correspondence: Address reprint requests to N. Yagi, Tel.: +81-791-58-0908; Fax: +81-791-58-0830; E-mail: yagi{at}spring8.or.jp.

Structural changes in frog skeletal muscle were studied using x-ray diffraction with a time resolution of 0.53–1.02 ms after a single electrical stimulus at 8°C. Tension began to drop at 6 ms (latency relaxation), reached a minimum at 8 ms, and then twitch tension developed. The intensity of the meridional reflection at 1/38.5 nm^-1, from troponin molecules on the thin filament, began to increase at 4–5 ms and reached a maximum at 12 ms. The meridional reflections based on the myosin 43-nm repeat began to decrease when the tension began to develop. The peak position of the third-order myosin meridional reflection began to shift toward the higher angle at 5 ms, reached a maximum shift (0.02%) at 10 ms, and then moved toward the lower angle. The intensity of the second actin layer line at 1/18 nm^-1 in the axial direction, which was measured at 12°C, began to rise at 5 ms, whereas the latency relaxation started at 3.5 ms. These results suggest that 1), the Ca²⁺-induced structural changes in the thin filament and a structural change in the thick filament have already taken place during latency relaxation; and 2), the Ca²⁺ regulation of the thin filament is highly cooperative.

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