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Structural Changes of Cross-Bridges on Transition from Isometric to Shortening State in Frog Skeletal Muscle

SPring-8/JASRI, 1-1-1 Kouto, Sayo, Hyogo, Japan

Correspondence: Address reprint requests to Naoto Yagi, SPring-8/JASRI, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan. E-mail: yagi{at}spring8.or.jp.

Structural changes in the myosin cross-bridges were studied by small-angle x-ray diffraction at a time resolution of 0.53 ms. A frog sartorius muscle, which was electrically stimulated to induce isometric contraction, was released by 1% in 1 ms, and then its length was decreased to allow steady shortening with tension of 30% of the isometric level. Intensity of all reflections reached a constant level in 5–8 ms. Intensity of the 7.2-nm meridional reflection and the (1,0) sampling spot of the 14.5-nm layer line increased after the initial release but returned to the isometric level during steady shortening. The 21.5-nm meridional reflection showed fast and slow components of intensity increase. The intensity of the 10.3-nm layer line, which arises from myosin heads attached to actin, decreased to a steady level in 2 ms, whereas other reflections took longer, 5–20 ms. The results show that myosin heads adapt quickly to an altered level of tension, and that there is a distinct structural state just after a quick release.