An x-ray diffraction study on early structural changes in skeletal muscle contraction.

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 degrees 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 approximately 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 approximately 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 degrees C, began to rise at 5 ms, whereas the latency relaxation started at 3.5 ms. These results suggest that 1), the Ca(2+)-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 Ca(2+) regulation of the thin filament is highly cooperative.