Time-resolved X-ray diffraction studies on the effect of slow length changes on tetanized frog skeletal muscle.

1. The mechanism of the enhancement and the deficit of isometric force by slow length changes in frog skeletal muscle was studied with the time-resolved X-ray diffraction technique, using intense X-rays of synchrotron radiation. 2. When a tetanized muscle was slowly stretched by 4% from sarcomere lengths 2.3-2.4 microns, the force rose to a peak during stretch and then decreased to a steady level 10-15% higher than that immediately before stretch. 3. The intensity of the 1,1 equatorial reflection decreased nearly linearly during stretch and then again increased after the completion of stretch, reaching a steady level 12 +/- 5% (mean +/- S.D., n = 11) lower than that immediately before stretch. The above 1,1 intensity change was roughly a mirror image of the force change. 4. The intensity of the 1,0 equatorial reflection showed no marked changes in response to a slow stretch, except for an initial transient increase observed occasionally. 5. If a tetanized muscle was slowly released by 4% from sarcomere lengths 2.3-2.4 microns, the steady force attained after the completion of release was lower than that immediately before release. 6. The 1,1 intensity increased slightly during release, while the 1,0 intensity did not change significantly. 7. The half-width of both the 1,0 and the 1,1 reflections did not change appreciably in response to slow length changes. 8. Slow length changes always produced changes in the spacing between the reflections as expected from the constant-volume behaviour of the myofilament lattice. 9. These results indicate that a slow stretch produces disordering of the myofilament lattice in such a way that the thin filaments are displaced from trigonal positions in the thick filament lattice. The resulting increase in the overall repulsion forces between the filaments may lead to the enhanced isometric force after stretch.