A real-time observation of X-ray diffraction from frog skeletal muscle during and after slow length changes.

X-ray diffraction patterns from a frog skeletal muscle were recorded in real time using an X-ray image intensifier and a CCD video camera with a time resolution of 1/60s. The muscle was stimulated and then released or stretched by 6% at a constant speed. The equatorial (1,1) reflection decreased in intensity during stretches, suggesting a conformational change of cross-bridges. The integrated intensity of the 3rd meridional reflection from the thick filament decreased during both releases and stretches. The decrease was larger in stretches than in releases and at higher velocities. The intensity change correlated well with the change in tension, showing that the axial mass distribution of cross-bridges becomes wider as the tension increases. The axial Bragg spacing of the 3rd meridional reflection tended to increase in stretches and decrease in releases by less than 0.1%, possibly reflecting the elasticity in the thick filament backbone. The only significant effect of a length change remaining at the end of the stimulation was the broadening of the 3rd meridional reflection, suggesting non-uniformity of sarcomere lengths or axial misalignment of thick filaments as causes of enhancement and deficit of tension.