Millisecond time-resolved changes in x-ray reflections from contracting muscle during rapid mechanical transients, recorded using synchrotron radiation.

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Abstract
Low-angle X-ray diffraction diagrams were recorded from frog sartorius muscles by using synchrotron radiation as a high-intensity X-ray source. This has enabled changes in some of the principal reflections of interest to be followed with a time resolution of 1 ms, during small but very rapid length changes imposed on a contracting muscle. The 143 .ANG. meridional reflection, which is believed to arise from a repeating pattern of myosin cross-bridges along the length of the muscle, shows large changes in intensity in these circumstances. During both rapid releases and rapid stretches, by amounts that produce a translation of actin and myosin filaments past each other by about 100 .ANG. and that are completed in about a millisecond (i.e., before significant cross-bridge detachment would be expected), an almost synchronous decrease in 143 .ANG. intensity occurs, by 50% or more. This is followed, in the case of quick releases, by a rapid partial recovery of intensity lasting 5-6 ms (which may represent cross-bridge release and reattachment) and then by a more gradual return to the normal isometric value. Quick stretches show only the slower return of intensity. Immediately after the length change, the initial drop in 143 .ANG. intensity can be reversed if the release (or stretch) is reversed. These changes provide evidence of a more direct kind than previously available that the active sliding of actin filaments past myosin filaments during contraction is produced by longitudinal movement of attached cross-bridges.