Millisecond time-resolved changes in x-ray reflections from contracting muscle during rapid mechanical transients, recorded using synchrotron radiation (cross-bridge movement/contraction mechanism/high-intensity x-ray beams)

Low-angle x-ray diffraction diagrams have been recorded from frog sartorius muscles by using synchrotron radia- tion 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-A 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 in- tensity 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 A and that are com- pleted in about a millisecond (i.e., before significant cross-bridge detachment would be expected), an almost synchronous decrease in 143-A intensity occurs, by 50% or more. This is followed, in the case of quick releases, by a rapid partial recovery of intensity last- ing 5-6 ms (which may represent cross-bridge release and reat- tachment) and then by a more gradual return to the normal iso- metric value. Quick stretches show only the slower return of intensity. Immediately after the length change, the initial drop in 143-A intensity can be reversed if the release (or stretch) is re- versed. These changes provide evidence of a more direct kind than has hitherto been available that the active sliding of actin filaments past myosin filaments during contraction is produced by longitu- dinal movement of attached cross-bridges.