Structural changes of
contractile proteins were examined by millisecond time-resolved two-dimensional x-ray
diffraction recordings during relaxation of skinned
skeletal muscle fibers from
rigor after caged ATP
photolysis. It is known that the initial dissociation of the
rigor actomyosin complex is followed by a period of transient active contraction, which is markedly prolonged in the presence of ADP by a mechanism yet to be clarified. Both single-headed (overstretched
muscle fibers with exogenous
myosin subfragment-1) and two-headed (fibers with full filament overlap) preparations were used. Analyses of various actin-based layer line reflections from both specimens showed the following: 1), The dissociation of the
rigor actomyosin complex was fast and only modestly decelerated by ADP and occurred in a single exponential manner without passing through any detectable transitory state. Its ADP sensitivity was greater in the two-headed preparation but fell short of explaining the large ADP effect on the transient active contraction. 2), The decay of the activated state of the
thin filament followed the time course of tension more closely in an ADP-dependent manner. These results suggest that the interplay between the reattached active
myosin heads and the
thin filament is responsible for the prolonged active contraction in the presence of ADP.
