We have used fluorescence polarizat

We have used fluorescence polarization to examine orientational changes of the 5- and 6-isomers of acetamidotetramethylrhodamine (ATR) covalently bound to SH-1 (Cys-707 of the myosin heavy chain) in single, skinned fibers from rabbit psoas muscle after rapid length steps or photolysis of caged nucleotides. Similar results were obtained with both the 5- and 6-isomers of ATR. After the photolysis of caged ATP, large and rapid changes in the fluorescence polarization signals were observed and were complete well before appreciable force had been generated. Changes in the fluorescence polarization signals after the photolysis of caged ADP were similar to those after the photolysis of caged ATP, despite an almost negligible change in force. The fluorescence polarization signals remained almost constant after rapid length steps in both rigor and active muscle fibers. These results suggest that structural changes at SH-1 monitored by 5- or 6-ATR are not associated directly with the force-generating event of muscle contraction, but may be involved in the communication pathway between the nucleotide and actin-binding sites of myosin.