TENSION RELAXATION INDUCED BY PULSE PHOTOLYSIS OF CAGED-ATP IN PARTIALLY CROSS-LINKED FIBERS FROM RABBIT PSOAS MUSCLE

Muscle contractile force is thought to be generated by ATP-induced con formational changes in myosin crossbridges, In the present study, we i nvestigated the response to ATP binding of force-bearing, attached cro ssbridges, For this investigation, shinned fibers, in which myosin hea ds were in part covalently crosslinked to thin filaments with a zero-l ength crosslinker, were prepared, Caged ATP [the P-3-1-(2-nitro)phenyl ethyl ester of ATP] was then pulse-photolyzed in these crosslinked fib ers, which retained ATP-induced ''rigor'' tension, and then the subseq uent tension changes were followed at 14-16 degrees C and ionic streng ths of 0.1-2 M, A rapid tension decrease was observed after the photol ysis in the partially crosslinked fibers, The rate of the decrease was not any different from that in the uncrosslinked fibers compared at i onic strength of 0.2 M, This and other results thus indicate a kinetic similarity in the crosslinked and uncrosslinked crossbridges in respo nse to ATP binding, These findings also suggest that ATP-induced struc tural changes take place in the attached crossbridges at a rate simila r to that of the ATP-induced dissociation of crossbridges from thin fi laments.