KINETICS OF RELAXATION FROM RIGOR OF PERMEABILIZED FAST-TWITCH SKELETAL FIBERS FROM THE RABBIT USING A NOVEL CAGED-ATP AND APYRASE

The complex time course of tension decay was investigated in fast-twit ch permeabilized rabbit muscle fibers when they were relaxed from the rigor state using photochemical generation of ATP. A novel caged ATP c ompound, the P-3-3',5'-dimethoxybenzoin ester of ATP (DMB-caged ATP), as well as the P-3-1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP) , have been used. DMB-caged ATP photolyzes at least three orders of ma gnitude more rapidly than NPE-caged ATP. The role of ADP on relaxation kinetics from rigor was examined by using apyrase to remove ADP from the rigor muscle solutions. The presence of P-i-sensitive states was i nvestigated from the effect of P-i on relaxation. Rigor tension was va ried enabling the influence of tension on the relaxation to be examine d, The time course of relaxation was faster with DMB-caged ATP compare d with NPE-caged ATP for concentrations of ATP released by photolysis greater than 0.7 mM. Most of the complexity in the relaxation tension records was caused by ADP. In the absence of ADP, tension decayed mono tonically after photochemical release of ATP in a process whose rate w as unaffected by P-i. In the presence of ADP, relaxation was more comp lex and tension passed through a maximum. A model invoking cooperative interactions involving ADP-containing myosin heads provides a reasona ble description of the data.