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-twitch permeabilized rabbit muscle fibers when they were relaxed from the rigor state using photochemical generation of ATP. A novel caged ATP compound, the P3–3',5'-dimethoxybenzoin ester of ATP (DMB-caged ATP), as well as the P3–1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP), have been used. DMB-caged ATP photolyzes at least three orders of magnitude 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 Pi-sensitive states was investigated from the effect of Pi on relaxation. Rigor tension was varied enabling the influence of tension on the relaxation to be examined. The time course of relaxation was faster with DMB-caged ATP compared 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 monotonically after photochemical release of ATP in a process whose rate was unaffected by Pi. In the presence of ADP, relaxation was more complex and tension passed through a maximum. A model invoking cooperative interactions involving ADP-containing myosin heads provides a reasonable description of the data.