Mechanical Transients Initiated by Photolysis of Caged ATP within Fibers of Insect Fibrillar Flight Muscle

A B ST R ACT Kinetics of the cross-bridge cycle in insect fibrillar flight muscle have been measured using laser pulse photolysis of caged ATP and caged inorganic phosphate (Pi) to produce rapid step increases in the concentration of ATP and P, within single glycerol-extracted fibers. Rapid photochemical liberation of 100 ~M-1 mM ATP from caged ATP within a fiber caused relaxation in the absence of Ca ~÷ and initiated an active contraction in the presence of ~ 30 ~M Ca 2÷. The apparent second order rate constant for detachment of rigor cross-bridges by ATP was between 5 × 104 and 2 x lO s M-~s -t. This rate is not appreciably sensitive to the Ca 2+ or P~ concentrations or to rigor tension level. The value is within an order of magnitude of the analogous reaction rate constant measured with isolated actin and insect myosin subfragment- 1 (1986.J. Muscle Res. Cell Motil. 7:179-192). In both the absence and presence of Ca ~÷ insect fibers showed evidence of transient cross-bridge reattachment after ATP-induced detachment from rigor, as found in corresponding experiments on rabbit psoas fibers. However, in contrast to results with rabbit fibers, tension traces of insect fibers starting at different rigor tensions did not converge to a common time course until late in the transients. This result suggests that the proportion of myosin cross-bridges that can reattach into force-generating states depends on stress or strain in the filament lattice. A steady 10-ram concentration of P~ markedly decreased the transient reattachment phase after caged ATP photolysis. P~ also decreased the amplitude of stretch activation after step stretches applied in the presence of Ca 2÷ and ATP. Photolysis of caged P~ during stretch activation abruptly terminated the development of tension. These results are consistent with a linkage between P, release and the steps leading to force production in the cross-bridge cycle.