The goal of the research program is to relate physiological and biochemical ideas of muscle contraction by measurement of biochemical rate constants of the actomyosin ATPase in a physiological active muscle preparation. The rate and distribution of exchange of labelled oxygen atoms between the solvent water and product phosphate of the ATPase reaction in muscle fibers will be monitored at varying calcium concentrations. This information should help distinguish whether calcium switches on sections of the thin filament fully or modifies attachment rate. Caged phosphate compounds are protected molecules which are biologically inert but release the active compound when irradiated with near-u.v. light. Using caged compounds and a high power pulsed laser it is possible to produce step concentration changes in ATP, ADP, Pi, and ATP analogs in the millimolar range within a skinned fiber. This technique has provided quantitative estimates of cross-bridge detachment, cross-bridge reattachment with force generation, and rate of ADP desorption in isometric rigor fibers. The experiments will be extended to analyze the strain dependence of the ADP dissociation rate, possible modification to the rate of force generation by (Pi) and effect of steady shortening on cross-bridge detachment and reattachment reactions. The thermodynamic relationship between chemical and mechanical reactions will be studied using steady and rapid temperature jumps produced by a pulsed holmium laser. Effects of calcium, alterations of pH buffer, and calcium chelating agent will quantitatively evaluate the contributions of troponin and cross-bridge cycling rates to the observed response. The significance of this research extends over a broad range of muscle physioloby and cell motility.