Time-resolved 31P nuclear magnetic resonance studies on isometric contraction of frog skeletal muscle

The time course of changes of inorganic phosphate (Pi) and phosphocreatine (PCr) was studied during and after brief isometric tetanic contractions using phosphorus nuclear magnetic resonance on isolated semitendinosus muscles of the bullfrog at 4 degrees C. We followed essentially the method of Dawson, Gadian and Wilkie and the signal-to-noise ratio was improved by using many muscles. Immediately following the relaxation of contraction, the fall of PCr was not sufficient to account for the Pi released. The level of PCr remained unaltered, or decreased further, in the early post-contractile period. The amount of Pi that appeared without PCr splitting was similar to the amount of myosin sites (0.3 mmol kg-1 muscle) irrespective of the duration of contraction. The post-contractile utilization of PCr was strongly temperature dependent. It was prolonged at 1 degree C, whereas it almost disappeared at 10 degrees C. When the filament overlap was much reduced by stretching the muscles the phenomenon became undetectable. All these results have indicated that the post-contractile utilization of PCr is closely associated with the actin-myosin ATPase cycle.