RESUMEN
The goal of this study was to separately determine ATP use by contractile and noncontractile processes in stimulated skeletal muscle. ATP use by tetanically stimulated bullfrog semitendinosus muscle was monitored at room temperature with in vivo 31P-nuclear magnetic resonance. Oxidative phosphorylation was inhibited by cyanide, and ATP use could therefore be calculated by accounting for ATP derived from the creatine kinase (CK) reaction (measured from decreases in phosphocreatine) and from glycolysis (estimated from decreases of intracellular pH). In unfatigued muscles stimulated at optimal length for force production, total ATP utilization (representing both contractile and noncontractile processes) was 2.5 +/- 0.09 (SE) mM/s (n = 6; 53% ATP from glycolysis, 47% from CK). In separate experiments, cross-bridge interactions between actin and myosin filaments were eliminated by increasing sarcomere length; therefore, with stimulation, residual ATP use reflected only noncontractile processes. In stimulated stretched muscles, ATP utilization was reduced compared with unstretched muscles to 1.07 +/- 0.08 mM/s (61% ATP from glycolysis, 39% from CK). These findings suggest that, during contraction near optimum length, a large proportion (approximately 43%) of ATP is used by noncontractile processes, with more ATP derived from glycolysis than from CK.