Oxygen uptake kinetics: old and recent lessons from experiments on isolated muscle in situ

The various mechanisms responsible for ATP resynthesis include phosphocreatine (PCr) hydrolysis, anaerobic glycolysis and oxidative phosphorylation. Among these, the latter represents the most important mechanism of energy provision. However, oxidative phosphorylation is characterized by a lower maximal power and a slow attainment of a steady state in response to increased metabolic demand. The rate of adjustment of oxidative metabolism during metabolic transitions, which can be evaluated on the basis of the analysis of O₂ uptake (O₂) kinetics, has implications for exercise tolerance and muscle fatigue. Analysis of O₂ kinetics represents a valid tool for the functional evaluation of healthy subjects, athletes and patients. Over the last 35 years experiments conducted on isolated muscle preparations in situ have allowed us to gain insights into several key aspects of skeletal muscle O₂ kinetics. Their main limiting factor resides in an intrinsic slowness of intracellular oxidative metabolism when adjusting to augmented metabolic needs. The rate of adjustment of oxidative phosphorylation in mitochondria can be functionally related to PCr hydrolysis occurring in the cytoplasm.