Muscle Respiration during Exercise1–3

The importance of oxygen as a limiting factor for the mitochondrial respiration in contracting skeletal muscle was studied. The oxygen tension was found to decrease in human gastrocnemius muscle during leg exercise, and the magnitude of the decrease was a function of the intensity as well as the frequency of the muscle contractions. Significant relationships were found between the intramuscular oxygen tension and various muscle metabolites such as the [phosphocreatine], the [ATP]/[ADP] ratio, and the [lactate]/[pyruvate] ratio. This suggests that the oxygen tension measured in the extracellular compartment is of importance for the intracellular energy, and redox state, in exercising muscle. These findings were confirmed in an experimental model where the oxygen delivery could be varied in a more predictable way. Thus, perfusing the contracting rat hindlimb at various oxygen partial pressures showed significant relationships between the oxygen delivery and the [phosphocreatine] and the [lactate] in the gas...