Metabolic recovery after exercise and the assessment of mitochondrial function in vivo in human skeletal muscle by means of 31P NMR.

It has been suggested that the rate of phosphocreatine resynthesis after exercise is an index of mitochondrial oxidative metabolism in intact muscle. To investigate this hypothesis, the time courses of metabolite recovery following mild and more severe dynamic exercise of human forearm muscle were compared by means of 31P NMR. Severe exercise resulted in greater net hydrolysis of phosphocreatine and greater intracellular acidosis than light exercise. The rate of phosphocreatine resynthesis was significantly slower during recovery from the more severe exercise. To explain this it was noted that, as a consequence of the high activity of creatine kinase in the sarcoplasm, the [phosphocreatine] at any time is a function of the intracellular pH. Calculations demonstrate that the difference between rates of phosphocreatine recovery after the two exercise protocols was primarily determined by the rates of recovery of the intracellular pH to normal rest values. It is concluded that the calculated rate of recovery of the cytosolic free [ADP] to its pre-exercise concentration may provide a more specific measure of mitochondrial oxidative activity.