The Effect of Acute and Chronic Exercise on Mitochondrial Respiratory Sensitivity to ADP in Human Skeletal Muscle

ADP/ATP is believed to either diffuse slowly between mitochondria and the cytosol or be substituted with fast diffusing creatine (Cr) and phosphocreatine (PCr) ‘phosphate shuttling’ catalyzed by mitochondrial creatine kinase. Previous studies assessing Cr-dependent and independent ADP-stimulated respiratory kinetics showed acute exercise had no effect or impairment on ADP/ATP diffusion but promoted phosphate shuttling. Conversely, chronic exercise impaired both models of energy exchange. In an attempt to reconcile these seemingly diverse findings, we employed a longitudinal study design to assess the acute and chronic effects of exercise on energy exchange in human skeletal muscle when modeling in vivo PCr/Cr conditions in vitro. Our findings demonstrate an impairment in energy exchange when modeling in vivo concentrations of PCr/Cr found during high intensity exercise, despite an increase in mitochondrial content. These results contradict classical models of endurance exercise adaptations which hypothesize that greater oxidative capacity coincides with improved energy exchange.