Effect of muscle glycogen on glucose, lactate and amino acid metabolism during exercise and recovery in human subjects

1. Eight subjects performed two-legged exercise, one leg with low and the o ther with normal muscle glycogen content. The purpose was to study the effe ct of low initial muscle glycogen content on the metabolic response during 1 h of exercise and 2 h of recovery. This model allows direct comparison of net fluxes of substrates and metabolites over the exercising legs receivin g the same arterial inflow. 2. Muscle glycogen breakdown during exercise was 60% lower in the leg with a reduced preexercise glycogen concentration and the rate of glucose uptake during exercise was 30% higher. 3. The amount of pyruvate that was oxidized during exercise was calculated to be approximately 450 mmol in the low-glycogen leg and 750 mmol in the no rmal-glycogen leg, which suggests more fat and amino acid oxidation in the low-glycogen leg. 4. During exercise, there was a significant release of amino acids not meta bolized in the muscle, e.g, tyrosine and phenylalanine, only from the low-g lycogen leg, suggesting an increased rate of net protein degradation in thi s leg. 5. The release of tyrosine and phenylalanine from the low-glycogen leg duri ng the exercise period and the change in their muscle concentrations yield a net tyrosine and phenylalanine production rate of 1.4 and 1.5 mmol h(-1), respectively. The net rate of protein degradation was then calculated to b e 7-12 g h(-1). 6. The results suggest that the observed differences in metabolism between the low-glycogen and the normal-glycogen leg are induced by the glycogen le vel per se, since the legs received the same arterial supply of hormones an d substrates.