Adaptive responses of rats to prolonged treatment with epinephrine

Rats were given a daily injection of L-epinephrine, 100 micrograms/100 g body wt, for 6 wk. The hearts of the epinephrine-treated animals were heavier (11.5%), and blood glucose and plasma insulin concentrations were lower than those of control rats. Acute responses to epinephrine were compared in the two groups. An increase in blood glucose and decreases in plasma insulin, liver glycogen, and muscle glycogen occurred in both groups. The magnitude of these responses were similar in the two groups except for the decrease in muscle glycogen, which was smaller in the chronic epinephrine-treatment group. There were no changes in respiratory capacity, citrate synthase or succinate dehydrogenase activities, or in cytochrome c concentration in skeletal muscle in response to 6 wk of epinephrine treatment. These results are compatible with the suggestion that catecholamines may play a role in some of the metabolic and cardiac adaptations to exercise training. However, they argue strongly against the hypothesis that catecholamines are responsible for inducing the increase in muscle mitochondria that occurs in response to exercise training.