Endurance exercise training causes adrenal medullary hypertrophy in young and old Fischer 344 rats

The purpose of the present investigation was to determine the effects of endurance exercise training on adrenal medullary volume and epinephrine content in young (5 month) and old (23 month) female Fischer 344 rats. Animals from each group underwent 10 weeks of treadmill running (60 minutes per day, 5 days per week). 72 hours following the last training session animals were killed and the adrenal glands removed for subsequent analysis. Plantaris muscle citrate synthase activity increased with training in both young and old animals (39.8% young; 36.4% old). Trained animals had larger adrenal medullary volumes (48% increase in young, and 18% in old) than untrained controls. Trained animals also had higher total adrenal medullary epinephrine content (36% increase in young, and 24% in old). There were no differences in adrenal medullary epinephrine or norepinephrine concentration (micrograms/microliters medulla). It was concluded that the training-induced increase in adrenal epinephrine content is due to an increase in the size of the medulla, and not to a greater medullary epinephrine concentration. Furthermore, similar responses to training occur in both old and young animals.