Norepinephrine turnover in various tissues at rest and during exercise: evidence for a training effect

This study quantitates the sympathetic response to an acute bout of exercise in specific tissues recruited during exercise in Fischer 344 rats (National Institute on Aging, Bethesda, MD), as well as determining any associated training adaptations. After 10 weeks of treadmill running, significant improvements were found for both endurance time and running speed eliciting maximal response in trained animals compared to controls (477% and 57%, respectively). Norepinephrine turnover (NEt) was determined by the administration of alpha-methyl tyrosine (250 mg/kg), a competitive inhibitor of tyrosine hydroxylase. Animals were killed at rest, after 30 minutes of submaximal (75% max) and at maximal exercise. Cardiac NEt values were 36.3, 141.5 and 528.1 ng/g/h for trained (Tr), 37.2, 134.5 and 638.7 ng/g/h for untrained (Un) animals at rest, submax and max, respectively. A similar response was found in the adrenals. However, the opposite was found with training in the liver. NEt values in Tr animals were greater (6.4, 80.5 and 161.8 ng/g/h) compared with Un (5.8, 25.5 and 75.8 ng/g/h) at rest, submax and max, respectively. It was concluded that training elicits a diminished sympathetic response in the heart and adrenals in response to an acute bout of exercise. Such a training effect was not found in the liver.