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It was the purpose of this investigation to examine any age-related changes in norepinephrine turnover (NEt) in four tissues at rest and during exercise. Fischer 344 rats 6 (n = 20) and 25 mo of age (n = 20) were received from the National Institute on Aging. NEt was determined at rest, during 30 min of submaximal exercise, and at maximal exercise by administration of alpha-methyl-p-tyrosine, a competitive inhibitor of tyrosine hydroxylase. Resting NE declined with age in both heart (38.2 vs. 30.5 ng.g-1.h-1) and liver (11.2 vs. 6.4 ng.g-1.h-1). NEt was greater in the older animals compared with the young animals in heart (120.9 vs. 169.5 ng.g-1.h-1), liver (23.1 vs. 38.9 ng.g-1.h-1), and adrenals (74.0 vs. 98.4 ng.mg-1.h-1) during submaximal exercise. In response to maximal exercise, NEt varied depending on age and tissue. It was concluded that, in response to exercise stress, the older animals generally demonstrated a higher NEt (reflecting elevated sympathetic activity) perhaps because of a decreased adrenergic receptor sensitivity and/or responsiveness.
Mazzeo et al. (Wed,) studied this question.