The importance of body composition to the increase in plasma norepinephrine appearance rate in elderly men.

Several studies have documented an increase in sympathetic nervous system (SNS) activity, as reflected by either plasma norepinephrine (NE) concentration or NE appearance rate, with aging. Because similar increases have been noted in young obese persons, and because adiposity increases with age, we hypothesized that body composition might be an important determinant of heightened SNS activity. Baseline SNS activity, energy expenditure, and responses to a standard formula "meal" were compared in 11 young (31.1 +/- 5 years) (m +/- SD) and 9 old (64.9 +/- 6.3 years) persons. Both baseline NE concentration, p less than .05, and the NE appearance rate, p less than .05, were increased in the elderly group. The percentage of body fat, p = .004, and age, p less than .02, were correlated independently with NE appearance rate but not with NE concentration. Although plasma NE increased after the meal in both groups, NE appearance increased in the young group only. We conclude that NE appearance rate is a better reflection of SNS activity than NE concentration. We also found that the percentage of body fat and age are independent determinants of baseline SNS activity, which together account for 52% of the variability in SNS activity, as reflected by NE appearance rate.