Effects of aging on catecholamine metabolism.

To determine the cause of the high plasma norepinephrine (NE) concentrations in elderly subjects, we measured apparent NE secretion, NE plasma clearance, and NE production in 14 young and 13 elderly normal subjects. Apparent NE secretion, estimated by isotope dilution analysis, was higher (P less than 0.01) in the elderly subjects [3.08 +/- 0.45 (+/- SEM) nmol/m2 X min] than in the young subjects (1.84 +/- 0.12 nmol/m2 X min). Plasma clearance of NE did not differ between the young (1470 +/- 120 ml/m2 X min) and the elderly (1295 +/- 153) subjects. NE production, estimated from NE metabolite excretion, was 9.66 +/- 0.8 nmol/m2 X min in the elderly subjects, not significantly different from that in the young subjects, who produced NE at a rate of 11.7 +/- 1.1 nmol/m2 X min. Excretion of the O-methyl derivative of NE normetanephrine was increased (P less than 0.01) in the aged, whereas excretion of the deaminated metabolites vanillylmandelic acid and dihydroxmandelic acid was decreased. Our data indicate that the rate at which NE enters the circulation is increased in the elderly, but NE production is normal. Taken together, these results suggest that in aging, there is an alteration in the local disposition of sympathetic neuronal NE.