Effects of Aging on Catecholamine Metabolism*

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Abstract
To determine the cause of the high plasma norepinephrine (NE) concentrations in elderly subjects, apparent NE secretion, NE plasma clearance and NE production were measured in 14 young and 13 elderly normal subjects. Apparent NE secretion, estimated by isotope dilution analysis, was higher (P < 0.01) in the elderly subjects [3.08 .+-. 0.45 (.+-. SEM [standard error of the mean]) nmol/m2 .cntdot. min] than in the young subjects (1.84 .+-. 0.12 nmol/m2 .cntdot. min). Plasma clearance of NE did not differ between the young (1470 .+-. 120 ml/m2 .cntdot. min) and the elderly (1295 .+-. 153) subjects. NE production, estimated from NE metabolite excretion, was 9.66 .+-. 0.8 nmol/m2 .cntdot. 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 .cntdot. min. Excretion of the O-methyl derivative of NE normetanephrine was increased (P < 0.01) in the aged; excretion of the deaminated metabolites vanillylmandelic acid and dihydroxymandelic acid was decreased. Apparently, the rate at which NE enters the circulation is increased in the elderly, but NE production is normal. Evidently, in aging, there is an alteration in the local disposition of sympathetic neuronal NE.