Age Differences in the Plasma Clearance Mechanisms for Epinephrine and Norepinephrine in Humans*

There is an age-related increase in plasma norepinephrine (NE) in humans that is due to both an increase in NE appearance into plasma and a decrease in plasma NE clearance. However, previous studies demonstrated no difference in plasma epinephrine (EPI) in young and old subjects, and the effect of aging on plasma EPI appearance and clearance is unclear. To study age differences in basal NE and EPI metabolism we infused eight young (aged 19-26 yr) and eight old (aged 64-74 yr) normal subjects with 3HNE or 3HEPI (15 microCi/m2 bolus dose plus 0.35 microCi/m2/min for 50 min) to achieve steady state conditions on separate days. The old subjects had higher arterialized plasma NE levels mean, 217 +/- 13 (+/- SE) vs. 149 +/- 12 pg/mL; P less than 0.005 and plasma NE appearance. In contrast, neither plasma EPI levels (98 +/- 8 vs. 104 +/- 10 pg/mL; P = NS) nor EPI appearance rates were different in the old and young subjects. The plasma clearance rates of EPI and NE were nearly identical in the young subjects (1.63 +/- 0.14 vs. 166 +/- 0.09 L/min X m2; P = NS). Plasma NE clearance was lower in the old compared to the young subjects (1.38 +/- 0.06 vs. 1.64 +/- 0.10 L/min X m2; P less than 0.05) and was lower than EPI plasma clearance in the same subjects. Although NE and EPI can be removed by both neuronal and nonneuronal uptake mechanisms, and mean plasma clearance values for NE and EPI are the same in the young, the age-related decline in catecholamine clearance is specific for NE. This finding implies a differential effect of age on a catecholamine removal mechanism that is specific for NE.