Enhanced plasma norepinephrine response to upright posture and oral glucose administration in elderly human subjects

Plasma norepinephrine (NE) levels in response to upright posture and to oral glucose ingestion were measured in healthy young and old (> 65 yr of age) subjects. Peak plasma NE concentrations with standing were higher in the elderly (1334 ± 146 pg/ml versus 855 ± 46; p < 0.05) and plasma NE remained elevated in the elderly compared with the young subjects even after 15 min of recumbent resting. Following oral glucose plasma NE rose higher in the elderly (79% compared with 32% in the young) and peaked later (120 min after ingestion compared with 60 min in the young). Cardiovascular changes with upright posture and with oral glucose were similar in young and old. Alterations in disappearance of NE from the circulation could not account for the greater elevations in plasma NE concentration in the elderly either, since the rates of fall in circulating NE levels following termination of an NE infusion were the same in both groups. The metabolic clearance rate of NE was unchanged. Thus, the plasma NE responses to stimulation by standing and by oral glucose ingestion are enhanced in elderly subjects.