Plasma norepinephrine levels of elderly men on a controlled sodium intake diet.

Plasma levels of norepinephrine were compared between young (n = 5) and old (n = 5) men maintained for five days on a diet providing a controlled and uniform sodium intake (2 mEq/kg body weight). The groups were compared after an overnight period of recumbency and fasting (10 hours) and after postural stimulation (20 minutes standing). Plasma norepinephrine concentrations were slightly, though not significantly, higher in older men after prolonged recumbency (older men, 232 +/- 39 pg/ml; younger men, 183 +/- 24 pg/ml). After postural stimulation, the plasma norepinephrine levels were elevated in both groups, but the measured increase of the older group was significantly greater than that of the younger group (older men, 607 +/- 72 pg/ml; younger men, 393 +/- 51 pg/ml; P less than 0.05). The results of this study suggest the greater response of plasma norepinephrine to postural stimulation observed in elderly men occurred even though the extracellular fluid volume statuses of the older group were comparable to those of the younger group. Whether the exaggerated plasma norepinephrine response of the older men represented a higher degree of sympathetic activity or a diminished plasma clearance of norepinephrine was not established.