Plasma Norepinephrine Levels Are Influenced by Sodium Intake, Glucocorticoid Administration, and Circadian Changes in Normal Man*

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Abstract
The influences of factors that may contribute to blood pressure regulation, including Na homeostasis, ACTH and circadian rhythm, on plasma norepinephrine (NE) levels were examined in normal man. Basal supine NE levels and their response to 2 h of upright posture were examined in 8 subjects studied after 5 days on either low (20 meq), normal (120 meq), or high (200 meq) Na diets. Na depletion increased, and Na loading decreased basal supine plasma NE. Low, but not high, Na diets had a significant effect on plasma NE responses to upright posture. However, more acute NA loading, with isotonic saline infusion for 4 h, had no effect on plasma NE. ACTH administration (40 U, i.m., for 31 days) had no significant effect on supine and upright plasma NE. However, dexamethasone administration (0.5 mg, orally, 4 times daily for 3 days) depressed (P < 0.05) both basal NE and its response to upright posture. Noctural and daytime recumbent secretory patterns of NE and blood pressure levels were also determined at 20-min intervals in 9 subjects with polygraphic monitoring to determine sleep stages. Mean plasma NE levels were lowest at 0400-0600 h (125-150 pg/ml), rose abruptly before awakening, and reached peak concentrations at 0900 h (340 .+-. 27 pg/ml). All subjects had 7 or more secretory peaks (NE, 25% above the mean 24-h concentration), most occurring between 0600-1800 h. Nocturnal secretion of NE was unrelated to rapid eye movement and non-rapid eye movement sleep, but was correlated (r = 0.63; P < 0.001) with mean arterial pressure. These findings demonstrate that several factors may have an important influence on the daily regulation of NE secretion. Chronic, but not acute, changes in Na balance alter basal and postural NE secretion. Glucocorticoid administration suppresses basal and posture-stimulated NE. Since ACTH did not alter plasma NE, this could represent a direct inhibitory effect of glucocorticoids on NE secretion. NE is released in secretory bursts, and displays a circadian biorhythm that is not sleep associated but correlates with changes in mean arterial pressure.
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