Effect of Sodium Intake on Plasma Catecholamines in Normal Subjects*

The effect of the state of sodium balance on the activity of the sympathetic nervous system has been evaluated previously by measuring urinary catecholamine excretion. Since urinary catecholamine may be affected by factors such as renal function or renal production of catecholamines, blood catecholamines may provide a better index of the activity of the sympathetic nervous system. The present study was undertaken to evaluate the effect of varying sodium intake on blood catecholamines. Thirteen normal subjects were studied for a period of 3 weeks in a metabolic ward. They received during the first, second, and third week 10, 100, and 200 meq sodium/day, respectively. On the seventh day of each week, when the patients had achieved sodium balance, urinary sodium excretion as well as blood levels of PRA, norepinephrine (NE), epinephrine (Ep), and dopamine (D) were measured in the supine position, at 5, 10, 15, and 20 min of upright posture, and at the end of 40 min of ambulation. The results show that: 1) blood levels of NE, Ep, and D as well as PRA were significantly higher during low sodium intake than during medium or high sodium intake, 2) as in the case of PRA, there was an inverse relationship between the blood levels of NE, Ep, and urinary sodium excretion; 3) upright posture produced a significant increment in the blood levels of NE which was not affected by sodium intake; and 4) the increment in PRA with posture was significantly greater during low sodium intake than with medium high sodium intake. The data demonstrate that: 1) the plasma levels of NE, Ep, and D are affected by the state of sodium balance, particularly during marked sodium depletion; and 2) meaningful interpretation of the significance of the blood levels of catecholamines should be made with reference to indices of sodium balance, such as urinary sodium excretion.