Transient enhancement of sympathetic nervous system activity by long-term restriction of sodium intake

To further investigate the relationship between salt intake and sympathetic nervous system activity, the short- and long-term effects of a low-salt diet (40 meq/day) were assessed in 10 normal subjects. Measurements of hemodynamic, hormonal, and other parameters were obtained on the day preceding institution of the low-salt diet (day 0) and on days 4, 7, 30, and 60 of the diet. Urinary sodium excretion was 178 +/- 10 meq/24 hr on day 0 and 31 +/- 4, 38 +/- 4, 45 +/- 6, and 47 +/- 7 meq/24 hr on days 4, 7, 30, and 60, respectively (all p less than .001 compared with day 0). Blood pressure, urinary potassium, serum electrolytes, and cardiac function (as assessed by echocardiography) were not modified by the 2 month low-salt diet. Plasma renin activity and plasma aldosterone were significantly elevated above control values throughout the entire period of the low-salt diet. In contrast, plasma norepinephrine concentration increased significantly only on days 4 and 7 (from 253 +/- 20 pg/ml on day 0 to 495 +/- 32 pg/ml, p less than .001, and 347 +/- 22 pg/ml, p less than .05, respectively), returning to baseline at days 30 (280 +/- 18 pg/ml) and 60 (262 +/- 18 pg/ml). Changes in plasma epinephrine paralleled those observed for norepinephrine. Similarly, resting heart rate and the blood pressure response to isometric exercise were significantly increased only on days 4 and 7 of the low-salt diet. These results suggest that sympathetic nervous system activity is enhanced only transiently during a sustained reduction in sodium intake.