The circadian rhythm of blood pressure and the effect of sa lt intake in salt-sensitive subjects

OBJECTIVE: To investigate the effect of salt intake on the circadian rhythm of blood pressure and the characteristics of ambulatory blood pressure (ABP) in salt-sensitive (SS) subjects. METHODS: Twenty-three normotensive subjects and forty-three patients with essential hypertension were included in this study. Salt sensitivity was determined with a chronic dietary salt loading test and an acute venous saline loading test, respectively. 24-hour ABP measurements were performed twice in normotensive subjects when they were on a high salt diet and when they had a low salt diet. Blood and urine samples were collected for measurement of plasma norepinephrine concentration (PNE), plasma renin activity (PRA), angiotensin II, aldosterone, erythrocyte sodium content and urinary sodium excretion. 24-hour ABP readings were also obtained in patients with essential hypertension when antihypertensive drugs were discontinued for at least 2 weeks. RESULTS: High salt intake attenuated the circadian rhythm of blood pressure in SS normotensive subjects, and the nocturnal decline in blood pressure was smaller in SS patients with essential hypertension than in salt-resistant (SR) patients. The level of PNE was higher and the suppression of PRA was smaller in SS subjects than in SR subjects when they had a high salt diet, and the urinary sodium excretion decreased and the erythrocyte sodium content increased significantly in SS subjects on high salt intake. CONCLUSIONS: High salt intake caused an abnormal circadian rhythm of blood pressure in SS subjects. The blunted nocturnal decline in blood pressure may be a characteristics of SS patients with essential hypertension.