The circadian rhythm of blood pressure and the effect of salt 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 es sential hypertension were included in this study. Salt sensitivity was dete rmined with a chronic dietary salt loading test and an acute venous saline loading test, respectively. 24-hour ABP measurements were performed twice i n normotensive subjects when they were on a high salt diet and when they ha d a low salt diet. Blood and urine samples were collected for measurement o f plasma norepinephrine concentration (PNE), plasma renin activity (PRA), a ngiotensin II, aldosterone, erythrocyte sodium content and urinary sodium e xcretion. 24-hour ABP readings were also obtained in patients with essentia l 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 wa s 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 conte nt increased significantly in SS subjects on high salt intake. Conclusions High salt intake caused an abnormal circadian rhythm of blood p ressure in SS subjects. The blunted nocturnal decline in blood pressure may be a characteristics of SS patients with essential hypertension.