Role of the sympathetic nervous system in the nocturnal fall in blood pressure.

To evaluate mechanisms responsible for differences between patients showing a nocturnal fall in blood pressure ("dippers") and those showing no such fall in blood pressure ("nondippers"), we performed 24-hour (h) ambulatory blood pressure monitoring in 25 patients with untreated essential hypertension who were 37-49 years of age (16 men and 9 women). The diagnosis of essential hypertension was based on the patients' history, physical examination, routine laboratory tests, and intravenous pyelography. Blood pressure was measured by sphygmomanometer and by noninvasive ambulatory monitoring for 24 h. Exercise was performed on a supine bicycle ergometer. The initial workload was 50 W and was increased progressively by 25 W at 3-min intervals. Plasma and urinary norepinephrine levels were measured by high-performance liquid chromatography. Dippers were defined as patients with a difference of > 10 mmHg in the systolic BP or > 5 mmHg in the diastolic BP between daytime and nighttime. Eleven patients were dippers and 14 patients were nondippers. There was a positive correlation between the nocturnal fall in blood pressure and the rise in blood pressure during exercise (r = 0.54, p < 0.01), and the increase during exercise was greater in dippers than in nondippers. There was also a significant positive correlation between the urinary norepinephrine level and the fall in blood pressure at night (r = 0.75, p < 0.01). A significant increase in plasma norepinephrine during exercise was found in dippers, as compared with nondippers. These results suggest that in patients with hypertension a nocturnal fall in blood pressure is closely related to the blood-pressure response to exercise, and that the attenuation of sympathetic nervous activity might play an important role in the nocturnal decrease in blood pressure.