Impaired nocturnal melatonin secretion in non-dipper hypertensive patients

Background: Melatonin plays a role in the biologic regulation of circadian rhythms, including sleep. Melatonin has also been shown to modulate vascular smooth muscle tone and to induce hemodynamic effects in humans. Objective: To evaluate whether melatonin plays a role in the circadian rhythm of blood pressure in hypertensive patients. Methods: Sixteen elderly patients with essential hypertension were evaluated. Patients were defined as either dippers (DIP, n = 8) or non-dippers (NDIP, n = 8) according to the nocturnal change in the mean arterial pressure (MAP). 6-Sulfatoxymelatonin (6-SMT), the main melatonin metabolite, was determined by enzyme-linked immunosorbent assay (ELISA) in two separate urine collections, one in the daytime and one during the night. Results: Both groups of DIP and NDIP hypertensives were comparable in regard to age and sex. During the night, the mean arterial pressure decreased by 10.3 ± 2.2% in the DIP and increased by 7.5 ± 1.7% in the NDIP group (p < 0.01). Daily 6-SMT was comparable in DIP (3.28 ± 0.87 μg/12 h) and NDIP (2.31 ± 0.68 μg/12 h) (p = 0.39). However, while the DIP presented the physiological nocturnal increase in urinary 6-SMT (mean 8.19 ± 1.68 μg/12 h), this surge of melatonin production was missing in NDIP in whom nocturnal urinary 6-SMT concentrations were not significantly different from daily levels (mean 2.56 ± 0.79 μg/12 h). The nocturnal change in urinary 6-SMT excretion was positively correlated to the nocturnal change in MAP (R = 0.54; p = 0.031). Conclusions: NDIP hypertensive patients differ from DIP hypertensives by having an impaired nocturnal melatonin secretion. Thus, melatonin may play a role in the circadian rhythm of blood pressure in hypertensive patients.