Aims: 

The aim of the study was to investigate the preventive effect of melatonin on the development of spontaneous hypertension and vascular reactivity of isolated blood vessels in young spontaneous hypertensive rats (SHRs) in early stage of hypertension development.

Methods: 

For 4 weeks, melatonin (12 mg/ kg/day) was added to the drinking water given to a group of 4- and 8-week-old SHRs and normotensive Wistar rats, and we compared them with groups of untreated SHRs and normotensive control Wistar rats. Blood pressure (BP) was measured by the tail-cuff technique. Rings of isolated thoracic aorta and mesenteric artery were set up for isometric tension recording.

Results: 

The treatment of rats with melatonin slows down hypertension development in SHRs (BP at the end of 8^th and 12^th week 173.8 ± 1.3 and 123.4 ± 1.1 mmHg, respectively, compared with 191.1 ± 3.0 and 143.4 ± 1.5 mmHg in untreated SHRs, p < 0.001), and did not significantly change BP in age-matched normotensive Wistar rats. Melatonin treatment had no significant effect either on the heart weight/body weight ratio or acetylcholine-induced relaxation in thoracic aorta. Vascular contractions to periarterial nerve stimulation and to exogenous noradrenaline were increased in SHRs. In both melatonin-treated SHR groups, contractions in the mesenteric artery induced by high frequency of stimulation were slightly inhibited. In the thoracic aorta, melatonin lowered sensitivity of vascular smooth muscle to exogenous noradrenaline.

Conclusion: 

Results showed that long lasting administration of melatonin to SHRs with developing spontaneous hypertension resulted in decreased increment of BP, and had only slight effect on adrenergically induced increase of vascular tone.