Aims: 

Plasma melatonin concentrations are changed in hypertensive patients, but the role of melatonin in hypertension development is not understood. The purpose of our study was to test the hypothesis that melatonin and melatonin receptor levels are changed in rats during L-NAME (NG-nitro-L-arginine methyl ester) induced hypertension. L-NAME blocks nitric oxide synthase activity and contributes to hypertension development.

Methods: 

Wistar rats (36) were treated with L-NAME (40 mg/kg) and another group (36) served as a control. Blood pressure was measured weekly. After 4 weeks, animals were sacrified over a 24h cycle in 4h intervals and plasma melatonin concentrations were measured using radioimmunoanalysis and the MT1 receptor density in aorta was quantified using western blot analysis.

Results: 

The L-NAME treatment induced the expected increase in blood pressure (178 ± 1 vs. control 118 ± 1 mmHg). We found a clear cut daily rhythm of melatonin levels in pineal gland and plasma in both groups. Pineal melatonin concentrations in L-NAME treated rats were higher than in controls (3377 ± 478 vs. 1752 ± 329 pg/pineal gland) in the middle of the dark period. No differences between both groups were found in plasma melatonin levels. Melatonin receptors did not exhibit clear daily rhythm neither in control nor L-NAME treated rats.

Conclusion: 

Results suggest that L-NAME treatment stimulates melatonin production through blocking of inhibitory effect of NO on adrenergic stimulation of melatonin biosynthesis. Increased utilization of melatonin in hypertensive rats may explain the absence of differences in plasma hormone concentration.

This work was supported by APVV project 20-022704 and project UK/238/2007.