Vasorelaxant effects of the chronic treatment with melatonin on mesentericartery and aorta of spontaneously hypertensive rats

Objective To investigate the effect of a chronic treatment with melatonin o n arterial pressure and a possible improvement of the vascular muscarinic a nd NO synthase (NOS) pathways in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. Design and methods Mean arterial pressure (MAP), systolic (SBP), diastolic blood pressure (DBP), and heart rate (HR) were evaluated in conscious rats treated with 30 mg/kg per day of melatonin during 4 weeks. Changes in MAP w ere evaluated following an intravenous injection of the NOS inhibitor NI-ni tro-L-arginine methyl ester (L-NAME). Relaxant effects of acetylcholine (Ac h), sodium nitroprusside (SNP), and the calcium ionophore A23187 were exami ned on mesenteric beds and aortic rings with or without treatment with mela tonin. Results Melatonin produced a significant reduction of MAP, SBP, DBP and HR in SHR (P < 0.05). L-NAME increased the MAP of melatonin-treated SHR by the same magnitude as that of WKY rats which was significantly higher than tha t of non-treated SHR (P < 0.05). Melatonin treatment improved the maximal r elaxation of mesenteric arteries to A23187 in SHR (P < 0.001) to the WKY le vel and caused a slight increament in Ach- and A23187-induced vasodilations in aorta from SHR and WKY rats (P < 0.05). Conclusion The present study showed that melatonin exerted a bradycardic an d an antihypertensive action in SHR. The enhancement by melatonin of the en dothelium-dependent vasodilation (Ach and/or A23187) in mesenteric artery a nd aorta from SHR and WKY rats and the higher increase in MAP following L-N AME treatment in melatonin-treated SHR suggest the contribution of an impro ved vascular NOS pathway activity in the hypotensive effect of melatonin. ( C) 2001 Lippincott Williams & Wilkins.