The inhibitory effects of melatonin on vascular reactivity : possible role of vasoactive mediators

الملخص EN

Melatonin 'MEL), the principal hormone of the vertebral pineal gland.

Elicits several neurobiological effects However, the effect of MEL.

On vascular tissues are still vague The first goal of the study was to investigate the effect of MEL.

on isolated rabbit aortic rings and its role in the vascular reactivity- to contractile agents, noradrenalin (NA) and phenylephrine (PHE) and relaxant agents (acetylcholine and sodium nitroprusside ).

In addition, the levels of nitric oxide (NO), Cyclic Granosin monophosphate (cG.MP).

total calcium lipid peroxides, superoxide dismutase (SOD) and glutathione (GSH) were also investigated in tissue homogenates of rabbit aortic rings preincubated (20min) in MRL with and without contractile agents.

Vaso results revealed that MEL has an endothelium dependent yaw-relaxant effect and potentiated significantly the vaso-relaxmt effect of acetylcholine: Moreover, MHL (10 4) had a significant inhibitory effect on the contractile responses of aortic rings to both NA and PHE In comparison win control tissue rings, the Level of lipid peroxide were significantly increased while the level of GSH, and SOD activities were significantly decreased in tissue homogenates of aortic rings pre-incubated (20min ) in NA or PHE .

in the level of NO and cGMP were significantly lower testis of XO and cCr\4P were significantly lower in tissue rings pretreated with NA and PHE respectively.

The levels of total calcium were significantly increased in only tissue rings pretreated with NA The levels of lipid peroxides were significantly decreased while the levels of GSH, NO and cGMP and SOD activities were significantly increased in tissue homogenates of aortic rings incubated (20min) in MEL(10 -4 M) in comparison to ring tissues incubated in NA or PHE alone In aortic ring incubated in MEL+PHE .

the levels of lipid peroxides were significantly lower while the levels of GSH and cGMP and SOD activities were significantly higher than their level in ring tissues incubated in PHE.

In aortic rings incubated in MEL+NA the levels of lipid penixides and total calcium were signifuitntly lower while the levels of NO were significantly higher than their levels in ring tissues incubated in NA alon.

It is concluded from these results that MEL has an endothelium dependent vasorelaxant effect and potentiated the endothelium dependent vasorelaxation induced by acetylcholine.

It inhibits the contractile responses of aortic rings to and PHE, These fffecis may be.

in part, due to re-dtalance the prooxidant/antioxidants system, lowered calcium content and elevated NO and Cgmp levels in vascular tissue.