AimTo investigate the effect of melatonin (MT) on the activity of inducible nitric oxide synthase in thoracic aorta of endotoxemia rats.MethodsSprague-Dawley rats were divided into four groups randomly: Vehicle group,Lipopolysaccharide (LPS) group(LPS 4 μg/g), LPS+MT group(MT5 μg/g, i.p.) which was given in twice, one injected 30 min before LPS and the other injected 60 min after LPS(4 μg/g ,i.p), and MT group.Six hours after LPS injection， rats were killed and thoracic aortic rings were prepared.The contraction response to phenylephrine and the endothelium-dependent relaxation response to acetylcholine before and after the incubation of aminoguanidine and Nω-nitro-L- arginine (L-NNA) were observed with the isolated artery rings technique.Inducible nitric oxide synthase activity in the artery tissues were detected.The ultrestructure of endothelium on the artery was observed by scanning electron microscope.ResultsCompared with LPS group, thoracic aortic rings in LPS+MT group exhibited an increased contraction response to phenylephrine.After incubation with aminoguanidine, the contractile response to phenylephrine increased significantly in LPS and LPS+MT group.Inducible nitric oxide synthase activity in LPS+MT group decreased markedly compared with that of LPS group in thoracic aorta.ConclusionThe protective role of MT on vascular activity in endotoxemia may be due to its properties to inhibit inducible nitric oxide synthase in a certain degree.