Role of endothelium in the secondary relaxation following contraction by various agonists in isolated rabbit superior mesenteric artery rings.

In isolated rabbit superior mesenteric artery rings, noradrenaline, phenylephrine and angiotensin II produce an initial contraction which is followed by a relaxation phase. No relaxation phase was observed following K+-induced contraction. This relaxation phase is significantly decreased for all agonists tested when endothelium intact rings are pretreated with methylene blue. Hydroquinone pretreatment also causes a significant reduction in the relaxation phase induced by noradrenaline and angiotensin II and completely abolished the response to phenylephrine. Removal of endothelium abolishes the secondary relaxation phase of noradrenaline and phenylephrine, but only reduces that elicited by angiotensin II. These results are taken as an indirect evidence indicating that the release of the endothelium-derived relaxing factor from endothelium of superior mesenteric artery rings by noradrenaline, phenylephrine and angiotensin II is probably one of the underlying mechanisms for the relaxation phases following the contractions elicited by these agonists.