Effects of Aminoguanidine on Norepinephrine-Induced Vascular Contraction in Renovascular Hypertensive Rats

Choi, Seok; Jun, Kyu Bae; Jun, Jae Yeoul; Yoon, Pyung Jin; Kim, Jun Soo; Kim, Myung Young; Mun, Hyun Sik; Yeum, Cheol Ho

Abstract

BACKGROUND
It has been established that hypertension is characterized by a dysfunctional endothelium. Among the endothelial factors that cause vasorelaxation, nitric oxide has been most widely known, which is synthesized by the enzyme nitric oxide synthase. This study was aimed to evaluate the role of the inducible nitric oxide synthase in chronic two-kidney, one clip (2K1C) hypertensive rats. METHODS: 2K1C hypertension was made by clipping the left renal artery and age- matched rats received a sham treatment served as control. In vitro experiments were preformed on intact and endothelium-denuded isolated thoracic aortic rings from lipopolysaccharide (LPS)-treated rats, in the presence of aminoguanidine alone, considered to be a selective inhibitor of the inducible nitric oxide synthase and of aminoguanidine and the nonselective nitric oxide synthase inhibitor Nomega-nitro-L-arginine (L-NNA). RESULTS: LPS treatment induced a shift to the right of concentration-response curves to norepinephrine in aortic rings with or without endothelium from sham- clipped control rats, while it did not modify in 2K1C hypertensive rats. In aortic rings with endothelium, aminoguanidine caused a significant shift of the norepinephrine concentration-response curve to the left in LPS-treated control rats, but had no effect in hypertensive rats. L-NNA caused an additional shift of the concentration-response curve to norepinephrine in both control and hypertensive rats, although the magnitude was diminished in hypertensive rats. In the endothelium-denuded rings, norepinephrine-induced contractions were enhanced by aminoguanidine in LPS- treated control rats, whereas no significant changes were observed in hypertensive rats. LPS treatment inhibited the relaxation response to acetylcholine in aortic rings from control rats, while it was without effect in hypertensive rats. L-arginine caused a dose-dependent relaxation in endothelium-denuded rings from LPS-treated rats. The relaxation response to L-arginine was attenuated by aminoguanidine in control rats, but no significant changes were noted in hypertensive rats. CONCLUSION: These results provide indirect evidence for an impaired activity of the inducible nitric oxide synthase in 2K1C hypertension, although involvement of an altered activity of constitutive nitric oxide synthase in the endothelium cannot be excluded.