Endothelium-independent and -dependent vasoactivity of 6-nitronorepinephrine

Vasoactivities of 6-nitronorepinephrine were investigated using rat aorta. 6-Nitronorepinephrine (> 100 microM) caused dose-dependent contraction in both endothelium-intact and -denuded aorta, although the latter showed greater contraction than the former. Prazosin (> 3 nM), an alpha1-adrenoceptor antagonist, attenuated significantly the 6-nitronorepinephrine-induced contractions, thereby suggesting the alpha1-adrenoceptor involvement. Aortic rings prepared from reserpine-pretreated rats showed the 6-nitronorepinephrine-induced a contraction to the extent similar to those from untreated rats, suggesting that endogenous norepinephrine does not play a role in the 6-nitronorepinephrine-induced contraction. 6-Nitronorepinephrine (> 10 microM) potentiated norepinephrine-induced contraction only in the presence of endothelium. The augmentation was attenuated by catalase (1200 U/ml). H2O2 (10-300 microM) augmented the norepinephrine-induced contraction only in the endothelium-intact rat aortic rings. 6-Nitronorepinephrine attenuated significantly acetylcholine-induced relaxation. Catalase prevented the 6-nitronorepinephrine-induced inhibition of the acetylcholine-induced relaxation. These results suggest that 6-nitronorepinephrine has a weak alpha1-adrenoceptor agonistic property and that the endothelium-dependent potentiation by 6-nitronorepinephrine of the norepinephrine-induced contraction is mediated through production of H2O2.