Urotensin-II (U-II), a cyclic peptide widely expressed in blood vessels, has diverse vascular actions that range from potent vasoconstriction to vasodilation. Although, U-II-induced vasodilation has been shown to be partially dependent on nitric oxide (NO), the involvement of vascular adventitia-derived NO, remains unknown. The present study aimed to elucidate the activation of U-II on L-arginine/NO pathway in isolated rat aortic adventitia. In adventitia of thoracic and abdominal aortas, the l-arginine/NO pathway was similarly characterized: the uptake of l-[(3)H]arginine was Na(+)-independent, with the peak occurring over around 40 min incubation; the total NO synthase (NOS) activity was mostly calcium-independent (>90%), and significantly inhibited by a specific iNOS inhibitor AMT; the production of NO metabolites nitrate and nitrite (NO(x)) was stimulated by L-arginine but not by D-arginine. In aortic adventitia exposed to rat U-II (10(-9) and 10(-8)M) for 6 h, the V(max) of l-[(3)H]arginine uptake over 40 min incubation was significantly increased, while the K(m) of l-[(3)H]arginine uptake showed no significant change. Besides, the iNOS mRNA level was up-regulated, the total NOS activity, largely calcium-independent, was significantly induced, and the NO(x) production was significantly stimulated by U-II. According to the same protocol as U-II, the positive control lipopolysaccharide (LPS, 10 microg/ml), which had been established to activate adventitial L-arginine/NO pathway, increased l-[(3)H]arginine uptake, iNOS activity and NO(x) production to a greater extent than U-II. In addition, the total NOS activities induced by 3 and 6h incubation of U-II and LPS were significantly inhibited by a specific inhibitor of protein synthesis, actinomycin D. In conclusion, the results showed that rat U-II activated L-arginine/NOS/NO pathway in rat aortic adventitia, suggesting a potential contributive role of adventitia-derived NO in the vasodilator response of U-II.