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Aim: We tested the hypothesis that bradykinin (BK)-induced relaxation of phenylephrine (PE) and endothelin-1 (ET-1) contractions can be differentially modulated by reactive oxygen species (ROS). Methods: Aortic rings isolated from Sprague-Dawley rats were used for the study. The contribution of ROS to PE(1×10-9-1×10-5 mol/L)- and ET-1 (1×10-10-1×10-8 mol/L)-induced contractions and the influence of ROS in B K (1×10-9-1×10mol/L) relaxation of PE (1×10-7 mol/L) or ET-1 (1×10-9 mol/L)-induced tension was evaluated in the aorta in the presence or absence of the following antioxidants: catalase (CAT, 300 U/mL), superoxide dismutase (SOD, 300 U/mL), and vitamin C (1×10-4 mol/L). Results: Tension generated by ET-1 (1 × 10-9 mol/L) or PE (1 × 10-7 mol/L) was differentially relaxed by BK(1 × 10-5 mol/L), producing a maximal relaxation of 75 %±5 % and 35±4%, respectively.The BK (1×10-5 mol/L)-induced relaxation of PE (1×10-7 mol/L) tension was signifi-cantly enhanced from 35%±4% (control) to 56%±9%, 60%±5%, and 49%±6% by SOD, CAT, and vitamin C, respectively (P<0.05, n=8). However, the relaxation of ET-1 (1×10-9 mol/L) tension was significantly attenuated from 75%±5% (control)to 37%±9%, 63%±4%, and 39%±7% by SOD, CAT, and vitamin C, respectively(P<0.05, n=8). On the other hand, CAT had no effect on PE-induced tension, while SOD enhanced PE-induced tension (36%,P<0.05, n=10) and vitamin C attenuated(66%, P<0.05, n=8) the tension induced by PE. By contrast, SOD or vitamin C had no effect, but CAT attenuated (44%, P<0.05, n=9) the tension induced by ET-1.Conclusion: We have demonstrated that O2- and H2O2 differentially modulate BK relaxation in an agonist-specific manner. O2- attenuates BK-induced relaxation of PE contraction, but contributes to the relaxation of ET-1 contraction. O2- seems to inhibit PE contraction, while H2O2 contributes to ET-1-induced contraction. Thus,ROS differentially modulate vascular tone depending on the vasoactive agent that is used to generate the tone.