Objectives-Increased formation of reactive oxygen species (ROS) has been identified as a causative factor in endothelial dysfunction by reducing NO bioavailability and uncoupling endothelial nitric oxide synthase (eNOS). However, the specific contribution of ROS to endothelial function is not well understood. Methods and Results-A major source of intracellular ROS is the NADPH oxidase (Nox) family of enzymes. The goal of the current study was to directly assess the contribution of NADPH oxidase derived superoxide to eNOS function by expressing Nox5, a single gene product that constitutively produces superoxide within cells. Paradoxically, we found that instead of inhibiting eNOS, coexpression of Nox5 increased NO release from both bovine and human endothelial cells. To establish the functional significance of this observation in intact blood vessels, the endothelium of mouse aorta was transduced with Nox5 or control adenoviruses. Nox5 potently inhibited Ach-induced relaxation and potentiated contractile responses to phenylephrine. In precontracted aortae, acute exposure to superoxide dismutase induced significant vascular relaxation in vessels exposed to Nox5 versus control and unmasked the ability of Nox5 to activate eNOS in blood vessel endothelium. (EC) has been identified as a causative factor in this process by reducing NO bioavailability, uncoupling eNOS via BH4 depletion or homodimer disruption, and also by altering redox-sensitive signaling cascades. However, elevated superoxide production is frequently accompanied by changes in blood pressure, cellular signaling, hormones, and the composition of the extracellular milieu, making it difficult to ascertain the independent effects of intracellular ROS. For example, endothelial function is reduced in animal models of type II diabetes, angiotensin-dependent hypertension, and atherosclerosis, and this deficit is accompanied by significant increases in superoxide formation. 2-4 However, whether increased superoxide is the causative factor, a participant, or requires the cooperation of other factors present in the extracellular milieu is not yet known. Conclusions-These

Activity of endothelial nitric oxide synthase (eNOS), a key determinant of cardiovascular is modulated by several factors, including reactive oxygen species (ROS). The goal of the current study was to assess the direct effects of intracellular ROS on endothelial function and the activity of eNOS. To achieve this, we utilized Nox5, a unique member of the NADPH oxidase family that can function as a single gene product. We transiently co‐expressed eNOS and Nox5 in COS‐7 cells and measured the amount of NO produced. Contrary to our original hypothesis, we found increased amounts of NO produced in cells expressing Nox5. In endothelial cells transduced with an adenovirus expressing Nox5, we observed dose‐dependent increases in NO production in cells expressing Nox5 using both NO‐specific chemiluminescence and a co‐culture cGMP bioassay. To establish the functional significance of this observation in blood vessels, the endothelium of mouse aorta was transduced with control or Nox5 adenoviruses. In precontracted blood vessels, acute exposure to SOD induced significant vascular relaxation in vessels exposed to Nox5 versus control and unmasked the ability of Nox5 to activate eNOS. To identify the underlying mechanism we next assessed whether Nox5 modifies eNOS phosphorylation, the intra‐versus extra‐cellular effects of Nox5 and the ability of Nox5 to activate a calcium‐independent eNOS mutant.