OBJECTIVE: NADPH oxidases are important sources of reactive oxygen species (ROS) in the vasculature. In phagocytic cells, the catalytic subunit of NADPH oxidase is a glycoprotein, gp91phox. However, vascular smooth muscle cells (VSMCs), which show prominent NADPH oxidase activity, lack gp91phox. Hence, we examined the role of Nox4, a gp91phox homologue, in superoxide production in mouse-cultured VSMCs. METHODS AND RESULTS: Incubation of VSMCs with NADPH increased ROS production whether detected by lucigenin-enhanced chemiluminescence or dichlorofluorescein. Superoxide production was inhibited by the NADPH oxidase inhibitors, diphenyleneiodonium and apocynin, but not by inhibitors of other potential sources of superoxide. In unstimulated VSMCs, phosphorothioate antisense oligonucleotides against Nox4 down-regulated mRNA expression of the subunit by 65% and attenuated superoxide production by 41% without affecting Nox1 expression. Interleukin-1beta (IL-beta) thrombin and platelet-derived growth factor (PDGF) also reduced Nox4 mRNA expression after 3 h without affecting Nox1 levels. Of these stimuli, only IL-beta reduced superoxide, but this effect was more rapid (< or =30 min) than its actions on Nox4. CONCLUSIONS: Under resting conditions, NADPH oxidase activity in VSMCs is largely dependent upon Nox4 expression. Proinflammatory mediators down-regulated Nox4 but did not affect Nox1 expression, so other factors must compensate to regulate superoxide production.