Rabbit thoracic aorta was assessed for the influence of the endothelium and nitric oxide (NO) on superoxide anion (SO) levels in the presence and absence of an inhibitor of superoxide dismutase. Aortic rings (0.5 cm) were incubated for 30 min at 37 degrees C in the presence or absence of diethyldithiocarbamate (DDC, 10 mM), a CuZn superoxide dismutase inhibitor. Rings were then placed in a solution containing lucigenin (250 microM) at 37 degrees C, and changes in amounts of SO over 10 min were determined by measuring chemiluminescence under basal and acetylcholine-stimulated conditions. Treatment with DDC markedly enhanced basal levels of SO, and the DDC-evoked levels were significantly reduced by the SO scavenger, Tiron (10 mM). Addition of acetylcholine (10 microM) to the assay did not significantly affect the levels of SO in either control or DDC-treated rings. Also, mechanical removal of the endothelium or pretreatment of the rings with the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) (300 microM), did not significantly affect the levels of SO in DDC-treated rings. In contrast, exogenous NO at 1 and 10 microM reduced the DDC-evoked SO levels by 54 and 77%, respectively. These data imply that the predominant sources of SO in the rabbit aorta are vascular components other than the endothelium and that endogenous superoxide dismutase modulates the level of SO. Although exogenous NO reduced aortic SO levels, neither basal nor acetylcholine-stimulated production of endogenous NO appears sufficient to reduce SO levels.