Nitric oxide and superoxide anion production during endothelial cell proliferation

We have previously reported that nitric oxide (NO) synthase activity, protein, and mRNA are increased in proliferating compared with postconfluent bovine aortic endothelial cells (BAEC). Because superoxide anion inactivates NO, in the present study, we have assessed the effect of proliferation on superoxide anion production by use of cytochrome c reduction. The superoxide anion production in proliferating cells was increased about threefold compared with postconfluent cells in both basal and calcium ionophore-stimulated conditions and exceeded the amount of released nitrite and nitrate (NOx) in all cases. A-23187 (1 microM) stimulated the superoxide anion production about twofold at all stages of confluence. Because superoxide anion can inactivate NO, we then assessed the effect of proliferation on NO bioactivity released in the conditioned medium, by use of RFL-6 cells (reporter cells very rich in guanylate cyclase, which on activation by NO generates guanosine 3',5'-cyclic monophosphate, second messenger of NO). In the absence of added superoxide dismutase (SOD) in the conditioned medium, the guanylate cyclase-stimulating activities evoked by A-23187 from proliferating and growth-arrested cells were similar, despite a greater NOx release in the former. When SOD (100 U/ml) was added in the conditioned medium, the guanylate cyclase-stimulating activity evoked by 1 microM A-23187 was increased approximately 10-fold and closely paralleled NOx release (i.e., was greater in supernatant of proliferating cells than in that of growth-arrested cells). Thus BAEC release more superoxide anion extracellularly than NO at all stages of confluence. Endothelium-derived superoxide anion is a major determinant of the breakdown of NO.