Regulation of bovine endothelial constitutive nitric oxide synthase by oxygen

Oxygen (O-2) may regulate pulmonary vascular resistance through changes in endothelial nitric oxide (NO) production. To determine whether constitutive NO synthase (cNOS) is regulated by O-2, we assessed cNOS expression and activity in bovine pulmonary artery endothelial cells exposed to different concentrations of O-2. In a time-dependent manner, changes in O-2 concentration from 95 to 3% produced a progressive decrease in cNOS mRNA and protein levels resulting in 4.8- and 4.3-fold reductions after 24 h, respectively, This correlated with changes in cNOS activity as determined by nitrite measurements, Compared with 20% O-2, cNOS activity was increased 1.5-fold in 95% O-2 and decreased 1.9-fold in 3% O-2. A decrease in O-2 concentration from 95 to 3% shortened cNOS mRNA half-life from 46 to 24 h and caused a 20-fold repression of cNOS gene transcription. Treatment with cycloheximide produced a threefold increase in cNOS mRNA at all O-2 concentrations, suggesting that cNOS mRNA expression is negatively regulated under basal condition. We conclude that O-2 upregulates cNOS expression through transcriptional and posttranscriptional mechanisms. A decrease in cNOS activity in the presence of low O-2 levels, therefore, may contribute to hypoxia-induced vasoconstriction in the pulmonary circulation.