Effect of graded hypoxia on the induction and function of inducible nitric oxide synthase in rat mesangial cells

Inducible nitric oxide synthase (iNOS) catalyzes the formation of nitric oxide (NO) from L-arginine and O₂. Although some O₂ is required for this reaction, it is uncertain whether biologically relevant levels of hypoxia alter this pathway. We examined the effects of graded hypoxia on several steps in the iNOS pathway in lipopolysaccharide (LPS)-stimulated rat glomerular mesangial cells: induction of iNOS mRNA, NO synthesis, NO oxidation to nitrite (NO₂^-) and nitrate (NO₃^-), and accumulation of cGMP. Cultured cells were incubated for 24 hours in airtight flasks containing O₂ (21%, 10%, 2.5%, and 0%), CO₂ (5%), and N₂ (balance), resulting in media PO₂ levels of 140±3, 85±1, 46±3 (moderate hypoxia), and 32±5 (severe hypoxia) mm Hg, respectively. During normoxia (PO₂, 85 to 140 mm Hg) LPS increased iNOS mRNA with associated increases in NO synthesis, NO₂^- and NO₃^- accumulation, and intracellular cGMP levels. In the absence of LPS, there was minimal NO synthesis and no detectable iNOS mRNA. Even during severe hypoxia, LPS elevated NO₂^- and NO₃^- relative to levels in unstimulated cells (P<.05), although to a lesser extent than during normoxia (P<.05). The induction of iNOS mRNA by LPS was preserved in hypoxia, and intracellular cGMP levels were similar at all levels of oxygen tension, indicating that iNOS induction and function were not altered by moderate or severe hypoxia. However, moderate hypoxia did alter the partitioning and oxidation of NO, favoring the appearance of NO in the 'headspace' (defined as the gas overlying the cells) and NO₃^- in the media. We conclude that although hypoxia can alter the partitioning and decomposition of NO, the induction of iNOS mRNA and the activity of its enzyme product are resistant to hypoxia (PO₂, >32 mm Hg).