Nitrite secures essential
nitric oxide (NO) bioavailability in
hypoxia at low endogenous concentrations, whereas it becomes toxic at high concentrations. We exposed brown trout to normoxic and hypoxic water in the absence and presence of added ambient
nitrite to decipher the cellular metabolism and effects of
nitrite at basal and elevated concentrations under different
oxygen regimes. We also tested hypotheses concerning the influence of
nitrite on branchial
nitric oxide synthase (NOS), Na(+)/K(+)-
ATPase (nka) and
heat shock protein (hsp70)
mRNA expression. Basal plasma and erythrocyte
nitrite levels were higher in
hypoxia than normoxia, suggesting increased NOS activity.
Nitrite exposure strongly elevated
nitrite concentrations in plasma, erythrocytes, heart tissue and white muscle, which was associated with an extensive metabolism of
nitrite to
nitrate and to
iron-nitrosylated and S-nitrosated compounds.
Nitrite uptake was slightly higher in
hypoxia than normoxia, and high internal
nitrite levels extensively converted blood
hemoglobin to
methemoglobin and
nitrosylhemoglobin.
Hypoxia increased inducible NOS (iNOS)
mRNA levels in the gills, which was overruled by a strong inhibition of iNOS expression by
nitrite in both normoxia and
hypoxia, suggesting negative-feedback regulation of iNOS gene expression by
nitrite. A similar inhibition was absent for neuronal NOS. Branchial NKA activity stayed unchanged, but
mRNA levels of the nkaα1a subunit increased with
hypoxia and
nitrite, which may have countered an initial NKA inhibition.
Nitrite also increased hsp70 gene expression, probably contributing to the cytoprotective effects of
nitrite at low concentrations.
Nitrite displays a concentration-dependent switch between positive and negative effects similar to other signaling molecules.