Nitric oxide availability in deeply hypoxic crucian carp: acute and chronic changes and utilization of ambient nitrite reservoirs

Recent research suggest that anoxia-tolerant fish transfer extracellular nitrite into the tissues, where it is used for nitric oxide (NO) generation, iron-nitrosylation, and S-nitrosation of proteins, as part of the cytoprotective response toward prolonged hypoxia and subsequent reoxygenation. We hypothesized that crucian carp take up ambient nitrite and use it as a source of cellular NO availability during hypoxia. Fish were exposed for 1 day to normoxia (Po2 > 140 mmHg) and deep hypoxia (1 < Po2 < 3 mmHg) at both low (< 0.2 μM) and moderately elevated (10 μM) ambient [nitrite] to decipher NO metabolites in plasma and several tissues. We also compared NO metabolite changes during acute (10 min) and chronic (1 day) exposures to three different O2 levels. Plasma [nitrite] decreased with decreasing [O2], while the cellular concentrations of nitrite and nitros(yl)ated compounds either increased or stayed constant, depending on O2 level and tissue type. Nitrite was notably increased in the heart during deep hypoxia, and the increase was amplified by elevated ambient [nitrite]. Raised nitrite also increased gill [nitrite] and decreased mRNA expression of an inducible nitric oxide synthase-2 gene variant. The data support that ambient nitrite is taken up across the gills to be distributed via the blood to the tissues, particularly the heart, where it assists in cytoprotection and other functions. Cardiac nitrite was not elevated in acutely exposed fish, revealing that the response requires time. NO metabolite levels were higher during acute than chronic exposures, possibly caused by increased swimming activity and stress in acutely exposed fish.