Metabolic fates and effects of nitrite in brown trout under normoxic and hypoxic conditions: blood and tissue nitrite metabolism and interactions with branchialNOS,Na+/K+-ATPaseandhsp70expression

Abstract

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 concentrationdependent switch between positive and negative effects similar to other signaling molecules.

Effects of nitrite exposure on haematological parameters, oxidative stress and apoptosis in juvenile turbot (Scophthalmus maximus)

“…Also, turbot exposed to nitrite in our study showed increased HSP70 level, which might reflect nitrite exposure caused tissue damage inducing the expression of HSP 70 (Deane and Woo, 2007;Jensen et al, 2015), and overexpression of HSP 70 entered blood (Madeira et al, 2012;SHENG et al, 2007).…”

Nitric oxide synthase (NOS) inhibitor L‐NAME activates inducible NOS/NO system and drives multidimensional regulation of Na+/K+‐ATPase in ionocyte epithelia of immersion‐stressed air‐breathing fish (Anabas testudineus Bloch)

“…Similar rise in NO production due to activation of iNOS expression has been demonstrated in hypoxic rainbow trout (Gerber et al, 2016, 2017). In addition, hypoxia has been shown to activate iNOS expression in brown trout (Jensen et al, 2015). Moreover, a protective role of iNOS against oxidative stress and ammonia tolerance has been reported in the hepatocytes of Magur catfish (Hasan et al, 2020).…”

“…The enzymatic actions of nitric oxide synthases (NOS) on l ‐arginine release NO in the presence of molecular oxygen and NADPH (Griffith & Stuehr, 1995). Among three NOS isoforms, constitutive neuronal NOS (nNOS) and inducible NOS (iNOS) are more active in teleost fishes (Andreakis et al, 2011; Cioni et al, 2019; Jensen et al, 2015). Generally, a small amount of NO is formed from l ‐arginine by nNOS, but during stress large amount of NO are released from inducible calcium‐independent iNOS in many cell types for a long period of time (Althaus, 2012; Choudhury & Saha, 2015).…”

“…As a short‐lived free radical, nitric oxide (NO) acts as gasotransmitter and performs autocrine/paracrine function to impose physiological actions in mammals and fishes (Ally et al, 2020; Althaus, 2012; Cioni et al, 2019; Jensen et al, 2015; Hasan et al, 2020). The enzymatic actions of nitric oxide synthases (NOS) on l ‐arginine release NO in the presence of molecular oxygen and NADPH (Griffith & Stuehr, 1995).…”

“…The NOS/NO system of fish responds to many environmental stressors such as hyperammonia (Choudhury & Saha, 2012a), temperature elevation (Jorgensen et al, 2014), desiccation stress (Choudhury & Saha, 2012b; Garofalo et al, 2015), hyperosmotic stress (Gerber et al, 2018) and hypoxia/anoxia (Imbrogno et al, 2014; Jensen et al, 2015; Sandvik et al, 2012; Hansen et al, 2016). Similarly, a role for NO in stress acclimation has been reported in fishes (Fago & Jensen, 2015; Hasan et al, 2020).…”