Uptake of nitrite, nitrate, and bromide in rainbow trout, Oncorhynchus mykiss: Effects on ionic balance

Effects of uptake into blood plasma of NO2-, NO3-, and Br- from 1 mM ambient concentrations were studied in rainbow trout (Oncorhynchus mykiss). Nitrite and bromide were concentrated in plasma, competing for the branchial chloride uptake mechanism. Nitrate appeared to be taken up passively, with plasma concentrations remaining below ambient [NO3-] after 8 days exposure. This limited uptake appeared central to the low toxicity of NO3-, and did not measurably influence electrolyte balance or haematology. Plasma [Br-] increased to 51 mM during 14 days, which was paralleled by a 1:1 stoichiometrical decrease in plasma [Cl-]. This was the only detected effect of Br- exposure and was tolerated without mortality. Nitrite-exposed trout fell into two distinct groups. Trout dying before 2 days of NO2- exposure quickly developed methaemoglobinaemia; high plasma [NO2-], [lactate], and [K+]; and low [Cl-]. In trout surviving up to 4 days, plasma [NO2-] and methaemoglobin rose more slowly, plasma [Cl-] decreased less, and extracellular lactate and potassium levels were not significantly elevated. In both groups, plasma [NO3-] rose to values comparable with plasma [NO2-] (about 3 mM), reflecting an internal conversion of nitrite to nitrate. Nitrite-exposure significantly decreased skeletal muscle [K+], whereas no significant changes were observed in cardiac muscle.