Isolated and combined exposure to ammonia and nitrite in rainbow trout (Oncorhynchus mykiss): effects on electrolyte status, blood respiratory properties and brain glutamine/glutamate concentrations

Abstract

Rainbow trout (Oncorhynchus mykiss) were exposed for up to 4 days to 100, 300 and 500 μM ammonia and 0, 300 and 600 μM nitrite. Each ammonia concentration was combined with each nitrite concentration, giving a total of nine exposure groups. High mortality was observed in trout exposed to 500 μM ammonia in combination with 600 μM nitrite. Other exposure groups only showed sporadic mortality. Interactive effects (i.e. synergism or antagonism) of ammonia and nitrite exposure were not observed on the physiological parameters measured. Ammonia and nitrite, however, both caused a significant and additive decrease in muscle potassium concentrations after 4 days of exposure. The decrease was approximately 50 μmol g−1 dry weight for both nitrite and ammonia at the highest exposure concentrations. Nitrite was accumulated in plasma to approximately twice the ambient concentration, which was associated with a small significant increase in blood [lactate]. Plasma ammonia concentration only increased significantly upon ammonia exposure. The extracellular osmolality and Cl−, Na+ and amino acid concentrations remained constant. A transient increase in haematocrit and blood haemoglobin concentration was observed in trout exposed to the highest ammonia concentration. Accumulation of nitrite induced methaemoglobin formation and thereby functional anaemia, while no changes in red cell organic phosphates occurred. Plasma glutamate oxaloacetate transaminase (GOT) activity increased after 4 days exposure to the highest ammonia concentration. No changes occurred in plasma glutamate pyruvate transaminase (GPT) activity. Brain glutamate concentrations decreased while the brain glutamine concentrations increased in trout exposed to ammonia, suggesting detoxification of ammonia by its reaction with glutamate to form glutamine. Brain [glutamate] did not decrease below a threshold of approximately 2.37 μmol g−1 wet weight.