Aspects of nitrogen metabolism in teleost and elasmobranch fishes

This thesis is an investigation of the metabolic differences in nitrogen metabolism of teleost and elasmobranch fishes using LC-ESI-MS/MS technology and 15N as a metabolic tracer. In a representative teleost Oncorhynchus mykiss, the metabolic fate of exogenous ammonia involves a broad labelling of the amino acid pool, including glutamine, glutamate and the branched-chain amino acids, whereas in the representative elasmobranch Squalus acanthias, the labelling is almost exclusively contained in glutamine, which is consistent with the need of elasmobranchs to synthesize urea using glutamine as the nitrogen donor. The primary route of entry for the exogenous ammonia was the gill, so to determine whether the route of entry affected the subsequent metabolism of ammonia I examined the incorporation of endogenous ammonia into the amino acid pool. It was found that in the teleost, the branched chain amino acids did not incorporate endogenous ammonia, unlike the situation for exogenous ammonia. This indicates a substantial capacity of the gills of both species to influence the fate of nitrogen. The direct measurement of nitrogen incorporation into the amino acid pool in both exogenous and endogenous experimental situations in both the teleost and elasmobranch system provides direct evidence of salvaging of essential amino acids and further elucidating the role of other amino acids, particularly glutamate and glutamine, in handling both exogenously and endogenously derived nitrogen in the form of ammonia. I further examined the plasma kinetics of amino acids in O. mykiss and S. acanthias and found that in both cases amino acid turnover was rapid, and when corrected for metabolic rate, turnover rates were similar to mammalian values. These studies are the first plasma kinetic parameters to be determined for individual amino acids in any elasmobranch and provide a much more robust data set for a teleost fish. Taken together, the data presented in this thesis provides a significant advance in the understanding of nitrogen metabolism in both elasmobranchs and teleosts.