Preliminary observations on the effects of water hardness on free taurine and other amino acids in plasma and muscle of channel catfish
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An interesting metabolic adjustment that fish make in their adaptation to a particular environment concerns the maintenance of the proper osmotic balance in their tissues. Free amino acids can be important isosmotic regulators; they contribute to intra- and extracellular fluid osmolality and are important in cell volume regulation. For this reason, a laboratory experiment was conducted to evaluate the effects of water ionic composition on the level of free amino acids in the plasma and muscles of juvenile channel catfish Ictalurus punctatus. Duplicate groups of fish were fed a commercial diet (40% crude protein) twice daily to apparent satiation. Fish were held undertwo different regimes of water hardness, 17.9 and 407 mg/L asCaCO3, which were considered soft and hard water, respectively. Blood and muscle samples were obtained from fish at the start and termination of the 4-week experiment. Mean plasma osmolarity was 294 and 273 mmol/kg for fish reared in hard and soft water, respectively. Analysis of the free amino acid concentrations of plasma and muscle with high-performance liquid chromatography revealed that the ionic environment had significant (P < 0.05) effects on the concentration of taurine and other amino acids. Circulating plasma amino acid concentrations indicated an increased release of taurine, glycine, serine, glutamine, and histidine in fish acclimated to soft water. In contrast, fish acclimated to hard water accumulated taurine, alanine, and tyrosine in the muscle free amino acid pool. Although the magnitude of the changes in amino acid level that occurred in the plasma and skeletal muscle indicates that taurine is the principal organic osmolyte in channel catfish, these changes are of less significance than the total osmotic concentration of plasma. However, the responsiveness of taurine to water hardness suggests an important role for this amino acid in regulating cell membrane permeability; it may also have physiological significance, as taurine mediates many other biological functions in fish. To our knowledge, the present study is the first to report the effects of water hardness on free amino acid profiles in the tissues of channel catfish. 2002 Taylor & Francis Group, LLC.
