Glutamine accumulation and up-regulation of glutamine synthetase activity in the swamp eel, Monopterus albus (Zuiew), exposed to brackish water

The swamp eel, Monopterus albus, is an air-breathing teleost which typically lives in freshwater but can also be found in estuaries, where it has to deal with ambient salinity fluctuations. Unlike other teleosts, its gills are highly degenerate. Hence, it may have uncommon osmoregulatory adaptations, but no information is available on its osmoregulatory capacity and mechanisms at present. In this study M. albus was exposed to a 5day progressive increase in salinity from freshwater (1 parts per thousand) to brackish water (25 parts per thousand) and subsequently kept in 25 parts per thousand water for a total of 4 days. The results indicate that M. albus switched from hyperosmotic hyperionic regulation in freshwater to a combination of osmoconforming and hypoosmotic hypoionic regulation in 25 parts per thousand water. Exposure to 25 parts per thousand water resulted in relatively large increases in plasma osmolality, [Na+] and [Cl-]. Consequently, fish exposed to 25 parts per thousand water had to undergo cell volume regulation through accumulation of organic osmolytes and inorganic ions. Increases in tissue free amino acid content were apparently the result of increased protein degradation, decreased amino acid catabolism, and increased synthesis of certain non-essential amino acids. Here we report for the first time that glutamine is the major organic osmolyte in M. albus. Glutamine content increased to a phenomenal level of > 12 mu molg(-1) and > 30 mu molg(-1) in the muscle and liver, respectively, of fish exposed to 25 parts per thousand water. There were significant increases in glutamine synthetase (GS) activity in muscle and liver of these fish. In addition, exposure to 25 parts per thousand water for 4 days led to significant increases in GS protein abundance in both muscle and liver, indicating that increases in the expression of GS mRNA could have occurred.