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

Abstract

SUMMARYThe 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 5 day progressive increase in salinity from freshwater (1‰) to brackish water ( . Consequently, fish exposed to 25‰ 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 μmol g -1 and >30 μmol g -1 in the muscle and liver, respectively, of fish exposed to 25‰ water. There were significant increases in glutamine synthetase (GS) activity in muscle and liver of these fish. In addition, exposure to 25‰ 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.

The freshwater Amazonian stingray, Potamotrygon motoro, up-regulates glutamine synthetase activity and protein abundance, and accumulates glutamine when exposed to brackish (15‰) water

“…Since P. motoro is akin to marine elasmobranchs, results obtained in this study may have implications that differ from those reported by Tok and colleagues (Tok et al, 2009) in the teleost, M. albus. In euryhaline freshwater or marine elasmobranchs, the OUC is the dominant pathway of urea production and GS is the key initial nitrogen-tapping enzyme for urea production via CPS III-based OUC.…”

“…Ammonia is also detoxified to glutamine through GS in vertebrate brains (Cooper and Plum, 1987), although increases in glutamine synthesis and GS activity in non-brain tissues are uncommon among vertebrates, except for several air-breathing fishes, which include M. albus (Tay et al, 2003;Ip et al, 2004b;Chew et al, 2005), Bostrychus sinensis (Ip et al, 2001a;Anderson et al, 2002), Oxyeleotris marmorata (Jow et al, 1999) and Misgurnus anguillicaudatus (Chew et al, 2001), exposed to terrestrial conditions or environmental ammonia. In contrast, Tok and colleagues (Tok et al, 2009) reported that acclimation to 25‰ water led to significant increases in GS activity and protein abundance in the liver of M. albus. In this study, we also demonstrated that GS activity and protein expression were upregulated in muscle and liver of P. motoro exposed to 15‰ water.…”

“…As for fish, exposure of the euryhaline brackish water B. sinensis ) and the euryhaline freshwater O. marmorata ) to seawater led to either no significant changes or very minor increases in glutamine content in various tissues. However, Tok and colleagues (Tok et al, 2009) reported recently that glutamine accumulated to high levels (12 and 37molg -1 in muscle and liver, respectively) in M. albus exposed to 25‰ water. In the present study, we also demonstrated significant increases in glutamine content in liver and muscle of P. motoro exposed to 15‰ water, although the level of glutamine accumulation was lower than that of M. albus.…”

“…Therefore, this study was undertaken to test the hypothesis that, like other euryhaline freshwater teleosts [Anguilla anguilla (Huggins and Colley, 1971), Cyprinus carpio (Hegab and Hanke, 1983), Salmo gairdneri (Kaushik and Luquet, 1979), Oreochromis mossambicus (Venkatachari, 1974;Assem and Hanke, 1983;Fiess et al, 2007), Anabas testudineus (Chang et al, 2007), Monopterus albus (Tok et al, 2009)] the ocellated river stingray, Potamotrygon motoro, accumulates free amino acids (FAAs), which act as organic osmolytes for cell volume regulation, in its tissues when exposed to 15‰ water. Fish were fed freshwater shrimp daily in this set of experiments in order to test the hypotheses that feeding would not induce ureotely (i.e.…”

Molecular cloning and expression of hepatopancreas glutamine synthetase in the Pacific white shrimp, Litopenaeus vannamei, induced by acute hypo-osmotic stress

“…Ammonium is then assimilated into nitrogenous organic compounds by the concerted effort of two enzymes that compose the GS and glutamate synthetase cycle (Teixeira and Fidalgo, 2009). This process is a defense mechanism in fish and indicates high levels of GS content (Tok et al, 2009). The gene sequence of GS has been cloned in many species (Fucci et al, 1995;Lai et al, 2011;Tanguy et al, 2005;Trapido-Rosenthal et al, 1993).…”