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

Abstract

SUMMARYThis study aimed to examine whether the stenohaline freshwater stingray, Potamotrygon motoro, which lacks a functional ornithine-urea cycle, would up-regulate glutamine synthetase (GS) activity and protein abundance, and accumulate glutamine during a progressive transfer from freshwater to brackish (15‰) water with daily feeding. Our results revealed that, similar to other freshwater teleosts, P. motoro performed hyperosmotic regulation, with very low urea concentrations in plasma and tissues, in freshwater. In 15‰ water, it was non-ureotelic and non-ureoosmotic, acting mainly as an osmoconformer with its plasma osmolality, [Na + ] and [Cl -] comparable to those of the external medium. There were significant increases in the content of several free amino acids (FAAs), including glutamate, glutamine and glycine, in muscle and liver, but not in plasma, indicating that FAAs could contribute in part to cell volume regulation. Furthermore, exposure of P. motoro to 15‰ water led to up-regulation of GS activity and protein abundance in both liver and muscle. Thus, our results indicate for the first time that, despite the inability to synthesize urea and the lack of functional carbamoyl phosphate synthetase III (CPS III) which uses glutamine as a substrate, P. motoro retained the capacity to up-regulate the activity and protein expression of GS in response to salinity stress. Potamotrygon motoro was not nitrogen (N) limited when exposed to 15‰ water with feeding, and there were no significant changes in the amination and deamination activities of hepatic glutamate dehydrogenase. In contrast, P. motoro became N limited when exposed to 10‰ water with fasting and could not survive well in 15‰ water without food.

Ammonia production, excretion, toxicity, and defense in fish: a review

“…Glutamine formation plays a major role in detoxifying exogenous and endogenous ammonia in non-ureogenic fishes, especially in the brain, during exposure to exogenous/environmental ammonia (Arillo et al, 1981; Dabrowska and Wlasow, 1986; Mommsen and Walsh, 1992; Peng et al, 1998; Lim et al, 2004a; Ip et al, 2005a; Veauvy et al, 2005; Wee et al, 2007; Wright et al, 2007; Tng et al, 2009; Sanderson et al, 2010) or after feeding (Wicks and Randall, 2002; Lim et al, 2004b). In certain fish species, like the swamp eel, M. albus (Tok et al, 2009), and the Amazonian freshwater stingray, Potamotrygon motoro (Ip et al, 2009), glutamine synthesized from endogenous ammonia can act as an important osmolyte for cell volume regulation during acclimation to high salinity. As for ureogenic and ureosmotic fishes, glutamine can act as a substrate for the synthesis of urea which is essential for osmoregulation in a hyperosmotic environment.…”

Expression Analysis of Glycerol Synthesis–Related Liver Transcripts in Rainbow Smelt (Osmerus mordax) Exposed to a Controlled Decrease in Temperature

“…An alternative explanation is that in cold smelt livers, GS may be upregulated as part of the general stress response and may serve to store nitrogen for biosynthesis (e.g., nucleotides and amino acids) in the spring. GS transcript levels were upregulated in liver of long-jaw mudsucker exposed to hypoxia (Gracey et al 2001), GS activities and protein levels were upregulated in liver of freshwater Amazonian stingray exposed to brackish water (Ip et al 2009), and GS transcript and protein levels were upregulated in liver of gulf toadfish during confinement stress (Kong et al 2000). Two glucocorticoid response elements were identified in the promoter region of the GS gene of gulf toadfish (Esbaugh and Walsh 2009), consistent with the contention that an increase in glutamine is a general stress response.…”

Branchial Na+:K+:2Cl− cotransporter 1 and Na+/K+-ATPase α-subunit in a brackish water-type ionocyte of the euryhaline freshwater white-rimmed stingray, Himantura signifer

“…Recently, Wong et al (2013) demonstrated that H. signifer might experience greater salinity-induced oxidative stress in fresh water than in brackish water, possibly related to its short history of fresh water invasion. In the laboratory, H. signifer can survive in fresh water (salinity 0.7) indefinitely or in brackish water (salinity 20) for at least two weeks (Tam et al, 2003; Ip et al, 2009). Unlike potamotrygonid stingrays, it possesses a functional urea cycle (Ip et al, 2003; Tam et al, 2003), despite having relatively low plasma urea concentrations (45–70 mmol l −1 ) in fresh water.…”