Nitrogen metabolism in the mudskipper, Periophthalmus cantonens: Changes in free amino acids and related compounds in various tissues under conditions of ammonia loading, with special reference to its high ammonia tolerance

Amphibious fishes: evolution and phenotypic plasticity

Amphibious fishes spend part of their life in terrestrial habitats. The ability to tolerate life on land has evolved independently many times, with more than 200 extant species of amphibious fishes spanning 17 orders now reported. Many adaptations for life out of water have been described in the literature, and adaptive phenotypic plasticity may play an equally important role in promoting favourable matches between the terrestrial habitat and behavioural, physiological, biochemical and morphological characteristics. Amphibious fishes living at the interface of two very different environments must respond to issues relating to buoyancy/gravity, hydration/desiccation, low/high O2 availability, low/high CO2 accumulation and high/low NH3 solubility each time they traverse the air-water interface. Here, we review the literature for examples of plastic traits associated with the response to each of these challenges. Because there is evidence that phenotypic plasticity can facilitate the evolution of fixed traits in general, we summarize the types of investigations needed to more fully determine whether plasticity in extant amphibious fishes can provide indications of the strategies used during the evolution of terrestriality in tetrapods.

Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes

With the development of air-breathing capabilities, some fishes can emerge from water, make excursions onto land or even burrow into mud during droughts. Air-breathing fishes have modified gill morphology and morphometry and accessory breathing organs, which would tend to reduce branchial ammonia excretion. As ammonia is toxic, air-breathing fishes, especially amphibious ones, are equipped with various strategies to ameliorate ammonia toxicity during emersion or ammonia exposure. These strategies can be categorized into (1) enhancement of ammonia excretion and reduction of ammonia entry, (2) conversion of ammonia to a less toxic product for accumulation and subsequent excretion, (3) reduction of ammonia production and avoidance of ammonia accumulation and (4) tolerance of ammonia at cellular and tissue levels. Active ammonia excretion, operating in conjunction with lowering of ambient pH and reduction in branchial and cutaneous NH₃ permeability, is theoretically the most effective strategy to maintain low internal ammonia concentrations. NH₃ volatilization involves the alkalization of certain epithelial surfaces and requires mechanisms to prevent NH₃ back flux. Urea synthesis is an energy-intensive process and hence uncommon among air-breathing teleosts. Aestivating African lungfishes detoxify ammonia to urea and the accumulated urea is excreted following arousal. Reduction in ammonia production is achieved in some air-breathing fishes through suppression of amino acid catabolism and proteolysis, or through partial amino acid catabolism leading to alanine formation. Others can slow down ammonia accumulation through increased glutamine synthesis in the liver and muscle. Yet, some others develop high tolerance of ammonia at cellular and tissue levels, including tissues in the brain. In summary, the responses of air-breathing fishes to ameliorate ammonia toxicity are many and varied, determined by the behaviour of the species and the nature of the environment in which it lives.

Regulation of amino acid metabolism as a defensive strategy in the brain of three freshwater teleosts in response to high environmental ammonia exposure

Many teleosts have evolved mechanisms to cope with ammonia toxicity in the brain when confronted with high environmental ammonia (HEA). In the present study, the possible role of conversion of accumulated ammonia to glutamine and other free amino acids in the brain of three freshwater teleosts differing in their sensitivities to ammonia was investigated. The detoxification mode of ammonia in brain is suggested to be through amination of glutamate to glutamine by the coupled activities of glutamate dehydrogenase (GDH), transaminase (aspartate aminotransaminase 'AST' and alanine aminotransaminase 'ALT') and glutamine synthetase (GSase). We investigated the metabolic response of amino acids in the brain of highly sensitive salmonid Oncorhynchus mykiss (rainbow trout), the less sensitive cyprinid Cyprinus carpio (common carp) and the highly resistant cyprinid Carassius auratus (goldfish) when exposed to 1mM ammonia (as NH4HCO3; pH 7.9) for 0 h (control), 3 h, 12 h, 24 h, 48 h, 84 h and 180 h. Results show that HEA exposure increased ammonia accumulation significantly in the brain of all the three species from 12h onwards. Unlike in trout, ammonia accumulation in carp and goldfish was restored to control levels (48-84h); which was accompanied with a significant increase in glutamine content as well as GSase activity. In trout, glutamine levels also increased (84-180 h) but GSase was not activated. The elevated glutamine level in trout was accompanied by a significant depletion of the glutamate pool in contrast to the stable glutamate levels seen in carp and goldfish. This suggests a simultaneous increase in the rate of glutamate formation to match with the demand of glutamine formation in cyprinids. The activity of GDH was elevated significantly in carp and goldfish but remained unaltered in trout. Also, the transaminase enzymes (AST and ALT) were elevated significantly in exposed carp and goldfish while only ALT was up-regulated in trout. Consequently, in carp and goldfish both aspartate and alanine were utilized under HEA, whereas only alanine was consumed in trout. With ammonia treatment, significant changes in concentrations of other amino acids also occurred. None of the species could detoxify brain ammonia into urea. This study suggests that protective strategies to combat ammonia toxicity in brain are more pronounced in carp and goldfish than in trout.

Induction of four glutamine synthetase genes in brain of rainbow trout in response to elevated environmental ammonia

The key strategy for coping with elevated brain ammonia levels in vertebrates is the synthesis of glutamine from ammonia and glutamate, catalyzed by glutamine synthetase (GSase). We hypothesized that all four GSase isoforms (Onmy-GS01-GS04) are expressed in the brain of the ammonia-intolerant rainbow trout Oncorhynchus mykiss and that cerebral GSase is induced during ammonia stress. We measured GSase activity and the mRNA expression of Onmy-GS01-GS04 in fore-, mid- and hindbrain and liver, as well as ammonia concentrations in plasma, liver and brain of fish exposed to 9 or 48 h of 0 (control) or 670 micromol l(-1) NH(4)Cl (75% of the 96 h-LC(50) value). The mRNA of all four GSase isoforms were detected in brain (not liver). After 9 h of NH(4)Cl exposure, brain, liver and plasma ammonia content were elevated by two- to fourfold over control values. Midbrain, hindbrain and liver GSase activities were 1.3- to 1.5-fold higher in ammonia-exposed fish relative to control fish. Onmy-GS01-GS04 mRNA levels in brain (not liver) of ammonia-exposed fish (9 h) were significantly elevated by two- to fourfold over control values. After 48 h of the NH(4)Cl treatment, ammonia content and GSase activity, but not mRNA levels, in all tissues examined remained elevated compared to control fish. Taken together, these findings indicate that all four GSase isoforms are constitutively expressed in trout brain and are inducible under high external ammonia conditions. Moreover, elevation of GSase activities in fore-, mid- and hindbrain in response to environmental ammonia underlines the importance of brain GSase in the ammonia-stress response.

High ambient ammonia promotes growth in a ureogenic goby, Mugilogobius abei

Mugilogobius abei has the ability to produce large amounts of urea when exposed to high ambient ammonia. Despite this metabolically costly approach, and reports of growth inhibition effects of ammonia on fish, M. abei exposed to ammonia shows no adverse effects on growth. To investigate this observation the growth of M. abei was measured at room temperatures for 8 weeks at a constant ration level under solitary and grouped conditions, in 20% SW with or without (control) 2 mM NH(4)Cl. Furthermore, pituitary mRNA levels of growth hormone, oxygen consumption, incorporation of external (15)N-ammonia into amino acid and protein fractions as well as behavioral activities were also examined. The specific growth rates of ammonia-exposed fish under grouped condition over the 8 weeks were significantly higher than those of control, while those rates under solitary condition were not significantly different between the treatments. The pituitary of ammonia-exposed fish had higher growth hormone mRNA than in control fish. The use of (15)N isotope revealed that M. abei can actively use external ammonia as a supplementary nitrogen source. Oxygen consumption of ammonia-exposed fish was significantly lower than that of control fish. Locomotor activity and aggressive behavior under grouped condition were significantly reduced in ammonia-exposed fish as compared to those of control. These combined alterations in the ammonia-exposed fish may result in the higher growth rates.

The African Sharptooth CatfishClarias gariepinusCan Tolerate High Levels of Ammonia in Its Tissues and Organs during Four Days of Aerial Exposure

The African sharptooth catfish Clarias gariepinus lives in freshwater, is an obligatory air breather, and can survive on land during drought. The objective of this study was to elucidate how C. gariepinus defends against ammonia toxicity when exposed to terrestrial conditions. During 4 d of aerial exposure, there was no accumulation of urea in its tissues, and the rate of urea excretion remained low. Thus, exposure to terrestrial conditions for 4 d did not induce ureogenesis or ureotely in C. gariepinus. Volatilization of NH(3) was not involved in excreting ammonia during aerial exposure. In addition, there were no changes in levels of alanine in the muscle, liver, and plasma of C. gariepinus; nor were there any changes in the glutamine levels in these tissues. However, there were extraordinarily high levels of ammonia in the muscle (14 micromol g(-1)), liver (18 micromol g(-1)), and brain (11 micromol g(-1)) of fish exposed to terrestrial conditions for 4 d. This is the first report on a fish adopting high tolerance of ammonia in cells and tissues as the single major strategy to defend against ammonia toxicity during aerial exposure. At present, it is uncertain how C. gariepinus tolerates such high levels of ammonia, especially in its brain, but it can be concluded that, contrary to previous reports on two air-breathing catfishes (Clarias batrachus and Heteropneustes fossilis) from India, C. gariepinus does not detoxify ammonia to urea or free amino acids on land.

The African lungfish, Protopterus dolloi, detoxifies ammonia to urea during environmental ammonia exposure

The African lungfish, Protopterus dolloi, was able to maintain a low level of blood plasma ammonia during exposure to high concentrations of environmental ammonia. After 6 d of exposure to 30 or 100 mM NH(4)Cl, the total ammonia concentrations in the blood plasma were 0.288 and 0.289 mM, respectively, which were only 1.7-fold greater than the control value of 0.163 mM. In addition, accumulation of ammonia occurred only in the muscle, but not in the liver. This was achieved in part through urea synthesis, as reflected by significant increases in urea contents in the muscle, liver, and plasma of the experimental animals. In contrast with plasma ammonia, the plasma urea concentrations of specimens exposed to 30 or 100 mM NH(4)Cl for 6 d increased 15.4-fold and 18.8-fold, respectively. Taken together, these results suggest that P. dolloi upregulated the rate of urea synthesis to detoxify ammonia during environmental ammonia exposure and that the increased rate of urea synthesis was fast enough to compensate for the rate of endogenous ammonia production plus the net influx of exogenous ammonia in these experimental animals. Simultaneously, there were increases in the rates of urea excretion in the experimental animals between day 2 and day 6 of environmental ammonia exposure. Interestingly, the rates of urea excretion in specimens exposed to 100 mM NH(4)Cl were lower than those exposed to 30 mM NH(4)Cl, despite the presumably greater load of ammonia to be detoxified to urea in the former situation. It would appear that P. dolloi was regulating the rate of urea excretion during ammonia exposure to retain urea, which might have some physiological functions under environmental stresses yet to be determined. There were decreases in the contents of glutamate, glutamine, and total free amino acids in the liver of the experimental animals, which indirectly suggest that a reduction in the rate of proteolysis and/or amino acid catabolism would have occurred that might lead to a decrease in ammonia production. Our results suggest that, unlike marine elasmobranchs and coelacanths, which synthesize and retain urea for osmoregulatory purposes, the ureogenic P. dolloi was adapted to synthesizing and excreting urea for the purpose of ammonia detoxification.

Postprandial increases in nitrogenous excretion and urea synthesis in the giant mudskipper Periophthalmodon schlosseri

The objective of this study was to determine the effects of feeding on the excretory nitrogen (N) metabolism of the giant mudskipper, Periophthalmodon schlosseri, with special emphasis on the role of urea synthesis in ammonia detoxification. The ammonia and urea excretion rates of P. schlosseri increased 1.70- and 1.92-fold, respectively, within the first 3 h after feeding on guppies. Simultaneously, there were significant decreases in ammonia levels in the plasma and the brain, and in urea contents in the muscle and liver, of P. schlosseri at 3 h post-feeding. Thus, it can be concluded that P. schlosseri was capable of unloading ammonia originally present in some of its tissues in anticipation of ammonia released from the catabolism of excess amino acids after feeding. Subsequently, there were significant increases in urea content in the muscle, liver and plasma (1.39-, 2.17- and 1.62-fold, respectively) at 6 h post-feeding, and the rate of urea synthesis apparently increased 5.8-fold between 3 h and 6 h. Increased urea synthesis might have occurred in the liver of P. schlosseri because the greatest increase in urea content was observed therein. The excess urea accumulated in the body at 6 h was completely excreted between 6 and 12 h, and the percentage of waste-N excreted as urea-N increased significantly to 26% during this period, but never exceeded 50%, the criterion for ureotely, meaning that P. schlosseri remained ammonotelic after feeding. By 24 h, 62.7% of the N ingested by P. schlosseri was excreted, out of which 22.6% was excreted as urea-N. This is the first report on the involvement of increased urea synthesis and excretion in defense against ammonia toxicity in the giant mudskipper, and our results suggest that an ample supply of energy resources, e.g. after feeding, is a prerequisite for the induction of urea synthesis. Together, increases in nitrogenous excretion and urea synthesis after feeding effectively prevented a postprandial surge of ammonia in the plasma of P. schlosseri as reported previously for other fish species. Consequently, contrary to previous reports, there were significant decreases in the ammonia content of the brain of P. schlosseri throughout the 24 h period post-feeding, accompanied by a significant decrease in brain glutamine content between 12 h and 24 h.

Strategies for surviving high concentrations of environmental ammonia in the swamp eel Monopterus albus

The swamp eel Monopterus albus lives in muddy ponds, swamps, canals, and rice fields in the tropics. It encounters high concentrations of environmental ammonia (HEA) during dry seasons or during agricultural fertilization in rice fields. This study aimed at determining the tolerance of M. albus to environmental ammonia and at elucidating the strategies that it adopts to defend against ammonia toxicity in HEA. In the laboratory, M. albus exhibited very high environmental ammonia tolerance; the 48-, 72-, and 96-h median lethal concentrations of total ammonia at pH 7.0 and 28 degrees C were 209.9, 198.7, and 193.2 mM, respectively. It was apparently incapable of actively excreting ammonia against a concentration gradient. In addition, it did not detoxify ammonia to urea, the excretion of which would lead to a loss of nitrogen and carbon, during ammonia loading. The high tolerance of M. albus to HEA was attributable partially to its exceptionally high tolerance to ammonia at the cellular and subcellular levels. During the 144 h of exposure to 75 mM NH(4)Cl at pH 7.0, the ammonia contents in the muscle, liver, brain, and gut of M. albus reached 11.49, 15.18, 6.48, and 7.51 mu mol g(-1), respectively. Such a capability allowed the accumulation of high concentrations of ammonia in the plasma (3.54 mu mol mL(-1)) of M. albus exposed to HEA, which would reduce the net influx of exogenous ammonia. Subsequent to the buildup of internal ammonia levels, M. albus detoxified ammonia produced endogenously to glutamine. The glutamine contents in the muscle and liver reached 10.84 and 17.06 mu mol g(-1), respectively, after 144 h of exposure to HEA, which happened to be the highest known for fish. Unlike urea, the storage of glutamine in the muscle during ammonia loading allowed its usage for anabolic purposes when the adverse environmental condition subsides. Glutamine synthetase activity increased significantly in the liver and gut (2.8- and 1.5-fold, respectively) of specimens exposed to HEA for 144 h. These results suggest that the liver was the main site of ammonia detoxification and the gut was more than a digestive/absorptive organ in M. albus. Monopterus albus did not undergo a reduction in amino acid catabolism during the first 24 h of ammonia exposure. However, assuming a total inhibition of excretion of endogenous ammonia, there was a deficit of -312 mu mol N between the reduction in nitrogenous excretion (3,360 mu mol N) and the retention of nitrogen (3,048 mu mol N) after 72 h of aerial exposure. The deficit became much greater after 144 h, reaching a value of -3,243 mu mol N. These results suggest that endogenous ammonia production in M. albus was suppressed in order to prevent the newly established internal steady state concentration of ammonia from rising to an intolerable level after an extended period of exposure to HEA.

African sharptooth catfish Clarias gariepinus does not detoxify ammonia to urea or amino acids but actively excretes ammonia during exposure to environmental ammonia

The African sharptooth catfish Clarias gariepinus lives in freshwater, is an obligatory air breather, and exhibits high tolerance of environmental ammonia. This study aimed at elucidating the strategies adopted by C. gariepinus to defend against ammonia toxicity during ammonia exposure. No carbamoyl phosphate synthetase (CPS) I or III activities were detected in the liver or muscle of the adult C. gariepinus. In addition, activities of other ornithine-urea cycle (OUC) enzymes, especially ornithine transcarbamylase, were low in the liver, indicating that adult C. gariepinus does not have a "functional" hepatic OUC. After being exposed to 50 or 100 mM NH4Cl for 5 d, there was no induction of hepatic OUC enzymes and no accumulation of urea in tissues of the experimental animals. In addition, the rate of urea excretion remained low and unchanged. Hence, ammonia exposure did not induce ureogenesis or ureotely in C. gariepinus as suggested elsewhere for another obligatory air-breathing catfish of the same genus, Clarias batrachus, from India. Surprisingly, the local C. batrachus did not possess any detectable CPS I or III activities in the liver or muscle as had been reported for the Indian counterpart. There were no changes in levels of alanine in the muscle, liver, and plasma of C. gariepinus exposed to 50 or 100 mM NH4Cl for 5 d; neither were there any changes in the glutamine levels in these tissues. Yet even after being exposed to 100 mM NH4Cl for 5 d, there was no significant increase in the level of ammonia in the muscle, which constitutes the bulk of the specimen. In addition, the level of ammonia accumulated in the plasma was relatively low compared to other tropical air-breathing fishes. More importantly, for all NH4Cl concentrations tested (10, 50, or 100 mM), the plasma ammonia level was maintained relatively constant (2.2-2.4 mM). These results suggest that C. gariepinus was able to excrete endogenous ammonia and infiltrated exogenous ammonia against a very steep ammonia gradient. When exposed to freshwater (pH 7.0) with or without 10 mM NH4Cl, C. gariepinus was able to excrete ammonia continuously to the external medium for at least 72 h. This was achieved while the plasma NH4+ and NH3 concentrations were significantly lower than those of the external medium. Diffusion trapping of NH3 through boundary layer acidification can be eliminated as the pH of the external medium became more alkaline instead. These results represent the first report on a freshwater fish (C. gariepinus) adopting active excretion of ammonia (probably NH4+) as a major strategy to defend against ammonia toxicity when exposed to environmental ammonia.

Effects of peritoneal injection of NH4HCO3 on nitrogen excretion and metabolism in the swamp eel Monopterus albus-- increased ammonia excretion with an induction of glutamine synthetase activity

Monopterus albus has to deal with high environmental ammonia concentrations during dry seasons and agricultural fertilization in rice fields. In this study, NH4HCO3 (10 micromol per g fish) was injected into the peritoneal cavity of M. albus, raising the level of ammonia in the body, in order to elucidate the strategies involved in defense against the toxicity of exogenous ammonia. During the subsequent 24 h after NH4HCO3 injection, there was a significant increase in the ammonia excretion rate, which indicates that the main strategy adopted by M. albus was to remove the majority of the exogenous ammonia through enhanced ammonia excretion. Exogenous ammonia was not detoxified into urea for excretion or accumulation. Six hours post-injection of NH4HCO3, ammonia content in the tissues built up significantly, especially in the brain, which suggests that M. albus had high tolerance of ammonia toxicity at the cellular and sub-cellular levels. By hour 12 post-injection, there were significant increases in the activities of glutamine synthetase in the muscle, liver, and gut, accompanied by significant increases in glutamine contents in the muscle and the liver. There was also a significant increase in the glutamine content in the brain at hour 6 post-injection of NH4HCO3. These results confirm the capability of M. albus to detoxify ammonia through glutamine synthesis. Overall, injection of NH4HCO3 had only minor effects on the contents of FAAs, other than glutamine, in tissues of M. albus because the majority (70%) of the injected ammonia was excreted within the 24-h period.

Role of ureogenesis in tackling problems of ammonia toxicity during exposure to higher ambient ammonia in the air-breathing walking catfishclarias batrachus

In the present study, the possible role of ureogenesis to avoid the accumulation of toxic ammonia to a lethal level under hyper-ammonia stress was tested in the air-breathing walking catfish Clarias batrachus by exposing the fish at 25 mM NH4Cl for 7 days. Excretion of ammonia by the NH4Cl-exposed fish was totally suppressed, which was accompanied by significant accumulation of ammonia in different body tissues. The walking catfish, which is otherwise predominantly ammoniotelic, turned totally towards ureotelism from ammoniotelism with a 5- to 6-fold increase of urea-N excretion during exposure to higher ambient ammonia. Stimulation of ureogenesis was accompanied with significant increase of some of the key urea cycle enzymes such as carbamyl phosphate synthetase (urea cycle-related), argininosuccinate synthetase and argininosuccinate lyase both in hepatic and non-hepatic tissues. Due to this unique physiological strategy of turning towards ureotelism from ammoniotelism via the induced urea cycle, this air-breathing catfish is able to survive in very high ambient ammonia, which they face in certain seasons of the year in the natural habitat.

A comparison of the effects of environmental ammonia exposure on the Asian freshwater stingray Himantura signifer and the Amazonian freshwater stingray Potamotrygon motoro

The white-edge whip tail ray Himantura signifer inhabits a freshwater environment but has retained the capability to synthesize urea de novo through the arginine-ornithine-urea cycle (OUC). The present study aimed to elucidate whether the capacity of urea synthesis in H. signifer could be upregulated in response to environmental ammonia exposure. When H. signifer was exposed to environmental ammonia, fairly high concentrations of ammonia were accumulated in the plasma and other tissues. This would subsequently reduce the net influx of exogenous ammonia by reducing the NH(3) partial pressure gradient across the branchial and body surfaces. There was also an increase in the OUC capacity in the liver. Since the ammonia produced endogenously could not be excreted effectively in the presence of environmental ammonia, it was detoxified into urea through the OUC. In comparison, the South American freshwater stingray Potamotrygon motoro, which has lost the capability to synthesize urea de novo, was unable to detoxify ammonia to urea during ammonia loading. No increase in glutamine was observed in the various tissues of H. signifer exposed to environmental ammonia despite a significant increase in the hepatic glutamine synthetase activity. These results indicate that the excess glutamine formed was channelled completely into urea formation through carbamoyl phosphate synthetase III. It has been reported elsewhere that both urea synthesis and urea retention were upregulated in H. signifer exposed to 20 per thousand water for osmoregulatory purposes. By contrast, for H. signifer exposed to environmental ammonia in freshwater, the excess urea formed was excreted to the external medium instead. This suggests that the effectiveness of urea synthesis de novo as a strategy to detoxify ammonia is determined not simply by an increase in the capacity of urea synthesis but, more importantly, by the ability of the animal to control the direction (i.e. absorption or excretion) and rate of urea transport. Our results suggest that such a strategy began to develop in those elasmobranchs, e.g. H. signifer, that migrate into a freshwater environment from the sea but not in those permanently adapted to a freshwater environment.

Unusual physiology of scale-less carp, Gymnocypris przewalskii, in Lake Qinghai: a high altitude alkaline saline lake

The scale-less carp (Gymnocypris przewalskii) inhabits Lake Qinghai located on the Qinghai-Tibet plateau (elevation, 3200 m) in western China. The lake waters are alkaline (pH 9.4, titratable alkalinity=30 mmol l(-1)), Mg(2+)-rich (18.7 mmol l(-1)), Ca(2+)-poor (0.30 mmol l(-1)) and saline (9 per thousand ). These fish make annual spawning migrations into freshwater rivers. We investigated the physiology of nitrogen excretion and ionoregulation of fish from the lake and river. Fish from both waters were ammonotelic, although ammonia-N excretion rates were lower in lake fish (175 vs. 344 micromol kg(-1) h(-1), P<0.05) resulting in unusually high levels of ammonia in blood plasma (2.23 vs. 0.32 mmol l(-1)), bile, liver, muscle and brain. Exposure to 0.4 mmol l(-1) total ammonia in lake water ([NH(3)]=0.16 mmol l(-1)) killed fish within 8 h. River fish survived exposure to 1.0 mmol l(-1) total ammonia in river water at pH 8.0 ([NH(3)]=0.023 mmol l(-1)) for 24 h suggesting high ammonia tolerance in lake fish. High glutamate dehydrogenase and glutamine synthetase activities in tissues probably allow the fish to alleviate ammonia toxicity by amino acid accumulation. Neither lake nor river fish relied on urea excretion to remove excess N. Urea-N excretion rates were below 20 micromol kg(-1) h(-1) for both groups, and levels of urea in plasma and tissues were moderate. When exposed to elevated ammonia, urea-N excretion increased slightly (approximately 50 micromol kg(-1) h(-1)) and liver and muscle urea levels increased in the river fish. Plasma ion levels were within the range typical of cyprinids, but river fish had significantly higher plasma [Na(+)] and [Cl(-)] and lower [K(+)] than fish from the lake. During 48-h lake-to-river water transfer, plasma Na(+) and Cl(-) levels rose significantly. Significantly higher Na(+)/K(+)-ATPase activity in the gills of river fish may be related to the higher plasma ion levels. Plasma [Mg(2+)] and [Ca(2+)] were tightly regulated despite the great differences in the lake and river water levels.

Role of amino acid metabolism in an air-breathing catfish, Clarias batrachus in response to exposure to a high concentration of exogenous ammonia

The air-breathing ureogenic walking catfish (Clarias batrachus) faces various environmental constraints throughout the year leading to the problem of accumulation of toxic ammonia. In the present study, the possible role of conversion of accumulated ammonia to various non-essential free amino acids (FAAs) was tested in this fish under hyper-ammonia stress caused by exposing the fish at 25 mM NH(4)Cl for 7 days. Significant accumulation of ammonia of approximately two- to threefold was observed in different tissues (except in the brain), which was accompanied with the significant accumulation of non-essential FAAs in the NH(4)Cl-exposed fish. There was approximately two- to threefold increase of non-essential FAAs in different tissues and in the plasma of the NH(4)Cl-exposed fish compared to the control fish after 7 days of exposure, which was mainly attributable to the increase of Asp, Ala, Gly, Glu, Gln and taurine (Tau) concentrations in general, with certain tissue-specific variations. This was also accompanied with significant increase of activity of certain amino acid metabolism-related enzymes such as the glutamine synthetase (approx. two- to threefold), glutamate dehydrogenase (ammonia utilizing direction) (approx. twofold), aspartate and alanine aminotransaminases (approx. twofold) mainly in the liver, kidney and muscle of the NH(4)Cl-exposed fish. Thus, it appears that the walking catfish has the capacity of active conversion of accumulated ammonia to non-essential FAAs under condition of high concentrations of external ammonia. However, the increase of urea excretion rate due to active conversion of ammonia to urea via the induced urea cycle appears to be quantitatively much more important pathway than the increase of tissue levels of FAAs in dealing with a severe ammonia load.

Swimming and ammonia toxicity in salmonids: the effect of sub lethal ammonia exposure on the swimming performance of coho salmon and the acute toxicity of ammonia in swimming and resting rainbow trout

This study tested the hypothesis that swimming exacerbates ammonia toxicity in fish. Both sub-lethal and acute toxicity testing was conducted in a swim tunnel on swimming and resting coho salmon and rainbow trout, respectively. The sub lethal tests on coho salmon also considered the compartmentalization of ammonia within the fish. Coho salmon showed a significant linear decrease in U(crit) both with increasing water ammonia (0, 0.02, 0.04 and 0.08 mg per l NH3) and increasing plasma ammonia. Data collected included plasma pH and ammonia, muscle pH and ammonia and muscle membrane potential. Based on results found in these experiments it was concluded that the reduction in swimming performance was due to both metabolic challenges as well as depolarization of white muscle. Acute toxicity testing on swimming and resting rainbow trout revealed that swimming at (60% U(crit) or approximately 2.2 body lengths/s) decreased the LC50 level from 207+/-21.99 mg N per l in resting fish to 32.38+/-10.81. The LC50 for resting fish was significantly higher than that for swimming fish. The acute value set forth by the US EPA at the same pH is 36.1 mg N per l and may not protect swimming fish. In addition the effect of water hardness on ammonia toxicity was considered. It was found that increased water calcium ameliorates ammonia toxicity in fish living in high pH water.

Glutamine synthetase in brain: effect of ammonia

Glutamine synthetase (GS) in brain is located mainly in astrocytes. One of the primary roles of astrocytes is to protect neurons against excitotoxicity by taking up excess ammonia and glutamate and converting it into glutamine via the enzyme GS. Changes in GS expression may reflect changes in astroglial function, which can affect neuronal functions. Hyperammonemia is an important factor responsible of hepatic encephalopathy (HE) and causes astroglial swelling. Hyperammonemia can be experimentally induced and an adaptive astroglial response to high levels of ammonia and glutamate seems to occur in long-term studies. In hyperammonemic states, astroglial cells can experience morphological changes that may alter different astrocyte functions, such as protein synthesis or neurotransmitters uptake. One of the observed changes is the increase in the GS expression in astrocytes located in glutamatergic areas. The induction of GS expression in these specific areas would balance the increased ammonia and glutamate uptake and protect against neuronal degeneration, whereas, decrease of GS expression in non-glutamatergic areas could disrupt the neuron-glial metabolic interactions as a consequence of hyperammonemia. Induction of GS has been described in astrocytes in response to the action of glutamate on active glutamate receptors. The over-stimulation of glutamate receptors may also favour nitric oxide (NO) formation by activation of NO synthase (NOS), and NO has been implicated in the pathogenesis of several CNS diseases. Hyperammonemia could induce the formation of inducible NOS in astroglial cells, with the consequent NO formation, deactivation of GS and dawn-regulation of glutamate uptake. However, in glutamatergic areas, the distribution of both glial glutamate receptors and glial glutamate transporters parallels the GS location, suggesting a functional coupling between glutamate uptake and degradation by glutamate transporters and GS to attenuate brain injury in these areas. In hyperammonemia, the astroglial cells located in proximity to blood-vessels in glutamatergic areas show increased GS protein content in their perivascular processes. Since ammonia freely crosses the blood-brain barrier (BBB) and astrocytes are responsible for maintaining the BBB, the presence of GS in the perivascular processes could produce a rapid glutamine synthesis to be released into blood. It could, therefore, prevent the entry of high amounts of ammonia from circulation to attenuate neurotoxicity. The changes in the distribution of this critical enzyme suggests that the glutamate-glutamine cycle may be differentially impaired in hyperammonemic states.

The effect of sub-lethal ammonia exposure on fed and unfed rainbow trout: the role of glutamine in regulation of ammonia

Many species of fishes have evolved mechanisms for coping with ammonia caused by either high ammonia environments or an inability to excrete nitrogenous wastes. Rainbow trout (Oncorhynchus mykiss), have not been known to have such a mechanism. The present study investigated whether rainbow trout can use amino acid synthesis and storage to cope with ammonia. Experiments were performed on fed and unfed rainbow trout under both control and elevated ammonia conditions (0 and 10 mgN/l (total ammonia nitrogen), pH 7.2). The results indicate that both feeding and ammonia exposure increased plasma ammonia significantly 6 h postprandial and post ammonia exposure. After 48 h the fed/ammonia exposed fish had plasma ammonia levels that were not significantly different than the fed/control fish. Plasma ammonia was reduced by more than 50%, attributable to ammonia being converted to glutamine in brain, liver and muscle tissue. Feeding alone also increased glutamine levels in brain tissue. Activity of glutamine synthetase in brain and liver was increased corresponding to an increase in glutamine concentrations when fish were exposed to ammonia. This is the first report showing that rainbow trout can detoxify endogenous and exogenous ammonia.

Nitrogen metabolism and excretion in the mangrove killifish Rivulus marmoratus I. The influence of environmental salinity and external ammonia

At a field site in Belize, mangrove killifish Rivulus marmoratus inhabit hypersaline waters (up to 48 ‰) containing approximately 1 mmol l–1 ammonia. We tested the hypotheses that R. marmoratus modify their nitrogen metabolism and excretion (i) by accumulating free amino acids (FAAs) and urea in the tissues during hyperosmotic stress and (ii) by shifting to ureotelism and accumulating FAAs during hyperammonia stress. Urea excretion (JUrea) (but not ammonia excretion, JAmm) displayed a diurnal pattern, with significantly less (75 %) urea excreted at night than during the day in both laboratory-reared clones and wild-caught killifish. When fish were exposed to hypersaline conditions (45 ‰ sea water), JUrea was significantly reduced and tissue urea and FAA levels were elevated compared with those of control fish (15 ‰ sea water). When R. marmoratus were exposed to 0, 1, 2, 5 and 10 mmol l–1 NH4Cl (pH 8) for 48 h, no differences were found in JUrea. Remarkably, prolonged exposure (10 days) to 5 mmol l–1 NH4Cl (pH 8) did not result in an elevation of tissue ammonia levels. In addition, tissue urea and total FAA levels did not differ between control and ammonia-exposed fish after ⩾4 days. We propose that the euryhaline R. marmoratus retain urea and FAAs within their tissues in response to extreme osmotic stress. In contrast to many ammonia-tolerant fishes, R. marmoratus do not shift to ureotelism during prolonged hyperammonia stress, nor do they convert nitrogenous wastes into FAAs. The data suggest that killifish continue to eliminate ammonia despite an unfavourable blood-to-water gradient, thereby avoiding accumulation of ammonia.

Ammonia detoxification and localization of urea cycle enzyme activity in embryos of the rainbow trout (Oncorhynchus mykiss) in relation to early tolerance to high environmental ammonia levels

The present study investigated the role of ammonia as a trigger for hatching, mechanisms of ammonia detoxification and the localization of urea cycle enzymes in the early life stages of freshwater rainbow trout (Oncorhynchus mykiss). The key urea cycle enzyme carbamoyl phosphate synthetase III was found exclusively in the embryonic body (non-hepatic tissues); related enzymes were distributed between the liver and embryonic body. ‘Eyed-up’ trout embryos were exposed either acutely (2h) to 10mmoll−1 NH4Cl or chronically (4 days) to 0.2mmoll−1 NH4Cl. Time to hatching was not affected by either acute or chronic NH4Cl exposure. Urea levels, but not ammonia levels in the embryonic tissues, were significantly higher than in controls after both acute and chronic NH4Cl exposure, whereas there were no significant changes in urea cycle enzyme activities. Total amino acid levels in the embryonic tissues were unaltered by chronic ammonia exposure, but levels of most individual amino acids and total amino acid levels in the yolk were significantly lower (by 34–58%) than in non-exposed controls. The data indicate that trout embryos have an efficient system to prevent ammonia accumulation in embryonic tissue, by conversion of ammonia to urea in embryonic tissues and through elevation of ammonia levels in the yolk.

Reduction in the rates of protein and amino acid catabolism to slow down the accumulation of endogenous ammonia: a strategy potentially adopted by mudskippers (Periophthalmodon schlosseri snd Boleophthalmus boddaerti) during aerial exposure in constant darkness

This study was designed to elucidate the strategies adopted by mudskippers to handle endogenous ammonia during aerial exposure in constant darkness. Under these conditions, specimens exhibited minimal locomotory activity, and the ammonia and urea excretion rates in both Periophthalmodon schlosseri and Boleophthalmus boddaerti decreased significantly. As a consequence, ammonia accumulation occurred in the tissues of both species of mudskipper. A significant increase in urea levels was found in the liver of P. schlosseri after 24h of aerial exposure, but no similar increase was seen in the tissues of B. boddaerti. It is unlikely that these two species of mudskipper detoxified ammonia to urea during aerial exposure since B. boddaerti does not possess a complete ornithine-urea cycle (OUC) and, although all the OUC enzymes were present in P. schlosseri, the activity of carbamoyl phosphate synthetase present in the liver mitochondria was too low to render the OUC functional for ammonia detoxification. Peritoneal injection of 15NH4Cl into P. schlosseri showed that this mudskipper was capable of incorporating some of the labelled ammonia into urea in its liver. However, aerial exposure did not affect this capability and did not induce detoxification of the accumulated ammonia to urea. Mudskippers exposed to terrestrial conditions and constant darkness did, however, show significant decreases in the total free amino acid content in the liver and blood, in the case of P. schlosseri and in the muscle of B. boddaerti. No changes in the alanine or glutamine content of the muscle were found in either species. Analyses of the balance between the reduction in nitrogenous excretion and the increase in nitrogenous accumulation further revealed that these two species of mudskipper were capable of reducing their protein and amino acid catabolic rates. Such adaptations constitute the most efficient way to avoid the build-up of internal ammonia, and would render unnecessary the detoxification of ammonia through energetically expensive pathways. This finding may be the first report of a teleost fish showing a reduction in proteolysis and amino acid catabolism in response to aerial exposure.

Functional ureogenesis in the gobiid fish Mugilogobius abei

To examine the transition to ureogenesis, the gobiid fish Mugilogobius abei was immersed in 2 mmol l(−)(1) NH(4)HCO(3) or a (15)N-labelled ammonia solution [1 mmol l(−)(1) ((15)NH(4))(2)SO(4), pH 8.0] for 4–8 days. When exposed to 2 mmol l(−)(1) NH(4)HCO(3) or (15)N-labelled ammonia solution for 4 days, the rate of urea excretion increased to seven times that of the control (in 20 % synthetic sea water) and remained at this level for 4 days. The proportion of nitrogen excreted as urea reached 62 % of total nitrogen excretion (ammonia-N + urea-N). (15)N-enrichment of the amide-N in glutamine in the tissues of fish exposed to (15)N-labelled ammonia was virtually the same as that of ammonia-N: i.e. approximately twice that of urea-N in the excreta and the tissues. Glutamine contents and glutamine synthetase activities in the liver and muscle increased greatly following exposure to ammonia. Urea and citrulline contents in the muscle and whole body of the exposed fish increased significantly, whereas uric acid contents remained unchanged. Carbamoyl phosphate synthetase III (CPSase III) mRNA expression and CPSase III activity were detected in the muscle, skin and gill, but levels were negligible in the liver. Furthermore, all other ornithine-urea cycle (O-UC) enzymes were also detected in muscle, skin and gill. Thus, M. abei clearly shows the transition from ammoniotely to ureotely under ammonia-loading condition and is able to produce urea mainly via the O-UC operating in multiple non-hepatic tissues as a means for ammonia detoxification.

High ammonia tolerance in fishes of the family Batrachoididae (Toadfish and Midshipmen)

Three fish species of the family Batrachoididae, the gulf toadfish (Opsanus beta), the oyster toadfish (Opsanus tau), and the plainfin midshipman (Porichthys notatus) demonstrated exceptionally high tolerances to elevated water ammonia with 96-h LC50 values of 9.75, 19.72 and 6 mM total ammonia, respectively. Using pH values we calculated the corresponding unionized ammonia (NH(3)) values to be 519, 691 and 101 µM, respectively. These values are well above typical values for most teleost fishes, but close to those of ureotelic fish examined to date. Following sublethal high ammonia exposure (HAE) blood and tissue (brain, liver and muscle) sampling confirmed that internal ammonia levels rose substantially in all three species, suggesting that they were not simply avoiding toxicity by impermeance to ammonia. The three species of batrachoidids can be characterized in the following manner with respect to the inabilities to synthesize and excrete urea, based on these studies and prior research: O. beta (fully ureotelic)>O. tau (moderately ureotelic)>P. notatus (ammoniotelic). While some of the high ammonia tolerance for O. beta and O. tau can be explained by their ability to detoxify it to urea, other mechanisms must be at play for P. notatus. Further experiments determined that all three species possess rather high activities of glutamine synthetase (GSase) in brain especially (60-180 U g(-1)), that glutamine accumulates in many tissues, and that LC50 values are correlated positively with brain GSase activity. Taken together, our results suggest that alternative/additional mechanisms for ammonia detoxification via urea synthesis must be considered to explain the exceptionally high ammonia tolerance of this group.

Immunolocalization of ion-transport proteins to branchial epithelium mitochondria-rich cells in the mudskipper (Periophthalmodon schlosseri)

The branchial epithelium of the mudskipper Periophthalmodon schlosseri is densely packed with mitochondria-rich (MR) cells. This species of mudskipper is also able to eliminate ammonia against large inward gradients and to tolerate extremely high environmental ammonia concentrations. To test whether these branchial MR cells are the sites of active ammonia elimination, we used an immunological approach to localize ion-transport proteins that have been shown pharmacologically to be involved in the elimination of NH(4)(+) (Na(+)/NH(4)(+) exchanger and Na(+)/NH(4)(+)-ATPase). We also investigated the role of carbonic anhydrase and boundary-layer pH effects in ammonia elimination by using the carbonic anhydrase inhibitor acetazolamide and by buffering the bath water with Hepes, respectively. In the branchial epithelium, Na(+)/H(+) exchangers (both NHE2- and NHE3-like isoforms), a cystic fibrosis transmembrane regulator (CFTR)-like anion channel, a vacuolar-type H(+)-ATPase (V-ATPase) and carbonic anhydrase immunoreactivity are associated with the apical crypt region of MR cells. Associated with the MR cell basolateral membrane and tubular system are the Na(+)/K(+)-ATPase and a Na(+)/K(+)/2Cl(−) cotransporter. A proportion of the ammonia eliminated by P. schlosseri involves carbonic anhydrase activity and is not dependent on boundary-layer pH effects. The apical CFTR-like anion channel may be serving as a HCO(3)(−) channel accounting for the acid-base neutral effects observed with net ammonia efflux inhibition.

Changes in free amino acid synthesis in the perfused liver of an air-breathing walking catfish, Clarias batrachus infused with ammonium chloride: a strategy to adapt under hyperammonia stress

The changes in the free amino acid (FAA) levels, the rate of efflux of FAAs from the perfused liver, and the activity of some enzymes related to amino acid metabolism such as glutamate dehydrogenase (GDH, both reductive amination and oxidative deamination), glutamine synthetase (GS), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) were studied in the liver of a freshwater air-breathing teleost, the walking catfish, Clarias batrachus, perfused with 5 and 10 mM NH(4)Cl. The level of the various non-essential FAAs increased significantly, with a total increase of about 150%, which was accompanied by a significant increase of both ammonia and urea-N in the perfused liver both with 5 and 10 mM NH(4)Cl. The rate of efflux of these non-essential FAAs from the perfused liver also increased significantly with a total increase of about 115% and 160% at 5 and 10 mM NH(4)Cl, respectively. The activity of the mentioned amino acid metabolism-related enzymes in the perfused liver also got stimulated, except for GDH in the ammonia forming direction and ALT, under a higher ammonia load. The activity (both tissue and specific) of GDH in the glutamate forming direction increased maximally, followed by AST and GS in a decreasing order. Owing to these physiological adaptive strategies related to amino acid metabolism along with the presence of a functional and regulatory urea cycle (reported earlier), it is believed that this catfish is able to survive in very high ambient ammonia or in the air or in the mud during habitat drying.

The mudskipper, Periophthalmodon schlosseri, actively transports NH4+ against a concentration gradient

Periophthalmodon schlosseri can maintain ammonia excretion rates and low levels of ammonia in its tissues when exposed to 8 and 30 mM NH4Cl, but tissue ammonia levels rise when the fish is exposed to 100 mM NH4Cl in 50% seawater. Because the transepithelial potential is not high enough to maintain the NH4+ concentration gradient between blood and water, ammonia excretion under such a condition would appear to be active. Branchial Na+-K+-ATPase activity is very high and can be activated by physiological levels of NH4+ instead of K+. Ammonia excretion by the fish against a concentration gradient is inhibited by the addition of ouabain and amiloride to the external medium. It is concluded that Na+-K+-ATPase and an Na+/H+ exchanger may be involved in the active excretion of ammonia across the gills. This unique ability of P. schlosseri to actively excrete ammonia is related to the special structure of its gills and allows the fish to continue to excrete ammonia while air exposed or in its burrow.

Increases in tissue free amino acid levels in response to prolonged emersion in marine crabs: an ammonia-detoxifying process efficient in the intertidal Carcinus maenas but not in the subtidal Necora puber

Carcinus maenas and Necora puber were exposed to air for 72 h and 18 h, respectively, at 18 C. Changes in the free amino acid (FAA) content of their muscle, hepatopancreas and haemolymph were recorded during air-exposure and subsequent reimmersion. Muscle and hepatopancreas urate contents and haemolymph serum protein levels were also measured during emersion. In air-exposed C. maenas, the muscle FAA pool increased significantly within the first 24 h of emersion. This increase was due to an increase in the non-essential amino acid (NEAA) pool only; the essential amino acid (EAA) pool did not change. In haemolymph, the EAA pool decreased during the first 24 h of emersion, whereas the FAA and NEAA pools did not change. However, in this compartment, glutamine levels increased throughout the air-exposure period. No significant changes in FAA, NEAA and EAA contents of the hepatopancreas were observed during the 72 h emersion. In air-exposed N. puber, the FAA pools of muscle and hepatopancreas did not change, although changes in the levels of some amino acids were observed during the 18 h emersion period. In this species, large increases in both the NEAA and EAA pools in the haemolymph were recorded. High levels of urate were observed in the muscle and hepatopancreas of immersed N. puber, but no significant changes occurred during emersion. In contrast, immersed C. maenas exhibited low levels of urate in both compartments, and hepatopancreas urate levels increased slightly during emersion. Haemolymph protein content did not change in air-exposed N. puber, whereas it increased in the haemolymph of 72 h emersed C. maenas. The origin of newly formed NEAAs and their role in ammonia detoxification, particularly in C. maenas, which is able to regulate its internal ammonia levels during such a prolonged emersion, are discussed.

Urea production and transport in teleost fishes

Teleosts appear to have retained the genes for the urea cycle enzymes. A few species express the full complement of enzymes and are ureotelic (e.g., Lake Magadi tilapia) or ammoniotelic (e.g., largemouth bass), whereas most species have low or non-detectable enzyme activities in liver tissue and excrete little urea (e.g., adult rainbow trout). It was surprising, therefore, to find the expression of four urea cycle enzymes during early life stages of rainbow trout. The urea cycle may play a role in ammonia detoxification during a critical time of development. Exposure to alkaline water (pH 9.0-9.5) or NH4Cl (0.2 mmol/l) increased urea excretion by several-fold in trout embryos, free embryos and alevin. Urea transport is either by passive simple diffusion or via carried-mediated transport proteins. Molecular studies have revealed that a specialised urea transport protein is present in kidney tissue of elasmobranchs, similar to the facilitated urea transporter found in the mammalian inner medulla of the kidney.

Effect of air exposure on nitrogen metabolism in the crab Cancer pagurus

In C. pagurus exposed to air for 18 h, blood ammonia content decreased within the 2 first hours, then increased at a relatively constant rate (25 microM/h); blood urate content increased at a lower rate (10 microM/h) and a classical blood acidosis was observed. In the cheliped muscle, a transient 22% decrease in GDH activity for ammonia formation and a 48% increase in GDH activity in the reverse reaction (glutamate synthesis) occurred following 6 and 12 h of emersion, respectively. Changes in LDH activity, used as an indicator of anaerobic potential of muscle, were not observed, except for an 18% increase in crabs exposed to air for 24 h. The increase in blood urate content, not known as a response to emersion in decapods, appeared to be different from that observed in response to hypoxia. The relatively low blood ammonia overload and the GDH increased activity for glutamate synthesis suggested that part of the produced ammonia was stored under a bound form in some tissues. The response of C. pagurus to air exposure is discussed on account of the Storey and Storey ('90) theory.