Muscle as the primary site of urea cycle enzyme activity in an alkaline lake-adapted tilapia, Oreochromis alcalicus grahami

Trichinella: Becoming a parasite

Phylogenetic evidence indicates that free-living nematodes gave rise to parasitic nematodes where parasitism evolved independently at least 15 times. The high level of genetic and biological diversity among parasites dictates an equally high level of diversity in the transition to parasitism. We previously hypothesized that horizontal gene transfer (HGT) played an important role in the evolution of parasitism among early ancestors of Trichinella, mediated by an interplay of ecological and evolutionary pathways that contributed to persistence and diversification. We propose that host selection may have been associated with the metabolism of ammonia and engender a new paradigm whereby the reprogrammed nurse cell is capable of generating cyanate thereby enabling the importance of the Trichinella cyanase in the longevity of the cell. Parasites and parasitism have revealed considerable resilience against a backdrop of climate change and environmental perturbation. Here we provide a putative link between key periods in the evolution of Trichinella and major geological and climatological events dating back 500 million years. A useful lens for exploring such ideas, the Stockholm Paradigm, integrates Ecological Fitting (a foundation for host colonization and diversification), the Oscillation Hypothesis (recurring shifts between trends in generalization and specialization relative to host range), the Geographic Mosaic Theory of Coevolution (microevolutionary co-adaptive processes), and the Taxon Pulse Hypothesis (alternating events of biotic expansion i.e., exploitation in evolutionary and ecological time). Here we examine how one or more of these interactive theories, in a phylogenetic-historical context and in conjunction with HGT, may help explain the scope and depth of diversity among Trichinella genotypes.

Analysis of metabolite differences between South Korean and Chinese yellow goosefish (Lophius litulon) using capillary electrophoresis time-of-flight mass spectrometry

The yellow goosefish is a benthic fish that belongs to the family Lophiidae and order Lophiiformes and is distributed in the Yellow and East China Seas. This study aimed to distinguish between yellow goosefish from different geographical origins by analyzing their metabolites. Capillary electrophoresis time-of-flight mass spectrometry was used to analyze metabolite profiles in the muscle tissues of yellow goosefish to distinguish between Korean and Chinese yellow goosefish. In total, 271 putative metabolites were extracted using 50% acetonitrile in water. Principal component analysis and orthogonal partial least squares discriminant analysis (OPLS-DA) were used to distinguish different geographical origins using the metabolite profiles obtained. The R2 and Q2 values of the OPLS-DA model were 0.856 and 0.695, respectively, indicating that the model was well-fitted and had good predictability. The heat map revealed that nucleic acid and amino compounds differed between the Korean and Chinese fish, and the variable importance in the projection scores obtained from OPLS-DA showed that there were geographical differences in the primary metabolites (5'-methylthioadenosine, adenosine, uridine 5-diphosphate, guanosine 5-diphosphate, urea, homocarnosine, O-acetylcarnitine, cycloleucine, cycloleucine S-adenosylmethionine, S-adenosylhomocysteine, ethanolamine, myo-inositol 1-phosphate), which were identified as potential candidate biomarkers.

The argininosuccinate synthetase can differentially regulate nitric oxide synthase in yellow catfish Pelteobagrus fulvidraco

Fish are at high risk of exposure to ammonia in aquaculture systems. When ammonia stress occurs, fish are more prone to disease outbreaks, but the mechanism is not very clear. The argininosuccinate synthetase (ASS) plays an important role in the regulation of urea synthesis and nitric oxide synthesis. We speculated that there must be some relationship between ASS expression and disease outbreak. In this study, ASS was cloned from the yellow catfish. The full-length cDNAs of ASS was 1558 bp, with open reading frames of 1236 bp. The mRNA expression of ASS gene was the highest in liver, kidney and brain. This study consists of two parts: 1) For ammonia challenge in vivo, yellow catfish (15.00 ± 1.50 g) were divided into control group, low ammonia group (1/10 96 h LC₅₀), and high ammonia group (1/2 96 h LC₅₀). The experiment continued for 192 h. The results showed that ammonia stress elevated serum ammonia content, and inhibited urea synthesis enzymes activities but up-regulated the expression levels of related genes except ARG, and induced arginine accumulation and nitric oxide synthase (nNOS and iNOS) different expression, and decreased resistance to Aeromonas hydrophage; 2) For ammonia challenge in vitro, the primary culture of liver cell was divided into four groups: control group, BPP group (Bj-BPP-10c was added as ASS activator), Amm group (96 h LC₅₀), and Amm + BPP group. The experiment continued for 96 h. The results showed that the Bj-BPP-10c can inhibit nNOS activity and improve cell survival rate, and enhance iNOS activity and immune response (lysozyme, complement, respiratory burst, and phagocytic index) by activate ASS when ammonia stress occurred. Our results indicated that targeted regulation of ASS can improve iNOS activity, and enhance the immune response of yellow catfish under ammonia stress.

Ammonia induces changes in carbamoyl phosphate synthetase I and its regulation of glutamine synthesis and urea cycle in yellow catfish Pelteobagrus fulvidraco

Fishes can adapt to certain levels of environmental ammonia in water, but the strategies utilized to defend against ammonia toxicity are not exactly the same. The carbamyl phosphate synthase I (CPS I) plays an important role in the regulation of glutamine synthesis and urea cycle, which are the most common strategies for ammonia detoxification. In this study, CPS I was cloned from the yellow catfish. The full-length cDNAs of the CPS I was 5 034 bp, with open reading frames of 4 461 bp. Primary amino acid sequence alignment of CPS I revealed conserved similarity between the functional domains of the yellow catfish CPS I protein with CPS I proteins of other animals. The mRNA expression of CPS I was significantly up-regulated in liver and kidney tissues after acute ammonia stress. The CPS I RNA interference (RNAi) down-regulated the mRNA expressions of CPS I and ornithine transcarbamylase (OTC), but up-regulated glutamine synthetase (GS) and glutamate dehydrogenase (GDH) expressions in primary culture of liver cell after acute ammonia stress. Similarly, the activity of enzymes related to urea cycle decreased significantly, while the activity of enzymes related to glutamine synthesis increased significantly. The results of RNAi in vitro suggested that when the urea cycle is disturbed, the glutamine synthesis will be activated to cope with ammonia toxicity.

Arginine metabolism and its functions in growth, nutrient utilization, and immunonutrition of fish

Fish have limited ability in endogenous biosynthesis of arginine. Arginine is an indispensable amino acid for fish, and the arginine requirement varies with fish species and fish size. Recent studies on fish have demonstrated that arginine influences nutrient metabolism, stimulates insulin release, is involved in nonspecific immune responses and antioxidant responses, and elevates disease resistance. Specifically, arginine can regulate energy homeostasis via modulating the adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) pathway, and also regulate protein synthesis via activating the target of rapamycin (TOR) signaling pathway. The present article reviews pertinent knowledge of arginine in fish, including dietary quantitative requirements, endogenous anabolism and catabolism, regulation of the endocrine and metabolic systems, and immune-regulatory functions under pathogenic challenge. Our findings showed that further data about the distribution of arginine after intake into specific cells, its sub-cellular sensor to initiate downstream signaling pathways, and its effects on fish mucosal immunity, especially the adaptive immune response against pathogenic infection in different species, are urgently needed.

An in vitro study of urea and ammonia production and transport by the intestinal tract of fed and fasted rainbow trout: responses to luminal glutamine and ammonia loading

Digestion of dietary protein in teleosts results in high ammonia levels within the intestinal chyme that may reach concentrations that are many-fold greater than blood plasma levels. We used in vitro gut sac preparations of the ammoniotelic rainbow trout (Oncorhynchus mykiss) to investigate the role of the intestine in producing and transporting ammonia and urea, with specific focus on feeding versus fasting, and on responses to loading of the lumen with 2 mmol L^-1 glutamine or 2 mmol L^-1 ammonia. Feeding increased not only ammonia production and both mucosal and serosal fluxes, but also increased urea production and serosal fluxes. Elevated urea production was accompanied by an increase in arginase activity but minimal CPS III activity, suggesting that urea may be produced by direct arginolysis. The ammonia production and serosal fluxes increased in fasted preparations with glutamine loading, indicating an ability of the intestinal tissue to deaminate glutamine and perhaps use it as an energy source. However, there was little evidence of urea production or transport resulting from the presence of glutamine. Furthermore, the intestinal tissues did not appear to convert surplus ammonia to urea as a detoxification mechanism, as urea production and serosal flux rates decreased in fed preparations, with minimal changes in fasted preparations. Nevertheless, there was indirect evidence of detoxification by another pathway, as ammonia production rate decreased with ammonia loading in fed preparations. Overall, our study suggests that intestinal tissues of rainbow trout have the ability to produce urea and detoxify ammonia, likely via arginolysis.

Adaptation of the carbamoyl-phosphate synthetase enzyme in an extremophile fish

Tetrapods and fish have adapted distinct carbamoyl-phosphate synthase (CPS) enzymes to initiate the ornithine urea cycle during the detoxification of nitrogenous wastes. We report evidence that in the ureotelic subgenus of extremophile fish Oreochromis Alcolapia, CPS III has undergone convergent evolution and adapted its substrate affinity to ammonia, which is typical of terrestrial vertebrate CPS I. Unusually, unlike in other vertebrates, the expression of CPS III in Alcolapia is localized to the skeletal muscle and is activated in the myogenic lineage during early embryonic development with expression remaining in mature fish. We propose that adaptation in Alcolapia included both convergent evolution of CPS function to that of terrestrial vertebrates, as well as changes in development mechanisms redirecting CPS III gene expression to the skeletal muscle.

Nutrition and metabolism of glutamate and glutamine in fish

Glutamate (Glu) and glutamine (Gln) comprise a large proportion of total amino acids (AAs) in fish in the free and protein-bound forms. Both Glu and Gln are synthesized de novo from other α-amino acids and ammonia. Although these two AAs had long been considered as nutritionally non-essential AAs for an aquatic animal, they must be included adequately in its diet to support optimal health (particularly intestinal health) and maximal growth. In research on fish nutrition, Glu has been used frequently as an isonitrogenous control on the basis of the assumption that this AA has no nutritional or physiological function. In addition, purified diets used for feeding fish generally lack glutamine. As functional AAs, Glu and Gln are major metabolic fuels for tissues of fish (including the intestine, liver, kidneys, and skeletal muscle), and play important roles not only in protein synthesis but also in glutathione synthesis and anti-oxidative reactions. The universality of Glu and Gln as abundant intracellular AAs depends on their enormous versatility in metabolism. Dietary supplementation with Glu and Gln to farmed fish can improve their growth performance, intestinal development, innate and adaptive immune responses, skeletal muscle development and fillet quality, ammonia removal, and the endocrine status. Glu (mainly as monosodium glutamate), glutamine, or AminoGut (a mixture of Glu and Gln) is a promising feed additive to reduce the use of fishmeal, while gaining the profitability of global aquaculture production. Thus, the concept of dietary requirements of fish for Glu and Gln is a paradigm shift in the nutrition of aquatic animals (including fish).

Fasting in the ureotelic Lake Magadi tilapia, Alcolapia grahami, does not reduce its high metabolic demand, increasing its vulnerability to siltation events

Lake Magadi, Kenya, is one of the most extreme aquatic environments on Earth (pH~10, anoxic to hyperoxic, high temperatures). Recently, increased water demand and siltation have threatened the viable hot springs near the margins of the lake where Alcolapia grahami, the only fish surviving in the lake, live. These Lake Magadi tilapia largely depend on nitrogen-rich cyanobacteria for food and are 100% ureotelic. Their exceptionally high aerobic metabolic rate, together with their emaciated appearance, suggests that they are energy-limited. Therefore, we hypothesized that during food deprivation, Magadi tilapia would economize their energy expenditure and reduce metabolic rate, aerobic performance and urea-N excretion. Surprisingly, during a 5-day fasting period, routine metabolic rates increased and swimming performance (critical swimming speed) was not affected. Urea-N excretion remained stable despite the lack of their N-rich food source. Their nitrogen use switched to endogenous sources as liver and muscle protein levels decreased after a 5-day fast, indicating proteolysis. Additionally, fish relied on carbohydrates with lowered muscle glycogen levels, but there were no signs indicating use of lipid stores. Gene expression of gill and gut urea transporters were transiently reduced as were gill rhesus glycoprotein Rhbg and Rhcg-2. The reduction in gill glutamine synthetase expression concomitant with the reduction in Rh glycoprotein gene expression indicates reduced nitrogen/ammonia metabolism, most likely decreased protein synthesis. Additionally, fish showed reduced plasma total CO₂, osmolality and Na⁺ (but not Cl^-) levels, possibly related to reduced drinking rates and metabolic acidosis. Our work shows that Lake Magadi tilapia have the capacity to survive short periods of starvation which could occur when siltation linked to flash floods covers their main food source, but their seemingly hardwired high metabolic rates would compromise long-term survival.

Differential Transcriptomic and Metabolomic Responses in the Liver of Nile Tilapia (Oreochromis niloticus) Exposed to Acute Ammonia

Ammonia is toxic to aquatic animal. Currently, only limited works were reported on the responses of aquatic animals after ammonia exposure using "omics" technologies. Tilapia suffers from the stress of ammonia-nitrogen during intensive recirculating aquaculture. Optimizing ammonia stress tolerance has become an important issue in tilapia breeding. The molecular and biochemical mechanisms of ammonia-nitrogen toxicity have not been understood comprehensively in tilapia yet. In this study, using RNA-seq and gas chromatograph system coupled with a Pegasus HT time-of-flight mass spectrometer (GC-TOF-MS) techniques, we investigated differential expressed genes (DEGs) and metabolomes in the liver at 6 h post-challenges (6 hpc) and 24 h post-challenges (24 hpc) under high concentration of ammonia-nitrogen treatment. We detected 2258 DEGs at 6 hpc and 315 DEGs at 24 hpc. Functional enrichment analysis indicated that DEGs were significantly associated with cholesterol biosynthesis, steroid and lipid metabolism, energy conservation, and mitochondrial tissue organization. Metabolomic analysis detected 31 and 36 metabolites showing significant responses to ammonia-nitrogen stress at 6 and 24 hpc, respectively. D-(Glycerol 1-phosphate), fumaric acid, and L-malic acid were found significantly down-regulated at both 6 and 24 hpc. The integrative analysis of transcriptomics and metabolomics suggested considerable alterations and precise control of gene expression at both physiological and molecular levels in response to the stress of ammonia-nitrogen in tilapia.

Modulation of lipid metabolism in juvenile yellow catfish (Pelteobagrus fulvidraco) as affected by feeding frequency and environmental ammonia

In the intensive culture systems, excessive feeding leads to ammonia accumulation, which results in lipid metabolism disorder. However, little information is available on the modulation of lipid metabolism in fish as affected by feeding frequency and ammonia stress. In this study, weight gain increased as feeding frequency increased from one to four times daily, but feed conversion ratio is opposite. The highest survival was found in ammonia group when fish was fed two times daily. Liver ammonia content increased as feeding frequency increased from one to four times daily, and the highest brain ammonia content was found when fish was fed four times daily. The highest liver 6-phospho-gluconate dehydrogenase (6PGD), fatty acid synthase (FAS), carnitine palmitoyltransferase (CPT), and lipoprotein lipase (LPL) contents were found in control group when fish was fed four times daily; in comparison, the highest liver 6PGD, FAS, CPT, and LPL contents were found in ammonia group when fish was fed two times daily. Liver 6PGD, FAS, CPT 1, SREBP-1, and PPARα mRNA expression in control group increased significantly as feeding frequency increased from one to four times daily, and the highest expression of 6PGD, G6PD, and FAS was observed in ammonia group when fish was fed two times daily. This study indicated that the optimal feeding frequency is two times daily when yellow catfish exposed to ammonia.

Effect of dietary alanyl-glutamine dipeptide against chronic ammonia stress induced hyperammonemia in the juvenile yellow catfish (Pelteobagrus fulvidraco)

Triplicate groups of juvenile yellow catfish (1.98 ± 0.01 g) were fed diets supplemented with 0% and 1% alanyl-glutamine dipeptide (AGD) for 56 days under three ammonia concentrations (0.01, 5.70 and 11.40 mg L^-1 total ammonia nitrogen). The results showed that ammonia poisoning could induce growth (weight gain and specific growth rate) and survival reduction, live ammonia and serum malondialdehyde accumulation, and subsequently lead to blood deterioration (serum total protein, albumin, globulin, alkaline phosphatase and acid phosphatase reduced), oxidative stress (superoxide dismutase and glutathione peroxidase activities declined), and induce down-regulation of antioxidant enzymes (SOD, GPX and GRX) genes transcription. However, dietary supplemented with 1% AGD could mitigate the adverse effect of ammonia poisoning on fish growth performance.

Differential expression of multiple glutamine synthetase genes in air-breathing magur catfish, Clarias magur and their induction under hyper-ammonia stress

The present study demonstrates the unique presence of three different gs genes (cmgs01, cmgs02, and cmgs03) in air-breathing ureogenic magur catfish (Clarias magur), which is otherwise reported to be encoded by a single gene in higher vertebrates. Of these three genes, two (cmgs01and cmgs03) were identified as 'liver' form, predominantly expressed in liver cells, and the third one as 'brain' form (cmgs02), expressed chiefly in brain cells. Molecular characterization studies have revealed conservation of homologous active site residues in all the three gs genes. In silico analysis, accompanied by GS enzyme assay and Western blot analysis of different GS isoforms in different subcellular fractions indicated the mitochondrial localization of cmGS01 and cmGS03 in liver and kidney cells and cytosolic localization of cmGS02 in brain cells. Further, exposure of magur catfish to high external ammonia (HEA; 25 mM NH₄Cl) led to a significant induction of multiple gs genes as evidenced by higher expression of different gs mRNAs at variable levels in different tissues. The cmgs01 and cmgs03 mRNA levels elevated significantly in liver, kidney, muscle, and gills, whereas the cmgs02 mRNA level increased considerably in the brain after 14 days of exposure to HEA. These increases in mRNA levels were associated with a significant rise in cmGS01 and cmGS03 proteins in liver, kidney, muscle, and gills, and the cmGS02 protein in the brain after 14 days of exposure to HEA. Therefore, it can be concluded that the unique differential expression of three gs genes and their induction under high ammonia level probably helps in detoxification of ammonia to glutamine and further to urea via the ornithine-urea cycle in ureogenic as well as non-ureogenic tissues of these magur catfish.

Ammonia and urea handling by early life stages of fishes

Nitrogen metabolism in fishes has been a focus of comparative physiologists for nearly a century. In this Review, we focus specifically on early life stages of fishes, which have received considerable attention in more recent work. Nitrogen metabolism and excretion in early life differs fundamentally from that of juvenile and adult fishes because of (1) the presence of a chorion capsule in embryos that imposes a limitation on effective ammonia excretion, (2) an amino acid-based metabolism that generates a substantial ammonia load, and (3) the lack of a functional gill, which is the primary site of nitrogen excretion in juvenile and adult fishes. Recent findings have shed considerable light on the mechanisms by which these constraints are overcome in early life. Perhaps most importantly, the discovery of Rhesus (Rh) glycoproteins as ammonia transporters and their expression in ion-transporting cells on the skin of larval fishes has transformed our understanding of ammonia excretion by fishes in general. The emergence of larval zebrafish as a model species, together with genetic knockdown techniques, has similarly advanced our understanding of ammonia and urea metabolism and excretion by larval fishes. It has also now been demonstrated that ammonia excretion is one of the primary functions of the developing gill in rainbow trout larvae, leading to new hypotheses regarding the physiological demands driving gill development in larval fishes. Here, we highlight and discuss the dramatic changes in nitrogen handling that occur over early life development in fishes.

The protective effects of taurine on acute ammonia toxicity in grass carp Ctenopharynodon idellus

The four experimental groups were carried out to test the response of grass carp Ctenopharyngodon idella to ammonia toxicity and taurine: group 1 was injected with NaCl, group 2 was injected with ammonium acetate, group 3 was injected with ammonium acetate and taurine, and group 4 was injected taurine. Fish in group 2 had the highest ammonia content in the liver and brain, and alanine, arginine, glutamine, glutamate and glycine contents in liver. Brain alanine and glutamate of fish in group 2 were significantly higher than those of fish in group 1. Malondialdehyde content of fish in group 2 was the highest, but superoxide dismutase and glutathione activities were the lowest. Although fish in group 2 had the lowest red cell count and hemoglobin, the highest alkaline phosphatase, complement C3, C4 and total immunoglobulin contents appeared in this group. In addition, superoxide dismutase and glutathione activities, red cell count and hemoglobin of fish in group 3 were significantly higher than those of fish in group 2, but malondialdehyde content is the opposite. This study indicates that ammonia exerts its toxic effects by interfering with amino acid transport, inducing reactive oxygen species generation and malondialdehyde accumulation, leading to blood deterioration and over-activation of immune response. The exogenous taurine could mitigate the adverse effect of high ammonia level on fish physiological disorder.

Ammonia toxicity induces glutamine accumulation, oxidative stress and immunosuppression in juvenile yellow catfish Pelteobagrus fulvidraco

A study was carried to test the response of yellow catfish for 28 days under two ammonia concentrations. Weight gain of fish exposure to high and low ammonia abruptly increased at day 3. There were no significant changes in fish physiological indexes and immune responses at different times during 28-day exposure to low ammonia. Fish physiological indexes and immune responses in the treatment of high ammonia were lower than those of fish in the treatment of low ammonia. When fish were exposed to high ammonia, the ammonia concentration in the brain increased by 19-fold on day 1. By comparison, liver ammonia concentration reached its highest level much earlier at hour 12. In spite of a significant increase in brain and liver glutamine concentration, there was no significant change in glutamate level throughout the 28-day period. The total superoxide dismutase (SOD), glutathione peroxidase (GPX) and glutathione reductase (GR) activities in the brain gradually decreased from hour 0 to day 28. Liver SOD, GPX and GR activities reached the highest levels at hour 12, and then gradually decreased. Thiobarbituric acid reactive substance brain and liver content gradually increased throughout the 28-day period. Lysozyme, acid phosphatase and alkaline phosphatase activities in the liver reached exceptionally low levels after day 14. This study indicated that glutamine accumulation in the brain was not the major cause of ammonia poisoning, the toxic reactive oxygen species is not fully counter acted by the antioxidant enzymes and immunosuppression is a process of gradual accumulation of immunosuppressive factors.

Evolution of urea transporters in vertebrates: adaptation to urea's multiple roles and metabolic sources

In the two decades since the first cloning of the mammalian kidney urea transporter (UT-A), UT genes have been identified in a plethora of organisms, ranging from single-celled bacteria to metazoans. In this review, focusing mainly on vertebrates, we first reiterate the multiple catabolic and anabolic pathways that produce urea, then we reconstruct the phylogenetic history of UTs, and finally we examine the tissue distribution of UTs in selected vertebrate species. Our analysis reveals that from an ancestral UT, three homologues evolved in piscine lineages (UT-A, UT-C and UT-D), followed by a subsequent reduction to a single UT-A in lobe-finned fish and amphibians. A later internal tandem duplication of UT-A occurred in the amniote lineage (UT-A1), followed by a second tandem duplication in mammals to give rise to UT-B. While the expected UT expression is evident in excretory and osmoregulatory tissues in ureotelic taxa, UTs are also expressed ubiquitously in non-ureotelic taxa, and in tissues without a complete ornithine-urea cycle (OUC). We posit that non-OUC production of urea from arginine by arginase, an important pathway to generate ornithine for synthesis of molecules such as polyamines for highly proliferative tissues (e.g. testis, embryos), and neurotransmitters such as glutamate for neural tissues, is an important evolutionary driving force for the expression of UTs in these taxa and tissues.

Rh proteins and NH4(+)-activated Na+-ATPase in the Magadi tilapia (Alcolapia grahami), a 100% ureotelic teleost fish

The small cichlid fish Alcolapia grahami lives in Lake Magadi, Kenya, one of the most extreme aquatic environments on Earth (pH ~10, carbonate alkalinity ~300 mequiv l(-1)). The Magadi tilapia is the only 100% ureotelic teleost; it normally excretes no ammonia. This is interpreted as an evolutionary adaptation to overcome the near impossibility of sustaining an NH3 diffusion gradient across the gills against the high external pH. In standard ammoniotelic teleosts, branchial ammonia excretion is facilitated by Rh glycoproteins, and cortisol plays a role in upregulating these carriers, together with other components of a transport metabolon, so as to actively excrete ammonia during high environmental ammonia (HEA) exposure. In Magadi tilapia, we show that at least three Rh proteins (Rhag, Rhbg and Rhcg2) are expressed at the mRNA level in various tissues, and are recognized in the gills by specific antibodies. During HEA exposure, plasma ammonia levels and urea excretion rates increase markedly, and mRNA expression for the branchial urea transporter mtUT is elevated. Plasma cortisol increases and branchial mRNAs for Rhbg, Rhcg2 and Na(+),K(+)-ATPase are all upregulated. Enzymatic activity of the latter is activated preferentially by NH4(+) (versus K(+)), suggesting it can function as an NH4(+)-transporter. Model calculations suggest that active ammonia excretion against the gradient may become possible through a combination of Rh protein and NH4(+)-activated Na(+)-ATPase function.

Expression pattern and biochemical properties of zebrafish N-acetylglutamate synthase

The urea cycle converts ammonia, a waste product of protein catabolism, into urea. Because fish dispose ammonia directly into water, the role of the urea cycle in fish remains unknown. Six enzymes, N-acetylglutamate synthase (NAGS), carbamylphosphate synthetase III, ornithine transcarbamylase, argininosuccinate synthase, argininosuccinate lyase and arginase 1, and two membrane transporters, ornithine transporter and aralar, comprise the urea cycle. The genes for all six enzymes and both transporters are present in the zebrafish genome. NAGS (EC 2.3.1.1) catalyzes the formation of N-acetylglutamate from glutamate and acetyl coenzyme A and in zebrafish is partially inhibited by L-arginine. NAGS and other urea cycle genes are highly expressed during the first four days of zebrafish development. Sequence alignment of NAGS proteins from six fish species revealed three regions of sequence conservation: the mitochondrial targeting signal (MTS) at the N-terminus, followed by the variable and conserved segments. Removal of the MTS yields mature zebrafish NAGS (zfNAGS-M) while removal of the variable segment from zfNAGS-M results in conserved NAGS (zfNAGS-C). Both zfNAGS-M and zfNAGS-C are tetramers in the absence of L-arginine; addition of L-arginine decreased partition coefficients of both proteins. The zfNAGS-C unfolds over a broader temperature range and has higher specific activity than zfNAGS-M. In the presence of L-arginine the apparent Vmax of zfNAGS-M and zfNAGS-C decreased, their Km(app) for acetyl coenzyme A increased while the Km(app) for glutamate remained unchanged. The expression pattern of NAGS and other urea cycle genes in developing zebrafish suggests that they may have a role in citrulline and/or arginine biosynthesis during the first day of development and in ammonia detoxification thereafter. Biophysical and biochemical properties of zebrafish NAGS suggest that the variable segment may stabilize a tetrameric state of zfNAGS-M and that under physiological conditions zebrafish NAGS catalyzes formation of N-acetylglutamate at the maximal rate.

Nitrogen metabolism of the intestine during digestion in a teleost fish, the plainfin midshipman (Porichthys notatus)

Digestion affects nitrogen metabolism in fish, as both exogenous and endogenous proteins and amino acids are catabolized, liberating ammonia in the process. Here we present a model of local detoxification of ammonia by the intestinal tissue of the plainfin midshipman (Porichthys notatus) during digestion, resulting in an increase in urea excretion of gastrointestinal origin. Corroborating evidence indicated whole-animal ammonia and urea excretion increased following feeding, and ammonia levels within the lumen of the midshipman intestine increased to high levels (1.8±0.4 μmol N g(-1)). We propose that this ammonia entered the enterocytes and was detoxified to urea via the ornithine-urea cycle (O-UC) enzymes, as evidenced by a 1.5- to 2.9-fold post-prandial increase in glutamine synthetase activity (0.14±0.05 and 0.28±0.02 μmol min(-1) g(-1) versus 0.41±0.03 μmol min(-1) g(-1)) and an 8.7-fold increase in carbamoyl phosphate synthetase III activity (0.3±1.2 versus 2.6±0.4 nmol min(-1) g(-1)). Furthermore, digestion increased urea production by isolated gastrointestinal tissue 1.7-fold, supporting our hypothesis that intestinal tissue synthesizes urea in response to feeding. We further propose that the intestinal urea may have been excreted into the intestinal lumen via an apical urea transporter as visualized using immunohistochemistry. A portion of the urea was then excreted to the environment along with the feces, resulting in the observed increase in urea excretion, while another portion may have been used by intestinal ureolytic bacteria. Overall, we propose that P. notatus produces urea within the enterocytes via a functional O-UC, which is then excreted into the intestinal lumen. Our model of intestinal nitrogen metabolism does not appear to be universal as we were unab le to activate the O-UC in the intestine of fed rainbow trout. However, literature values suggest that multiple fish species could follow this model.

Insights into the in vivo regulation of glutamate dehydrogenase from the foot muscle of an estivating land snail

Land snails, Otala lactea, survive in seasonally hot and dry environments by entering a state of aerobic torpor called estivation. During estivation, snails must prevent excessive dehydration and reorganize metabolic fuel use so as to endure prolonged periods without food. Glutamate dehydrogenase (GDH) was hypothesized to play a key role during estivation as it shuttles amino acid carbon skeletons into the Krebs cycle for energy production and is very important to urea biosynthesis (a key molecule used for water retention). Analysis of purified foot muscle GDH from control and estivating conditions revealed that estivated GDH was approximately 3-fold more active in catalyzing glutamate deamination as compared to control. This kinetic difference appears to be regulated by reversible protein phosphorylation, as indicated by ProQ Diamond phosphoprotein staining and incubations that stimulate endogenous protein kinases and phosphatases. The increased activity of the high-phosphate form of GDH seen in the estivating land snail foot muscle correlates well with the increased use of amino acids for energy and increased synthesis of urea for water retention during prolonged estivation.

The relationship between NMDA receptor function and the high ammonia tolerance of anoxia-tolerant goldfish

Acute ammonia toxicity in vertebrates is thought to be characterized by a cascade of deleterious events resembling those associated with anoxic/ischemic injury in the central nervous system. A key event is the over-stimulation of neuronal N-methyl-D-aspartate (NMDA) receptors, which leads to excitotoxic cell death. The similarity between the responses to acute ammonia toxicity and anoxia suggests that anoxia-tolerant animals such as the goldfish (Carassius auratus Linnaeus) may also be ammonia tolerant. To test this hypothesis, the responses of goldfish were compared with those of the anoxia-sensitive rainbow trout (Oncorhynchus mykiss Walbaum) during exposure to high external ammonia (HEA). Acute toxicity tests revealed that goldfish are ammonia tolerant, with 96 h median lethal concentration (LC(50)) values of 199 μmol l(-1) and 4132 μmol l(-1) for NH(3) and total ammonia ([T(Amm)]=[NH(3)]+[NH(4)(+)]), respectively. These values were ~5-6 times greater than corresponding NH(3) and T(Amm) LC(50) values measured in rainbow trout. Further, the goldfish readily coped with chronic exposure to NH(4)Cl (3-5 mmol l(-1)) for 5 days, despite 6-fold increases in plasma [T] to ~1300 μmol l(-1) and 3-fold increases in brain [T(Amm)] to 6700 μmol l(-1). Muscle [T(Amm)] increased by almost 8-fold from ~900 μmol kg(-1) wet mass (WM) to greater than 7000 μmol kg(-1) WM by 48 h, and stabilized. Although urea excretion rates (J(Urea)) increased by 2-3-fold during HEA, the increases were insufficient to offset the inhibition of ammonia excretion that occurred, and increases in urea were not observed in the brain or muscle. There was a marked increase in brain glutamine concentration at HEA, from ~3000 μmol kg(-1) WM to 15,000 μmol kg(-1) WM after 48 h, which is consistent with the hypothesis that glutamine production is associated with ammonia detoxification. Injection of the NMDA receptor antagonists MK801 (0.5-8 mg kg(-1)) or ethanol (1-8 mg kg(-1)) increased trout survival time by 1.5-2.0-fold during exposure to 2 mmol l(-1) ammonia, suggesting that excitotoxic cell death contributes to ammonia toxicity in this species. In contrast, similar doses of MK801 or ethanol had no effect on ammonia-challenged (8-9.5 mmol l(-1) T(Amm)) goldfish survival times, suggesting that greater resistance to excitotoxic cell death contributes to the high ammonia-tolerance of the goldfish. Whole-cell recordings measured in isolated brain slices of goldfish telencephalon during in vitro exposure to 5 mmol l(-1) or 10 mmol l(-1) T(Amm) reversibly potentiated NMDA receptor currents. This observation suggested that goldfish neurons may not be completely resistant to ammonia-induced excitotoxicity. Subsequent western blot and densitometric analyses revealed that NMDA receptor NR1 subunit abundance was 40-60% lower in goldfish exposed to 3-5 mmol l(-1) T(Amm) for 5 days, which was followed by a restoration of NR1 subunit abundance after 3 days recovery in ammonia-free water. We conclude that the goldfish brain may be protected from excitotoxicity by downregulating the abundance of functional NMDA receptors during periods when it experiences increased internal ammonia.

Phylogenetic aspects of carbamoyl phosphate synthetase in lungfish: a transitional enzyme in transitional fishes

Carbamoyl phosphate synthetase (CPS) catalyses the formation of carbamoyl phosphate from glutamine or ammonia, bicarbonate and ATP. There are three different isoforms of CPS that play vital roles in two metabolic pathways, pyrimidine biosynthesis (CPS II) and arginine/urea biosynthesis (CPS I and CPS III). Gene duplication has been proposed as the evolutionary mechanism creating this gene family with CPS II likely giving rise to the CPS I/III clade. In the evolutionary history of this gene family it is still undetermined when CPS I diverged from CPS III on the path to terrestriality in the vertebrates. Transitional organisms such as lungfishes are of particular interest because they are capable of respiring via gills and with lungs and therefore can be found in both aquatic and terrestrial environments. Notably, enzymatic characterization of the mitochondrial CPS isoforms in this transitional group has not led to clear conclusions. In order to determine which CPS isoform is present in transitional animals, we examined partial sequences for liver CPS amplified from five species of lungfish, and a larger fragment of CPS from one lungfish species (Protopterus annectens) and compared them to CPS isoforms from other fish and mammals. Enzyme activities for P. annectens liver were also examined. While enzyme activities did not yield a clear distinction between isoforms (virtually equal activities were obtained for either CPS I or III), CPS sequences from the lungfishes formed a monophyletic clade within the CPS I clade and separate from the CPS III clade of other vertebrates. This finding implies that the mitochondrial isoform of CPS in lungfish is derived from CPS I and is likely to have a physiological function similar to CPS I. This finding is important because it supports the hypothesis that lungfish employ a urea cycle similar to terrestrial air-breathing vertebrates.

Niche dimensions in fishes: an integrative view

Current shifts in ecosystem composition and function emphasize the need for an understanding of the links between environmental factors and organism fitness and tolerance. The examples discussed here illustrate how recent progress in the field of comparative physiology may provide a better mechanistic understanding of the ecological concepts of the fundamental and realized niches and thus provide insights into the impacts of anthropogenic disturbance. Here we argue that, as a link between physiological and ecological indicators of organismal performance, the mechanisms shaping aerobic scope and passive tolerance set the dimensions of an animal's niche, here defined as its capacity to survive, grow, behave, and interact with other species. We demonstrate how comparative studies of cod or killifish populations in a latitudinal cline have unraveled mitochondrial mechanisms involved in establishing a species' niche, performance, and energy budget. Riverine fish exemplify how the performance windows of various developmental stages follow the dynamic regimes of both seasonal temperatures and river hydrodynamics, as synergistic challenges. Finally, studies of species in extreme environments, such as the tilapia of Lake Magadi, illustrate how on evolutionary timescales functional and morphological shifts can occur, associated with new specializations. We conclude that research on the processes and time course of adaptations suitable to overcome current niche limits is urgently needed to assess the resilience of species and ecosystems to human impact, including the challenges of global climate change.

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

Many fishes are ammonotelic but some species can detoxify ammonia to glutamine or urea. Certain fish species can accumulate high levels of ammonia in the brain or defense against ammonia toxicity by enhancing the effectiveness of ammonia excretion through active NH4+transport, manipulation of ambient pH, or reduction in ammonia permeability through the branchial and cutaneous epithelia. Recent reports on ammonia toxicity in mammalian brain reveal the importance of permeation of ammonia through the blood-brain barrier and passages of ammonia and water through transporters in the plasmalemma of brain cells. Additionally, brain ammonia toxicity could be related to the passage of glutamine through the mitochondrial membranes into the mitochondrial matrix. On the other hand, recent reports on ammonia excretion in fish confirm the involvement of Rhesus glycoproteins in the branchial and cutaneous epithelia. Therefore, this review focuses on both the earlier literature and the up-to-date information on the problems and mechanisms concerning the permeation of ammonia, as NH(3), NH4+ or proton-neutral nitrogenous compounds, across mitochondrial membranes, the blood-brain barrier, the plasmalemma of neurons, and the branchial and cutaneous epithelia of fish. It also addresses how certain fishes with high ammonia tolerance defend against ammonia toxicity through the regulation of the permeation of ammonia and related nitrogenous compounds through various types of membranes. It is hoped that this review would revive the interests in investigations on the passage of ammonia through the mitochondrial membranes and the blood-brain barrier of ammonotelic fishes and fishes with high brain ammonia tolerance, respectively.

Reciprocal enhancement of uptake and toxicity of cadmium and calcium in rainbow trout (Oncorhynchus mykiss) liver mitochondria

The interactive effects of cadmium (Cd) and calcium (Ca) on energy metabolism in rainbow trout liver mitochondria were studied to test the prediction that Ca would protect against Cd-induced mitochondrial liability. Isolated rainbow trout liver mitochondria were energized with malate and glutamate and exposed to increasing concentrations (5-100 microM) of Cd and Ca singly and in combination at 15 degrees C. Accumulation of Cd and Ca in the mitochondria and mitochondrial respiration (oxygen consumption) rates were measured. Additionally, un-energized mitochondria were incubated with low doses (1 microM) of Cd and Ca singly and in combination, with time-course measurements of cation accumulation/binding and oxygen consumption rates. In energized actively phosphorylating mitochondria, the uptake rates of both Cd and Ca were dose-dependent and enhanced when administered concurrently. Upon low-dose incubation, Cd accumulation was rapid and peaked in 5 min, while no appreciable uptake of Ca occurred. Functionally, the resting (state 4, ADP-limited) respiration rate was not affected in the dose-response exposure, but it decreased remarkably on low-dose incubation. Adenosine diphosphate (ADP)-stimulated respiration (state 3) rate was impaired dose-dependently with maximal inhibitions (at the highest dose, 100 microM) of 32, 64 and 73% for Ca, Cd, and combined exposures, respectively. The combined effects of Ca and Cd suggested synergistic (more than additive) action and partial additivity of effects at low and higher doses of the two cations, respectively. Moreover, on a molar basis, Cd was twice as toxic as Ca to rainbow trout liver mitochondria and when combined, approximately 90% of the effects were attributable to Cd. The coupling efficiency, as measured by respiratory control ratio (RCR) and phosphorylation efficiency, measured as ADP/O ratio, both decreased as the exposure dosage and duration increased. In addition, Cd and Ca exposure decreased mitochondrial proton leak (state 4+ respiration) rates on prolonged exposure possibly by inhibiting processes that generate mitochondrial membrane potential, the force that drives proton leak. Overall these data suggest that the widely accepted theme that Ca and Cd are competitive antagonists does not hold for mitochondrial effects and that Cd and Ca cooperate to impair oxidative phosphorylation in rainbow trout liver mitochondria.

Ammonia and urea transporters in gills of fish and aquatic crustaceans

The diversity of mechanisms of ammonia and urea excretion by the gills and other epithelia of aquatic organisms, especially fish and crustaceans, has been studied for decades. Although the decades-old dogma of ;aquatic species excrete ammonia' still explains nitrogenous waste excretion for many species, it is clear that there are many mechanistic variations on this theme. Even within species that are ammonoteles, the process is not purely ;passive', often relying on the energizing effects of proton and sodium-potassium ATPases. Within the ammonoteles, Rh (Rhesus) proteins are beginning to emerge as vital ammonia conduits. Many fishes are also known to be capable of substantial synthesis and excretion of urea as a nitrogenous waste. In such species, members of the UT family of urea transporters have been identified as important players in urea transport across the gills. This review attempts to draw together recent information to update the mechanisms of ammonia and urea transport by the gills of aquatic species. Furthermore, we point out several potentially fruitful avenues for further research.

Is the alkaline tide a signal to activate metabolic or ionoregulatory enzymes in the dogfish shark (Squalus acanthias)?

Experimental metabolic alkalosis is known to stimulate whole-animal urea production and active ion secretion by the rectal gland in the dogfish shark. Furthermore, recent evidence indicates that a marked alkaline tide (systemic metabolic alkalosis) follows feeding in this species and that the activities of the enzymes of the ornithine-urea cycle (OUC) for urea synthesis in skeletal muscle and liver and of energy metabolism and ion transport in the rectal gland are increased at this time. We therefore evaluated whether alkalosis and/or NaCl/volume loading (which also occurs with feeding) could serve as a signal for activation of these enzymes independent of nutrient loading. Fasted dogfish were infused for 20 h with either 500 mmol L(-1) NaHCO3 (alkalosis + volume expansion) or 500 mmol L(-1) NaCl (volume expansion alone), both isosmotic to dogfish plasma, at a rate of 3 mL kg(-1) h(-1). NaHCO3 infusion progressively raised arterial pH to 8.28 (control = 7.85) and plasma [HCO3-] to 20.8 mmol L(-1) (control = 4.5 mmol L(-1)) at 20 h, with unchanged arterial P(CO2), whereas NaCl/volume loading had no effect on blood acid-base status. Rectal gland Na+,K+-ATPase activity was increased 50% by NaCl loading and more than 100% by NaHCO3 loading, indicating stimulatory effects of both volume expansion and alkalosis. Rectal gland lactate dehydrogenase activity was elevated 25% by both treatments, indicating volume expansion effects only, whereas neither treatment increased the activities of the aerobic enzymes citrate synthase, NADP-isocitrate dehydrogenase, or the ketone body-utilizing enzyme beta-hydroxybutyrate dehydrogenase in the rectal gland or liver. The activity of ornithine-citrulline transcarbamoylase in skeletal muscle was doubled by NaHCO3 infusion, but neither treatment altered the activities of other OUC-related enzymes (glutamine synthetase, carbamoylphosphate synthetase III). We conclude that both the alkaline tide and salt loading/volume expansion act as signals to activate some but not all of the elevated metabolic pathways and ionoregulatory mechanisms needed during processing of a meal.

Inversion of allosteric effect of arginine on N-acetylglutamate synthase, a molecular marker for evolution of tetrapods

Background

The efficient conversion of ammonia, a potent neurotoxin, into non-toxic metabolites was an essential adaptation that allowed animals to move from the aquatic to terrestrial biosphere. The urea cycle converts ammonia into urea in mammals, amphibians, turtles, snails, worms and many aquatic animals and requires N-acetylglutamate (NAG), an essential allosteric activator of carbamylphosphate synthetase I (CPSI) in mammals and amphibians, and carbamylphosphate synthetase III (CPSIII) in fish and invertebrates. NAG-dependent CPSI and CPSIII catalyze the formation of carbamylphosphate in the first and rate limiting step of ureagenesis. NAG is produced enzymatically by N-acetylglutamate synthase (NAGS), which is also found in bacteria and plants as the first enzyme of arginine biosynthesis. Arginine is an allosteric inhibitor of microbial and plant NAGS, and allosteric activator of mammalian NAGS.

Results

Information from mutagenesis studies of E. coli and P. aeruginosa NAGS was combined with structural information from the related bacterial N-acetylglutamate kinases to identify four residues in mammalian NAGS that interact with arginine. Substitutions of these four residues were engineered in mouse NAGS and into the vertebrate-like N-acetylglutamate synthase-kinase (NAGS-K) of Xanthomonas campestris, which is inhibited by arginine. All mutations resulted in arginine losing the ability to activate mouse NAGS, and inhibit X. campestris NAGS-K. To examine at what point in evolution inversion of arginine effect on NAGS occur, we cloned NAGS from fish and frogs and examined the arginine response of their corresponding proteins. Fish NAGS were partially inhibited by arginine and frog NAGS were activated by arginine.

Conclusion

Difference in arginine effect on bacterial and mammalian NAGS most likely stems from the difference in the type of conformational change triggered by arginine binding to these proteins. The change from arginine inhibition of NAGS to activation was gradual, from complete inhibition of bacterial NAGS, to partial inhibition of fish NAGS, to activation of frog and mammalian NAGS. This change also coincided with the conquest of land by amphibians and mammals.

Contribution of dietary arginine to nitrogen utilisation and excretion in juvenile sea bass (Dicentrarchus labrax) fed diets differing in protein source

The role of dietary arginine in affecting nitrogen utilisation and excretion was studied in juvenile European sea bass (Dicentrarchus labrax) fed for 72 days with diets differing in protein sources (plant protein-based (PM) and fish-meal-based (FM)). Fish growth performance and nitrogen utilisation revealed that dietary Arg surplus was beneficial only in PM diets. Dietary Arg level significantly affected postprandial plasma urea concentrations. Hepatic arginase activity increased (P<0.05) in response to dietary Arg surplus in fish fed plant protein diets; conversely ornithine transcarbamylase activity was very low and inversely related to arginine intake. No hepatic carbamoyl phosphate synthetase III activity was detected. Dietary arginine levels did not affect glutamate dehydrogenase activity. A strong linear relationship was found between liver arginase activity and daily urea-N excretion. Dietary Arg excess reduced the proportion of total ammonia nitrogen excreted and increased the contribution of urea-N over the total N excretion irrespective of dietary protein source. Plasma and excretion data combined with enzyme activities suggest that dietary Arg degradation via hepatic arginase is a major pathway for ureagenesis and that ornithine-urea cycle is not completely functional in juvenile sea bass liver.

Sequence, expression and evolutionary relationships of carbamoyl phosphate synthetase I in the toadXenopus laevis

The sequence of carbamoyl phosphate synthetase I (CPSase I) cDNA and expression of the enzyme in liver of the toad Xenopus laevis are reported. CPSase I mRNA increases 6-fold when toads are exposed to high salinity for extended periods of time. The deduced 1,494-amino acid sequence of the CPSase I is homologous to other CPSases and reveals a domain structure and conserved amino acids common to other CPSases. A serine residue (S287) is present where there is a cysteine residue required for glutamine-dependent activity in CPSase Types III and II (Type I CPSases utilize only ammonia as nitrogen-donating substrate). A sequence of DNA 964 bases upstream from the ATG start codon for the CPSase I gene is also reported. Phylogenetic analysis for 30 CPSase isoforms, including X. laevis CPSase I, across a wide spectrum of phyla is reported and discussed. The results are consistent with the views that eukaryotic CPSase II as a multifunctional complex evolved from prokaryotic CPSase II and that CPSase I in terrestrial vertebrates and CPSase III in fishes arose from eukaryotic CPSase II by independent events after the divergence of plants in eukaryotic evolution.

Shifting Patterns of Nitrogen Excretion and Amino Acid Catabolism Capacity during the Life Cycle of the Sea Lamprey (Petromyzon marinus)

The jawless fish, the sea lamprey (Petromyzon marinus), spends part of its life as a burrow-dwelling, suspension-feeding larva (ammocoete) before undergoing a metamorphosis into a free swimming, parasitic juvenile that feeds on the blood of fishes. We predicted that animals in this juvenile, parasitic stage have a great capacity for catabolizing amino acids when large quantities of protein-rich blood are ingested. The sixfold to 20-fold greater ammonia excretion rates (J(Amm)) in postmetamorphic (nonfeeding) and parasitic lampreys compared with ammocoetes suggested that basal rates of amino acid catabolism increased following metamorphosis. This was likely due to a greater basal amino acid catabolizing capacity in which there was a sixfold higher hepatic glutamate dehydrogenase (GDH) activity in parasitic lampreys compared with ammocoetes. Immunoblotting also revealed that GDH quantity was 10-fold and threefold greater in parasitic lampreys than in ammocoetes and upstream migrant lampreys, respectively. Higher hepatic alanine and aspartate aminotransferase activities in the parasitic lampreys also suggested an enhanced amino acid catabolizing capacity in this life stage. In contrast to parasitic lampreys, the twofold larger free amino acid pool in the muscle of upstream migrant lampreys confirmed that this period of natural starvation is accompanied by a prominent proteolysis. Carbamoyl phosphate synthetase III was detected at low levels in the liver of parasitic and upstream migrant lampreys, but there was no evidence of extrahepatic (muscle, intestine) urea production via the ornithine urea cycle. However, detection of arginase activity and high concentrations of arginine in the liver at all life stages examined infers that arginine hydrolysis is an important source of urea. We conclude that metamorphosis is accompanied by a metabolic reorganization that increases the capacity of parasitic sea lampreys to catabolize intermittently large amino acid loads arising from the ingestion of protein rich blood from their prey/hosts. The subsequent generation of energy-rich carbon skeletons can then be oxidized or retained for glycogen and fatty acid synthesis, which are essential fuels for the upstream migratory and spawning phases of the sea lamprey's life cycle.

The dogfish shark (Squalus acanthias) increases both hepatic and extrahepatic ornithine urea cycle enzyme activities for nitrogen conservation after feeding

Urea not only is utilized as a major osmolyte in marine elasmobranchs but also constitutes their main nitrogenous waste. This study investigated the effect of feeding, and thus elevated nitrogen intake, on nitrogen metabolism in the Pacific spiny dogfish Squalus acanthias. We determined the activities of ornithine urea cycle (O-UC) and related enzymes in liver and nonhepatic tissues. Carbamoyl phosphate synthetase III (the rate-limiting enzyme of the O-UC) activity in muscle is high compared with liver, and the activities in both tissues increased after feeding. The contribution of muscle to urea synthesis in the dogfish body appears to be much larger than that of liver when body mass is considered. Furthermore, enhanced activities of the O-UC and related enzymes (glutamine synthetase, ornithine transcarbamoylase, arginase) were seen after feeding in both liver and muscle and were accompanied by delayed increases in plasma urea, trimethylamine oxide, total free amino acids, alanine, and chloride concentrations, as well as in total osmolality. The O-UC and related enzymes also occurred in the intestine but showed little change after feeding. Feeding did not change the rate of urea excretion, indicating strong N retention after feeding. Ammonia excretion, which constituted only a small percentage of total N excretion, was raised in fed fish, while plasma ammonia did not change, suggesting that excess ammonia in plasma is quickly ushered into synthesis of urea or protein. In conclusion, we suggest that N conservation is a high priority in this elasmobranch and that feeding promotes ureogenesis and growth. Furthermore, exogenous nitrogen from food is converted into urea not only by the liver but also by the muscle and to a small extent by the intestine.

The little skate Raja erinacea exhibits an extrahepatic ornithine urea cycle in the muscle and modulates nitrogen metabolism during low-salinity challenge

Urea synthesis via the hepatic ornithine urea cycle (OUC) has been well described in elasmobranchs, but it is unknown whether OUC enzymes are also present in extrahepatic tissues. Muscle and liver urea, trimethylamine oxide (TMAO), and other organic osmolytes, as well as selected OUC enzymes (carbamoyl phosphate synthetase III, ornithine transcarbamoylase, arginase, and the accessory enzyme glutamine synthetase), were measured in adult little skates (Raja erinacea) exposed to 100% or 75% seawater for 5 d. Activities of all four OUC enzymes were detected in the muscle. There were no changes in muscle OUC activities in skates exposed to 75% seawater; however, arginase activity was significantly lower in the liver, compared to controls. Urea, TMAO, and several other osmolytes were significantly lower in the muscle of little skates exposed to 75% seawater, whereas only glycerophosphorylcholine was significantly lower in the liver. Urea excretion rates were twofold higher in skates exposed to 75% seawater. Taken together, these data suggest that a functional OUC may be present in the skeletal muscle tissues of R. erinacea. As well, enhanced urea excretion rates and the downregulation of the anchor OUC enzyme, arginase, in the liver may be critical in regulating tissue urea content under dilute-seawater stress.

Discordance between genetic structure and morphological, ecological, and physiological adaptation in Lake Magadi tilapia

The Magadi tilapia (Alcolapia grahami, formerly Oreochromis alcalicus grahami) is a remarkable example of teleost life in an extreme environment. Typical conditions include water pH=10, titration alkalinity>300 mM, osmolality=525 mOsm, temperatures ranging from 23 degrees to 42 degrees C, and O(2) levels fluctuating diurnally between extreme hyperoxia and anoxia. A number of relatively small tilapia populations are present in various thermal spring lagoons around the margin of the lake separated by kilometers of solid trona crust (floating Na(2)CO(3)) underlain by anoxic water. Despite the apparent isolation of different populations, annual floods may provide opportunities for exchange of fish across the surface of the trona and subsequent gene flow. To assess the question of isolation among Lake Magadi populations, we analyzed the variable control region of the mitochondrial DNA (mtDNA) from six lagoons. A total of seven mtDNA haplotypes, including three common haplotypes, were observed in all six populations. Several of the Lake Magadi populations showed haplotype frequencies indicative of differentiation, while others showed very little. However, differentiation among lagoon populations was discordant with their geographical distribution along the shoreline. All populations exhibited the unusual trait of 100% ureotelism but specialized morphological and physiological characteristics were observed among several of the lagoon systems. In addition, distinct differences were observed in the osmolality among the lagoons with levels as high as 1,400-1,700 mOsm kg(-1), with corresponding differences in the natural levels of whole-body urea. These levels of osmotic pressure proved fatal to fish from less alkaline systems but remarkably were also fatal to the fish that inhabited lagoons with this water chemistry. Upon more detailed inspection, specific adaptations to differential conditions in the lagoon habitat were identified that allowed survival of these cichlids. Additional evidence against potential for gene flow among lagoons despite the sharing of common mtDNA haplotypes was that the osmolality of floodwaters following a heavy rain showed lethal levels exceeding 1,700 mOsm kg(-1). In isolation, different mtDNA haplotypes would be predicted to go to fixation in different populations due to rapid generation times and the small effective population sizes in a number of lagoons. We propose a model of balancing selection to maintain common mtDNA sequences through a common selection pressure among lagoons that is based on microhabitats utilized by the tilapia.

Molecular biology of major components of chloride cells

Current understanding of chloride cells (CCs) is briefly reviewed with emphasis on molecular aspects of their channels, transporters and regulators. Seawater-type and freshwater-type CCs have been identified based on their shape, location and response to different ionic conditions. Among the freshwater-type CCs, subpopulations are emerging that are implicated in the uptake of Na(+), Cl(-) and Ca(2+), respectively, and can be distinguished by their shape of apical crypt and affinity for lectins. The major function of the seawater CC is transcellular secretion of Cl(-), which is accomplished by four major channels and transporters: (1). CFTR Cl(-) channel, (2). Na(+),K(+)-ATPase, (3). Na(+)/K(+)/2Cl(-) cotransporter and (4). a K(+) channel. The first three components have been cloned and characterized, but concerning the K(+) channel that is essential for the continued generation of the driving force by Na(+),K(+)-ATPase, only one candidate is identified. Although controversial, freshwater CCs seem to perform the uptake of Na(+), Cl(-) and Ca(2+) in a manner analogous to but slightly different from that seen in the absorptive epithelia of mammalian kidney and intestine since freshwater CCs face larger concentration gradients than ordinary epithelial cells. The components involved in these processes are beginning to be cloned, but their CC localization remains to be established definitively. The most important yet controversial issue is the mechanism of Na(+) uptake. Two models have been postulated: (i). the original one involves amiloride-sensitive electroneutral Na(+)/H(+) exchanger (NHE) with the driving force generated by Na(+),K(+)-ATPase and carbonic anhydrase (CA) and (ii). the current model suggests that Na(+) uptake occurs through an amiloride-sensitive epithelial sodium channel (ENaC) electrogenically coupled to H(+)-ATPase. While fish ENaC remains to be identified by molecular cloning and database mining, fish NHE has been cloned and shown to be highly expressed on the apical membrane of CCs, reviving the original model. The CC is also involved in acid-base regulation. Analysis using Osorezan dace (Tribolodon hakonensis) living in a pH 3.5 lake demonstrated marked inductions of Na(+),K(+)-ATPase, CA-II, NHE3, Na(+)/HCO(3)(-) cotransporter-1 and aquaporin-3 in the CCs on acidification, leading to a working hypothesis for the mechanism of Na(+) retention and acid-base regulation.

Dogmas and controversies in the handling of nitrogenous wastes: osmoregulation during early embryonic development in the marine little skate Raja erinacea; response to changes in external salinity

Marine elasmobranchs retain relatively high levels of urea to counterbalance the osmotic strength of seawater. Oviparous species, such as the little skate Raja erinacea, release encapsulated embryos that hatch after about 9 months on the seafloor. To study the ureosmotic capability of skate embryos, we measured a variety of possible osmolytes and ornithine-urea cycle (OUC) enzyme activities in little skate embryos, and determined their physiological response to dilute seawater (75% SW) exposure relative to controls (100% SW). The urea:trimethylamine oxide (TMAO) + other osmolytes ratio was 2.3-2.7:1. At the earliest stage of development investigated (4 months), there were significant levels of the key OUC enzyme, carbamoyl phosphate synthetase III, as well as ornithine transcarbamoylase, arginase and glutamine synthetase, providing evidence for a functional OUC. Embryos (4 and 8 months) survived and recovered from exposure to 5 days of 75% SW. There was a significant increase in the rate of urea excretion (five- to tenfold), no change in OUC enzyme activities, and significant decreases in the tissue content of urea, TMAO and other osmolytes in embryos exposed to 75% SW compared to 100% SW. Taken together, the data indicate that little skate embryos synthesize and retain urea, as well as a suite of other osmolytes, in order to regulate osmotic balance with the external environment. Interestingly, these ureosmotic mechanisms are in place as early as 4 months, around the time at which the egg capsule opens and the embryo is in more direct contact with the external environment.

Pulsatile urea excretion in the gulf toadfish: mechanisms and controls

Opsanus beta expresses a full complement of ornithine-urea cycle (OUC) enzymes and is facultatively ureotelic, reducing ammonia-N excretion and maintaining urea-N excretion under conditions of crowding/confinement. The switch to ureotelism is keyed by a modest rise in cortisol associated with a substantial increase in cytosolic glutamine synthetase for trapping of ammonia-N and an upregulation of the capacity of the mitochondrial OUC to use glutamine-N. The entire day's urea-N production is excreted in 1 or 2 short-lasting pulses, which occur exclusively through the gills. The pulse event is not triggered by an internal urea-N threshold, is not due to pulsatile urea-N production, but reflects pulsatile activation of a specific branchial excretion mechanism that rapidly clears urea-N from the body fluids. A bidirectional facilitated diffusion transporter, with pharmacological similarity to the UT-A type transporters of the mammalian kidney, is activated in the gills, associated with an increased trafficking of dense-cored vesicles in the pavement cells. An 1814 kB cDNA ('tUT') coding for a 475-amino acid protein with approximately 62% homology to mammalian UT-A's has been cloned and facilitates phloretin-sensitive urea transport when expressed in Xenopus oocytes. tUT occurs only in gill tissue, but tUT mRNA levels do not change over the pulse cycle, suggesting that tUT regulation occurs at a level beyond mRNA. Circulating cortisol levels consistently decline prior to a pulse event and rise thereafter. When cortisol is experimentally clamped at high levels, natural pulse events are suppressed in size but not in frequency, an effect mediated through glucocorticoid receptors. The cortisol decline appears to be permissive, rather than the actual trigger of the pulse event. Fluctuations in circulating AVT levels do not correlate with pulses; and injections of AVT (at supraphysiological levels) elicit only minute urea-N pulses. However, circulating 5-hydroxytryptamine (5-HT) levels fluctuate considerably and physiological doses of 5-HT cause large urea-N pulse events. When the efferent cranial nerves to the gills are sectioned, natural urea pulse events persist, suggesting that direct motor output from the CNS to the gill is not the proximate control.

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.

Physiological adaptations of the gut in the Lake Magadi tilapia, Alcolapia grahami, an alkaline- and saline-adapted teleost fish

We describe the gut physiology of the Lake Magadi tilapia (Alcolapia grahami), specifically those aspects associated with feeding and drinking while living in water of unusually high carbonate alkalinity (titratable base=245 mequiv l(-1)) and pH (9.85). Drinking of this highly alkaline lake water occurs at rates comparable to or higher than those seen in marine teleosts. Eating and drinking take place throughout the day, although drinking predominates during hours of darkness. The intestine directly intersects the esophagus at the anterior end of the stomach forming a 'T', and the pyloric sphincter, which comprises both smooth and striated muscle, is open when the stomach is empty and closed when the stomach is full. This unique configuration (a functional trifurcation) allows imbibed alkaline water to bypass the empty stomach, thereby avoiding a reactive mixing with acidic gastric fluids, and minimizes interference with a full stomach. No titratable base was present in the stomach, where the mean pH was 3.55, but the intestine was progressively more alkaline (foregut 6.96, midgut 7.74, hindgut 8.12, rectum 8.42); base levels in the intestinal fluid were comparable to those in lake water. The gut was highly efficient at absorbing water (76.6%), which accompanied the absorption of Na(+) (78.5%), titratable base (80.8%), and Cl(-) (71.8%). The majority of Na(+), base and water absorption occurred in the foregut by an apparent Na(+) plus base co-transport system. Overall, more than 70% of the intestinal flux occurred via Na(+) plus base co-transport, and less than 30% by Na(+) plus Cl(-) co-transport, a very different situation from the processes in the intestine of a typical marine teleost.

Mechanism of acid adaptation of a fish living in a pH 3.5 lake

Despite unfavorable conditions, a single species of fish, Osorezan dace, lives in an extremely acidic lake (pH 3.5) in Osorezan, Aomori, Japan. Physiological studies have established that this fish is able to prevent acidification of its plasma and loss of Na(+). Here we show that these abilities are mainly attributable to the chloride cells of the gill, which are arranged in a follicular structure and contain high concentrations of Na(+)-K(+)-ATPase, carbonic anhydrase II, type 3 Na(+)/H(+) exchanger (NHE3), type 1 Na(+)-HCO(3)(-) cotransporter, and aquaporin-3, all of which are upregulated on acidification. Immunohistochemistry established their chloride cell localization, with NHE3 at the apical surface and the others localized to the basolateral membrane. These results suggest a mechanism by which Osorezan dace adapts to its acidic environment. Most likely, NHE3 on the apical side excretes H(+) in exchange for Na(+), whereas the electrogenic type 1 Na(+)-HCO(3)(-) cotransporter in the basolateral membrane provides HCO(3)(-) for neutralization of plasma using the driving force generated by Na(+)-K(+)-ATPase and carbonic anhydrase II. Increased expression of glutamate dehydrogenase was also observed in various tissues of acid-adapted dace, suggesting a significant role of ammonia and bicarbonate generated by glutamine catabolism.

A second glutamine synthetase gene with expression in the gills of the gulf toadfish (Opsanus beta)

We characterized the expression of the nitrogen metabolism enzyme glutamine synthetase [GSase; L-glutamate: ammonia ligase (ADP-forming), E.C. 6.3.1.2] in tissues of the gulf toadfish Opsanus beta subjected to unconfined (ammonotelic) and confined (ureotelic) conditions. Enzymological results demonstrate that mass-specific GSase activities rank in the order of brain > liver > stomach approximately kidney > intestine > gill > heart/spleen > muscle. When tissue mass is used to calculate a glutamine synthetic potential, the liver has the greatest, followed by muscle > stomach and intestine, with minor contributions from the remaining tissues. Additionally, during confinement stress, GSase activity increases significantly only in liver (fivefold) and muscle (twofold), tissues that previously showed significant expression of the other enzymes of urea synthesis. Western analyses of samples on SDS gels demonstrated that GSase-specific protein content reflected enzyme activity, and all tissues except muscle had a single, similarly sized GSase subunit of 49.4 kDa; muscle showed staining of two bands of 36.8 and 98.9 kDa, which may possibly result from another gene product or post-translational modification. RT-PCR and RACE-PCR revealed the presence of a second GSase cDNA from gill tissue that shares only 73% nucleotide and amino acid sequence similarity with the GSase cDNA previously cloned from liver, and that lacks a mitochondrial leader-targeting sequence. RT-PCR and restriction digestion experiments demonstrated that mRNA from the original 'liver' GSase is expressed in all tissues examined (liver, gill, stomach, intestine, kidney, brain and muscle), whereas the new 'gill' form shows expression primarily in the gill. Gill GSase activity shows apparently exclusive expression in the soluble compartment, while other tissues expressing the 'liver' form show both cytoplasmic and mitochondrial activities. Phylogenetic analysis of a number of GSases demonstrates that the toadfish gill GSase has a greater affinity for a clade that includes the Xenopus GSase genes and one of two Fugu GSase genes, than it has for a clade containing the toadfish liver GSase and other described teleost GSase genes. The results are discussed in the context of a prior hypothesis on an ammonia-trapping mechanism in the gill of the toadfish.

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.

Excretory nitrogen metabolism in the juvenile axolotl Ambystoma mexicanum: differences in aquatic and terrestrial environments

The fully grown but nonmetamorphosed (juvenile) axolotl Ambystoma mexicanum was ureogenic and primarily ureotelic in water. A complete ornithine-urea cycle (OUC) was present in the liver. Aerial exposure impeded urea (but not ammonia) excretion, leading to a decrease in the percentage of nitrogen excreted as urea in the first 24 h. However, urea and not ammonia accumulated in the muscle, liver, and plasma during aerial exposure. By 48 h, the rate of urea excretion recovered fully, probably due to the greater urea concentration gradient in the kidney. It is generally accepted that an increase in carbamoyl phosphate synthetase activity is especially critical in the developmental transition from ammonotelism to ureotelism in the amphibian. Results from this study indicate that such a transition in A. mexicanum would have occurred before migration to land. Aerial exposure for 72 h exhibited no significant effect on carbamoyl phosphate synthetase-I activity or that of other OUC enzymes (with the exception of ornithine transcarbamoylase) from the liver of the juvenile A. mexicanum. This supports our hypothesis that the capacities of OUC enzymes present in the liver of the aquatic juvenile axolotl were adequate to prepare it for its invasion of the terrestrial environment. The high OUC capacity was further supported by the capability of the juvenile A. mexicanum to survive in 10 mM NH(4)Cl without accumulating amino acids in its body. The majority of the accumulating endogenous and exogenous ammonia was detoxified to urea, which led to a greater than twofold increase in urea levels in the muscle, liver, and plasma and a significant increase in urea excretion by hour 96. Hence, it can be concluded that the juvenile axolotl acquired ureotelism while submerged in water, and its hepatic capacity of urea synthesis was more than adequate to handle the toxicity of endogenous ammonia during migration to land.

Glutamine synthetase expression in liver, muscle, stomach and intestine ofBostrichthys sinensisin response to exposure to a high exogenous ammonia concentration

A previous study provided evidence that the adaptive strategy used by the teleost fish Bostrichthys sinensis (sleeper) for detoxifying ammonia during extended periods of air exposure was to synthesize and store glutamine,primarily in the muscle, accompanied by an increase in glutamine synthetase(GSase) activity in liver. The aim of the present study was to assess the effect on GSase expression in various tissues of exposure of B. sinensis to exogenous ammonia. Exogenous ammonia increases internal ammonia concentrations in fish, mimicking environmental situations such as air exposure that preclude loss of ammonia across the gills, and thus triggering alternative mechanisms for ammonia detoxification. The results reveal relatively high levels of GSase activity, not only in liver but also,unexpectedly, in muscle, and even higher levels in intestine and, in particular, stomach. Exposure to ammonia results in significant increases in GSase activity, GSase protein and GSase mRNA levels in all of these tissues except stomach. The amino acid sequences of GSases from liver and stomach deduced from the cDNA sequences are essentially identical and are &gt;97 %identical to the amino acid sequences of GSases from Gulf toadfish(Opsanus beta) and marble goby (Oxyeleotris marmoratus).

Effect of alkalinity (pH 10) on ureogenesis in the air-breathing walking catfish, Clarias batrachus

Exposure of fish to alkaline conditions inhibits the rate of ammonia excretion, leading to ammonia accumulation and toxicity. The purpose of this study was to determine the role of ureogenesis via the urea cycle, to avoid the accumulation of ammonia to a toxic level during chronic exposure to alkaline conditions, for the air-breathing walking catfish, Clarias batrachus, where a full complement of urea cycle enzyme activity has been documented. The walking catfish can survive in water with a pH up to 10. At a pH of 10 the ammonia excretion rate by the walking catfish decreased by approximately 75% within 6 h. Although there was a gradual improvement of ammonia excretion rate by the alkaline-exposed fish, the rate remained 50% lower, even after 7 days. This decrease of ammonia excretion was accompanied by a significant accumulation of ammonia in plasma and body tissues (except in the brain). Urea-N excretion for alkaline-exposed fish increased 2.5-fold within the first day, which was maintained until day 3 and was then followed by a slight decrease to maintain a 2-fold increase in the urea-N excretion rate, even after 7 days. There was also a higher accumulation of urea in plasma and other body tissues (liver, kidney, muscle and brain). The activity of glutamine synthetase and three enzymes operating in the urea cycle (carbamyl phosphate synthetase, argininosuccinate synthetase, argininosuccinate lyase) increased significantly in hepatic and extra-hepatic tissue, such as the kidney and muscle in C. batrachus, during exposure to alkaline water. A significant increase in plasma lactate concentration noticed during alkaline exposure possibly helped in the maintenance of the acid-base balance. It is apparent that the stimulation of ureogenesis via the induced urea cycle is one of the major physiological strategies adopted by the walking catfish (C. batrachus) during chronic exposure to alkaline water, to avoid the in vivo accumulation of ammonia to a toxic level in body tissues and for the maintenance of pH homeostasis.