mRNA expression analysis of the physiological responses to ammonia infusion in rainbow trout

Expression of ion transport proteins and routine metabolism in juveniles of tropical gar (Atractosteus tropicus) exposed to ammonia

Tropical gar (Atractosteus tropicus) thrives in aquatic habitats with high levels of total nitrogen (TAN) and unionized ammonia (NH₃). However, the tolerance of TAN and NH₃, the excretion mechanisms involved, and the effects of these chemicals on routine metabolism are still unknown. Therefore, our objectives were to assess the acute toxicity of TAN and NH₃ in A. tropicus juveniles after a 96-h exposure (LC50-96 h) to NH₄Cl and after chronic exposure to two concentrations (15% and 30% of LC50-96 h TAN) for 12 days, as well as to evaluate the transcriptional effects associated with Rhesus proteins (rhag, rhbg, rhcg) and ion transporters (NHE, NKA, NKCC, and CFTR) in gills and skin; and to determine the effects of TAN and NH3 on routine metabolism through oxygen consumption (μM g^-1 h^-1) and gill ventilation frequency (beats min^-1). LC50-96 h values were 100.20 ± 11.21 mg/L for TAN and 3.756 ± 0.259 mg/L for NH₃. The genes encoding Rhesus proteins and ion transporters in gills and skin showed a differential expression according to TAN concentrations and exposure time. Oxygen consumption on day 12 showed significant differences between treatments with 15% and 30% TAN. Gill ventilation frequency on day 12 was higher in fish exposed to 30% TAN. In conclusion, A. tropicus juveniles are highly tolerant to TAN, showing upregulation of the genes involved in TAN excretion through gills and skin, which affects routine oxygen consumption and energetic cost. These findings are relevant for understanding adaptations in the physiological response of a tropical ancestral air-breathing fish.

The skin of adult rainbow trout is not a significant site of ammonia clearance from the blood

We hypothesized that the skin acts as an extrabranchial route for ammonia excretion in adult rainbow trout (Oncorhynchus mykiss) following high environmental ammonia (HEA) exposure. Trunks of control or HEA-exposed trout were perfused with saline containing 0 or 1 mmol l^-1 NH₄ ⁺ . Cutaneous ammonia excretion rates increased 2.5-fold following HEA exposure, however there was no difference in rates between trunks perfused with 0 or 1 mmol l^-1 NH₄ ⁺ . The skin is therefore capable of excreting its own ammonia load, but it does not clear circulating ammonia from the plasma.

Skeletal Muscle and the Effects of Ammonia Toxicity in Fish, Mammalian, and Avian Species: A Comparative Review Based on Molecular Research

Typically, mammalian and avian models have been used to examine the effects of ammonia on skeletal muscle. Hyperammonemia causes sarcopenia or muscle wasting, in mammals and has been linked to sarcopenia in liver disease patients. Avian models of skeletal muscle have responded positively to hyperammonemia, differing from the mammalian response. Fish skeletal muscle has not been examined as extensively as mammalian and avian muscle. Fish skeletal muscle shares similarities with avian and mammalian muscle but has notable differences in growth, fiber distribution, and response to the environment. The wide array of body sizes and locomotion needs of fish also leads to greater diversity in muscle fiber distribution and growth between different fish species. The response of fish muscle to high levels of ammonia is important for aquaculture and quality food production but has not been extensively studied to date. Understanding the differences between fish, mammalian and avian species’ myogenic response to hyperammonemia could lead to new therapies for muscle wasting due to a greater understanding of the mechanisms behind skeletal muscle regulation and how ammonia effects these mechanisms. This paper provides an overview of fish skeletal muscle and ammonia excretion and toxicity in fish, as well as a comparison to avian and mammalian species.

The Rhesus glycoprotein Rhcgb is expendable for ammonia excretion and Na+ uptake in zebrafish (Danio rerio)

In zebrafish (Danio rerio), the ammonia-transporting Rhesus glycoprotein Rhcgb is implicated in mechanisms of ammonia excretion and Na⁺ uptake. In particular, Rhcgb is thought to play an important role in maintaining ammonia excretion in response to alkaline conditions and high external ammonia (HEA) exposure, in addition to facilitating Na⁺ uptake via a functional metabolon with the Na⁺/H⁺-exchanger Nhe3b, specifically under low Na⁺ conditions. In the present study, we hypothesized that CRISPR/Cas9 knockout of rhcgb would reduce ammonia excretion and Na⁺ uptake capacity, particularly under the conditions listed above that have elicited increases in Rhcgb-mediated ammonia excretion and/or Na⁺ uptake. Contrary to this hypothesis, however, larval and juvenile rhcgb knockout (KO) mutants showed no reductions in ammonia excretion or Na⁺ uptake under any of the conditions tested in our study. In fact, under control conditions, rhcgb KO mutants generally displayed an increase in ammonia excretion, potentially due to increased transcript abundance of another rh gene, rhbg. Under alkaline conditions, rhcgb KO mutants were also able to maintain ammonia excretion, similar to wild-type fish, and stimulation of ammonia excretion after HEA exposure also was not affected by rhcgb KO. Surprisingly, ammonia excretion and Na⁺ uptake were unaffected by rhcgb or nhe3b KO in juvenile zebrafish acclimated to normal (800 μmol/L) or low (10 μmol/L) Na⁺ conditions. These results demonstrate that Rhcgb is expendable for ammonia excretion and Na⁺ uptake in zebrafish, highlighting the plasticity and flexibility of these physiological systems in this species.

Cortisol modulates metabolism and energy mobilization in wild-caught pumpkinseed (Lepomis gibbosus)

Acute elevation of cortisol via activation of the hypothalamic-pituitary-interrenal (HPI) axis aids the fish in dealing with a stressor. However, chronic elevation of cortisol has detrimental effects and has been studied extensively in lab settings. However, data pertaining to wild teleosts are lacking. Here, we characterized the metabolic consequences of prolonged cortisol elevation (96 h) in wild-caught pumpkinseed (Lepomis gibbosus). Pumpkinseed were implanted with cocoa butter alone (sham) or containing cortisol (25 mg kg^-1 body weight), and at 24, 48, 72, and 96 h, tissue samples were collected, whole-body ammonia excretion was determined, and whole-organism metabolism was assessed using intermittent flow respirometry. Cortisol-treated pumpkinseed exhibited the highest plasma cortisol concentration at 24 h post-implantation, with levels decreasing over the subsequent time points although remaining higher than in sham-treated fish. Cortisol-treated fish exhibited higher standard and maximal metabolic rates than sham-treated fish, but the effect of cortisol treatment on aerobic scope was negligible. Indices of energy synthesis/mobilization, including blood glucose concentrations, hepatosomatic index, hepatic glycogen concentrations, and ammonia excretion rates, were higher in cortisol-treated fish compared with controls. Our work suggests that although aerobic scope was not diminished by prolonged elevation of cortisol levels, higher metabolic expenditures may be of detriment to the animal's performance in the longer term.

Physiological trade-offs, acid-base balance and ion-osmoregulatory plasticity in European sea bass (Dicentrarchus labrax) juveniles under complex scenarios of salinity variation, ocean acidification and high ammonia challenge

In this era of global climate change, ocean acidification is becoming a serious threat to the marine ecosystem. Despite this, it remains almost unknown how fish will respond to the co-occurrence of ocean acidification with other conventional environmental perturbations typically salinity fluctuation and high ammonia threat. Therefore, the present work evaluated the interactive effects of elevated pCO₂, salinity reduction and high environmental ammonia (HEA) on the ecophysiological performance of European sea bass (Dicentrarchus labrax). Fish were progressively acclimated to seawater (32 ppt), to brackish water (10 ppt) and to hyposaline water (2.5 ppt). Following acclimation to different salinities for at least two weeks, fish were exposed to CO₂-induced water acidification representing present-day (control pCO₂, 400 μatm, LoCO₂) and future (high pCO₂, 1000 μatm, HiCO₂) sea-surface CO₂ level for 3, 7 and 21 days. At the end of each exposure period, fish were challenged with HEA for 6 h (1.18 mM representing 50% of 96 h LC₅₀). Results show that, in response to the individual HiCO₂ exposure, fish within each salinity compensated for blood acidosis. Fish subjected to HiCO₂ were able to maintain ammonia excretion rate (J_amm) within control levels, suggesting that HiCO₂ exposure alone had no impact on J_amm at any of the salinities. For 32 and 10 ppt fish, up-regulated expression of Na⁺/K⁺-ATPase was evident in all exposure groups (HEA, HiCO₂ and HEA/HiCO₂ co-exposed), whereas Na⁺/K⁺/2Cl^- co-transporter was up-regulated mainly in HiCO₂ group. Plasma glucose and lactate content were augmented in all exposure conditions for all salinity regimes. During HEA and HEA/HiCO₂, J_amm was inhibited at different time points for all salinities, which resulted in a significant build-up of ammonia in plasma and muscle. Branchial expressions of Rhesus glycoproteins (Rhcg isoforms and Rhbg) were upregulated in response to HiCO₂ as well as HEA at 10 ppt, with a more moderate response in 32 ppt groups. Overall, our findings denote that the adverse effect of single exposures of ocean acidification or HEA is exacerbated when present together, and suggests that fish are more vulnerable to these environmental threats at low salinities.

Physiological insights into largemouth bass (Micropterus salmoides) survival during long-term exposure to high environmental ammonia

Waterborne ammonia is an environmental pollutant that is toxic to all aquatic animals. However, ammonia induced toxicity as well as compensatory mechanisms to defend against high environmental ammonia (HEA) are not well documented at present for largemouth bass (Micropterus salmoides), a high value fish for culture and sport fisheries in the United States. To provide primary information on the sensitivity of this species to ammonia toxicity, a 96 h-LC₅₀ test was conducted. Thereafter, responses at physiological, ion-regulatory and transcript levels were determined to get insights into the underlying adaptive strategies to ammonia toxicity. For this purpose, fish were progressively exposed to HEA (8.31 mg/L representing 25% of 96 h-LC₅₀) for 3, 7, 14, 21 and 28 days. Temporal effects of HEA on oxygen consumption rate (MO₂), ammonia and urea dynamics, plasma ions (Na⁺, Cl^- and K⁺), branchial Na⁺/K⁺-ATPase (NKA) and H⁺-ATPase activity, muscle water content (MWC), energy store (glycogen, lipid and protein) as well as branchial mRNA expression of Rhesus (Rh) glycoproteins were assessed. Probit analysis showed that 96 h-LC₅₀ of (total) ammonia (as NH₄HCO₃) at 25 °C and pH 7.8 was 33.24 mg/L. Results from sub-lethal end-points shows that ammonia excretion rate (J_amm) was strongly inhibited after 7 days of HEA, but was unaffected at 3, 14 and 21 days. At 28 days fish were able to increase J_amm efficiently and concurrently, plasma ammonia re-established to the basal level. Urea production was increased as evidenced by a considerable elevation of plasma urea, but urea excretion rate remained unaltered. Expression of Rhcg isoform (Rhcg2) mRNA was up-regulated in parallel with restored or increased J_amm, suggesting its ammonia excreting role in largemouth bass. Exposure to HEA also displayed pronounced augmentations in NKA activity, exemplified by a rise in plasma [Na⁺]. Furthermore, [K⁺], [Cl^-] and MWC homeostasis were disrupted followed by recovery to the control levels. H⁺-ATPase activity was elevated but NKA did not appear to function preferentially as a Na⁺/NH₄⁺-ATPase. From 14 days onwards MO₂ was depressed, potentially an attempt towards minimizing catabolism. Glycogen content in liver and muscle were temporarily depleted, whereas a remarkable increment in protein was evident at the last exposure period. Overall, these data suggest that ammonia induced toxicity can disturb several biological processes in largemouth bass, however, it can adapt to the long-term sub-lethal ammonia concentrations by activating various components of ammonia excretory, ion-regulatory and metabolic pathways.

Transcriptomic evidence of adaptive tolerance to high environmental ammonia in mudskippers

Mudskippers are typical amphibious fishes and possess various strategies to ameliorate ammonia toxicity during exposure to environmental ammonia. The present study aimed to provide transcriptomic evidence through profiling the gill and liver transcriptomes of Boleophthalmus pectinirostris (BP) and Periophthalmus magnuspinnatus (PM), which were subjected to treatment with high environmental ammonia for up to 72 h. The results of gene function annotation showed that most of the differentially expressed genes were involved in metabolic pathways. After ammonia exposure, the protein and amino acid metabolism related genes in mudskippers were down-regulated, and PM had more down-regulated genes than BP. The expression levels of several representative genes involved in ammonia excretion in the gill were commonly increased. Interestingly, NH₄⁺ transporting and H⁺ excreting related genes, including Na⁺/K⁺(NH₄⁺)/2Cl^- cotransporter (nkcc), Na+/K+(NH4+)-ATPase (nka), carbonic anhydrase 2 (ca2), H⁺-ATPase, Na⁺/H⁺ (NH₄⁺)-exchanger (nhe), and carbonic anhydrase 15 (ca15), were up-regulated more significantly in BP than PM; however, the transcription levels of Rhesus glucoprotein b (Rhbg) and Rhesus glucoprotein c1 (Rhcg1), which constitute the NH₃ transporting channels, were up-regulated more significantly in PM than BP. Furthermore, the present study provides molecular evidence for how mudskippers adopt partial amino acid catabolism to decrease the production of endogenous ammonia under high environmental ammonia loading.

The effects of high environmental ammonia on the structure of rainbow trout hierarchies and the physiology of the individuals therein

Our goals were: (i) to determine whether sublethal concentrations of water-borne ammonia would prevent the formation of a dominance hierarchy, or alter its structure, in groups of 4 juvenile trout; (ii) to investigate the behavioral and physiological responses of individuals of different social rank exposed to a concentration of ammonia that still allowed hierarchy formation. Social hierarchies were created by using a technique in which a food delivery system that created competition also served to isolate individual fish for respirometry. Groups of 4 fish were exposed to elevated ammonia (NH₄HCO₃) 12 h before first feeding; aggression was recorded by video camera during morning feedings. Experimental ammonia concentrations were 700, 1200 and 1500 μmol L^-1 at pH 7.3, 12 °C (9.8, 16.8, and 21.0 mg L^-1 as total ammonia-N, or 0.0515, 0.0884, and 0.1105 mg L^-1 as NH₃-N). Aggression was severely reduced by 1200 and abolished by 1500 μmol L^-1 total ammonia, such that hierarchies did not form. However, groups exposed to 700 μmol L^-1 total ammonia still formed stable hierarchies but displayed lower levels of aggression in comparison to control hierarchies. Exposure continued for 11 days. Physiological parameters were recorded on day 5 (end of period 1) and day 10 (end of period 2), while feeding and plasma cortisol were measured on day 11. In control hierarchies, dominant (rank 1) trout generally exhibited higher growth rates, greater increases in condition factor, higher food consumption, and lower cortisol levels than did fish of ranks 2, 3, and 4. In comparison to controls, the 700 μmol L^-1 total ammonia hierarchies generally displayed lower growth, lower condition factor increases, lower O₂ consumption, lower cortisol levels, but similar feeding patterns, with smaller physiological differences amongst ranks during period 1. Effects attenuated during period 2, as aggression and physiological measures returned towards control levels, indicating both behavioral and physiological acclimation to ammonia. These disturbances in social behavior and associated physiology occurred at an ammonia concentration in the range of regulatory significance and relevance to aquaculture.

Ammonia exposure affects the mRNA and protein expression levels of certain Rhesus glycoproteins in the gills of climbing perch

The freshwater climbing perch, Anabas testudineus, is an obligate air-breathing and euryhaline teleost capable of active ammonia excretion and tolerant of high concentrations of environmental ammonia. As Rhesus glycoproteins (RhGP/Rhgp) are known to transport ammonia, this study aimed to obtain the complete cDNA coding sequences of various rhgp isoforms from the gills of A. testudineus, and to determine their mRNA and protein expression levels during 6 days of exposure to 100 mmol l^-1 NH₄Cl. The subcellular localization of Rhgp isoforms in the branchial epithelium was also examined in order to elucidate the type of ionocyte involved in active ammonia excretion. Four rhgp (rhag, rhbg, rhcg1 and rhcg2) had been identified from the gills of A. testudineus They had conserved amino acid residues for NH₄⁺ binding, NH₄⁺ deprotonation, channel gating and lining of the vestibules. Despite inwardly directed NH₃ and NH₄⁺ gradients, there were significant increases in the mRNA expression levels of the four branchial rhgp in A. testudineus at certain time points during 6 days of ammonia exposure, with significant increases in the protein abundances of Rhag and Rhcg2 on day 6. Immunofluorescence microscopy revealed a type of ammonia-inducible Na⁺/K⁺-ATPase α1c-immunoreactive ionocyte with apical Rhag and basolateral Rhcg2 in the gills of fish exposed to ammonia for 6 days. Hence, active ammonia excretion may involve NH₄⁺ entering the ionocyte through the basolateral Rhcg2 and being excreted through the apical Rhag, driven by a transapical membrane electrical potential generated by the apical cystic fibrosis transmembrane conductance regulator Cl^- channel, as suggested previously.

Molecular characterization of two Rhesus glycoproteins from the euryhaline freshwater white-rimmed stingray, Himantura signifer, and changes in their transcript levels and protein abundance in the gills, kidney, and liver during brackish water acclimation

Himantura signifer is a freshwater stingray which inhabits rivers in Southeast Asia. It is ammonotelic in fresh water, but retains the capacities of urea synthesis and ureosmotic osmoregulation to survive in brackish water. This study aimed to elucidate the roles of Rhesus glycoproteins (Rhgp), which are known to transport ammonia, in conserving nitrogen (N) in H. signifer during brackish water acclimation when N became limited resulting from increased hepatic urea synthesis. The complete coding sequence of rhbg from H. signifer consisted of 1383 bp, encoding 460 amino acids with an estimated molecular mass of 50.5 kDa, while that of rhcg comprised 1395 bp, encoding for 464 amino acids with an estimated molecular mass of 50.8 kDa. The deduced amino sequences of Rhbg and Rhcg contained ammonia binding sites, which could recruit NH₄⁺ to be deprotonated, and a hydrophobic pore with two histidine residues, which could mediate the transport of NH₃. Our results indicated for the first time that brackish water acclimation resulted in significant decreases in the expression levels of rhbg/Rhbg and rhcg/Rhcg in the gills of H. signifer, which offered a mechanistic explanation of brackish water-related decreased ammonia excretion reported elsewhere. Furthermore, rhbg/Rhbg expression levels increased significantly in the liver of H. signifer during brackish water acclimation, indicating that the ammonia produced by extra-hepatic tissues and released into the blood could be channeled into the liver for increased urea synthesis. Overall, these results lend support to the proposition that H. signifer becomes N-limited upon utilizing urea as an osmolyte in brackish water.

Pre-acclimation to low ammonia improves ammonia handling in common carp (Cyprinus carpio) when exposed subsequently to high environmental ammonia

We tested whether exposing fish to low ammonia concentrations induced acclimation processes and helped fish to tolerate subsequent (sub)lethal ammonia exposure by activating ammonia excretory pathways. Common carp (Cyprinus carpio) were pre-exposed to 0.27mM ammonia (∼10% 96h LC₅₀) for 3, 7 and 14days. Thereafter, each of these pre-exposed and parallel naïve groups were exposed to 1.35mM high environmental ammonia (HEA, ∼50% 96h LC₅₀) for 12h and 48h to assess the occurrence of ammonia acclimation based on sub-lethal end-points, and to lethal ammonia concentrations (2.7mM, 96h LC₅₀) in order to assess improved survival time. Results show that fish pre-exposed to ammonia for 3 and 7days had a longer survival time than the ammonia naïve fish. However, this effect disappeared after prolonged (14days) pre-exposure. Ammonia excretion rate (J_amm) was strongly inhibited (or even reversed) in the unacclimated groups during HEA. On the contrary, after 3days the pre-exposure fish maintained J_amm while after 7days these pre-acclimated fish were able to increase J_amm efficiently. Again, this effect disappeared after 14days of pre-acclimation. The efficient ammonia efflux in pre-acclimated fish was associated with the up-regulation of branchial mRNA expression of ammonia transporters and exchangers. Pre-exposure with ammonia for 3-7days stimulated an increment in the transcript level of gill Rhcg-a and Rhcg-b mRNA relative to the naïve control group and the up-regulation of these two Rhcg homologs was reinforced during subsequent HEA exposure. No effect of pre-exposure was noted for Rhbg. Relative to unacclimated fish, the transcript level of Na⁺/H⁺ exchangers (NHE-3) was raised in 3-7days pre-acclimated fish and remained higher during the subsequent HEA exposure while gill H⁺-ATPase activities and mRNA levels were not affected by pre-acclimation episodes. Likewise, ammonia pre-acclimated fish with or without HEA exposure displayed pronounced up-regulation in Na⁺/K⁺-ATPase activity and mRNA expression relative to the corresponding ammonia naïve groups. Overall, these data suggest that ammonia acclimation was evident for both lethal and the sub-lethal endpoints through priming mechanisms in ammonia excretory transcriptional processes, but these acclimation effects were transient and disappeared after prolonged pre-exposure.

A broader look at ammonia production, excretion, and transport in fish: a review of impacts of feeding and the environment

For nearly a century, researchers have studied ammonia production and excretion in teleost fish. Stemming from past investigations a body of knowledge now exists on various aspects including biochemical mechanisms of ammonia formation and specific routes and tissues used for ammonia transport, culminating in a current detailed theoretical model of branchial transport, including the molecular identities of the moieties involved. However, typical studies examining ammonia balance use routine laboratory conditions and fasted fish. While avoiding additional variables that influence nitrogen balance, these studies are arguably idealistic and do not reflect the natural variety of conditions that fish encounter. Further studies have revealed the impacts of extrinsic factors (e.g. salinity, pH, temperature) on ammonia handling in fasted fish whereas others have explored intrinsic factors, such as life history and developmental impacts. One routine challenge for ammonia balance that fish encounter is feeding and digestion. Fortunately, many new studies have revealed the impact of feeding and digestion on several aspects of ammonia balance; from production to excretion and to transport, and several have done so incorporating supplemental extrinsic and/or intrinsic factors. Together, these complex studies reveal realistic responses to multifactorial challenges encountered by animals in the wild and begin to provide a holistic view of ammonia balance in freshwater teleost fish.

Ammonia as a Potential Neurotoxic Factor in Alzheimer's Disease

Ammonia is known to be a potent neurotoxin that causes severe negative effects on the central nervous system. Excessive ammonia levels have been detected in the brain of patients with neurological disorders such as Alzheimer disease (AD). Therefore, ammonia could be a factor contributing to the progression of AD. In this review, we provide an introduction to the toxicity of ammonia and putative ammonia transport proteins. We also hypothesize how ammonia may be linked to AD. Additionally, we discuss the evidence that support the hypothesis that ammonia is a key factor contributing to AD progression. Lastly, we summarize the old and new experimental evidence that focuses on energy metabolism, mitochondrial function, inflammatory responses, excitatory glutamatergic, and GABAergic neurotransmission, and memory in support of our ammonia-related hypotheses of AD.

Differential modulation of ammonia excretion, Rhesus glycoproteins and ion-regulation in common carp (Cyprinus carpio) following individual and combined exposure to waterborne copper and ammonia

The main objective of this study was to understand the mode of interaction between waterborne copper (Cu) and high environmental ammonia (HEA) exposure on freshwater fish, and how they influence the toxicity of each other when present together. For this purpose, individual and combined effects of Cu and HEA were examined on selected physiological and ion-regulatory processes and changes at transcript level in the common carp (Cyprinus carpio). Juvenile carp were exposed to 2.6μM Cu (25% of the 96h LC50value) and to 0.65mM ammonia (25% of the 96h LC50value) singly and as a mixture for 12h, 24h, 48h, 84h and 180h. Responses such as ammonia (Jamm) and urea (Jurea) excretion rate, plasma ammonia and urea, plasma ions (Na(+), Cl(-) and K(+)), muscle water content (MWC) as well as branchial Na(+)/K(+)-ATPase (NKA) and H(+)-ATPase activity, and branchial mRNA expression of NKA, H(+)-ATPase, Na(+)/H(+) exchanger (NHE-3) and Rhesus (Rh) glycoproteins were investigated under experimental conditions. Results show that Jamm was inhibited during Cu exposure, while HEA exposed fish were able to increase excretion efficiently. In the combined exposure, Jamm remained at the control levels indicating that Cu and HEA abolished each other's effect. Expression of Rhcg (Rhcg-a and Rhcg-b) mRNA was upregulated during HEA, thereby facilitated ammonia efflux out of gills. On the contrary, Rhcg-a transcript level declined following Cu exposure which might account for Cu induced Jamm inhibition. Likewise, Rhcg-a was also down-regulated in Cu-HEA co-exposed fish whilst a temporary increment was noted for Rhch-b. Fish exposed to HEA displayed pronounced up-regulation in NKA expression and activity and stable plasma ion levels. In both the Cu exposure alone and combined Cu-HEA exposure, ion-osmo homeostasis was adversely affected, exemplified by the significant reduction in plasma [Na(+)] and [Cl(-)], and elevated plasma [K(+)], along with an elevation in MWC. These changes were accompanied by a decline in NKA activity. Gill H(+)-ATPase mRNA levels and activities were not affected by either Cu or HEA or both. Likewise, NHE-3 expression remained unaltered but tended to be numerically higher during HEA exposure. Overall, these data suggest that at equitoxic concentrations (25% of 96h LC50), the individual effect of Cu is more harmful while HEA induces quicker adaptive responses. Our findings also denote a competitive mode of interaction, exemplified by the inhibition of HEA -mediated adaptive responses in the presence of Cu.

Physiological and molecular responses of the goldfish (Carassius auratus) kidney to metabolic acidosis, and potential mechanisms of renal ammonia transport

Relative to the gills, the mechanisms by which the kidney contributes to ammonia and acid-base homeostasis in fish are poorly understood. Goldfish were exposed to a low pH environment (pH 4.0, 48 h), which induced a characteristic metabolic acidosis and an increase in total plasma [ammonia] but reduced plasma ammonia partial pressure (PNH3). In the kidney tissue, total ammonia, lactate and intracellular pH remained unchanged. The urinary excretion rate of net base under control conditions changed to net acid excretion under low pH, with contributions from both the NH4 (+) (∼30%) and titratable acidity minus bicarbonate (∼70%; TA-HCO3 (-)) components. Inorganic phosphate (Pi), urea and Na(+) excretion rates were also elevated while Cl(-) excretion rates were unchanged. Renal alanine aminotransferase activity increased under acidosis. The increase in renal ammonia excretion was due to significant increases in both the glomerular filtration and the tubular secretion rates of ammonia, with the latter accounting for ∼75% of the increase. There was also a 3.5-fold increase in the mRNA expression of renal Rhcg-b (Rhcg1) mRNA. There was no relationship between ammonia secretion and Na(+) reabsorption. These data indicate that increased renal ammonia secretion during acidosis is probably mediated through Rhesus (Rh) glycoproteins and occurs independently of Na(+) transport, in contrast to branchial and epidermal models of Na(+)-dependent ammonia transport in freshwater fish. Rather, we propose a model of parallel H(+)/NH3 transport as the primary mechanism of renal tubular ammonia secretion that is dependent on renal amino acid catabolism.

Ammonia excretion in the Atlantic hagfish (Myxine glutinosa) and responses of an Rhc glycoprotein

Hagfishes, the most ancient of the extant craniates, demonstrate a high tolerance for a number of unfavorable environmental conditions, including elevated ammonia. Proposed mechanisms of ammonia excretion in aquatic organisms include vesicular NH4+ transport and release by exocytosis in marine crabs, and passive NH3 diffusion, active NH4+ transport, and paracellular leakage of NH3 or NH4+ across the gills of fishes. Recently, an emerging paradigm suggests that Rhesus glycoproteins play a vital role in ammonia transport in both aquatic invertebrates and vertebrates. This study has identified an Rh glycoprotein ortholog from the gills of Atlantic hagfish. The hagfish Rhcg shares a 56–60% amino acid identity to other vertebrate Rhcg cDNAs. Sequence information was used to produce an anti-hagfish Rhcg (hRhcg) antibody. We have used hRhcg to localize protein expression to epithelial cells of the gill and the skin. In addition, we have quantified hRhcg expression following exposure to elevated plasma ammonia levels. Animals exposed to a 3 mmol/kg NH4Cl load resulted in significantly elevated plasma ammonia concentrations compared with controls for up to 4 h postinjection. This correlated with net ammonia excretion rates that were also significantly elevated for up to 4 h postinjection. Rhcg mRNA expression in both the gill and skin was significantly elevated by 15 min and 1 h, respectively, and hRhcg protein expression in gills was significantly elevated at 2, 4, and 8 h postinjection. These results demonstrate a potential role for Rhcg in the excretion of ammonia in the Atlantic hagfish.

Interactive effect of high environmental ammonia and nutritional status on ecophysiological performance of European sea bass (Dicentrarchus labrax) acclimated to reduced seawater salinities

We investigated the interactive effect of ammonia toxicity, salinity challenge and nutritional status on the ecophysiological performance of European sea bass (Dicentrarchus labrax). Fish were progressively acclimated to normal seawater (32ppt), to brackish water (20ppt and 10ppt) and to hyposaline water (2.5ppt). Following acclimation to different salinities for two weeks, fish were exposed to high environmental ammonia (HEA, 20mg/L ∼1.18mM representing 50% of 96h LC50 value for ammonia) for 12h, 48h, 84h and 180h, and were either fed (2% body weight) or fasted (unfed for 7 days prior to HEA exposure). Biochemical responses such as ammonia (Jamm) and urea excretion rate, plasma ammonia, urea and lactate, plasma ions (Na(+), Cl(-) and K(+)) and osmolality, muscle water content (MWC) and liver and muscle energy budget (glycogen, lipid and protein), as well as branchial Na(+)/K(+)-ATPase (NKA) and H(+)-ATPase activity, and branchial mRNA expression of NKA and Na(+)/K(+)/2Cl(-) co-transporter (NKCC1) were investigated in order to understand metabolic and ion- osmoregulatory consequences of the experimental conditions. During HEA, Jamm was inhibited in fasted fish at 10ppt, while fed fish were still able to excrete efficiently. At 2.5ppt, both feeding groups subjected to HEA experienced severe reductions and eventually a reversion in Jamm. Overall, the build-up of plasma ammonia in HEA exposed fed fish was much lower than fasted ones. Unlike fasted fish, fed fish acclimated to lower salinities (10ppt-2.5ppt) could maintain plasma osmolality, [Na(+)], [Cl(-)] and MWC during HEA exposure. Thus fed fish were able to sustain ion-osmotic homeostasis which was associated with a more pronounced up-regulation in NKA expression and activity. At 2.5ppt both feeding groups activated H(+)-ATPase. The expression of NKCC1 was down-regulated at lower salinities in both fed and fasted fish, but was upregulated within each salinity after a few days of HEA exposure. Though an increment in plasma lactate content and a decline in energy stores were noted for both feeding regimes, the effect was more severe in feed deprived fish. Overall, several different physiological processes were disturbed in fasted sea bass during HEA exposure while feeding alleviated adverse effects of high ammonia and salinity challenge. This suggests that low food availability can render fish more vulnerable to external ammonia, especially at reduced seawater salinities.

Rh protein expression in branchial neuroepithelial cells, and the role of ammonia in ventilatory control in fish

Bill Milsom has made seminal contributions to our understanding of ventilatory control in a wide range of vertebrates. Teleosts are particularly interesting, because they produce a 3rd, potentially toxic respiratory gas (ammonia) in large amounts. Fish are well known to hyperventilate under high environmental ammonia (HEA), but only recently has the potential role of ammonia in normal ventilatory control been investigated. It is now clear that ammonia can act directly as a ventilatory stimulant in trout, independent of its effects on acid-base balance. Even in ureotelic dogfish sharks, acute elevations in ammonia cause increases in ventilation. Peripherally, the detection of elevated ammonia resides in gill arches I and II in trout, and in vitro, neuroepithelial cells (NECs) from these arches are sensitive to ammonia, responding with elevations in intracellular Ca(2+) ([Ca(2+)]i). Centrally, hyperventilatory responses to ammonia correlate more closely with concentrations of ammonia in the brain than in plasma or CSF. After chronic HEA exposure, ventilatory responsiveness to ammonia is lost, associated with both an attenuation of the [Ca(2+)]i response in NECs, and the absence of elevation in brain ammonia concentration. Chronic exposure to HEA also causes increases in the mRNA expression of several Rh proteins (ammonia-conductive channels) in both brain and gills. "Single cell" PCR techniques have been used to isolate the individual responses of NECs versus other gill cell types. We suggest several circumstances (post-feeding, post-exercise) where the role of ammonia as a ventilatory stimulant may have adaptive benefits for O2 uptake in fish.

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.

Compensatory responses in common carp (Cyprinus carpio) under ammonia exposure: additional effects of feeding and exercise

Ammonia is an environmental pollutant that is toxic to all aquatic animals. The toxic effects of ammonia can be modulated by other physiological processes such as feeding and swimming. In this study, we wanted to examine these modulating effects in common carp (Cyprinus carpio). Fish were either fed (2% body weight) or starved (unfed for seven days prior to the sampling), and swimming at a sustainable, routine swimming speed or swum to exhaustion, while being exposed chronically (up to 28 days) to high environmental ammonia (HEA, 1 mg/L ~58.8 μmol/L as NH4Cl at pH 7.9). Swimming performance (critical swimming speed, Ucrit) and metabolic responses such as oxygen consumption rate (MO2), ammonia excretion rate (Jamm), ammonia quotient, liver and muscle energy budget (glycogen, lipid and protein), plasma ammonia and lactate, as well as plasma ion concentrations (Na(+), Cl(-), K(+) and Ca(2+)) were investigated in order to understand metabolic and iono-regulatory consequences of the experimental conditions. Cortisol plays an important role in stress and in both the regulation of energy and the ion homeostasis; therefore plasma cortisol was measured. Results show that during HEA, Jamm was elevated to a larger extent in fed fish and they were able to excrete much more efficiently than the starved fish. Consequently, the build-up of ammonia in plasma of HEA exposed fed fish was much slower. MO2 increased considerably in fed fish after exposure to HEA and was further intensified during exercise. During exposure to HEA, the level of cortisol in plasma augmented in both the feeding regimes, but the effect of HEA was more pronounced in starved fish. Energy stores dropped for both fed and the starved fish with the progression of the exposure period and further declined when swimming to exhaustion. Overall, fed fish were less affected by HEA than starved fish, and although exercise exacerbated the toxic effect in both feeding treatments, this was more pronounced in starved fish. This suggests that fish become more vulnerable to external ammonia during exercise, and feeding protects the fish against the adverse effects of high ammonia and exercise.

Ammonia transport across the skin of adult rainbow trout (Oncorhynchus mykiss) exposed to high environmental ammonia (HEA)

Recent molecular evidence points towards a capacity for ammonia transport across the skin of adult rainbow trout. A series of in vivo and in vitro experiments were conducted to understand the role of cutaneous ammonia excretion (J amm) under control conditions and after 12-h pre-exposure to high environmental ammonia (HEA; 2 mmol/l NH4HCO3). Divided chamber experiments with bladder-catheterized, rectally ligated fish under light anesthesia were performed to separate cutaneous J amm from branchial, renal, and intestinal J amm. Under control conditions, cutaneous J amm accounted for 4.5 % of total J amm in vivo. In fish pre-exposed to HEA, plasma total ammonia concentration increased 20-fold to approximately 1,000 μmol/l, branchial J amm increased 1.5- to 2.7-fold, and urinary J amm increased about 7-fold. Urinary J amm still accounted for less than 2 % of total J amm. Cutaneous J amm increased 4-fold yet amounted to only 5.7 % of total J amm in these fish. Genes (Rhcg1, Rhcg2, Rhbg, NHE-2, v-type H(+)-ATPase) known to be involved in ammonia excretion at the gills of trout were all expressed at the mRNA level in the skin, but their expression did not increase with HEA pre-exposure. In vitro analyses using [(14)C] methylamine (MA), an ammonia analog which is transported by Rh proteins, demonstrated that MA permeability in isolated skin sections was higher in HEA pre-exposed fish than in control fish. The addition of basolateral ammonia (1,000 μmol/l) to this system abolished this increase in permeability, suggesting ammonia competition with MA for Rh-mediated transport across the skin of HEA pre-exposed trout; this did not occur in skin sections from control trout. Moreover, in vitro J amm by the skin of fish which had been pre-exposed to HEA was also higher than in control fish in the absence of basolateral ammonia, pointing towards a possible cutaneous ammonia loading in response to HEA. In vitro MA permeability was reduced upon the addition of amiloride (10(-4) mol/l), but not phenamil (10(-5) mol/l) suggesting a role for a Na/H-exchanger (NHE) in cutaneous ammonia transport, as has been previously described in the skin of larval fish. Overall, it appears that under control conditions and in response to HEA pre-exposure, the skin makes only a very minor contribution to total J amm, but the observed increases in cutaneous J amm in vivo and in cutaneous J amm and MA permeability in vitro demonstrate the capacity for ammonia transport in the skin of adult trout. It remains unclear if this capacity may become significant under certain environmental challenges or if it is merely a remnant of cutaneous transport capacity from early life stages in these fish.

Genomic and phenotypic response of hornyhead turbot exposed to municipal wastewater effluents

Laboratory tests with marine flatfish were conducted to investigate associations among gene expression, higher biological responses and wastewater effluent exposure. In the present study, male hornyhead turbot (Pleuronichthys verticalis) were exposed to environmentally realistic (0.5%) and higher (5%) concentrations of chemically enhanced advanced-primary (PL) and full-secondary treated (HTP) effluents from two southern California wastewater treatment plants (WWTP). Hepatic gene expression was examined using a custom low-density microarray. Alterations in gene expression (vs. controls) were observed in fish exposed to both effluent types. Fish exposed to 0.5% PL effluent showed changes in genes involved in the metabolism of xenobiotics, steroids, and lipids, among other processes. Fish exposed to 5% PL effluent showed expression changes in genes involved in carbohydrate metabolism, stress responses, xenobiotic metabolism, and steroid synthesis, among others. Exposure to 5% HTP effluent changed the expression of genes involved in lipid, glutathione and xenobiotic metabolism, as well as immune responses. Although no concentration-dependent patterns of response to effluent exposure were found, significant Spearman correlations were observed between the expression of 22 genes and molecular and/or higher biological responses. These results indicate that microarray gene expression data correspond to higher biological responses and should be incorporated in studies assessing fish health after exposure to complex environmental mixtures.

Modulation of Rh glycoproteins, ammonia excretion and Na+ fluxes in three freshwater teleosts when exposed chronically to high environmental ammonia

We investigated relationships among branchial unidirectional Na(+) fluxes, ammonia excretion, urea excretion, plasma ammonia, plasma cortisol, and gill transporter expression and function in three freshwater fish differing in their sensitivity to high environmental ammonia (HEA). The highly ammonia-sensitive salmonid Oncorhynchus mykiss (rainbow trout), the less ammonia-sensitive cyprinid Cyprinus carpio (common carp) and the highly ammonia-resistant cyprinid Carassius auratus (goldfish) were exposed chronically (12-168 h) to 1 mmol l(-1) ammonia (as NH4HCO3; pH 7.9). During HEA exposure, carp and goldfish elevated ammonia excretion (JAmm) and Na(+) influx rates ( ) while trout experienced higher plasma ammonia (TAmm) and were only able to restore control rates of JAmm and . All three species exhibited increases in Na(+) efflux rate ( ). At the molecular level, there was evidence for activation of a 'Na(+)/NH4(+) exchange metabolon' probably in response to elevated plasma cortisol and TAmm, though surprisingly, some compensatory responses preceded molecular responses in all three species. Expression of Rhbg, Rhcg (Rhcg-a and Rhcg-b), H(+)-ATPase (V-type, B-subunit) and Na(+)/K(+)-ATPase (NKA) mRNA was upregulated in goldfish, Rhcg-a and NKA in carp, and Rhcg2, NHE-2 (Na(+)/H(+) exchanger) and H(+)-ATPase in trout. Branchial H(+)-ATPase activity was elevated in goldfish and trout, and NKA activity in goldfish and carp, but NKA did not appear to function preferentially as a Na(+)/NH4(+)-ATPase in any species. Goldfish alone increased urea excretion rate during HEA, in concert with elevated urea transporter mRNA expression in gills. Overall, goldfish showed more effective compensatory responses towards HEA than carp, while trout were least effective.

Cutaneous nitrogen excretion in the African clawed frog Xenopus laevis: effects of high environmental ammonia (HEA)

Ammonia is a highly toxic molecule and often introduced in considerable amounts into aquatic environments due to anthropogenic activities. Many aquatic and semi-aquatic amphibians utilize, in addition to their kidneys, the skin for osmoregulation and nitrogen excretion. In the present study the effects of prolonged (7-21 days) exposure to high environmental ammonia (HEA, 1 mmol l(-1) NH4Cl) on cutaneous nitrogen excretion and gene expression of key-transporters involved in nitrogen excretion and acid-base regulation were investigated in the fully aquatic African clawed frog, Xenopus laevis. The study revealed that X. laevis excretes predominately ammonia of which approximately 50% is excreted via the skin. Both the ventral and dorsal skin were capable to generate a net ammonia efflux, which was significantly activated by 10 mmol l(-1) of the phosphodiesterase blocker theophylline. The obtained data further suggest that the ammonia efflux was promoted by an acidification of the unstirred boundary layer, likely generated by an apical localized V-ATPase, with NH3 being transported via cutaneous expressed ammonia transporters, Rhbg and Rhcg. Prolonged HEA exposure did significantly reduce the net-flux rates over the ventral skin with Vmax changing from 256 nmol cm(-2) h(-1) in control frogs to 196 nmol cm(-2) h(-1) in HEA exposed animals. Further, prolonged HEA exposure caused a decrease in mRNA expression levels of the ammonia transporter Rhbg, Na(+)/K(+)-ATPase (α-subunit) and V-ATPase (subunit H) in the ventral and dorsal skin and the kidney. In contrast, Rhcg expression levels were unaffected by HEA in skin tissues.

Molecular and physiological responses to long-term sublethal ammonia exposure in Atlantic salmon (Salmo salar)

The objective of this study was to determine the underlying physiological and molecular responses to long-term sublethal ammonia exposure in Atlantic salmon (Salmo salar) parr. Previous studies have predominately focused on mechanisms during acute, short-term exposure. For that purpose Atlantic salmon parr were exposed to four ammonia concentrations between 4 and 1800 μmol l(-1) total ammonia nitrogen (TAN), and subjected to two feeding regimes for 15 weeks. Elevated environmental ammonia and full feeding strength caused an initial increase in plasma ammonia levels ([T(amm)]) after 22 days of exposure, which thereafter declined and remained similar to the control animals towards the end of the study. On the other hand, a progressive decrease in plasma urea levels was evident throughout the entire exposure period and depended on the concentration of environmental ammonia, with the largest decrease in urea levels observed at the highest ammonia concentrations (1700 and 1800 μmol l(-1) TAN). We hypothesized that the successful adaptation to long-term elevated ammonia levels would involve an increased capacity for carrier-facilitated branchial excretion. This hypothesis was strengthened by the first evidence of an up-regulation of branchial transcription of the genes encoding the Rhesus (Rh) glycoproteins, Rhcg1 and Rhcg2, urea transporter (UT) and aquaporin 3a (Aqp3a), during long-term exposure. Of the Rhesus glycoprotein (Rh) mRNAs, Rhcg1 was up-regulated at all tested ammonia levels, while Rhcg2 showed a concentration-sensitive increase. Increased transcription levels of V-type H(+)-ATPase (H(+)-ATPase) were observed at the highest ammonia concentrations (1700 and 1800 μmol l(-1) TAN) and coincided with an up-regulation of Rhcg2 at these concentrations. Transcription of UT and Aqp3a was increased after 15 weeks of exposure to low ammonia levels (470 and 480 μmol l(-1) TAN). A significant increase in brain glutamine (Gln) concentration was observed for full fed Atlantic salmon after 22 days and in fish with restricted feeding after 105 days of exposure to 1800 and 1700 μmol l(-1) TAN, respectively, without any concomitant decrease in brain glutamate (Glu) concentrations. These results suggest that Gln synthesis is an ammonia detoxifying strategy employed in the brain of Atlantic salmon parr during long-term sublethal ammonia exposure. Full feed strength had an additive effect on plasma [T(amm)], while the restricted feeding regime postponed the majority of the observed physiological and molecular responses. In conclusion, Atlantic salmon parr adapts to the long-term sublethal ammonia concentrations with increased branchial transcription levels of ammonia and urea transporting proteins and ammonia detoxification in the brain.

Seven things fish know about ammonia and we don't

In this review we pose the following seven questions related to ammonia and fish that represent gaps in our knowledge. 1. How is ammonia excretion linked to sodium uptake in freshwater fish? 2. How much does branchial ammonia excretion in seawater teleosts depend on Rhesus (Rh) glycoprotein-mediated NH(3) diffusion? 3. How do fish maintain ammonia excretion rates if branchial surface area is reduced or compromised? 4. Why does high environmental ammonia change the transepithelial potential across the gills? 5. Does high environmental ammonia increase gill surface area in ammonia tolerant fish but decrease gill surface area in ammonia intolerant fish? 6. How does ammonia contribute to ventilatory control? 7. What do Rh proteins do when they are not transporting ammonia? Mini reviews on each topic, which are able to present only partial answers to each question at present, are followed by further questions and/or suggestions for research approaches targeted to uncover answers.

The interactive effects of ammonia exposure, nutritional status and exercise on metabolic and physiological responses in gold fish (Carassius auratus L.)

This study aimed to elucidate the physiological effects of high environmental ammonia (HEA) following periods of feeding (2% body weight) and starvation (unfed for 7 days prior to sampling) in gold fish (Carassius auratus). Both groups of fish were exposed to HEA (1 mg/L; Flemish water quality guideline) for 0 h (control), 3 h, 12 h, 1 day, 4 days, 10 days, 21 days and 28 days. Measurements of weight gain (%), oxygen consumption (MO2), ammonia excretion rate, ammonia quotient (AQ), critical swimming speeds (Ucrit), plasma and muscle ammonia accumulation, plasma lactate, liver and muscle glycogen, lipid and protein content were done at various time intervals during the experimental periods. Overall, ammonia excretion rates, plasma ammonia accumulation and AQ were significantly affected by food regime in ammonia free water. HEA, the additional challenge in the present study, significantly altered all the studied parameters among fed and starved groups in days-dependent manner. Results show that weight gain (%), MO2, Ucrit, protein content in liver and muscle, and glycogen content in muscle among starved fish under HEA were considerably reduced compared to control and fed fish. Additionally a remarkable increase in plasma ammonia level, muscle ammonia, lactate accumulation and AQ was seen. However in fed fish, MO2, ammonia excretion rate, AQ and lactate level augmented after exposure to HEA. These results indicate that starved fish appeared more sensitive to HEA than fed fish. Furthermore, as expected, the toxic effect of ammonia exposure in both feeding treatments was exacerbated when imposed to exhaustive swimming (swum at 3/4th Ucrit). Such effects were more pronounced in starved fish. This suggests that starvation can instigate fish more vulnerable to external ammonia during exercise. Therefore, it was evident from our study that feeding ameliorates ammonia handling and reduces its toxicity during both routine and exhaustive swimming. Moreover, recovery was observed for some physiological parameters (e.g. MO2, ammonia excretion, Ucrit, plasma ammonia) during the last exposure periods (21-28 days) while for others (e.g. growth, tissue glycogen and protein content, muscle ammonia) effects only became apparent at this time. In the future, these results need to be considered in ecological context as fish in ammonia polluted may experience different phenomenon (starvation and exercise) simultaneously.

Branchial and extra-branchial ammonia excretion in goldfish (Carassius auratus) following thermally induced gill remodeling

Under cold acclimated conditions, goldfish (Carassius auratus) express an interlamellar cell mass (ILCM) which limits diffusive ion loss but may also impede branchial ammonia excretion (J(amm)). In the present study, goldfish were subjected to a 2-week 5 or 25 °C acclimation in order to modulate the degree of ILCM gill coverage and determine potential effects on J(amm). 25 °C-fish displayed gill coverage which was significantly lower than the 5 °C-fish, though the ILCM was not completely absent in these fish. 5 °C-fish demonstrated J(amm) values approximately 60% lower than those of 25 °C-fish. The magnitude of anterior (branchial) J(amm) strongly correlated with gill coverage (r(2)=0.83), suggesting that the ILCM may impede branchial J(amm). Divided chamber experiments demonstrated that relative to the 25 °C-fish, 5 °C-fish relied more upon posterior routes of excretion. In response to high external ammonia (HEA; 1.5mM NH(4)HCO(3)) exposures, 25 °C-fish displayed ammonia uptake while 5 °C-fish maintained excretion against HEA, suggesting that the ILCM may act as a barrier preventing ammonia uptake. In summary, the ILCM appears to impede branchial J(amm), such that 5 °C-rely more on extra-branchial routes of excretion. We hypothesize that gill remodeling in these fish may be intimately tied to physiological adjustments on the whole-body scale.

Revisiting the effects of crowding and feeding in the gulf toadfish, Opsanus beta: the role of Rhesus glycoproteins in nitrogen metabolism and excretion

Models of branchial transport in teleosts have been reshaped by the recent discovery of Rhesus (Rh) glycoproteins, a family of proteins that facilitate the movement of NH3 across cell membranes. This study examines the effects of crowding and feeding on ammonia excretion in gulf toadfish (Opsanus beta) within the context of Rh glycoproteins and the ammonia-fixing enzyme, glutamine synthetase (GS). Four Rh isoforms (Rhag, Rhbg, Rhcg1 and Rhcg2) were isolated from toadfish. Tissue distributions showed higher levels of mRNA expression in the gills and liver, moderate levels in the intestine and lower levels in the stomach. Crowding significantly lowered branchial Rh expression and ammonia excretion rates in fasted toadfish. A comparison of Rh expression in the digestive tract revealed relatively low levels of Rhcg1 and Rhcg2 in the stomach and high mRNA abundance of Rhbg, Rhcg1 and Rhcg2 in the intestine of fasted, crowded toadfish. We speculate that these trends may reduce secretion and enhance absorption, respectively, to minimize the amount of ammonia that is lost through gastrointestinal routes. By contrast, these patterns of expression were modified in response to an exogenous ammonia load via feeding. Post-prandial ammonia excretion rates were elevated twofold, paralleled by similar increases in branchial Rhcg1 mRNA, gastric Rhcg1 mRNA and mRNA of all intestinal Rh isoforms. These changes were interpreted as an attempt to increase post-prandial ammonia excretion rates into the environment owing to a gradient created by elevated circulating ammonia concentrations and acidification of the digestive tract. Overall, we provide evidence that toadfish modulate both the expression of Rh isoforms and urea synthesis pathways to tightly control and regulate nitrogen excretion.

Interactions between cortisol and Rhesus glycoprotein expression in ureogenic toadfish, Opsanus beta

In their native environment, gulf toadfish excrete equal quantities of ammonia and urea. However, upon exposure to stressful conditions in the laboratory (i.e. crowding, confinement or air exposure), toadfish decrease branchial ammonia excretion and become ureotelic. The objective of this study was to determine the influences of cortisol and ammonia on ammonia excretion relative to expression of Rhesus (Rh) glycoproteins and the ammonia-fixing enzyme, glutamine synthetase (GS). In vivo infusions and/or injections were used to manipulate corticosteroid activity and plasma ammonia concentrations in ureotelic toadfish. Metyrapone treatment to lower circulating cortisol levels resulted in a 3.5-fold elevation of ammonia excretion rates, enhanced mRNA expression of two of the toadfish Rh isoforms (Rhcg1 and Rhcg2), and decreased branchial and hepatic GS activity. Correspondingly, cortisol infusion decreased ammonia excretion 2.5-fold, a change that was accompanied by reduced branchial expression of all toadfish Rh isoforms (Rhag, Rhbg, Rhcg1 and Rhcg2) and a twofold increase in hepatic GS activity. In contrast, maintenance of high circulating ammonia levels by ammonia infusion enhanced ammonia excretion and Rh expression (Rhag, Rhbg and Rhcg2). Toadfish treated with cortisol showed an attenuated response to ammonia infusion with no change in Rh mRNA expression or GS activity. In summary, the evidence suggests that ammonia excretion in toadfish is modulated by cortisol-induced changes in both Rh glycoprotein expression and GS activity.

Ammonia excretion in the freshwater planarian Schmidtea mediterranea

In aquatic invertebrates metabolic nitrogenous waste is excreted predominately as ammonia. Very little is known, however, of the underlying mechanisms of ammonia excretion, particularly in freshwater species. Our results indicate that in the non-parasitic freshwater planarian Schmidtea mediterranea ammonia excretion depends on an acidification of the apical unstirred layer of the body surface and consequent ammonia trapping. Buffering of the environment to a pH of 7 or higher decreased excretion rate. Inhibitor experiments suggested further that the excretion mechanism involves the participation of the V-type H+-ATPase and carbonic anhydrase and possibly also the Na+/K+-ATPase and Na+/H+ exchangers (NHEs). Alkalinization (pH 8.5, 2 days) of the environment led to a 1.9-fold increase in body ammonia levels and to a down-regulation of V-ATPase (subunit A) and Rh-protein mRNA. Further, a two day exposure to non-lethal ammonia concentrations (1 mmol L-1) caused a doubling of body ammonia levels and led to an increase in Rh-protein and Na+/K+-ATPase (α-subunit) mRNA expression levels. In-situ hybridization studies indicated a strong mRNA expression of the Rh-protein in the epidermal epithelium. The ammonia excretion mechanism proposed for S. mediterranea reveals striking similarities to the current model suggested to function in gills of freshwater fish.

Digestion of a single meal affects gene expression of ion and ammonia transporters and glutamine synthetase activity in the gastrointestinal tract of freshwater rainbow trout

Experiments on freshwater rainbow trout, Oncorhynchus mykiss, demonstrated how digestion affected the transcriptional expression of gastrointestinal transporters following a single satiating meal (~3% body mass ration) after a 1-week fast. Quantitative real-time polymerase chain reaction was employed to measure the relative mRNA expression of three previously cloned and sequenced transporters [H(+)-K(+)-ATPase (HKA), Na(+)/HCO(3)(-) cotransporter (NBC), and the Rhesus glycoprotein (Rhbg1; an ammonia transporter)] over a 24-h time course following feeding. Plasma total ammonia increased about threefold from pre-feeding levels to 288 μmol l(-1), whereas total ammonia levels in chyme supernatant reached a sixfold higher value (1.8 mmol l(-1)) than plasma levels. Feeding did not appear to have a statistically significant effect on the relative mRNA expression of the gastric HKA or Rhbg1. However, the relative mRNA expression of gastric NBC was increased 24 h following the ingestion of a meal. Along the intestinal tract, feeding increased the relative mRNA expression of Rhbg1, but had no effect on the expression of NBC. Expression of the gastric HKA was undetectable in the intestinal tract of freshwater rainbow trout. Digestion increased the activity of glutamine synthetase in the posterior intestine at 12 and 24 h following feeding. This study is among the first to show that there are digestion-associated changes in gene expression and enzyme activity in the gastrointestinal tract of teleost fish illustrating the dynamic plasticity of this organ. These post-prandial changes occur over the relative short-term duration of digesting a single meal.

Roles of cortisol and carbonic anhydrase in acid-base compensation in rainbow trout, Oncorhynchus mykiss

Fish compensate for acid-base disturbances primarily by modulating the branchial excretion of acid-base equivalents, with a supporting role played by adjustment of urinary acid excretion. The present study used metabolic acid-base disturbances in rainbow trout, Oncorhynchus mykiss, to evaluate the role played by cortisol in stimulating compensatory responses. Trout infused with acid (an iso-osmotic solution of 70 mmol L(-1) HCl), base (140 mmol L(-1) NaHCO(3)) or saline (140 mmol L(-1) NaCl) for 24 h exhibited significant elevation of circulating cortisol concentrations. Acid infusion significantly increased both branchial (by 328 μmol kg(-1) h(-1)) and urinary (by 5.9 μmol kg(-1) h(-1)) net acid excretion, compensatory responses that were eliminated by pre-treatment of trout with the cortisol synthesis inhibitor metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone). The significant decrease in net acid excretion (equivalent to enhanced base excretion) of 203 μmol kg(-1) h(-1) detected in base-infused trout was unaffected by metyrapone treatment. Acid- and base-infusions also were associated with significant changes in the relative mRNA expression of branchial and renal cytosolic carbonic anhydrase (tCAc) and renal membrane-linked CA IV (tCA IV). Cortisol treatment caused changes in CA gene expression that tended to parallel those observed with acid but not base infusion. For example, significant increases in renal relative tCA IV mRNA expression were detected in both acid-infused (~2x) and cortisol-treated (~10x) trout, whereas tCA IV mRNA expression was significantly reduced (~5x) in base-infused fish. Despite changes in CA gene expression in acid- or base-infused fish, neither acid nor base infusion affected CAc protein levels in the gill, but both caused significant increases in branchial CA activity. Cortisol treatment similarly increased branchial CA activity in the absence of an effect on branchial CAc protein expression. Taken together, these findings provide support for the hypothesis that in rainbow trout, cortisol is involved in mediating acid-base compensatory responses to a metabolic acidosis, and that cortisol exerts its effects at least in part through modulation of CA.

Rh glycoprotein expression is modulated in pufferfish (Takifugu rubripes) during high environmental ammonia exposure

Rhesus (Rh) protein involvement in ammonia transport processes in freshwater fish has received considerable attention; however, parallel investigations in seawater species are scant. We exposed pufferfish to high environmental ammonia (HEA; 1 and 5 mmol l–1 NH4HCO3) and evaluated the patterns of ammonia excretion and gill Rh mRNA and protein expression. Gill H+-ATPase, NHE1, NHE2, NHE3, Na+/K+-ATPase (NKA), Na+/K+/2Cl– co-transporter (NKCC1) mRNA, H+-ATPase activity, NKA protein and activity, were also quantified. Activation of NKA by NH4+ was demonstrated in vitro. The downregulation of Rhbg mRNA and simultaneous upregulations of Rhcg1, H+-ATPase, NHE3, NKA, NKCC1 mRNA, H+-ATPase activity, and NKA protein and activity levels suggested that during HEA, ammonia excretion was mediated mainly by mitochondria-rich cells (MRCs) driven by NKA with basolateral NH4+ entry via NKA and/or NKCC1, and apical NH3 extrusion via Rhcg1. Reprotonation of NH3 by NHE3 and/or H+-ATPase would minimise back flux through the Rh channels. Downregulated Rhbg and Rhag mRNA observed in the gill during HEA suggests a coordinated protective response to minimise the influx of external ammonia via the pavement cells and pillar cells, respectively, while routing ammonia excretion through the MRCs. Exposure to hypercapnia (1% CO2 in air) resulted in downregulated gill and erythrocyte Rhag mRNA. Surprisingly, Rhag, Rhbg, Rhcg1 and Rhcg2 proteins responded to both hypercapnia and HEA with changes in their apparent molecular masses. A dual NH3/CO2 transport function of the pufferfish Rh proteins is therefore suggested. The results support and extend an earlier proposed model of pufferfish gill ammonia excretion that was based on immunolocalisation of the Rh proteins. Passive processes and/or Rhbg and Rhcg2 in the pavement cells may maintain basal levels of plasma ammonia but elevated levels may require active excretion via NKA and Rhcg1 in the MRCs.

Functional characterization of Rhesus glycoproteins from an ammoniotelic teleost, the rainbow trout, using oocyte expression and SIET analysis

Recent experimental evidence from rainbow trout suggests that gill ammonia transport may be mediated in part via Rhesus (Rh) glycoproteins. In this study we analyzed the transport properties of trout Rh proteins (Rhag, Rhbg1, Rhbg2, Rhcg1, Rhcg2, Rh30-like) expressed in Xenopus oocytes, using the radiolabeled ammonia analogue [14C]methylamine, and the scanning ion electrode technique (SIET). All of the trout Rh proteins, except Rh30-like, facilitated methylamine uptake. Uptake was saturable, with Km values ranging from 4.6 to 8.9 mmol l−1. Raising external pH from 7.5 to 8.5 resulted in 3- to 4-fold elevations in Jmax values for methylamine; Km values were unchanged when expressed as total or protonated methylamine. Efflux of methylamine was also facilitated in Rh-expressing oocytes. Efflux and influx rates were stimulated by a pH gradient, with higher rates observed with steeper H+ gradients. NH4Cl inhibited methylamine uptake in oocytes expressing Rhbg1 or Rhcg2. When external pH was elevated from 7.5 to 8.5, the Ki for ammonia against methylamine transport was 35–40% lower when expressed as total ammonia or NH4+, but 5- to 6-fold higher when expressed as NH3. With SIET we confirmed that ammonia uptake was facilitated by Rhag and Rhcg2, but not Rh30-like proteins. Ammonia uptake was saturable, with a comparable Jmax but lower Km value than for total or protonated methylamine. At low substrate concentrations, the ammonia uptake rate was greater than that of methylamine. The Km for total ammonia (560 μmol l−1) lies within the physiological range for trout. The results are consistent with a model whereby NH4+ initially binds, but NH3 passes through the Rh channels. We propose that Rh glycoproteins in the trout gill are low affinity, high capacity ammonia transporters that exploit the favorable pH gradient formed by the acidified gill boundary layer in order to facilitate rapid ammonia efflux when plasma ammonia concentrations are elevated.

Rhesus glycoprotein and urea transporter genes in rainbow trout embryos are upregulated in response to alkaline water (pH 9.7) but not elevated water ammonia

Recent studies have shown that genes for the putative ammonia transporter, Rhesus glycoproteins (Rh) and the facilitated urea transporter (UT) are expressed before hatching in rainbow trout (Oncorhychus mykiss Walbaum) embryos. We tested the hypothesis that Rh and UT gene expressions are regulated in response to environmental conditions that inhibit ammonia excretion during early life stages. Eyed-up embryos (22 days post-fertilization (dpf)) were exposed to control (pH 8.3), high ammonia (1.70 mmol l(-1) NH4HCO3) and high pH (pH 9.7) conditions for 48h. With exposure to high water ammonia, ammonia excretion rates were reversed, tissue ammonia concentration was elevated by 9-fold, but there were no significant changes in mRNA expression relative to control embryos. In contrast, exposure to high water pH had a smaller impact on ammonia excretion rates and tissue ammonia concentrations, whereas mRNA levels for the Rhesus glycoprotein Rhcg2 and urea transporter (UT) were elevated by 3.5- and 5.6-fold, respectively. As well, mRNAs of the genes for H+ATPase and Na+/H+ exchanger (NHE2), associated with NH3 excretion, were also upregulated by 7.2- and 13-fold, respectively, in embryos exposed to alkaline water relative to controls. These results indicate that the Rhcg2, UT and associated transport genes are regulated in rainbow trout embryos, but in contrast to adults, there is no effect of high external ammonia at this stage of development.

A new paradigm for ammonia excretion in aquatic animals: role of Rhesus(Rh) glycoproteins

Ammonia excretion at the gills of fish has been studied for 80 years, but the mechanism(s) involved remain controversial. The relatively recent discovery of the ammonia-transporting function of the Rhesus (Rh) proteins, a family related to the Mep/Amt family of methyl ammonia and ammonia transporters in bacteria, yeast and plants, and the occurrence of these genes and glycosylated proteins in fish gills has opened a new paradigm. We provide background on the evolution and function of the Rh proteins, and review recent studies employing molecular physiology which demonstrate their important contribution to branchial ammonia efflux. Rhag occurs in red blood cells,whereas several isoforms of both Rhbg and Rhcg occur in many tissues. In the branchial epithelium, Rhcg appears to be localized in apical membranes and Rhbg in basolateral membranes. Their gene expression is upregulated during exposure to high environmental ammonia or internal ammonia infusion, and may be sensitive to synergistic stimulation by ammonia and cortisol. Rhcg in particular appears to be coupled to H+ excretion and Na+uptake mechanisms. We propose a new model for ammonia excretion in freshwater fish and its variable linkage to Na+ uptake and acid excretion. In this model, Rhag facilitates NH3 flux out of the erythrocyte, Rhbg moves it across the basolateral membrane of the branchial ionocyte, and an apical “Na+/NH +4 exchange complex” consisting of several membrane transporters (Rhcg, V-type H+-ATPase, Na+/H+ exchanger NHE-2 and/or NHE-3, Na+ channel) working together as a metabolon provides an acid trapping mechanism for apical excretion. Intracellular carbonic anhydrase(CA-2) and basolateral Na+/HCO –3cotransporter (NBC-1) and Na+/K+-ATPase play indirect roles. These mechanisms are normally superimposed on a substantial outward movement of NH3 by simple diffusion, which is probably dependent on acid trapping in boundary layer water by H+ ions created by the catalysed or non-catalysed hydration of expired metabolic CO2. Profitable areas for future investigation of Rh proteins in fish are highlighted: their involvement in the mechanism of ammonia excretion across the gills in seawater fish, their possible importance in ammonia excretion across the skin, their potential dual role as CO2 transporters,their responses to feeding, and their roles in early life stages prior to the full development of gills.