A carrier enzyme basis for ammonium excretion in teleost gill. NH+4-stimulated Na-dependent ATPase activity in Opsanus beta

Exploring the versatility of the perfused crustacean gill as a model for transbranchial transport processes

The study of transbranchial ion and gas transport of water-breathing animals has long been a useful means of modeling transport processes of higher vertebrate organs through comparative physiology. The molecular era of biological research has brought forward valuable information detailing shifts in gene expression related to environmental stress and the sub-cellular localization of transporters; however, purely molecular studies can cause hypothetical transport mechanisms and hypotheses to be accepted without any direct physiological proof. Isolated perfused gill experiments are useful for testing most of these hypotheses and can sometimes be used outright to develop a well-supported working model for transport processes relating to an animal's osmoregulation, acid-base balance, nitrogen excretion, and respiratory gas exchange as well as their sensitivity to pollutants and environmental stress. The technique allows full control of internal hemolymph-like saline as well as the ambient environmental fluid compositions and can measure the electrophysiological properties of the gill as well as the transport rates of ions and gases as they traverse the gill epithelium. Additives such as pharmaceuticals or peptides as well as the exclusion of ions from the media are commonly used to identify the importance of specific transporters to transport mechanisms. The technique can also be used to identify the penetrance, retention, and localization of pollutants within the gill epithelium or to explore the uptake and metabolism of nutrients directly from the ambient environment. While this technique can be applied to virtually any isolatable organ, the anatomy and rigidity of the decapod crustacean gill make it an ideal candidate for most experimental designs.

Ammonia exposure impairs lateral-line hair cells and mechanotransduction in zebrafish embryos

Ammonia (including NH₃ and NH₄⁺) is a major pollutant of freshwater environments. However, the toxic effects of ammonia on the early stages of fish are not fully understood, and little is known about the effects on the sensory system. In this study, we hypothesized that ammonia exposure can cause adverse effects on embryonic development and impair the lateral line system of fish. Zebrafish embryos were exposed to high-ammonia water (10, 15, 20, 25, and 30 mM NH₄Cl; pH 7.0) for 96 h (0-96 h post-fertilization). The body length, heart rate, and otic vesicle size had significantly decreased with ≥15 mM NH₄Cl, while the number and function of lateral-line hair cells had decreased with ≥10 mM NH₄Cl. The mechanoelectrical transduction (MET) channel-mediated Ca²⁺ influx was measured with a scanning ion-selective microelectrode technique to reveal the function of hair cells. We found that NH₄⁺ (≥5 mM NH₄Cl) entered hair cells and suppressed the Ca²⁺ influx of hair cells. Neomycin and La³⁺ (MET channel blockers) suppressed NH₄⁺ influx, suggesting that NH₄⁺ enters hair cells via MET channels in hair bundles. In conclusion, this study showed that ammonia exposure (≥10 mM NH₄Cl) can cause adverse effects in zebrafish embryos, and lateral-line hair cells are sensitive to ammonia exposure.

Functional re-organization of the gills of metamorphosing sea lamprey (Petromyzon marinus): preparation for a blood diet and the freshwater to seawater transition

Sea lamprey (Petromyzon marinus) begin life as filter-feeding larvae (ammocoetes) before undergoing a complex metamorphosis into parasitic juveniles, which migrate to the sea where they feed on the blood of large-bodied fishes. The greater protein intake during this phase results in marked increases in the production of nitrogenous wastes (N-waste), which are excreted primarily via the gills. However, it is unknown how gill structure and function change during metamorphosis and how it is related to modes of ammonia excretion, nor do we have a good understanding of how the sea lamprey's transition from fresh water (FW) to sea water (SW) affects patterns and mechanisms of N-waste excretion in relation to ionoregulation. Using immunohistochemistry, we related changes in the gill structure of larval, metamorphosing, and juvenile sea lampreys to their patterns of ammonia excretion (J_amm) and urea excretion (J_urea) in FW, and following FW to artificial seawater (ASW) transfer. Rates of J_amm and J_urea were low in larval sea lamprey and increased in feeding juvenile, parasitic sea lamprey. In freshwater-dwelling ammocoetes, immunohistochemical analysis revealed that Rhesus glycoprotein C-like protein (Rhcg-like) was diffusely distributed on the lamellar epithelium, but following metamorphosis, Rhcg-like protein was restricted to SW mitochondrion-rich cells (MRCs; ionocytes) between the gill lamellae. Notably, these interlamellar Rhcg-like proteins co-localized with Na⁺/K⁺-ATPase (NKA), which increased in expression and activity by almost tenfold during metamorphosis. The distribution of V-type H⁺-ATPase (V-ATPase) on the lamellae decreased following metamorphosis, indicating it may have a more important role in acid-base regulation and Na⁺ uptake in FW, compared to SW. We conclude that the re-organization of the sea lamprey gill during metamorphosis not only plays a critical role in allowing them to cope with greater salinity following the FW-SW transition, but that it simultaneously reflects fundamental changes in methods used to excrete ammonia.

Is the dendritic organ of the striped eel catfish Plotosus lineatus an ammonia excretory organ?

The dendritic organ (DO) is a salt secretory organ in the Plotosidae marine catfishes. The potential role of the DO in ammonia excretion was investigated by examining the effects of salinity [brackishwater (BW 3‰), seawater (SW 34‰) and hypersaline water (HSW 60‰)] acclimation and DO ligation on ammonia excretion and ammonia transporter expression by immunohistochemistry (IHC), immunoblotting (IB) and qPCR. Ammonia flux rates (J_Amm) were significantly lower in BW compared to SW and HSW. DO ligation resulted in a significantly lower J_Amm in SW but not BW fish. IHC demonstrated apical and basolateral localization of Rhesus-associated glycoprotein (Rhag-like) and Rhbg-like proteins, respectively, in parenchymal cells of the DO acini. In the gills, which are the primary site of ammonia excretion in teleost fishes, IHC showed an apical localization of Rhag-like protein in some Na⁺/K⁺-ATPase (NKA) immunoreactive (IR) cells limited to a few interlamellar regions of the filament and, in both apical and basolateral membranes of pillar cells irrespective of treatment group. In gills, the distribution of NKA-IR cells showed no salinity and/or ligation dependency. IB of Rhag and Rhbg-like proteins was found only in the gills and expression levels did not change with salinity but ligation in BW decreased Rhbg-like levels. Although Rhcg was not detected with heterologous antibodies, rhcg1 mRNA expression was detected in both gills and DO. HSW was associated with the lowest expression in DO and ligations in SW and BW were without effect on branchial expression levels. Taken together these results indicate the DO potentially has a physiological role in ammonia excretion under SW conditions.

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.

An animal homolog of plant Mep/Amt transporters promotes ammonia excretion by the anal papillae of the disease vector mosquito Aedes aegypti

The transcripts of three putative ammonia (NH3/NH4 (+)) transporters, Rhesus-like glycoproteins AeRh50-1, AeRh50-2 and Amt/Mep-like AeAmt1 were detected in the anal papillae of larval Aedes aegypti Quantitative PCR studies revealed 12-fold higher transcript levels of AeAmt1 in anal papillae relative to AeRh50-1, and levels of AeRh50-2 were even lower. Immunoblotting revealed AeAmt1 in anal papillae as a pre-protein with putative monomeric and trimeric forms. AeAmt1 was immunolocalized to the basal side of the anal papillae epithelium where it co-localized with Na(+)/K(+)-ATPase. Ammonium concentration gradients were measured adjacent to anal papillae using the scanning ion-selective electrode technique (SIET) and used to calculate ammonia efflux by the anal papillae. dsRNA-mediated reductions in AeAmt1 decreased ammonia efflux at larval anal papillae and significantly increased ammonia levels in hemolymph, indicating a principal role for AeAmt1 in ammonia excretion. Pharmacological characterization of ammonia transport mechanisms in the anal papillae suggests that, in addition to AeAmt1, the ionomotive pumps V-type H(+)-ATPase and Na(+)/K(+)-ATPase as well as NHE3 are involved in ammonia excretion at the anal papillae.

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.

Ammonia excretion in Caenorhabditis elegans: mechanism and evidence of ammonia transport of the Rhesus protein CeRhr-1

The soil-dwelling nematode Caenorhabditis elegans is a bacteriovorous animal, excreting the vast majority of its nitrogenous waste as ammonia (25.3±1.2 µmol gFW(-1) day(-1)) and very little urea (0.21±0.004 µmol gFW(-1) day(-1)). Although these roundworms have been used for decades as genetic model systems, very little is known about their strategy to eliminate the toxic waste product ammonia from their bodies into the environment. The current study provides evidence that ammonia is at least partially excreted via the hypodermis. Starvation reduced the ammonia excretion rates by more than half, whereas mRNA expression levels of the Rhesus protein CeRhr-2, V-type H(+)-ATPase (subunit A) and Na(+)/K(+)-ATPase (α-subunit) decreased correspondingly. Moreover, ammonia excretion rates were enhanced in media buffered to pH 5 and decreased at pH 9.5. Inhibitor experiments, combined with enzyme activity measurements and mRNA expression analyses, further suggested that the excretion mechanism involves the participation of the V-type H(+)-ATPase, carbonic anhydrase, Na(+)/K(+)-ATPase, and a functional microtubule network. These findings indicate that ammonia is excreted, not only by apical ammonia trapping, but also via vesicular transport and exocytosis. Exposure to 1 mmol l(-1) NH4Cl caused a 10-fold increase in body ammonia and a tripling of ammonia excretion rates. Gene expression levels of CeRhr-1 and CeRhr-2, V-ATPase and Na(+)/K(+)-ATPase also increased significantly in response to 1 mmol l(-1) NH4Cl. Importantly, a functional expression analysis showed, for the first time, ammonia transport capabilities for CeRhr-1 in a phylogenetically ancient invertebrate system, identifying these proteins as potential functional precursors to the vertebrate ammonia-transporting Rh-glycoproteins.

Light induces changes in activities of Na(+)/K(+)-ATPase, H(+)/K(+)-ATPase and glutamine synthetase in tissues involved directly or indirectly in light-enhanced calcification in the giant clam, Tridacna squamosa

The objective of this study was to determine the effects of 12 h of exposure to light, as compared with 12 h of exposure to darkness (control), on enzymatic activities of transporters involved in the transport of NH(+) 4 or H(+), and activities of enzymes involved in converting NH(+) 4 to glutamate/glutamine in inner mantle, outer mantle, and ctenidia of the giant clam, Tridacna squamosa. Exposure to light resulted in a significant increase in the effectiveness of NH(+) 4 in substitution for K(+) to activate Na(+)/K(+)-ATPase (NKA), manifested as a significant increase in the Na(+)/NH(+) 4-activated-NKA activity in the inner mantle. However, similar phenomena were not observed in the extensible outer mantle, which contained abundant symbiotic zooxanthellae. Hence, during light-enhanced calcification, H(+) released from CaCO3 deposition could react with NH3 to form NH(+) 4 in the extrapallial fluid, and NH(+) 4 could probably be transported into the shell-facing inner mantle epithelium through NKA. Light also induced an increase in the activity of glutamine synthetase, which converts NH(+) 4 and glutamate to glutamine, in the inner mantle. Taken together, these results explained observations reported elsewhere that light induced a significant increase in pH and a significant decrease in ammonia concentration in the extrapallial fluid, as well as a significant increase in the glutamine concentration in the inner mantle, of T. squamosa. Exposure of T. squamosa to light also led to a significant decrease in the N-ethylmaleimide (NEM)-sensitive-V-H(+)-ATPase (VATPase) in the inner mantle, and significant increases in the Na(+)/K(+)-activated-NKA, H(+)/NH(+) 4-activated-H(+)/K(+)-ATPase, and NEM-sensitive-VATPase activities in ctenidia, indicating that light-enhanced calcification might perturb Na(+) homeostasis and acid/base balance in the hemolymph, and might involve the active uptake of NH(+) 4 from the environment. This is the first report on light having direct enhancing effects on activities of certain transporters/enzymes related to light-enhanced calcification in the inner mantle and ctenidia of T. squamosa.

Hypo-osmotic stress-induced physiological and ion-osmoregulatory responses in European sea bass (Dicentrarchus labrax) are modulated differentially by nutritional status

We investigated the impact of nutritional status on the physiological, metabolic and ion-osmoregulatory performance of European sea bass (Dicentrarchus labrax) when acclimated to seawater (32 ppt), brackish water (20 and 10 ppt) and hyposaline water (2.5 ppt) for 2 weeks. Following acclimation to different salinities, fish were either fed or fasted (unfed for 14 days). Plasma osmolality, [Na(+)], [Cl(-)] and muscle water content were severely altered in fasted fish acclimated to 10 and 2.5 ppt in comparison to normal seawater-acclimated fish, suggesting ion regulation and acid-base balance disturbances. In contrast to feed-deprived fish, fed fish were able to avoid osmotic perturbation more effectively. This was accompanied by an increase in Na(+)/K(+)-ATPase expression and activity, transitory activation of H(+)-ATPase (only at 2.5 ppt) and down-regulation of Na(+)/K(+)/2Cl(-) gene expression. Ammonia excretion rate was inhibited to a larger extent in fasted fish acclimated to low salinities while fed fish were able to excrete efficiently. Consequently, the build-up of ammonia in the plasma of fed fish was relatively lower. Energy stores, especially glycogen and lipid, dropped in the fasted fish at low salinities and progression towards the anaerobic metabolic pathway became evident by an increase in plasma lactate level. Overall, the results indicate no osmotic stress in both feeding treatments within the salinity range of 32 to 20 ppt. However, at lower salinities (10-2.5 ppt) feed deprivation tends to reduce physiological, metabolic, ion-osmo-regulatory and molecular compensatory mechanisms and thus limits the fish's abilities to adapt to a hypo-osmotic environment.

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.

Rh vs pH: the role of Rhesus glycoproteins in renal ammonia excretion during metabolic acidosis in a freshwater teleost fish

Increased renal ammonia excretion in response to metabolic acidosis is thought to be a conserved response in vertebrates. We tested the hypothesis that Rhesus (Rh) glycoproteins in the kidney of the freshwater common carp Cyprinus carpio play a critical role in regulating renal ammonia excretion during chronic metabolic acidosis. Exposure to water pH 4.0 (72 h) resulted in a classic metabolic acidosis with reduced plasma pHa, [HCO3-], no change in PCO2, and large changes in renal function. Urine [NH4+] as well as [titratable acidity–HCO3-] rose significantly over the acid exposure, but the profound reduction (5-fold) in urine flow rates eliminated the expected elevations in renal ammonia excretion. Low urine flow rates may be a primary strategy to conserve ions, as urinary excretion of Na+, Cl- and Ca2+ were significantly lower during the acid exposure relative to the control period. Interestingly, renal Rhcg1 mRNA and protein levels were elevated in acid relative to control groups, along with mRNA levels of several ion transporters, including the Na+/H+ exchanger (NHE3), H+ATPase and Na+/K+ATPase (NKA). Immunofluorescence microscopy showed a strong apical Rhcg1 signal in distal tubules. Taken together, these data show that renal Rh glycoproteins and associated ion transporters are responsive to metabolic acidosis, but conservation of ions through reduced urine flow rates takes primacy over renal acid-base regulation in the freshwater C. carpio. We propose that an “acid/base-ion balance” compromise explains the variable renal responses to metabolic acidosis in freshwater teleosts.

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.

Waterborne copper exposure inhibits ammonia excretion and branchial carbonic anhydrase activity in euryhaline guppies acclimated to both fresh water and sea water

Inhibition of ammonia excretion (J(amm)) is a common response to Cu exposure in freshwater (FW) and seawater (SW) organisms. To determine the mechanism of this response, a euryhaline species of guppy (Poecilia vivipara) was exposed to 20 μg Cu/l in FW (0 ppt) and SW (25 ppt) for 96 h. In both salinities, Cu transiently inhibited ammonia excretion (J(amm)) followed by a full recovery by the end of the 96 h exposure. The activities of Na(+)/K(+)-ATPase, H(+)-ATPase, and carbonic anhydrase (CA) were examined in the gills at 12 and 96 h of Cu exposure. In both salinity acclimations, CA activity was significantly inhibited following 12h of Cu exposure in P. vivipara, marking the first in vivo evidence of Cu-induced inhibition of CA in fish. Moreover, the inhibition and recovery of this enzyme were correlated with the inhibition and recovery of J(amm) in both salinity acclimations. The blockade of CA potentially acts as a common mechanism of J(amm) inhibition in FW and SW. There were no significant effects on Na(+)/K(+)-ATPase or H(+)-ATPase activity at either time point or salinity. However, H(+)-ATPase activity was upregulated at 96 h relative to the 12h time point, potentially involving this enzyme in re-establishing J(amm).

Ammonia transport by terrestrial and aquatic insects

Ammonia, an end product from amino acid and nucleic acid metabolism, is highly toxic for most animals. This review will provide an update on nitrogen metabolism in terrestrial and aquatic insects with emphasis on ammonia generation and transport. Aspects that will be discussed include metabolic pathways of nitrogenous compounds, the origin of ammonia and other nitrogenous waste products, ammonia toxicity, putative ammonia transporters as well as ammonia transport processes known in insects. Ammonia transport mechanisms in the mosquito Aedes aegypti, the tobacco hornworm Manduca sexta and the locust Schistocerca gregaria will be discussed in detail while providing additional, novel data.

A nose-to-nose comparison of the physiological and molecular responses of rainbow trout to high environmental ammonia in seawater versus freshwater

Steelhead rainbow trout acclimated to either freshwater (FW) or seawater (SW) were exposed to high environmental ammonia (HEA, 1000 μmol l(-1) NH(4)HCO(3), pH 7.8-8.0) for 24 h. SW trout restored ammonia excretion more rapidly (3-6 h versus 9-12 h in FW), despite higher production rates and lower plasma pH. Plasma total ammonia levels stabilized at comparable levels below the external HEA concentration, and blood acid-base disturbances were small at both salinities. The electrochemical gradients for NH(4)(+) entry (F(NH(4))(+)) were the same in the two salinities, but only because FW trout allowed their transepithelial potential to rise by ∼15 mV during HEA exposure. Elevation of plasma [cortisol] during HEA exposure was more prolonged in SW fish. Plasma [glucose] increased in SW, but decreased in FW trout. Plasma [urea-N] also decreased in FW, in concert with elevated urea transporter (UT) mRNA expression in the gills. Of 13 branchial transporters, baseline mRNA expression levels were higher for Rhcg1, NHE2, NKCC1a and UT, and lower for NBC1 and NKA-α1a in SW trout, whereas NKA-α1b, NHE3, CA2, H(+)-ATPase, Rhag, Rhbg and Rhcg2 did not differ. Of the Rh glycoprotein mRNAs responding to HEA, Rhcg2 was greatly upregulated in both FW and SW, Rhag decreased only in SW and Rhcg1 decreased only in FW. H(+)-ATPase mRNA increased in FW whereas NHE2 mRNA increased in SW; NHE3 did not respond, and V-type H(+)-ATPase activity declined in SW during HEA exposure. Branchial Na(+),K(+)-ATPase activity was much higher in SW gills, but could not be activated by NH(4)(+). Overall, the more effective response of SW trout was explained by differences in physical chemistry between SW and FW, which greatly reduced the plasma NH(3) tension gradient for NH(3) entry, as well as by the higher [Na(+)] in SW, which favoured Na(+)-coupled excretion mechanisms. At a molecular level, responses in SW trout showed subtle differences from those in FW trout, but were very different than in the SW pufferfish. Upregulation of Rhcg2 appears to play a key role in the response to HEA in both FW and SW trout, and NH(4)(+) does not appear to move through Na(+),K(+)-ATPase.

Mechanisms of transepithelial ammonia excretion and luminal alkalinization in the gut of an intestinal air-breathing fish, Misgurnus anguilliacaudatus

The weatherloach, Misgurnus angulliacaudatus, is an intestinal air-breathing, freshwater fish that has the unique ability to excrete ammonia through gut volatilization when branchial and cutaneous routes are compromised during high environmental ammonia or air exposure. We hypothesized that transepithelial gut NH4+ transport is facilitated by an apical Na+/H+ (NH4+) exchanger (NHE) and basolateral Na+/K+(NH4+)-ATPase, and that gut boundary layer alkalinization (NH4+ => NH3 + H+) is facilitated by apical HCO3- secretion through a Cl-/HCO3- anion exchanger. This was tested using a pharmacological approach with anterior (digestive) and posterior (respiratory) intestine preparations mounted in pH-stat equipped Ussing chambers. The anterior intestine had a markedly higher conductance, short circuit current and net base (Jbase) and ammonia excretions rates (Jamm) than posterior intestine. In anterior intestine, HCO3- accounted for 70% Jbase. In the presence of an imposed serosal-mucosal ammonia gradient, both NHE and Na+/K+-ATPase inhibitors EIPA (0.1mM) and ouabain (0.1mM) significantly inhibit Jamm in the anterior intestine, although only the former in the posterior intestine. In addition, the anion exchange inhibitor DIDS significantly reduced Jbase in anterior intestine although only at a high dose (1mM). Carbonic anhydrase does not appear to be associated with gut alkalization under these conditions since etoxzolamide was without effect on Jbase. Membrane fluidity of the posterior intestine was low suggesting low permeability, which was also reflected in a lower mucosal-serosal Jamm in the presence of an imposed gradient in contrast to the anterior intestine. To conclude although the posterior intestine is highly modified for gas exchange, it is the anterior intestine that is the likely site of ammonia excretion and alkalinization leading to ammonia volatilization in the gut.

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.

Na⁺,K⁺-ATPase activity in the posterior gills of the blue crab, Callinectes ornatus (Decapoda, Brachyura): modulation of ATP hydrolysis by the biogenic amines spermidine and spermine

We investigated the effect of the exogenous polyamines spermine, spermidine and putrescine on modulation by ATP, K⁺, Na⁺, NH₄⁺ and Mg²⁺ and on inhibition by ouabain of posterior gill microsomal Na⁺,K⁺-ATPase activity in the blue crab, Callinectes ornatus, acclimated to a dilute medium (21‰ salinity). This is the first kinetic demonstration of competition between spermine and spermidine for the cation sites of a crustacean Na⁺,K⁺-ATPase. Polyamine inhibition is enhanced at low cation concentrations: spermidine almost completely inhibited total ATPase activity, while spermine inhibition attained 58%; putrescine had a negligible effect on Na⁺,K⁺-ATPase activity. Spermine and spermidine affected both V and K for ATP hydrolysis but did not affect ouabain-insensitive ATPase activity. ATP hydrolysis in the absence of spermine and spermidine obeyed Michaelis-Menten behavior, in contrast to the cooperative kinetics seen for both polyamines. Modulation of V and K by K⁺, Na⁺, NH₄⁺ and Mg²⁺ varied considerably in the presence of spermine and spermidine. These findings suggest that polyamine inhibition of Na⁺,K⁺-ATPase activity may be of physiological relevance to crustaceans that occupy habitats of variable salinity.

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.

Ammonia excretion in rainbow trout (Oncorhynchus mykiss): evidence for Rh glycoprotein and H+-ATPase involvement

Branchial ammonia transport in freshwater teleosts is not well understood. Most studies conclude that NH(3) diffuses out of the gill and becomes protonated to NH(4)(+) in an acidified gill boundary layer. Rhesus (Rh) proteins are new members of the ammonia transporter superfamily and rainbow trout possess genes encoding for Rh30-like1 and Rhcg2. We identified seven additional full-length trout Rh cDNA sequences: one Rhag and two each of Rhbg, Rhcg1, and Rh30-like. The mRNA expression of Rhbg, Rhcg1, and Rhcg2 was examined in trout tissues (blood, brain, eye, gill, heart, intestine, kidney, liver, muscle, skin, spleen) exposed to high external ammonia (HEA; 1.5 mmol/l NH(4)HCO(3), pH 7.95, 15 degrees C). Rhbg was expressed in all tissues, Rhcg1 was expressed in brain, gill, liver, and skin, and Rhcg2 was expressed in gill and skin. Brain Rhbg and Rhcg1 were downregulated, blood Rh30-like and Rhag were downregulated, and skin Rhbg and Rhcg2 were upregulated with HEA. After an initial uptake of ammonia into the fish during HEA, excretion was reestablished, coinciding with upregulations of gill Rh mRNA in the pavement cell fraction: Rhcg2 at 12 and 48 h, and Rhbg at 48 h. NHE2 expression remained unchanged, but upregulated H(+)-ATPase (V-type, B-subunit) and downregulated carbonic anhydrase (CA2) expression and activity were noted in the gill and again expression changes occurred in pavement cells, and not in mitochondria-rich cells. Together, these results indicate Rh glycoprotein involvement in ammonia transport and excretion in the rainbow trout while underscoring the significance of gill boundary layer acidification by H(+)-ATPase.

K+ and NH4(+) modulate gill (Na+, K+)-ATPase activity in the blue crab, Callinectes ornatus: fine tuning of ammonia excretion

To better comprehend the mechanisms of ionic regulation, we investigate the modulation by Na+, K+, NH4(+) and ATP of the (Na+, K+)-ATPase in a microsomal fraction from Callinectes ornatus gills. ATP hydrolysis obeyed Michaelis-Menten kinetics with KM=0.61+/-0.03 mmol L(-1) and maximal rate of V=116.3+/-5.4 U mg(-1). Stimulation by Na+ (V=110.6+/-6.1 U mg(-1); K0.5=6.3+/-0.2 mmol L(-1)), Mg2+ (V=111.0+/-4.7 U mg(-1); K0.5=0.53+/-0.03 mmol L(-1)), NH4(+) (V=173.3+/-6.9 U mg(-1); K0.5=5.4+/-0.2 mmol L(-1)) and K+ (V=116.0+/-4.9 U mg(-1); K0.5=1.5+/-0.1 mmol L(-1)) followed a single saturation curve, although revealing site-site interactions. In the absence of NH4(+), ouabain (K(I)=74.5+/-1.2 micromol L(-1)) and orthovanadate inhibited ATPase activity by up to 87%; the inhibition patterns suggest the presence of F0F1 and K+-ATPases but not Na+-, V- or Ca2+-ATPase as contaminants. (Na+, K+)-ATPase activity was synergistically modulated by K+ and NH4(+). At 10 mmol L(-1) K+, increasing NH4(+) concentrations stimulated maximum activity to V=185.9+/-7.4 U mg(-1). However, at saturating NH4(+) (50 mmol L(-1)), increasing K+ concentrations did not stimulate activity further. Our findings provide evidence that the C. ornatus gill (Na+, K+)-ATPase may be particularly well suited for extremely efficient active NH4(+) excretion. At elevated NH4(+) concentrations, the enzyme is fully active, regardless of hemolymph K+ concentration, and K+ cannot displace NH4(+) from its exclusive binding sites. Further, the binding of NH4(+) to its specific sites induces an increase in enzyme apparent affinity for K+, which may contribute to maintaining K+ transport, assuring that exposure to elevated ammonia concentrations does not lead to a decrease in intracellular potassium levels. This is the first report of modulation by ammonium ions of C. ornatus gill (Na+, K+)-ATPase, and should further our understanding of NH4(+) excretion in benthic crabs.

A kinetic study of the gill (Na+, K+)-ATPase, and its role in ammonia excretion in the intertidal hermit crab, Clibanarius vittatus

To better comprehend the role of gill ion regulatory mechanisms, the modulation by Na(+), K(+), NH(4)(+) and ATP of (Na(+), K(+))-ATPase activity was examined in a posterior gill microsomal fraction from the hermit crab, Clibanarius vittatus. Under saturating Mg(2+), Na(+) and K(+) concentrations, two well-defined ATP hydrolyzing sites were revealed. ATP was hydrolyzed at the high-affinity sites at a maximum rate of V=19.1+/-0.8 U mg(-1) and K(0.5)=63.8+/-2.9 nmol L(-1), obeying cooperative kinetics (n(H)=1.9); at the low-affinity sites, hydrolysis obeyed Michaelis-Menten kinetics with K(M)=44.1+/-2.6 mumol L(-1) and V=123.5+/-6.1 U mg(-1). Stimulation by Na(+) (V=149.0+/-7.4 U mg(-1); K(M)=7.4+/-0.4 mmol L(-1)), Mg(2+) (V=132.0+/-5.3 U mg(-1); K(0.5)=0.36+/-0.02 mmol L(-1)), NH(4)(+) (V=245.6+/-9.8 U mg(-1); K(M)=4.5+/-0.2 mmol L(-1)) and K(+) (V=140.0+/-4.9 U mg(-1); K(M)=1.5+/-0.1 mmol L(-1)) followed a single saturation curve and, except for Mg(2+), obeyed Michaelis-Menten kinetics. Under optimal ionic conditions, but in the absence of NH(4)(+), ouabain (K(I)=117.3+/-3.5 mumol L(-1)) and orthovanadate inhibited up to 67% of the ATPase activity. The inhibition studies performed suggest the presence of F(0)F(1), V- and P-ATPases, but not Na(+)-, K(+)- or Ca(2+)-ATPases as contaminants in the gill microsomal preparation. (Na(+), K(+))-ATPase activity was synergistically modulated by NH(4)(+) and K(+). At 20 mmol L(-1) K(+), a maximum rate of V=290.8+/-14.5 U mg(-1) was seen as NH(4)(+) concentration was increased up to 50 mmol L(-1). However, at fixed NH(4)(+) concentrations, no additional stimulation was found for increasing K(+) concentrations (V=135.2+/-4.1 U mg(-1) and V=236.6+/-9.5 U mg(-1) and for 10 and 30 mmol L(-1) NH(4)(+), respectively). This is the first report to detail ionic modulation of gill (Na(+), K(+))-ATPase in C. vittatus, revealing an asymmetrical, synergistic stimulation of the enzyme by K(+) and NH(4)(+), as yet undescribed for other (Na(+), K(+))-ATPases, and should provide a better understanding of NH(4)(+) excretion in pagurid crabs.

Sub-lethal plasma ammonia accumulation and the exercise performance of salmonids

The proposal that plasma ammonia accumulation might impair the swimming performance of fish was first made over a decade ago, and has now proven to be the case for a number of salmonid species. The first experimental evidence was indirect, when a negative linear relationship between plasma ammonia concentrations and maximum sustainable swimming speed (U(crit)) was found following the exposure of brown trout (Salmo trutta) to sub-lethal concentrations of copper in soft acidic water. Since then, negative linear relationships between plasma ammonia concentration and U(crit) have been demonstrated following exposure of brown trout, rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch) to elevated water ammonia. For brown trout, the relationships between plasma ammonia and U(crit) were remarkably similar following either exposure to elevated water ammonia or to sub-lethal copper. This indicates that the impairment of swimming performance resulting from exposure to sub-lethal concentrations of heavy metals may be attributable in large part to an accumulation of endogenous ammonia. The negative relationship between plasma ammonia concentration and U(crit) was similar in size-matched rainbow and brown trout but, under similar regimes of ammonia exposure, rainbow trout were able to maintain a significantly lower plasma ammonia concentration, revealing inter-specific differences in ammonia permeability and/or transport. One primary mechanism by which ammonia accumulation may impair exercise performance is a partial depolarisation of membrane potential in tissues such as the brain and white muscle. This may prejudice the co-ordination of swimming movements and reduce or abolish the development of muscle tension, thus, compromising swimming efficiency and performance at the top end of the range.

The expression of gill Na, K-ATPase in milkfish, Chanos chanos, acclimated to seawater, brackish water and fresh water

Juvenile milkfish Chanos chanos (Forsskål, 1775) were transferred from a local fish farm to fresh water (FW; 0 per thousand ), brackish water (BW; 10 per thousand, 20 per thousand ) and seawater (SW; 35 per thousand ) conditions in the laboratory and reared for at least two weeks. The blood and gill of the fish adapted to various salinities were analyzed to determine the osmoregulatory ability of this euryhaline species. No significant difference was found in plasma osmolality, sodium or chloride concentrations of milkfish adapted to various salinities. In FW, the fish exhibited the highest specific activity of Na, K-ATPase (NKA) in gills, while the SW group was found to have the lowest. Relative abundance of branchial NKA alpha-subunit revealed similar profiles. However, in contrary to other euryhaline teleosts, i.e. tilapia, salmon and eel, the naturally SW-dwelling milkfish expresses higher activity of NKA in BW and FW. Immunocytochemical staining has shown that most Na, K-ATPase immunoreactive (NKIR) cells in fish adapted to BW and SW were localized to the filaments with very few on the lamellae. Moreover, in FW-adapted milkfish, the number of NKIR cells found on the lamellae increased significantly. Such responses as elevated NKIR cell number and NKA activity are thought to improve the osmoregulatory capacity of the milkfish in hyposaline environments.

Sodium turnover rate determines sensitivity to acute copper and silver exposure in freshwater animals

The mechanisms of acute copper and silver toxicity in freshwater organisms appear similar. Both result in inhibition of branchial sodium (and chloride) uptake initiating a cascade of effects leading to mortality. The inhibition of the branchial Na/K-ATPase in the basolateral membrane is generally accepted as the key component responsible for the reduced sodium uptake. We propose that branchial carbonic anhydrase and the apical sodium channel may also be important targets for both copper and silver exposure. Several attempts have been made to predict metal sensitivity. A prominent example is the geochemical-biotic ligand model. The geochemical-biotic ligand modeling approach has been successful in explaining variations in tolerance to metal exposure for specific groups of animals exposed at different water chemistries. This approach, however, cannot explain the large observed variation in tolerance to these metals amongst different groups of freshwater animals (i.e. Daphnia vs. fish). Based on the detailed knowledge of physiological responses to acute metal exposure, the present review offers an explanation for the observed variation in tolerance. Smaller animals are more sensitive than large animals because they exhibit higher sodium turnover rates. The same relative inhibition of sodium uptake results in faster depletion of internal sodium in animals with higher sodium turnover. We present a way to improve predictions of acute metal sensitivity, noting that sodium turnover rate is the key predictor for variation in acute copper and silver toxicity amongst groups of freshwater animals. We suggest that the presented sodium turnover model is used in conjunction with the Biotic Ligand Model for risk management decisions.

Ammonia excretion and urea handling by fish gills: present understanding and future research challenges

In fresh water fishes, ammonia is excreted across the branchial epithelium via passive NH(3) diffusion. This NH(3) is subsequently trapped as NH(4)(+) in an acidic unstirred boundary layer lying next to the gill, which maintains the blood-to-gill water NH(3) partial pressure gradient. Whole animal, in situ, ultrastructural and molecular approaches suggest that boundary layer acidification results from the hydration of CO(2) in the expired gill water, and to a lesser extent H(+) excretion mediated by apical H(+)-ATPases. Boundary layer acidification is insignificant in highly buffered sea water, where ammonia excretion proceeds via NH(3) diffusion, as well as passive NH(4)(+) diffusion due to the greater ionic permeability of marine fish gills. Although Na(+)/H(+) exchangers (NHE) have been isolated in marine fish gills, possible Na(+)/NH(4)(+) exchange via these proteins awaits evaluation using modern electrophysiological and molecular techniques. Although urea excretion (J(Urea)) was thought to be via passive diffusion, it is now clear that branchial urea handling requires specialized urea transporters. Four urea transporters have been cloned in fishes, including the shark kidney urea transporter (shUT), which is a facilitated urea transporter similar to the mammalian renal UT-A2 transporter. Another urea transporter, characterized but not yet cloned, is the basolateral, Na(+) dependent urea antiporter of the dogfish gill, which is essential for urea retention in ureosmotic elasmobranchs. In ureotelic teleosts such as the Lake Magadi tilapia and the gulf toadfish, the cloned mtUT and tUT are facilitated urea transporters involved in J(Urea). A basolateral urea transporter recently cloned from the gill of the Japanese eel (eUT) may actually be important for urea retention during salt water acclimation. A multi-faceted approach, incorporating whole animal, histological, biochemical, pharmacological, and molecular techniques is required to learn more about the location, mechanism of action, and functional significance of urea transporters in fishes.

Modulation by ammonium ions of gill microsomal (Na+,K+)-ATPase in the swimming crab Callinectes danae: a possible mechanism for regulation of ammonia excretion

The modulation by Na(+), K(+), NH(4)(+) and ATP of the (Na(+),K(+))-ATPase in a microsomal fraction from Callinectes danae gills was analyzed. ATP was hydrolyzed at high-affinity binding sites at a maximal rate of V=35.4+/-2.1 Umg(-1) and K(0.5)=54.0+/-3.6 nM, obeying cooperative kinetics (n(H)=3.6). At low-affinity sites, the enzyme hydrolyzed ATP obeying Michaelis-Menten kinetics with K(M)=55.0+/-3.0 microM and V=271.5+/-17.2 Umg(-1). This is the first demonstration of a crustacean (Na(+),K(+))-ATPase with two ATP hydrolyzing sites. Stimulation by sodium (K(0.5)=5.80+/-0.30 mM), magnesium (K(0.5)=0.48+/-0.02 mM) and potassium ions (K(0.5)=1.61+/-0.06 mM) exhibited site-site interactions, while that by ammonium ions obeyed Michaelis-Menten kinetics (K(M)=4.61+/-0.27 mM). Ouabain (K(I)=147.2+/-7.microM) and orthovanadate (K(I)=11.2+/-0.6 microM) completely inhibited ATPase activity, indicating the absence of contaminating ATPase and/or neutral phosphatase activities. Ammonium and potassium ions synergistically stimulated the enzyme, increasing specific activities up to 90%, suggesting that these ions bind to different sites on the molecule. The presence of each ion modulates enzyme stimulation by the other. The modulation of (Na(+),K(+))-ATPase activity by ammonium ions, and the excretion of NH(4)(+) in benthic crabs are discussed.

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

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

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

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

The linkage between Na+ uptake and ammonia excretion in rainbow trout: kinetic analysis, the effects of (NH4)2SO4 and NH4HCO3 infusion and the influence of gill boundary layer pH

The nature of the linkage between between branchial ammonia excretion (JAmm) and unidirectional Na+ influx (JNain) was studied in the freshwater rainbow trout (Oncorhynchus mykiss). Arterial plasma total [ammonia], PNH3 and JAmm were all elevated approximately threefold by intravascular infusion for 24 h with either 70 mmol l-1 (NH4)2SO4 or 140 mmol l-1 NH4HCO3 at a rate of approximately 400 micromol kg-1 h-1. Both treatments markedly stimulated JNain. NH4HCO3 induced metabolic alkalosis in the blood plasma, whereas (NH4)2SO4 caused a slight metabolic acidosis. Experiments with Hepes-buffered water (5 mmol l-1) under control conditions demonstrated that increases in gill boundary layer pH were associated with decreases in both JNain and JAmm. Thus, the stimulation of JNain caused by ammonium loading was not simply a consequence of a Na+-coupled H+ extrusion mechanism activated by internal acidosis or by alkalosis in the gill boundary layer. Indeed, there was no stimulation of net acidic equivalent excretion accompanying NH4HCO3 infusion. Michaelis-Menten kinetic analysis by acute variation of water [Na+] demonstrated that both infusions caused an almost twofold increase in JNamax but no significant change in Km, indicative of an increase in transporter number or internal counterion availability without an alteration in transporter affinity for external Na+. The increase in JNain was larger with (NH4)2SO4 than with NH4HCO3 infusion and in both cases lower than the increase in JAmm. Additional evidence of quantitative uncoupling was seen in the kinetics experiments, in which acute changes in JNain of up to threefold had negligible effects on JAmm under either control or ammonium-loaded conditions. In vitro measurements of branchial Na+/K+-ATPase activity demonstrated no effect of NH4+ concentration over the concentration range observed in vivo in infused fish. Overall, these results are consistent with a dominant role for NH3 diffusion as the normal mechanism of ammonia excretion, but indicate that ammonium loading directly stimulates JNain, perhaps by activation of a non-obligatory Na+/NH4+ exchange rather than by an indirect effect (e.g. Na+-coupled H+ excretion) mediated by altered internal or external acid-base status.

Cyanide exposure affects the production and excretion of ammonia by the mudskipper Boleophthalmus boddaerti

The concentrations of ammonia in the plasma of the mudskipper Boleophthalmus boddaerti exposed to cyanide for 1-6 days were significantly greater than the respective values of the controls. This was due to an increase in the production of NH3 in the muscle and an increase in the retention of NH3 and/or NH4+ in the blood of the cyanide-exposed fish when compared to controls. Cyanide exposure significantly increased the specific activity of muscle AMP deaminase. Since adenylosuccinate synthetase and lyase were also present in the muscle, exposure to cyanide might increase the production of NH3 from the catabolism of purine nucleotides. B. boddaerti exposed to cyanide excreted significantly less ammonia than the control fish. Results indicate changes in the permeability of the epithelial surfaces of the cyanide-exposed fish to NH3 and/or NH4+. Since the tissues and organs needed time to activate the inducible cyanide detoxification mechanisms, the increase in the production of NH3 might be an important defensive mechanism for B. boddaerti during the early phase of cyanide exposure.

Transport mechanisms of seawater teleost chloride cells: an inclusive model of a multifunctional cell

This review assembles recent information on seawater-type chloride cells of marine teleost fish and evaluates the secretion of Na+, Cl-, K+, H+ and NH4+ and the absorption of Ca2+. The evidence for the distribution (apical vs basolateral) and the abundance of the various ion pumps, cotransporters, channels and exchangers is assessed and an inclusive model is constructed. Relationships among the transport systems are presented to suggest that many, if not all, of these systems may be operating simultaneously in individual, multifunctional chloride cells.

Do amiloride and ouabain affect ammonia fluxes in perfused Carcinus gill epithelia?

The effects of inhibitors on the efflux of ammonia (from the basolateral to the apical side, Jb----a) were studied in preparation of isolated Carcinus gills immersed in dilute seawater (DSW) that was identical to the perfusion solution. Adding 10(-4) M amiloride to the solution bathing the gill preparations reduces the efflux of ammonia by 29% relative to the control value. Under experimental conditions, it appears that only about 1% of the amiloride-sensitive influx of Na+ (Ja----b) can be exchanged with NH4+ on an equimolar basis. The ammonium ion is apparently transported at the basolateral side by a carrier-mediated process. Kinetic analyses of the influx of ammonium ions revealed a Km of 36.99 microM and a maximum velocity (Vmax) equal to 19.6 mumol g-1.h-1. Basolaterally applied ouabain (5 x 10(-3) M) and NaCN (10(-3) M) reduced the efflux of ammonia by 46.7 and 42.2%, respectively, suggesting an interaction of NH4+ with the basolaterally located Na+/K+ exchanger in which NH4+ appears to be able to substitute for K+.

Characterization of transport Na(+)-ATPases in gills of freshwater tilapia : Evidence for branchial Na(+)/H (+) (-NH4 (+)), ATPase activity in fish gills

Branchial plasma membranes from the freshwater cichlid teleostOreochromis mossambicus (tilapia) contain two Na(+)-dependent ATPases: Na(+)/K(+) ATPase, and an amiloride-sensitive ATPase which is postulated to operate as a Na(+)/H(+) (-NH4 (+)) ATPase. It is suggested that both enzyme activities are located in the basolateral membrane system of the chloride cells. K(+) has opposing effects on the two enzymes: it stimulates Na(+)/K(+) ATPase and inhibits Na(+)/H(+) (-NH4 (+)) ATPase activity. Na(+)/H(+) ATPase appears more sensitive to NH4 (+) at low concentrations than Na(+)/K(+) ATPase and the stimulatory effect by NH4 (+) ions on the first enzyme could be important in facilitating NH4 (+) excretion by tilapia gills under physiological conditions.In vitro maximum stimulation by NH4 (+) is similar for the two enzymes (200%). In contrast to Na(+)/K(+) ATPase, Na(+)/H(+) ATPase activity is inhibited by supra-physiological (>20 mM) concentrations of NH4 (+).