Effects of adaptation to sea water, 170% sea water and to fresh water on activities and subcellular distribution of branchial Na+?K+-ATPase, low- and high affinity Ca++-ATPase, and ouabain-insensitive ATPase inGillichthys mirabilis

Effect of combined stress (salinity and temperature) in European sea bass Dicentrarchus labrax osmoregulatory processes

European sea bass Dicentrarchus labrax undertake seasonal migrations to estuaries and lagoons that are characterized by fluctuations in environmental conditions. Their ability to cope with these unstable habitats is undeniable, but it is still not clear how and to what extent salinity acclimation mechanisms are affected at temperatures higher than in the sea. In this study, juvenile sea bass were pre-acclimated to seawater (SW) at 18°C (temperate) or 24°C (warm) for 2weeks and then transferred to fresh water (FW) or SW at the respective temperature. Transfer to FW for two weeks resulted in decreased blood osmolalities and plasma Cl^- at both temperatures. In FW warm conditions, plasma Na⁺ was ~15% lower and Cl^- was ~32% higher than in the temperate-water group. Branchial Na⁺/K⁺-ATPase (NKA) activity measured at the acclimation temperature (V_apparent) did not change according to the conditions. Branchial Na⁺/K⁺-ATPase activity measured at 37°C (V_max) was lower in warm conditions and increased in FW compared to SW conditions whatever the considered temperature. Mitochondrion-rich cell (MRC) density increased in FW, notably due to the appearance of lamellar MRCs, but this increase was less pronounced in warm conditions where MRC's size was lower. In SW warm conditions, pavement cell apical microridges are less developed than in other conditions. Overall gill morphometrical parameters (filament thickness, lamellar length and width) differ between fish that have been pre-acclimated to different temperatures. This study shows that a thermal change affects gill plasticity affecting whole-organism ion balance two weeks after salinity transfer.

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.

Changes in gill H+-ATPase and Na+/K+-ATPase expression and activity during freshwater acclimation of Atlantic salmon (Salmo salar)

Few studies have examined changes in salmon gill ion transporter expression during the transition from seawater to freshwater, a pivotal moment in the salmonid life cycle. Seawater-acclimated Atlantic salmon were transferred to freshwater and blood and gill tissue were sampled over 30 days of acclimation. Salmon held in seawater had stable plasma osmolality and sodium and chloride levels throughout the experiment. Following freshwater exposure, plasma sodium and chloride levels and total osmolality decreased significantly before returning towards control levels over time. Gill H(+)-ATPase activity increased by more than 45% 14 days after exposure to freshwater, whereas H(+)-ATPase mRNA levels were not affected by the salinity change. Within 4 days of freshwater exposure, gill Na(+)/K(+)-ATPase activity increased ∼43% over control levels, remaining significantly higher until the 30 day sampling group when it declined back to control levels. This increase in activity was associated with a more than 7-fold increase in Na(+)/K(+)-ATPase isoform α1a mRNA level and a ∼60% decrease in Na(+)/K(+)-ATPase isoform β1b mRNA level. The mRNA levels of Na(+)/K(+)-ATPase isoforms α1c and α3 did not change as a result of freshwater exposure. The time courses for mRNA expression of the small membrane protein FXYD 11 and the β1-subunit were very similar, with levels increasing significantly 7 days following freshwater exposure before subsiding back to control levels at 30 days. Taken together, these data suggest an important role for Na(+)/K(+)-ATPase in freshwater acclimation in Atlantic salmon.

Water chemistry alters gene expression and physiological end points of chronic waterborne copper exposure in zebrafish, Danio rerio

This is the first study to implement a genomic approach to ascertain both transcriptional and functional end points of chronic Cu toxicity in fish associated with experimentally manipulated water chemistries. Over 21 d, zebrafish acclimated to softwater (Na(+) = 0.06 mM, Ca(2+) = 0.08 mM, Mg(2+) = 0.03 mM) were exposed to the following: soft-water (Ctrl); 12 microg L(-1) Cu (Cu); 3.3 mM Na(+) (Na); 3.3 mM Na(+) + 12 microg L(-1) Cu (Na + Cu); 3.3 mM Ca(2+) (Ca); or 3.3 mM Ca(2+) + 12 microg L(-1) Cu (Ca + Cu). Although effective at reducing Cu load in all tissues, Na(+) in the presence of Cu did not decrease the degree of oxidative damage, particularly in the gill and gut. In contrast, Ca + Cu treatment decreased Cu accumulation in gill, but not liver or gut, with no reduction in oxidative damage. Transcriptional analysis of candidate genes (atp7a, ctr1, ECaC, esr1) showed principally a down regulation of transcripts with the Cu only treatment, while Ca + Cu treatment restored some of the genes to control levels. Conversely, the Na + Cu treatment had a strong, opposing affect when compared to that of Cu alone. Zebrafish Affymetrix GeneChips revealed significantly clustered patterns of expression. Changes in expression induced by Cu appeared to be opposite to the majority of the other treatments. Our data on the preventative or enhancing effects of Na(+) and Ca(2+) both alone and in the presence of Cu, may, in the future, facilitate the incorporation of gene expression end points into a biotic ligand model predicting chronic Cu toxicity in this tropical model species of genomic importance.

Wild Arctic Char (Salvelinus alpinus) Upregulate Gill Na+,K+‐ATPase during Freshwater Migration

The successful acclimation of eurhyhaline fishes from seawater to freshwater requires the gills to stop actively secreting ions and start actively absorbing ions. Gill Na(+),K(+)-ATPase is known to be an integral part of the active ion secretion model of marine fishes, but its importance in the active ion uptake model of freshwater fishes is less clear. This study, conducted in the high Arctic, examines gill Na(+),K(+)-ATPase regulation in wild anadromous arctic char returning to freshwater from the ocean. Gill Na(+),K(+)-ATPase activity, protein expression, and mRNA expression of Na(+),K(+)-ATPase isoforms alpha 1a and alpha 1b were monitored in arctic char at three points along their migration route to and from Somerset Island, Nunavut, Canada: out at sea (Whaler's Point), in seawater near the river mouth (Nat's Camp), and after entering the Union River. Arctic char collected from the Union River had more than twofold greater gill Na(+),K(+)-ATPase activity. This was associated with a significant increase (threefold) in Na(+),K(+)-ATPase isoform alpha 1a mRNA expression and a significant increase in plasma sodium and osmolality levels compared with seawater char. Compared with char sampled from Whaler's Point, Na(+),K(+)-ATPase isoform alpha 1b mRNA expression was decreased by approximately 50% in char sampled at Nat's Camp and the Union River. These results suggest that the upregulation of gill Na(+),K(+)-ATPase activity is involved in freshwater acclimation of arctic char and implicate a role for Na(+),K(+)-ATPase isoform alpha 1a in this process. In addition, we discuss evidence that arctic char go through a preparatory phase, or "reverse smoltification," before entering freshwater.

Branchial chloride cells in sea bass (Dicentrarchus labrax) adapted to fresh water, seawater, and doubly concentrated seawater

Branchial chloride cells (CC) were studied in sea bass (Dicentrarchus labrax) maintained in seawater (SW: 35 per thousand) or gradually adapted to and subsequently maintained in fresh water (0.2 per thousand) or doubly concentrated seawater (DSW: 70 per thousand). Changes were observed in the location, number, and structure of CCs, that were discriminated by light, scanning, and transmission electron microscopy, as well as by immunofluorescence on the basis of their high Na(+)/K(+)-ATPase antigen content. The number of CCs increased in both fresh water and doubly concentrated seawater compared to control fish maintained in SW. In both experimental conditions, these cells were found on the gill filament (as in control fish) and even on the lamellae, especially in hypersaline conditions. Structural changes concerned the shapes and sizes of CCs and their apical outcrops and particularly the structures of their functional complexes (mitochondria, tubular system, and endoplasmic reticulum), which developed significantly in DSW adapted fish. The changes in the expression of the Na(+)/K(+)-ATPase were evaluated by assessing the enzyme's density at the ultrastructural level following immunogold labeling. This parameter was significantly higher in doubly concentrated seawater. The adaptative significance of the quantitative and morphofunctional changes in branchial chloride cells is discussed in relation to the original osmoregulatory strategy of this marine euryhaline teleost.

Osmoregulation and salinity effects on the expression and activity of Na+,K(+)-ATPase in the gills of European sea bass, Dicentrarchus labrax (L.)

The European sea bass, Dicentrarchus labrax, tolerates salinities ranging from freshwater (FW) to hypersaline conditions. In two experiments, we analysed changes in plasma ions, muscle water content (MWC), gill Na+,K(+)-ATPase activity, and alpha-subunit mRNA expression during the course of acclimation from 15 ppt salt water to FW or high salinity seawater (HSSW). In Experiment 1, fish (6.2 +/- 1.1 g) were acclimated from 15 ppt to either FW, 5, 15, 25, 50, or 60 ppt SW and sampled after 10 days. Gill Na+,K(+)-ATPase activity was stimulated in FW- and in 50 and 60 ppt SW-groups relative to the 15 ppt control group. In Experiment 2, subgroups of fish (89 +/- 7 g) were transferred from 15 ppt SW to FW or 50 ppt SW, and sampled 1, 2, 4, and 10 days later. Plasma osmolality, [Na+] and [Cl-] decreased in the FW-group and increased in the HSSW-group one day after transfer and lasting until day 10. This was accompanied by a pronounced increase in MWC in the FW-group and an insignificant decrease in the HSSW-group. The plasma [Na+]:[Cl-]-ratio increased markedly in the FW-group and decreased slightly in the HSSW-group, suggesting acid-base balance disturbances after transfer. Gill Na+,K(+)-ATPase activity was unchanged in 15 ppt SW but doubled in FW- and HSSW-groups after transfer. In both groups, this was preceded by a 2- to 5-fold elevation of the gill alpha-subunit Na+,K(+)-ATPase mRNA level. Thus increased expression of alpha-subunit mRNA is part of the molecular mechanism of both FW and SW acclimation in sea bass. Gill Na+,K(+)-ATPase Na(+)-, K(+)-, and ouabain-affinity were similar in fish acclimated to FW, 15 ppt, and HSSW, suggesting that identical isoforms of the catalytic subunit of the enzyme are expressed irrespective of salinity.

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.

Effects of salinity on chloride cells and Na+, K(+)-ATPase activity in the teleost Gillichthys mirabilis

1. Longjawed mudsuckers, Gillichthys mirabilis, in 30 ppt seawater (SW) were transferred to 1.5, 30 and 60 ppt SW. 2. In the first 1-3 days after transfer, plasma chloride level and plasma osmolarity rose in the 60 ppt SW fish, and decreased in the 1.5 ppt SW fish. 3. By day 21, however, plasma chloride and osmolarity were at or near the levels seen in the controls (30 ppt). 4. Branchial and jawskin Na+, K(+)-ATPase activities were high in all salinities, and did not differ significantly among treatments. 5. The vital fluorescent stains DASPEI and anthroylouabain were used to detect mitochondria and Na+, K(+)-ATPase, respectively, in chloride cells. 6. Both stains indicated that jawskin chloride cell density did not differ among treatment groups. 7. In contrast, chloride cell size increased significantly with increasing salinity. 8. The chloride cells of fish in 60 ppt SW were noticeably angular in outline, whereas those of both the 1.5 and 30 ppt SW fish were circular. 9. The results are discussed in relation to the ion transport requirements encountered in the intertidal habitat of the mudsucker.

Effects of ambient ion concentrations on gill ATPases in fresh water eel,Anguilla anguilla

Branchial activities of Na(+),K(+)-ATPase (ouabain sensitive), Mg(2+) ATPase (ouabain insensitive) and kinetic analysis of high and low affinity Ca(2+) ATPase were measured inAnguilla anguilla that had been acclimated to demineralized water (DW, Ca < 10 μM), freshwater (FW, Ca = 2 mM), and Low calcium freshwater (L-Ca, Ca = 0.9 mM). Na(+),K(+)-ATPase activity decreased while ouabain insensitive activity increased when ambient Ca(2+) decreased. Two kinetic forms of Ca(2+) ATPase could be resolved in each environmental condition. The stimulation coefficients of both sites or enzymes were not affected by ambient Ca(2+) concentrations. The maximal velocity of both the high and the low affinity Ca(2+) ATPase was increased when external Ca(2+) was decreased during acclimation. The low affinity Ca(2+) ATPase and the Mg(2+) stimulated enzyme could be a non specific enzyme accepting either Ca(2+) or Mg(2+). Results are compared with previous results in the literature and in relation to the branchial morphology and ionic exchanges in fish.

Influence of salinity on the energetics of gill and kidney of Atlantic salmon (Salmo salar)

The effect of seawater acclimation and adaptation to various salinities on the energetics of gill and kidney of Atlantic salmon (Salmo salar) was examined. Smolts and non-smolts previously reared in fresh water were exposed to a rapid increase in salinity to 30 ppt. Plasma osmolarity, [Na(+)], [Cl(-)], [K(+)] and [Mg(++)] increased in both groups but were significantly lower in smolts than non-smolts. Gill Na(+), K(+)-ATPase specific activity, initially higher in smolts, increased in both groups after 18 days in seawater. Kidney Na(+), K(+)-ATPase specific activity was not affected by salinity in either group. Gill and kidney citrate synthase specific activity was not affected by seawater exposure in smolts but decreased in non-smolts. In a second experiment, Atlantic salmon smolts reared in fresh water were acclimated to 0, 10 or 30 ppt seawater for 3 months at a temperature of 13-14°C. Gill Na(+), K(+)-ATPase was positively correlated with salinity, displaying 2.5- and 5-fold higher specific activity at 10 and 30 ppt, respectively, than at 0 ppt. Kidney Na(+), K(+)-ATPase specific activity was not significantly affected by environmental salinity. Citrate synthase and cytochrome c oxidase specific activities in gill were slightly (6-13%) lower at 10 ppt than at 0 and 30 ppt, whereas kidney activities were lowest at 30 ppt. Oxygen consumption of isolated gill filaments was significantly higher when incubated in isosmotic saline and at 30 ppt than at 0 ppt, but was not affected by the prior acclimation salinity. The results indicate that although high salinity induces increased gill Na(+), K(+)-ATPase activity, it does not induce substantial increases in metabolic capacity of gill or kidney.

Effects of epinephrine on branchial and renal calcium handling in the rainbow trout

Acute exposure of rainbow trout (Salmo gairdneri) to low external calcium (25 microM) caused an immediate but transient increase in plasma epinephrine concentration that may have been related to a concomitant depression of blood pH. Intra-arterial infusion of epinephrine at normal ambient calcium levels (0.35 mM) for 4 h caused circulating levels of epinephrine to rise from 2.9 X 10(-9) to 8.0 X 10(-8) M but did not affect norepinephrine levels, or branchial unidirectional calcium fluxes. Active (ATP-dependent) calcium transport across basolateral plasma membranes prepared from gill epithelial cells was not affected by pretreatment of fish with epinephrine or by direct application of epinephrine or cAMP, in vitro. Epinephrine infusion elevated urine flow rate, decreased urine pH, and increased urine phosphate levels significantly. Net renal calcium efflux increased significantly as a result of the increased urine flow rate. It is concluded that epinephrine does not stimulate branchial calcium uptake or renal conservation of calcium in rainbow trout at normal external calcium levels and therefore we cautiously suggest that epinephrine is unlikely to be involved in calcium balance during periods of exposure to low external calcium. Instead, epinephrine may play a role in compensating the acid-base disturbances and the increased branchial water influx that are associated with exposure to low ambient calcium.

The effect of pH and/or calcium-enriched freshwater on gill Ca2+-ATPase activity and osmotic water inflow in rainbow trout (Salmo gairdneri)

Rainbow trout (Salmo gairdneri) were exposed to pH 5.0-5.1, 6.6 and/or calcium-enriched freshwater for 14 days. Hematocrit, gill Ca2+-ATPase enzyme activities, gill osmotic water inflow, plasma calcium and osmolarity were measured. No significant changes in plasma calcium ion levels were found. The typical increase in hematocrit usually associated with exposure of fish to acidified water was not found in the present study and is discussed. Plasma osmolarity decreased in fish exposed to calcium-enriched freshwater (60 mg Ca2+ X 1(-1) ) in comparison to fish exposed to control freshwater conditions (2 mg Ca2+ X 1(1) ), irrespective of the pH level. Gill Ca2+-ATPase enzyme activities were measured for both low affinity (3 mM Ca2+) and high affinity (100 microM) activity. Exposure of rainbow trout to low pH (pH 5.0-5.1) did not affect the specific activity of Ca2+-ATPase enzyme. However, low affinity Ca2+-ATPase activity in fish exposed to calcium-enriched freshwater did show a significant reduction. The increase in gill osmotic water permeability in fish exposed to calcium-enriched freshwater is interpreted as a result of the increase in osmolarity of the ambient media.

Ca2+-dependent phosphatase and Ca2+-dependent ATPase activities in plasma membranes of eel gill epithelium--III. Stimulation of branchial high-affinity Ca2+-ATPase activity during prolactin-induced hypercalcemia in American eels

Infusions of ovine prolactin for 10 days induced hypercalcemia in unfed American eels, Anguilla rostrata LeSueur, that tentatively was related to stimulation of branchial Ca2+-uptake mechanisms. Analysis of ATPase activities in the plasma membranes of the branchial epithelium in prolactin treated eels showed a specific stimulation of high-affinity Ca2+-ATPase. The results of this study form further evidence that the high-affinity Ca2+-ATPase activity represents the Ca2+-pump of the branchial epithelium.

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

1. Branchial Na+K+-ATPase specific activity is some 20% greater in hyposaline adapted Opsanus beta than in SW specimens. 2. Ouabain insensitive ATPase (Mg2+-ATPase) specific activities were similar, while whole body activity differences in low salinity and SW adapted fish could be accounted for by the 30% difference in extractable gill protein. 3. NH+4 ion was 15% more effective at dephosphorylation of the microsomal Na-dependent phosphoenzyme than either Rb+ or K+, and revealed a maximal ATPase affinity (Km = 0.2 mM) within the physiological range of blood [K+]. 4. Similar properties as pH optima, ATP and Mg2+ Km's, ouabain sensitivity, percent recoveries and subcell distribution indicated that the NH+4-stimulation acts through the Na+ K+-ATPase carrier enzyme and may be responsible for the Na+/NH+4 exchange in Opsanus beta.

Separation by velocity sedimentation of the gill epithelial cells and their ATPases activities in the seawater adapted eel Anguilla anguilla L

The separation of cell populations by sedimentation was carried out on heterogeneous suspensions of branchial cells of the seawater adapted eel Anguilla anguilla. The cell sedimentation rates vary mainly in relation to size and permit the separation of enriched fractions of chloride cells and respiratory cells. The method is described and discussed. The homogeneity of the separated populations was checked by microscopy and particle counting. An enrichment of 95% by volume concentration of the two main cell types was obtained. The separated cells had good viability (85-95% viable). This was controlled by Trypan blue exclusion tests. The adenosine triphosphatase activities of the different cell populations were measured and compared.

Ca2+-dependent phosphatase and ATPase activities in eel gill plasma membranes--I. Identification of Ca2+-activated ATPase activities with non-specific phosphatase activities

The characteristics of Ca2+-activated ATPase activities previously often postulated as components for the calcium transporting system in fish gills do not fulfil the requirements of a transport Ca2+-ATPase. The chelation of Ca2+- or Mg2+-ions is a prerequisite for the adenosinephosphate esters to serve as substrate for gill plasma membrane phosphatases. Ca2+-activated ATP hydrolysis results from the activity of a heterogeneous pool of phosphatases located in the plasma membranes of the branchial epithelium, as is concluded from substrate specificity tests and the effects of various inhibitors on these hydrolytic activities. In the present study only non-specific phosphatases could be shown.