Isoform expression of Na+-K+-ATPase alpha-subunit in gills of the teleost Oreochromis mossambicus

Melatonin integrates multidimensional regulation of Na+/K+-ATPase in ionocytes and promotes stress and ease response in hypoxia-induced air-breathing fish: lessons from integrative approach

As circadian regulator, melatonin is involved in many physiological processes including ionosmotic regulation in fishes. Na⁺/K⁺-ATPase (NKA), an ubiquitous Na⁺/K⁺ transporter in ionocyte epithelia that drives electrochemical Na⁺ gradients and systemic osmotic integration, is a target of stress in fish. However, it is not certain how melatonin regulates NKA functions in ionocyte epithelia and how it modulates the adaptive response such as stress and ease response in fish particularly in hypoxia condition. We, thus, examined the short-term in vivo action of melatonin on the dynamics of NKA regulation in branchial, renal and intestinal ionocytes of hypoxia-induced air-breathing fish (Anabas testudineus Bloch). Interestingly, we found a rise in plasma melatonin in fish when kept for 30 min of forced submergence in water and that indicates a role for melatonin in hypoxia tolerance. A fall in blood [Na⁺ _, K⁺] occurred in these hypoxic fish which later showed a recovery after melatonin treatment. Similarly, melatonin favored the fall in NKA activity in branchial and renal epithelia of hypoxic fish, though it remarkably stimulated its activities in non-stressed fish. Likewise, melatonin that produced differential pattern of mRNA expression in nkaα1-subunit isoforms (nkaα1a, nkaα1b and nkaα1c) and melatonin receptor isoforms (mtnr1a, mtnr1bb, mtnr1bb _x1x2 ) in the tested ionocyte epithelia, showed reversed expression in hypoxic fish. In addition, the rise in NKAα-protein abundance in branchial and renal epithelia of melatonin-treated hypoxic fish indicated a recovery action of melatonin. A higher NKAα-immunoreactivity was found in the immunohistochemical and immunofluorescent images of branchial ionocytes and renal proximal and distal ionocytes of hypoxic fish treated with melatonin. Furthermore, an activation of PKA and PKG-dependent phosphorylation was found in branchial epithelia of hypoxic fish. The generated integrative parabola model showed that melatonin has a maximum targeted action on NKA function in the renal epithelia, suggesting its lead role in the integration of ionosmotic balance during the recovery or ease response. Over all, the data indicate a multidimensional and preferential action of melatonin on NKA regulation in fish ionocytes that integrate the recovery action against hypoxia, thus pointing to a major role for melatonin in stress and ease response in this fish.

Nitric oxide synthase (NOS) inhibitor L-NAME activates inducible NOS/NO system and drives multidimensional regulation of Na+ /K+ -ATPase in ionocyte epithelia of immersion-stressed air-breathing fish (Anabas testudineus Bloch)

Nitric oxide (NO) has been implicated in Na⁺  homeostatic control in water-breathing fishes. It is, however, uncertain whether air-breathing fish relies on NO to coordinate Na⁺ /K⁺ -ATPase (NKA)-driven Na⁺  transport during acute hypoxemia. We, thus, examined the action of nitric oxide synthase (NOS) inhibitor, L-NAME on NO availability, inducible NOS (iNOS) protein abundance and the regulatory dynamics of NKA in osmoregulatory epithelia of Anabas testudineus kept at induced hypoxemia. As expected in nonstressed fish, in vivo L-NAME (100 ng g^-1 ) challenge for 30 min declined NO production in serum (40%) and osmoregulatory tissues (average 51.6%). Surprisingly, the magnitude of such reduction was less in hypoxemic fish after L-NAME challenge due to the net gain of NO (average 23.7%) in these tissues. Concurrently, higher iNOS protein abundance was found in branchial and intestinal epithelia of these hypoxemic fish. In nonstressed fish, L-NAME treatment inhibited the NKA activity in branchial and intestinal epithelia while stimulating its activity in renal epithelia. Interestingly in hypoxemic fish, L-NAME challenge restored the hypoxemia-inhibited NKA activity in branchial and renal epithelia. Similar recovery response was evident in the NKAα protein abundance in immunoblots and immunofluorescence images of branchial epithelia of these fish. Analysis of Nkaα1 isoform transcript abundance (Nkaα1a, α1b, α1c) also showed spatial and preferential regulation of Nkaα1 isoform switching. Collectively, the data indicate that L-NAME challenge activates iNOS/NO system in the branchial ionocyte epithelia of hypoxemia-stressed Anabas and demands multidimensional regulation of NKA to restore the Na⁺  transport rate probably to defend against acute hypoxemia.

Characterization and expression of Na+/K+-ATPase in gills and kidneys of the Teleost fish Oreochromis mossambicus, Oreochromis urolepis hornorum and their hybrids in response to salinity challenge

Tilapia (Oreochromis mossambicus, O. urolepis hornorum, their hybrids O. mossambicus♀ × O. hornorum♂ and O. hornorum♀ × O. mossambicus♂) were exposed to a high salinity environment to evaluate their osmoregulatory responses. The plasma osmolality of all the tilapia species were elevated with the salinity challenge. The activities of Na⁺/K⁺-ATPase (NKA) in both the gill and kidney showed a similar increased change tendency compared with the control. The distribution of NKA α1 mRNA in all the examined tissues suggested that NKA α1 has a possible housekeeping role for this isoform. The amount of NKA α1 mRNA in the gill and kidney was elevated in the four fishes with similar expression patterns after transfer from freshwater to seawater. The NKAα1 mRNA expression levels in the gill reached their peak level at 24 h after transfer (P < 0.01) compared to the freshwater group, following decreases in the pretreatment level at 48 h (P > 0.05). However, the NKAα1 mRNA expression levels in the kidney were not significantly affected with increasing environmental salinity (P > 0.05). The differences in the responses to saltwater challenge may be associated with differences in saltwater tolerance between the four tilapia. The drastic increase in the plasma osmolality, NKA activities and mRNA expression suggested that the hybrids (O. mossambicus♀ × O. hornorum♂) possess heterosis in salinity responsiveness compared to that of both the parents, indicating a maternal effect on the salinity tolerance of the tilapia hybrids. This study provides a theoretical basis to further study the mechanism of fish osmoregulation response to salinity challenge.

Does Japanese medaka (Oryzias latipes) exhibit a gill Na(+)/K(+)-ATPase isoform switch during salinity change?

Some euryhaline teleosts exhibit a switch in gill Na(+)/K(+)-ATPase (Nka) α isoform when moving between fresh water (FW) and seawater (SW). The present study tested the hypothesis that a similar mechanism is present in Japanese medaka and whether salinity affects ouabain, Mg(2+), Na(+) and K(+) affinity of the gill enzyme. Phylogenetic analysis classified six separate medaka Nka α isoforms (α1a, α1b, α1c, α2, α3a and α3b). Medaka acclimated long-term (>30 days) to either FW or SW had similar gill expression of α1c, α2, α3a and α3b, while both α1a and α1b were elevated in SW. Since a potential isoform shift may rely on early changes in transcript abundance, we conducted two short-term (1-3 days) salinity transfer experiments. FW to SW acclimation induced an elevation of α1b and α1a after 1 and 3 days. SW to FW acclimation reduced α1b after 3 days with no other α isoforms affected. To verify that the responses were typical, additional transport proteins were examined. Gill ncc and nhe3 expression were elevated in FW, while cftr and nkcc1a were up-regulated in SW. This is in accordance with putative roles in ion-uptake and secretion. SW-acclimated medaka had higher gill Nka V max and lower apparent K m for Na(+) compared to FW fish, while apparent affinities for K(+), Mg(2+) and ouabain were unchanged. The present study showed that the Japanese medaka does not exhibit a salinity-induced α isoform switch and therefore suggests that Na(+) affinity changes involve altered posttranslational modification or intermolecular interactions.

Functional roles of Na(+)/K(+)-ATPase in active ammonia excretion and seawater acclimation in the giant mudskipper, Periophthalmodon schlosseri

The giant mudskipper, Periophthalmodon schlosseri, is an amphibious fish that builds burrows in the mudflats. It can actively excrete ammonia through its gills, and tolerate high environmental ammonia. This study aimed to examine the effects of seawater (salinity 30; SW) acclimation and/or environmental ammonia exposure on the kinetic properties of Na⁺/K⁺-ATPase (Nka) from, and mRNA expression and protein abundance of nka/Nka α–subunit isoforms in, the gills of P. schlosseri pre-acclimated to slightly brackish water (salinity 3; SBW). Our results revealed that the Nka from the gills of P. schlosseri pre-acclimated to SBW for 2 weeks had substantially higher affinity to (or lower K_m for) K⁺ than NH⁺₄, and its affinity to NH⁺₄ decreased significantly after 6-days exposure to 75 mmol l⁻¹ NH₄Cl in SBW. Hence, Nka transported K⁺ selectively to maintain intracellular K⁺ homeostasis, instead of transporting NH⁺₄ from the blood into ionocytes during active NH⁺₄ excretion as previously suggested. Two nkaα isoforms, nkaα1 and nkaα3, were cloned and sequenced from the gills of P. schlosseri. Their deduced amino acid sequences had K⁺ binding sites identical to that of Nkaα1c from Anabas testudineus, indicating that they could effectively differentiate K⁺ from NH⁺₄. Six days of exposure to 75 mmol l⁻¹ NH₄Cl in SBW, or to SW with or without 50 mmol l⁻¹ NH₄Cl led to significant increases in Nka activities in the gills of P. schlosseri. However, a significant increase in the comprehensive Nkaα protein abundance was observed only in the gills of fish exposed to 50 mmol l⁻¹ NH₄Cl in SW. Hence, post-translational modification could be an important activity modulator of branchial Nka in P. schlosseri. The fast modulation of Nka activity and concurrent expressions of two branchial nkaα isoforms could in part contribute to the ability of P. schlosseri to survive abrupt transfer between SBW and SW or abrupt exposure to ammonia.

Brain Na+/K+-ATPase α-subunit isoforms and aestivation in the African lungfish, Protopterus annectens

This study aimed to clone and sequence Na (+) / K (+)-ATPase (nka) α-subunit isoforms from, and to determine their mRNA expression levels and protein abundance in the brain of the African lungfish, Protopterus annectens during the induction, maintenance and arousal phases of aestivation in air. We obtained the full cDNA sequences of nkaα1, nkaα2 and nkaα3 from the brain of P. annectens. Phylogenetic analysis of their deduced amino acid sequences revealed that they are closer to the corresponding NKA α-subunits of tetrapods than to those of fishes. The mRNA expression of these three nkaα isoforms showed differential changes in the brain of P. annectens during the three phases of aestivation. After 12 days of aestivation, there was a significant increase in the protein abundance of Nkaα1 in the brain of P. annectens. This could be an important response to maintain cellular Na(+) and K(+) concentrations and regulate cell volume during the early maintenance phase of aestivation. On the other hand, the mRNA expression of nkaα2 decreased significantly in the brain of P. annectens after 6 months of aestivation, which could be a result of a suppression of transcriptional activities to reduce energy expenditure. The down-regulation of mRNA expression of nkaα1, nkaα2 and nkaα3 and the overall protein abundance of Nka α-subunit isoforms in the brain of P. annectens after 1 day of arousal from 6 months of aestivation were novel observations, and it could be an adaptive response to restore blood pressure and/or to prevent brain oedema.

Variations in the expression of vasotocin and isotocin receptor genes in the gilthead sea bream Sparus aurata during different osmotic challenges

The dynamic changes in mRNA expression levels for vasotocin (AVT) and isotocin (IT) receptor gene levels were assessed in a time-course response study in immature male specimens of the gilthead sea bream (Sparus aurata) submitted to hyper- (55‰ salinity) and hypo-osmotic (5‰ salinity) challenges. Two different cDNAs for the AVT receptor and one for the IT receptor (V1a2-type and V2-type AVTR, and ITR, respectively) were cloned by screening an S. aurata brain cDNA library. Genes for these receptors were expressed differentially and is nearly ubiquitously in 26 of the examined tissues. In the gills, both environmental salinity challenges up-regulated AVTR V1a2-type gene expression concomitantly with mRNA expression protein activity of Na(+), K(+)-ATPase gene expression and protein, whereas the AVTR V2-type and cystic fibrosis transmembrane conductance regulator (CFTR) mRNA levels were associated with mRNAs environmental salinity, indicating a possible connection between AVTRs and these transporters. In kidney, AVTR V1a2-type gene expression peaked rapidly and lasted only a short time (12-24h) in response to both osmotic challenges. In contrast, AVTR V2-type mRNA levels were enhanced in specimens exposed to hyperosmotic conditions, whereas they decreased under hypoosmotic environments, suggesting an antidiuretic role related to the vasoconstriction function. In the hypothalamus, only the expression of the AVTR V2-type gene was enhanced at 7 and 14 days under both experimental conditions. In the liver, both AVTRs had increased mRNA levels, with the upregulation of their AVTR V2-type gene increasing faster than the V1a2-type. The ITR gene was not sensitive to variations of external salinity in any of the analyzed tissues. Our results demonstrate the involvement of the vasotocinergic, but not the isotocinergic, pathway as well as the hypothalamic function, in the adjustments of both osmoregulatory and metabolic processes after osmotic challenges.

Quantitative molecular phenotyping of gill remodeling in a cichlid fish responding to salinity stress

A two-tiered label-free quantitative (LFQ) proteomics workflow was used to elucidate how salinity affects the molecular phenotype, i.e. proteome, of gills from a cichlid fish, the euryhaline tilapia (Oreochromis mossambicus). The workflow consists of initial global profiling of relative tryptic peptide abundances in treated versus control samples followed by targeted identification (by MS/MS) and quantitation (by chromatographic peak area integration) of validated peptides for each protein of interest. Fresh water acclimated tilapia were independently exposed in separate experiments to acute short-term (34 ppt) and gradual long-term (70 ppt, 90 ppt) salinity stress followed by molecular phenotyping of the gill proteome. The severity of salinity stress can be deduced with high technical reproducibility from the initial global label-free quantitative profiling step alone at both peptide and protein levels. However, an accurate regulation ratio can only be determined by targeted label-free quantitative profiling because not all peptides used for protein identification are also valid for quantitation. Of the three salinity challenges, gradual acclimation to 90 ppt has the most pronounced effect on gill molecular phenotype. Known salinity effects on tilapia gills, including an increase in the size and number of mitochondria-rich ionocytes, activities of specific ion transporters, and induction of specific molecular chaperones are reflected in the regulation of abundances of the corresponding proteins. Moreover, specific protein isoforms that are responsive to environmental salinity change are resolved and it is revealed that salinity effects on the mitochondrial proteome are nonuniform. Furthermore, protein NDRG1 has been identified as a novel key component of molecular phenotype restructuring during salinity-induced gill remodeling. In conclusion, besides confirming known effects of salinity on gills of euryhaline fish, molecular phenotyping reveals novel insight into proteome changes that underlie the remodeling of tilapia gill epithelium in response to environmental salinity change.

Salinity acclimation modulates copper toxicity in the sheepshead minnow, Cyprinodon variegatus

The sheepshead minnow (Cyprinodon variegatus) is able to withstand a wide range of salinities. Salinity acclimation involves physiological and biochemical changes, which may influence how organisms respond to a stressor. The present study investigated effects of salinity acclimation on subsequent Cu toxicity. In experiment 1, fish were acclimated to a hyposmotic, isosmotic, or hyperosmotic salinity for 14 d and then exposed at these salinities to 16.6 µM Cu(2+) for 12 h. Survival differed during this Cu challenge; fish acclimated to 2.5 ppt salinity were much more sensitive to Cu than those acclimated to 10.5 or 18.5 ppt seawater. In experiment 2, fish were exposed to 14.6 µM Cu(2+) for 6 h after the 14-d salinity acclimation. Whole-body Cu, whole-body Na, liver lipid peroxidation (LPO), liver catalase activity, and liver glucose levels were determined before and after Cu exposure. Prior to Cu exposure, the acclimation groups differed only for liver glucose levels, which were higher in the 2.5 ppt acclimated fish than in the others. These same 2.5 ppt acclimated fish were markedly affected by Cu, having increased whole-body Cu and liver LPO and decreased whole-body Na levels. Copper exposure had generally insignificant effects for the 10.5 ppt and the 18.5 ppt acclimated fish. This study showed that even in euryhaline fish, salinity acclimation can have a drastic effect on Cu toxicity.

Combined effects of high environmental ammonia, starvation and exercise on hormonal and ion-regulatory response in goldfish (Carassius auratus L.)

Due to eutrophication, high environmental ammonia (HEA) has become a frequent problem in aquatic environments, especially in agricultural or densely populated areas. During certain periods, e.g. winter, feed deprivation may occur simultaneously in natural waters. Additionally, under such stressful circumstances, fish may be enforced to swim at a high speed in order to catch prey, avoid predators and so on. Consequently, fish need to cope with all these stressors by altering physiological processes which in turn are controlled by genes expression. Therefore, in the present study, ammonia toxicity was tested in function of nutrient status (fed versus starved) and swimming performance activity (routine versus exhaustive). Goldfish, a relatively tolerant cyprinid, were exposed to HEA (1 mg/L; Flemish water quality guideline for surface water) for a period of 3 h, 12 h, 1 day, 4 days, 10 days, 21 days and 28 days and were either fed (2% body weight) or starved (kept unfed for 7 days prior to sampling). Results showed that the activity of Na⁺/K⁺-ATPase in the gills was stimulated by HEA and disturbance in ion balance was obvious with increases in plasma [Na⁺], [Cl⁻] and [Ca²⁺] after prolonged exposure. Additionally, osmoregulation and metabolism controlling hormones like cortisol and thyroid hormones (T3 and T4) were investigated to understand adaptive responses. The expression kinetics of growth, stress and osmo-regulatory representative genes such as Insulin-like growth factor 1 (IGF-I), growth hormone receptor (GHR), thyroid hormone receptor β (THRβ), prolactin receptor (PRLR), cortisol receptor (CR) and Na⁺/K⁺-ATPase α(3) were examined. Overall effect of HEA was evident since Na⁺/K⁺-ATPase activity, plasma cortisol, Na⁺ and Ca²⁺ concentration, expression level of CR and Na⁺/K⁺-ATPase α₃ mRNA in fed and starved fish were increased. On the contrary, transcript level of PRLR was reduced after 4 days of HEA; additionally T3 level and expression of GHR, IGF-I and THRβ genes were decreased following 10-21 days of HEA. Starvation, the additional challenge in the present study, significantly increased plasma cortisol level and CR transcript level under HEA compared to the fed exposed and control fish. Furthermore, a remarkable reduction in T3 and mRNA levels of THRβ, IGF-I and GHR genes was observed under starvation. The toxic effects in both feeding treatments were exacerbated when imposed to exhaustive swimming with more pronounced effects in starved fish. This confirms that starvation makes fish more vulnerable to external ammonia, especially during exercise.

Ion regulation in fish gills: recent progress in the cellular and molecular mechanisms

Fish encounter harsh ionic/osmotic gradients on their aquatic environments, and the mechanisms through which they maintain internal homeostasis are more challenging compared with those of terrestrial vertebrates. Gills are one of the major organs conducting the internal ionic and acid-base regulation, with specialized ionocytes as the major cells carrying out active transport of ions. Exploring the iono/osmoregulatory mechanisms in fish gills, extensive literature proposed several models, with many conflicting or unsolved issues. Recent studies emerged, shedding light on these issues with new opened windows on other aspects, on account of available advanced molecular/cellular physiological approaches and animal models. Respective types of ionocytes and ion transporters, and the relevant regulators for the mechanisms of NaCl secretion, Na(+) uptake/acid secretion/NH(4)(+) excretion, Ca(2+) uptake, and Cl(-) uptake/base secretion, were identified and functionally characterized. These new ideas broadened our understanding of the molecular/cellular mechanisms behind the functional modification/regulation of fish gill ion transport during acute and long-term acclimation to environmental challenges. Moreover, a model for the systematic and local carbohydrate energy supply to gill ionocytes during these acclimation processes was also proposed. These provide powerful platforms to precisely study transport pathways and functional regulation of specific ions, transporters, and ionocytes; however, very few model species were established so far, whereas more efforts are needed in other species.

Intestinal osmoregulatory acclimation and nitrogen metabolism in juveniles of the freshwater marble goby exposed to seawater

The objective of this study was to elucidate the role of the intestine from juveniles of the marble goby, Oxyeleotris marmorata, during seawater (SW) exposure. It has been reported elsewhere that SW-exposed juvenile O. marmorata exhibits hypoosmotic and hypoionic regulation, with the induction of branchial Na(+)/K(+)-ATPase (NKA), Na(+):K(+):2Cl(-) cotransporter (NKCC), and cystic fibrosis transmembrane receptor-like chloride channels. Here, we report that SW exposure also led to significant increases in the activity and protein abundance of NKA in, and probably an increase in Na(+) uptake through, its intestine. Additionally, there was an increase in apical NKCC immunoreactivity in the intestinal epithelium, indicating that there could be increased Cl(-) uptake through the intestine. These results suggest that absorption of ions, and hence water, from the intestinal lumen could be an essential part of the osmoregulatory process in juvenile O. marmorata during exposure to SW. Furthermore, there were significant increases in the glutamate content, and the aminating activity and protein abundance of glutamate dehydrogenase (GDH) in the intestine of fish exposed to SW. Since the intestinal glutamine synthetase activity and protein abundance decreased significantly, and the intestinal glutamine content remained unchanged, in the SW-exposed fish, excess glutamate formed via increased GDH activity in the intestine could be channeled to other organs to facilitate the increased synthesis of amino acids. Taken together, our results indicate for the first time that, besides absorbing ions and water during SW exposure, the intestine of juvenile O. marmorata also participated in altered nitrogen metabolism in response to salinity changes.

Expression regulation of Na+-K+-ATPase α1-subunit subtypes in zebrafish gill ionocytes

In zebrafish ( Danio rerio), six distinct Na+-K+-ATPase (NKA) α1-subunit genes have been identified, and four of them, zatp1a1a.1, zatp1a1a.2, zatp1a1a.4, and zatp1a1a.5, are expressed in embryonic skin where different types of ionocytes appear. The present study attempted to test a hypothesis of whether these NKA α1 paralogues are specifically expressed and function in respective ionocytes. Double fluorescence in situ hybridization analysis demonstrated the specific expression of zatp1a1a.1, zatp1a1a.2, and zatp1a1a. 5 in NKA-rich (NaR) cells, Na+-Cl− cotransporter (NCC)-expressing cells, and H+-ATPase-rich (HR) cells, respectively, based on the colocalization of the three NKA α1 genes with marker genes of the respective ionocytes (epithelial Ca2+ channel in NaR cells; NCC in NCC cells; and H+-ATPase and Na+/H+ exchanger 3b in HR cells). The mRNA expression (by real-time PCR) of zatp1a1a.1, zatp1a1a.2, and zatp1a1a.5 were, respectively, upregulated by low-Ca2+, low-Cl−, and low-Na+ freshwater, which had previously been reported to stimulate uptake functions of Ca2+, Cl−, and Na+. However, zatp1a1a.4 was not colocalized with any of the three types of ionocytes, nor did its mRNA respond to the ambient ions examined. Taken together, zATP1a1a.1, zATP1a1a.2, and zATP1a1a.5 may provide driving force for Na+-coupled cotransporter activity specifically in NaR, NCC, and HR cells, respectively.

Glycogen phosphorylase in glycogen-rich cells is involved in the energy supply for ion regulation in fish gill epithelia

The molecular and cellular mechanisms behind glycogen metabolism and the energy metabolite translocation between mammal neurons and astrocytes have been well studied. A similar mechanism is proposed for rapid mobilization of local energy stores to support energy-dependent transepithelial ion transport in gills of the Mozambique tilapia ( Oreochromis mossambicus). A novel gill glycogen phosphorylase isoform (tGPGG), which catalyzes the initial degradation of glycogen, was identified in branchial epithelial cells of O. mossambicus. Double in situ hybridization and immunocytochemistry demonstrated that tGPGG mRNA and glycogen were colocalized in glycogen-rich cells (GRCs), which surround ionocytes (labeled with a Na+-K+-ATPase antiserum) in gill epithelia. Concanavalin-A (a marker for the apical membrane) labeling indicated that GRCs and mitochondria-rich cells share the same apical opening. Quantitative real-time PCR analyses showed that tGPGG mRNA expression levels specifically responded to environmental salinity changes. Indeed, the glycogen content, glycogen phosphorylase (GP) protein level and total activity, and the density of tGPGG-expressing cells (i.e., GRCs) in fish acclimated to seawater (SW) were significantly higher than those in freshwater controls. Short-term acclimation to SW caused an evident depletion in the glycogen content of GRCs. Taken altogether, tGPGG expression in GRCs is stimulated by hyperosmotic challenge, and this may catalyze initial glycogen degradation to provide the adjacent ionocytes with energy to carry out iono- and osmoregulatory functions.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Expression of a duplicate Na,K-ATPase beta(1)-isoform in the European eel (Anguilla anguilla)

Recent studies on teleost fish have suggested that their genomes have undergone ancient polyploidization events resulting in the duplication of the genome. A duplicate copy of the Na,K-ATPase beta(1)-isoform (called beta(233)) has been identified in the European eel (Anguilla anguilla). The beta(233)-isoform shares high levels of nucleotide (74.8%) and amino acid (69.9%) homology with the eel beta(1)-subunit as well as other vertebrate beta(1)-sequences. Compared with the widely expressed beta(1)-isoform, expression of beta(233)-mRNA is mainly restricted to epithelial tissues. Seawater acclimation induced increases in beta(233)-mRNA levels in kidney, gill, and intestine of migratory "silver" but not the nonmigratory "yellow" adult eels, suggesting that the factors responsible for this upregulation are themselves developmentally regulated. Expression of a variably glycosylated 40- to 52-kDa beta(233)-protein in both gill "chloride" and intestinal epithelial cells suggests that the beta(233)-isoform of Na,K-ATPase may play an important functional role in the major osmoregulatory tissues of euryhaline fish such as the eel.

Synthesis of gill Na(+)-K(+)-ATPase in Atlantic salmon smolts: differences in alpha-mRNA and alpha-protein levels

Several parameters were analyzed to determine the mechanisms responsible for the enhancement of the gill Na(+)-K(+)-ATPase activity of Atlantic salmon smolts. A major alpha-subunit transcript of 3.7 kb was revealed by Northern blot in both parr and smolt gills when hybridized with two distinct cDNA probes. The alpha-mRNA abundance demonstrated an increase to maximal levels in smolts at an early stage of the parr-smolt transformation. This was followed by a gradual rise in alpha-protein levels, revealed by Western blots with specific antibodies and by an increase in gill Na(+)-K(+)-ATPase hydrolytic activity, both only reaching maximum levels a month later, at the peak of the transformation process. Parr fish experienced a decrease in alpha-mRNA abundance and had basal levels of alpha-protein and enzyme activity. Measurement of the binding of [(3)H]ouabain to Na(+)-K(+)-ATPase was characterized in smolts and parr gill membranes showing more than a twofold elevation in smolts and was of high affinity in both groups (dissociation constant = 20-23 nM). Modulation of the enzyme due to increased salinity was also observed in seawater-transferred smolts, as demonstrated by an increase in alpha-mRNA levels after 24 h with a rise in Na(+)-K(+)-ATPase activity occurring only after 11 days. No qualitative change in alpha-expression was revealed at either the mRNA or protein level. Immunological identification of the alpha-protein was performed with polyclonal antibodies directed against the rat alpha-specific isoforms, revealing that parr, freshwater, and seawater smolts have an alpha(3)-like isoform. This study shows that the increase in Na(+)-K(+)-ATPase activity in smolt gills depends first on an increase in the alpha-mRNA expression and is followed by a slower rise in alpha-protein abundance that eventually leads to a higher synthesis of Na(+)-K(+) pumps.