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

Female tilapia, Oreochromis sp. mobilised energy differently for growth and reproduction according to living environment

This study was conducted to investigate the energy mobilisation preference and ionoregulation pattern of female tilapia, Oreochromis sp. living in different environments. Three different treatments of tilapia as physiology compromising model were compared; tilapia cultured in recirculating aquaculture system (RAS as Treatment I-RAS), tilapia cultured in open water cage (Treatment II-Cage) and tilapia transferred from cage and cultured in RAS (Treatment III-Compensation). Results revealed that tilapia from Treatment I and III mobilised lipid to support gonadogenesis, whilst Treatment II tilapia mobilised glycogen as primary energy for daily exercise activity and reserved protein for growth. The gills and kidney Na+/K+ ATPase (NKA) activities remained relatively stable to maintain homeostasis with a stable Na+ and K+ levels. As a remark, this study revealed that tilapia strategized their energy mobilisation preference in accessing glycogen as an easy energy to support exercise metabolism and protein somatogenesis in cage culture condition, while tilapia cultured in RAS mobilised lipid for gonadagenesis purposes.

Cell-based homologous expression system for in-vitro characterization of environmental effects on transmembrane peptide transport in fish

All organisms encounter environmental changes that lead to physiological adjustments that could drive evolutionary adaptations. The ability to adjust performance in order to cope with environmental changes depends on the organism's physiological plasticity. These adjustments can be reflected in behavioral, physiological, and molecular changes, which interact and affect each other. Deciphering the role of molecular adjustments in physiological changes will help to understand how multiple levels of biological organization are synchronized during adaptations. Transmembrane transporters, which facilitate a cell's interaction with its surroundings, are prime targets for molecular studies of the environmental effects on an organism's physiology. Fish are subjected to environmental fluctuations and exhibit different coping mechanisms. To study the molecular adjustments of fish transporters to their external surrounding, suitable experimental systems must be established. The Mozambique tilapia (Oreochromis mossambicus) is an excellent model for environmental stress studies, due to its extreme salinity tolerance. We established a homologous cellular-based expression system and uptake assay that allowed us to study the effects of environmental conditions on transmembrane transport. We applied our expression system to investigate the effects of environmental conditions on the activity of PepT2, a transmembrane transporter critical in the absorption of dietary peptides and drugs. We created a stable, modified fish cell-line, in which we exogenously expressed the tilapia PepT2, and tested the effects of water temperature and salinity on the uptake of a fluorescent di-peptide, β-Ala-Lys-AMCA. While temperature affected only Vmax, medium salinity had a bi-directional effect, with significantly reduced Vmax in hyposaline conditions and significantly increased Km in hypersaline conditions. These assays demonstrate the importance of suitable experimental systems for fish ecophysiology studies. Furthermore, our in-vitro results show how the effect of hypersaline conditions on the transporter activity can explain expression shifts seen in the intestine of saltwater-acclimated fish, emphasizing the importance of complimentary studies in better understanding environmental physiology. This research highlights the advantages of using homologous expression systems to study environmental effects encountered by fish, in a relevant cellular context. The presented tools and methods can be adapted to study other transporters in-vitro.

Evidence for the involvement of branchial Vacuolar-type H+-ATPase in the acidification of the external medium by the West African lungfish, Protopterus annectens, exposed to ammonia-loading conditions

African lungfishes are obligatory air-breathers with exceptionally high environmental ammonia tolerance. They can lower the pH of the external medium during exposure to ammonia-loading conditions. This study aimed to demonstrate the possible involvement of branchial vacuolar-type H⁺-ATPase (Vha) in the ammonia-induced acidification of the external medium by the West African lungfish, Protopterus annectens, and to examine whether its capacity to acidify the medium could be augmented after exposure to 100 mmol l^-1 NH₄Cl for six days. Two full coding cDNA sequences of Vha subunit B (atp6v1b), atp6v1b1 and atp6v1b2, were obtained from the internal gills of P. annectens. The sequence of atp6v1b1 comprised 1548 bp, encoding 515 amino acids (57.4 kDa), while that of atp6v1b2 comprised 1536 bp, encoding 511 amino acids (56.6 kDa). Using a custom-made antibody reactive to both isoforms, immunofluorescence microscopy revealed the collective localization of Atp6v1b (atp6v1b1 and atp6v1b2) at the apical or the basolateral membrane of two different types of branchial Na⁺/K⁺-ATPase-immunoreactive ionocyte. The ionocytes labelled apically with Atp6v1b presumably expressed Atp6v1b1 containing a PDZ-binding domain, indicating that the apical Vha was positioned to transport H⁺ to the external medium. The expression of Atp6v1b was regulated post-transcriptionally, as the protein abundance of Atp6v1b and Vha activity increased significantly in the gills of fish exposed to 100 mmol l^-1 NH₄Cl for six days. Correspondingly, the fish exposed to ammonia had a greater capacity to acidify the external medium, presumably to decrease the ratio of [NH₃] to [NH₄⁺] in order to reduce the influx of exogenous NH₃.

Effects of single and dual-stressor elevation of environmental temperature and PCO2 on metabolism and acid-base regulation in the Louisiana red swamp crayfish, Procambarus clarkii

Elevation of temperature and CO₂ levels within the world's aquatic environments is expected to cause numerous physiological challenges to their inhabitants. While effects on marine ecosystems have been well studied, freshwater ecosystems have rarely been examined using a dual-stressor approach leaving our understanding of its inhabitants upon these challenges unclear. We aimed to identify the affects of elevated temperature and hypercapnia in isolation and in combination on the metabolic and acid-base regulatory processes of a freshwater crayfish, Procambarus clarkii. Crayfish were exposed to freshwater conditions that may be prevalent by the year 2100 and metabolic responses were determined after 14-days of exposure. In addition, changes in branchial mRNA expression of acid-base linked transporters were investigated. Interactions between exposure conditions influenced extracellular pH as well as the nitrogen physiology and routine metabolic rate of the crayfish. Crayfish exposed to individual and combined elevations in temperature and/or hypercapnia maintained an extracellular pH similar to that of control crayfish. Dual-stressor exposed crayfish seem to elevate the importance of ammonium as an excretable acid-equivalent based on an overall increase in the branchial mRNA expression of transporters related to ammonia excretion including the Na⁺/K⁺-ATPase, Rhesus-protein, and the V-type H⁺-ATPase. Overall, hypercapnia and dual-stressor conditions caused a metabolic depression that may have long-lasting consequences such as limited locomotion, growth, and reproduction. Future generations of crayfish given the chance to adapt over several generations may ameliorate these consequences.

Effects of seawater acclimation on two Na+/K+-ATPase α-subunit isoforms in the gills of the marble goby, Oxyeleotris marmorata

The marble goby, Oxyeleotris marmorata, is a freshwater teleost, but can acclimate progressively to survive in seawater (salinity 30). As an obligatory air-breather, it can also survive long periods of emersion. Two isoforms of Na⁺/K⁺-ATPase (nka) α-subunit, nkaα1 and nkaα3, but not nkaα2, had been cloned from the gills of O. marmorata. The cDNA sequence of nkaα1 consisted of 3069 nucleotides, coding for 1023 amino acids (112.5 kDa), whereas nkaα3 consisted of 2976 nucleotides, coding for 992 amino acids (109.5 kDa). As only one form of branchial Nkaα1 was identified using molecular cloning in this study, O. marmorata lacks specific freshwater- and seawater-type Nkaα isoforms as demonstrated by some other euryhaline fish species. The nkaα1 transcript level was about 2.5-fold higher than that of nkaα3 in the gills of freshwater O. marmorata. During exposure to seawater, the branchial transcript level of nkaα1 increased significantly on day 1 (~3.3-fold) and day 6 (~2.6-fold). By contrast, the branchial transcript level of nkaα3 increased significantly on day 1 (~2.6-fold), but not on day 6, of seawater exposure. Six days of exposure to seawater also led to significant increases in protein abundances of Nkaα1 (~6.9-fold) and Nkaα3 (~2.8-fold) in the gills of O. marmorata. Hence, the mRNA and protein expressions of both nkaα1/Nkaα1 and nkaα3/Nkaα3 were up-regulated in O. marmorata during seawater acclimation. This could explain why V_max increases but K_m for Na⁺ and K⁺ remain unchanged in Nka extracted from the gills of O. marmorata acclimated to seawater as reported previously.

The Non-ureogenic Stinging Catfish, Heteropneustes fossilis, Actively Excretes Ammonia With the Help of Na+/K+-ATPase When Exposed to Environmental Ammonia

The stinging catfish, Heteropneustes fossilis, can tolerate high concentrations of environmental ammonia. Previously, it was regarded as ureogenic, having a functional ornithine-urea cycle (OUC) that could be up-regulated during ammonia-loading. However, contradictory results indicated that increased urea synthesis and switching to ureotelism could not explain its high ammonia tolerance. Hence, we re-examined the effects of exposure to 30 mmol l^–1 NH₄Cl on its ammonia and urea excretion rates, and its tissue ammonia and urea concentrations. Our results confirmed that H. fossilis did not increase urea excretion or accumulation during 6 days of ammonia exposure, and lacked detectable carbamoyl phosphate synthetase I or III activity in its liver. However, we discovered that it could actively excrete ammonia during exposure to 8 mmol l^–1 NH₄Cl. As active ammonia excretion is known to involve Na⁺/K⁺-ATPase (Nka) indirectly in several ammonia-tolerant fishes, we also cloned various nkaα-subunit isoforms from the gills of H. fossilis, and determined the effects of ammonia exposure on their branchial transcripts levels and protein abundances. Results obtained revealed the presence of five nkaα-subunit isoforms, with nkaα1b having the highest transcript level. Exposure to 30 mmol l^–1 NH₄Cl led to significant increases in the transcript levels of nkaα1b (on day 6) and nkaα1c1 (on day 1 and 3) as compared with the control. In addition, the protein abundances of Nkaα1c1, Nkaα1c2, and total NKAα increased significantly on day 6. Therefore, the high environmental ammonia tolerance of H. fossilis is attributable partly to its ability to actively excrete ammonia with the aid of Nka.

Acute Exposure to key Aquaculture Environmental Stressors Impaired the Aerobic Metabolism of Carassius auratus gibelio

Carassius auratus gibelio is an omnivore favored for its flavor and is commonly used as a benthic species in traditional pond polyculture. This study investigated the effects of common aquaculture stressors, such as high ammonia, high nitrite, high pH, and hypoxia on the aerobic metabolism of C. auratus gibelio. The results showed that the standard metabolic rate (SMR) was positively correlated with ammonia, nitrite, and pH, while the maximum metabolic rate (MMR) was negatively correlated with all four stressors. Thus, aerobic scope (AS) was reduced when C. auratus gibelio was exposed to high ammonia, high nitrite, high pH, and hypoxia. The peak of post-prandial O₂ consumption was positively correlated with nitrite, pH, and the occurrence of the peak metabolic rate post-prandial was delayed in high ammonia, high nitrite, hypoxia, and high pH conditions. These findings indicated that, in experimental conditions, exposure to these environmental stressors can influence aerobic metabolism in C. auratus gibelio. With more energy required to maintain standard metabolic rates, less will be available for growth. While the C. auratus gibelio is one of the most hypoxia tolerance species, the reduction we observed in AS caused by stressors that commonly occur in ponds and in nature will likely affect growth in ponds and fitness in nature. These data have provided insight into the optimal, fitness-maximizing thresholds for these common stressors in this species of interest.

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.

Air-breathing and excretory nitrogen metabolism in fishes

During water-land transition, ancient fishes acquired the ability to breathe air, but air-breathing engendered problems in nitrogenous waste excretion. Nitrogen is a fundamental component of amino acids, proteins, and nucleic acids, and the degradation of these nitrogen-containing compounds releases ammonia. Ammonia is toxic and must be removed. Fishes in water excrete ammonia as the major nitrogenous waste through gills, but gills of air-breathing fishes are modified for air-breathing or largely replaced by air-breathing organs. Notably, fishes emerged from water can no longer excrete ammonia effectively because of a lack of water to flush the gills. Hence, ancient fishes that participated in water-land transition must have developed means to deal with ammonia toxicity. Extant air-breathing fishes, particularly amphibious ones, can serve as models to examine adaptations which might have facilitated the emergence of ancient fishes from water. Some of these fishes can actively emerge from water and display complex behaviors on land, while a few can burrow into mud and survive for years during drought. Many of them are equipped with mechanisms to ameliorate ammonia toxicity during emersion. In this review, the mechanisms adopted by air-breathing fishes to deal with ammonia toxicity during emersion were organized into seven disparate strategies. In addition, eight extant air-breathing fishes with distinctive terrestrial behaviors and peculiar natural habitats were selected to describe in detail how these seven strategies could be adopted in disparate combinations to ameliorate ammonia toxicity during emersion.

Mudskippers and Their Genetic Adaptations to an Amphibious Lifestyle

Mudskippers are the largest group of amphibious teleost fish that are uniquely adapted to live on mudflats. During their successful transition from aqueous life to terrestrial living, these fish have evolved morphological and physiological modifications of aerial vision and olfaction, higher ammonia tolerance, aerial respiration, improved immunological defense against terrestrial pathogens, and terrestrial locomotion using protruded pectoral fins. Comparative genomic and transcriptomic data have been accumulated and analyzed for understanding molecular mechanisms of the terrestrial adaptations. Our current review provides a general introduction to mudskippers and recent research advances of their genetic adaptations to the amphibious lifestyle, which will be helpful for understanding the evolutionary transition of vertebrates from water to land. Our insights into the genomes and transcriptomes will also support molecular breeding, functional identification, and natural compound screening.

Na+/H+ Exchanger 3 Is Expressed in Two Distinct Types of Ionocyte, and Probably Augments Ammonia Excretion in One of Them, in the Gills of the Climbing Perch Exposed to Seawater

The freshwater climbing perch, Anabas testudineus, is an euryhaline teleost and an obligate air-breather with the ability to actively excrete ammonia. Members of the Na+/H+ exchanger (NHE) family help maintain intracellular pH homeostasis and ionic balance through the electroneutral exchange of Na+ and H+. This study aimed to obtain, from the gills of A. testudineus, the full cDNA coding sequence of nhe3, and to determine the effects of exposure to seawater or 100 mmol l-1 of NH4Cl in fresh water on its mRNA and protein expression levels. Efforts were also made to elucidate the type of ionocyte that Nhe3 was associated with in the branchial epithelium of A. testudineus. The transcript level and protein abundance of nhe3/Nhe3 were very low in the gills of freshwater A. testudineus, but they increased significantly in the gills of fish acclimated to seawater. In the gills of fish exposed to seawater, Nhe3 was expressed in two distinct types of seawater-inducible Na+/K+-ATPase (Nka)-immunoreactive ionocytes. In Nkaα1b-immunoreactive ionocytes, Nhe3 had an apical localization. As these ionocytes also expressed apical Rhcg1 and basolateral Rhcg2, which are known to transport ammonia, they probably participated in proton-facilitated ammonia excretion in A. testudineus during seawater acclimation. In Nkaα1c-immunoreactive ionocytes, Nhe3 was atypically expressed in the basolateral membrane, and its physiological function is uncertain. For A. testudineus exposed to NH4Cl in fresh water, the transcript and protein expression levels of nhe3/Nhe3 remained low. In conclusion, the branchial Nhe3 of A. testudineus plays a greater physiological role in passive ammonia transport and acid-base balance during seawater acclimation than in active ammonia excretion during environmental ammonia exposure.

Identification and distribution of neuronal nitric oxide synthase and neurochemical markers in the neuroepithelial cells of the gill and the skin in the giant mudskipper, Periophthalmodon schlosseri

Mudskippers are amphibious fishes living in mudflats and mangroves. These fishes hold air in their large buccopharyngeal-opercular cavities where respiratory gas exchange takes place via the gills and higher vascularized epithelium lining the cavities and also the skin epidermis. Although aerial ventilation response to changes in ambient gas concentration has been studied in mudskippers, the localization and distribution of respiratory chemoreceptors, their neurochemical coding and function as well as physiological evidence for the gill or skin as site for O₂ and CO₂ sensing are currently not known. In the present study we assessed the distribution of serotonin, acetylcholine, catecholamines and nitric oxide in the neuroepithelial cells (NECs) of the mudskipper gill and skin epithelium using immunohistochemistry and confocal microscopy. Colocalization studies showed that 5-HT is coexpressed with nNOS, Na⁺/K⁺-ATPase, TH and VAChT; nNOS is coexpressed with Na⁺/K⁺-ATPase and TH in the skin. In the gill 5-HT is coexpressed with nNOS and VAhHT and nNOS is coexpressed with Na⁺/K⁺-ATPase and TH. Acetylcholine is also expressed in chain and proximal neurons projecting to the efferent filament artery and branchial smooth muscle. The serotonergic cells c labeled with VAChT, nNOS and TH, thus indicating the presence of NEC populations and the possibility that these neurotransmitters (other than serotonin) may act as primary transmitters in the hypoxic reflex in fish gills. Immunolabeling with TH antibodies revealed that NECs in the gill and the skin are innervated by catecholaminergic nerves, thus suggesting that these cells are involved in a central control of branchial functions through their relationships with the sympathetic branchial nervous system. The Na⁺/K⁺-ATPase in mitochondria-rich cells (MRCs), which are most concentrated in the gill lamellar epithelium, is colabeled with nNOS and associated with TH nerve terminals. TH-immunopositive fine varicosities were also associated with the numerous capillaries in the skin surface and the layers of the swollen cells. Based on the often hypercapnic and hypoxic habitat of the mudskippers, these fishes may represent an attractive model for pursuing studies on O₂ and CO₂ sensing due to the air-breathing that increases the importance of acid/base regulation and the O₂-related drive including the function of gasotransmitters such as nitric oxide that has an inhibitory (regulatory) function in ionoregulation.

Na+, K+-ATPase β1 subunit associates with α1 subunit modulating a "higher-NKA-in-hyposmotic media" response in gills of euryhaline milkfish, Chanos chanos

The euryhaline milkfish (Chanos chanos) is a popular aquaculture species that can be cultured in fresh water, brackish water, or seawater in Southeast Asia. In gills of the milkfish, Na⁺, K⁺-ATPase (i.e., NKA; sodium pump) responds to salinity challenges including changes in mRNA abundance, protein amount, and activity. The functional pump is composed of a heterodimeric protein complex composed of α- and β-subunits. Among the NKA genes, α1-β1 isozyme comprises the major form of NKA subunits in mammalian osmoregulatory organs; however, most studies on fish gills have focused on the α1 subunit and did not verify the α1-β1 isozyme. Based on the sequenced milkfish transcriptome, an NKA β1 subunit gene was identified that had the highest amino acid homology to β233, a NKA β1 subunit paralog originally identified in the eel. Despite this high level of homology to β233, phylogenetic analysis and the fact that only a single NKA β1 subunit gene exists in the milkfish suggest that the milkfish gene should be referred to as the NKA β1 subunit gene. The results of accurate domain prediction of the β1 subunit, co-localization of α1 and β1 subunits in epithelial ionocytes, and co-immunoprecipitation of α1 and β1 subunits, indicated the formation of a α1-β1 complex in milkfish gills. Moreover, when transferred to hyposmotic media (fresh water) from seawater, parallel increases in branchial mRNA and protein expression of NKA α1 and β1 subunits suggested their roles in hypo-osmoregulation of euryhaline milkfish. This study molecularly characterized the NKA β1 subunit and provided the first evidence for an NKA α1-β1 association in gill ionocytes of euryhaline teleosts.

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.

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.

Ammonia exposure increases the expression of Na(+):K (+):2Cl (-) cotransporter 1a in the gills of the giant mudskipper, Periophthalmodon schlosseri

The giant mudskipper, Periophthalmodon schlosseri, is an obligate air-breathing teleost that can actively excrete ammonia against high concentrations of environmental ammonia. This study aimed to clone and sequence the Na (+) :K (+) :2Cl (-) cotransporter 1 (nkcc1) from the gills of P. schlosseri, and to determine the effects of ammonia exposure on its mRNA expression and protein abundance after pre-acclimation to slightly brackish water (salinity 3; SBW) for 2 weeks. The complete coding cDNA sequences of nkcc1a consisted of 3453 bp, coding for 1151 amino acid with an estimated molecular mass of 125.4 kDa. Exposure to 75 mmol l(-1) NH4Cl in SBW had no effect on the mRNA expression of nkcc1a. However, western blotting revealed a significant increase in the protein abundance of multiple T4-immunoreactive bands of molecular mass 170-250 kDa in the gills of P. schlosseri exposed to ammonia. Furthermore, immunofluorescence microscopy demonstrated the colocalization of the increased T4-immunoreactive protein with Na(+)/K(+)-ATPase (Nka) α-subunit to the basolateral membrane of certain ionocytes in the gills of the ammonia-exposed fish. As Nkcc1 is known to have a basolateral localization, it can be concluded that ammonia exposure led to an increase in the expression of glycosylated Nkcc1, the molecular masses of which were reduced upon enzymatic deglycosylation, in the gills of P. schlosseri. The dependency on post-transcriptional and post-translational regulation of branchial Nkcc1 in P. schlosseri would facilitate prompt responses to changes in environmental condition. As NH4 (+) can replace K(+), NH4 (+) could probably enter ionocytes through the basolateral Nkcc1a during active ammonia excretion, but increased influx of Na(+), NH4 (+) and 2Cl(-) would alter the transmembrane Na(+) gradient. Consequently, exposure of P. schlosseri to ammonia would also result in an increase in branchial activity of Nka with decreased NH4 (+) affinity so as to maintain intracellular Na(+) and K(+) homeostasis as reported elsewhere.