Prolactin regulates luminal bicarbonate secretion in the intestine of the sea bream (Sparus aurata L.)

Na+/HCO3- cotransporter 1 (nbce1) isoform gene expression during smoltification and seawater acclimation of Atlantic salmon

The life history of Atlantic salmon (Salmo salar) includes an initial freshwater phase (parr) that precedes a springtime migration to marine environments as smolts. The development of osmoregulatory systems that will ultimately support the survival of juveniles upon entry into marine habitats is a key aspect of smoltification. While the acquisition of seawater tolerance in all euryhaline species demands the concerted activity of specific ion pumps, transporters, and channels, the contributions of Na⁺/HCO₃^- cotransporter 1 (Nbce1) to salinity acclimation remain unresolved. Here, we investigated the branchial and intestinal expression of three Na⁺/HCO₃^- cotransporter 1 isoforms, denoted nbce1.1, -1.2a, and -1.2b. Given the proposed role of Nbce1 in supporting the absorption of environmental Na⁺ by ionocytes, we first hypothesized that expression of a branchial nbce1 transcript (nbce1.2a) would be attenuated in salmon undergoing smoltification and following seawater exposure. In two separate years, we observed spring increases in branchial Na⁺/K⁺-ATPase activity, Na⁺/K⁺/2Cl^- cotransporter 1, and cystic fibrosis transmembrane regulator 1 expression characteristic of smoltification, whereas there were no attendant changes in nbce1.2a expression. Nonetheless, branchial nbce1.2a levels were reduced in parr and smolts within 2 days of seawater exposure. In the intestine, gene transcript abundance for nbce1.1 increased from spring to summer in the anterior intestine, but not in the posterior intestine or pyloric caeca, and nbce1.1 and -1.2b expression in the intestine showed season-dependent transcriptional regulation by seawater exposure. Collectively, our data indicate that tissue-specific modulation of all three nbce1 isoforms underlies adaptive responses to seawater.

Osmoregulatory actions of prolactin in the gastrointestinal tract of fishes

In fishes, prolactin (Prl) signaling underlies the homeostatic regulation of hydromineral balance by controlling essential solute and water transporting functions performed by the gill, gastrointestinal tract, kidney, urinary bladder, and integument. Comparative studies spanning over 60 years have firmly established that Prl promotes physiological activities that enable euryhaline and stenohaline teleosts to reside in freshwater environments; nonetheless, the specific molecular and cellular targets of Prl in ion- and water-transporting tissues are still being resolved. In this short review, we discuss how particular targets of Prl (e.g., ion cotransporters, tight-junction proteins, and ion pumps) confer adaptive functions to the esophagus and intestine. Additionally, in some instances, Prl promotes histological and functional transformations within esophageal and intestinal epithelia by regulating cell proliferation. Collectively, the demonstrated actions of Prl in the gastrointestinal tract of teleosts indicate that Prl operates to promote phenotypes supportive of freshwater acclimation and to inhibit phenotypes associated with seawater acclimation. We conclude our review by underscoring that future investigations are warranted to determine how growth hormone/Prl-family signaling evolved in basal fishes to support the gastrointestinal processes underlying hydromineral balance.

The gaseous gastrointestinal tract of a seawater teleost, the English sole (Parophrys vetulus)

There has been considerable recent progress in understanding the respiratory physiology of the gastrointestinal tract (GIT) in teleosts, but the respiratory conditions inside the GIT remain largely unknown, particularly the luminal PCO₂ and PO₂ levels. The GIT of seawater teleosts is of special interest due to its additional function of water absorption linked to HCO₃^- secretion, a process that may raise luminal PCO₂ levels. Direct measurements of GIT PCO₂ and PO₂ using micro-optodes in the English sole (Parophrys vetulus; anaesthetized, artificially ventilated, 10-12 °C) revealed extreme luminal gas levels. Luminal PCO₂ was 14-17 mmHg in the stomach and intestinal segments of fasted sole, considerably higher than arterial blood levels of 5 mmHg. Moreover, feeding, which raised intestinal HCO₃^- concentration, also raised luminal PCO₂ to 34-50 mmHg. All these values were higher than comparable measurements in freshwater teleosts, and also greater than environmental CO₂ levels of concern in aquaculture or global change scenarios. The PCO₂ values in subintestinal vein blood draining the GIT of fed fish (28 mmHg) suggested some degree of equilibration with high luminal PCO₂, whereas subintestinal vein PO₂ levels were relatively low (9 mmHg). All luminal sections of the GIT were virtually anoxic (PO₂ ≤ 0.3 mmHg), in both fasted and fed animals, a novel finding in teleosts.

Cloning and molecular characterization of PRL and PRLR from turbot (Scophthalmus maximus) and their expressions in response to short-term and long-term low salt stress

The pituitary hormone prolactin (PRL) regulates salt and water homeostasis by altering ion retention and water uptake through peripheral osmoregulatory organs. To understand the role of PRL and its receptor (PRLR) in hypoosmoregulation of turbot (Scophthalmus maximus), we characterized the PRL and PRLR gene and analyzed the tissue distribution of the two genes and their gene transcriptional patterns in the main expressed tissues under long-term and short-term low salt stress. The PRL cDNA is 1486 bp in length, incorporating an ORF of 636 bp with a putative primary structure of 211 residues. And the PRLR cDNA is 2849 bp in length, incorporating an ORF of 1944 bp with a putative primary structure of 647 residues. The deduced amino acid sequences of these two genes shared highly conserved structures with those from other teleosts. Quantitative real-time PCR results showed that PRL transcripts were strongly expressed in the pituitary and very weakly in brain, but were hardly expressed in other tissues. PRLR transcripts were most abundant in the kidney, to a lesser extent in the gill, intestine, brain, and spleen, and at low levels in the pituitary and other tissues examined. The expression of PRL in the pituitary increased after short-term or long-term low salt stress, and the highest expression level appeared 12 h after stress (P < 0.05). And there is no significant difference between both low salt group (5 ppt and 10 ppt) at each sampling point. The variation of PRLR expression in gill under short-term low salt stress is similar to that of PRL gene in pituitary, with highest value in 12 h (P < 0.05). However, the expression under long-term low salt stress was significantly higher than control group even than 12 h group under 5 ppt (P < 0.05). The expression of PRLR in the kidney increased first and then decreased after low salt stress, and the highest value also appeared in 12 h after stress and there was no significant difference between the salinity groups. After long-term low salt stress, the expression level also increased significantly (P < 0.05), but it was flat with 24 h, which was lower than 12 h. The variation of PRLR expression in the intestine was basically consistent with that in the kidney. The difference was that the expression level of 24 h after stress in the 5 ppt group was significantly higher than that of the 10 ppt group (P < 0.05). After a comprehensive analysis of the expression levels of the two genes, it can be found that the expression level increased and peaked at 12 h after short-term low salt stress, indicating that this time point is the key point for the regulation of turbot in response to low salt stress. This also provides very important information for studying the osmotic regulation of turbot. In addition, our results also showed that the expression of PRLR was stable in the kidney and intestine after long-term low salt stress, while the expression in the gill was much higher than short-term stress. It suggested that PRL and its receptors mainly exert osmotic regulation function in the gill under long-term low salt stress. At the same time, such a result also brings a hint for the low salt selection of turbot, focusing on the regulation of ion transport in the gill.

Molecular Targets of Natriuretic Action of Prolactin in the Rat Model of Cholestasis of Pregnancy

We analyzed the expression of molecular targets of natriuretic action of prolactin in different layers of the kidney in the rat model of cholestasis of pregnancy. Sodium bicarbonate cotransporter NBCe1 was most sensitive to the conditions of cholestasis and cholestasis of pregnancy: the expression NBCe1 mRNA and protein in the renal outer medulla decreased in comparison with the normal. All forms of cholestasis affected the mRNA expression of sodium-potassium chloride co-transporter NCC, α-subunit of the ENaCα epithelial sodium channel, and Nedd4-2 ubiquitin ligase in different layers of the kidney. The obtained data suggest that prolactin provides fine tuning of various sodium transporters in different parts of the nephron under pathological conditions.

Increased intestinal carbonate precipitate abundance in the sea bream (Sparus aurata L.) in response to ocean acidification

Marine fish contribute to the carbon cycle by producing mineralized intestinal precipitates generated as by-products of their osmoregulation. Here we aimed at characterizing the control of epithelial bicarbonate secretion and intestinal precipitate presence in the gilthead sea bream in response to predicted near future increases of environmental CO₂. Our results demonstrate that hypercapnia (950 and 1800 μatm CO₂) elicits higher intestine epithelial HCO₃^- secretion ex vivo and a subsequent parallel increase of intestinal precipitate presence in vivo when compared to present values (440 μatm CO₂). Intestinal gene expression analysis in response to environmental hypercapnia revealed the up-regulation of transporters involved in the intestinal bicarbonate secretion cascade such as the basolateral sodium bicarbonate co-transporter slc4a4, and the apical anion transporters slc26a3 and slc26a6 of sea bream. In addition, other genes involved in intestinal ion uptake linked to water absorption such as the apical nkcc2 and aquaporin 1b expression, indicating that hypercapnia influences different levels of intestinal physiology. Taken together the current results are consistent with an intestinal physiological response leading to higher bicarbonate secretion in the intestine of the sea bream paralleled by increased luminal carbonate precipitate abundance and the main related transporters in response to ocean acidification.

Molecular and functional regionalization of bicarbonate secretion cascade in the intestine of the European sea bass (Dicentrarchus labrax)

In marine fish the intestinal HCO₃^- secretion is the key mechanism to enable luminal aggregate formation and water absorption. Using the sea bass (Dicentrarchus labrax), the present study aimed at establishing the functional and molecular organization of different sections of the intestine concerning bicarbonate secretion and Cl^- movements. The proximal intestinal regions presented similar HCO₃^- secretion rates, while differences were detected in the molecular expression of the transporters involved and on regional HCO₃^- concentrations. The anterior region presented significantly higher Na⁺/K⁺-ATPase activity, Cl^- transepithelial transport and basolateral slc4a4, apical slc26a6 and slc26a3 expression levels. In the mid intestine, the total HCO₃^- content was significantly increased in the fluid as in the carbonate aggregates. In the rectum no HCO₃^- secretion was observed and was characterized by the diminished HCO₃^- total content, residual molecular expression of slc4a4, slc26a6 and slc26a3, higher H⁺-ATPase activity and expression, suggesting the existence of a different bicarbonate handling mechanism. The possible regulation of HCO₃^- secretion by extracellular HCO₃^- and increased intracellular cAMP levels were also investigated. cAMP did not affect HCO₃^- secretion, although Cl^- secretion was enhanced by cftr. HCO₃^- secretion rise due to the HCO₃^- basolateral increment showed that at resting levels slc4a4 was not a limiting step for secretion. The transcellular/intracellular dependence of apical HCO₃^- secretion differed between the proximal regions. In conclusion, intestinal HCO₃^- secretion has a functional region-dependent organization that was not reflected by the anterior-posterior regionalization on HCO₃^- secretion and expression profiles of chloride/water absorption related genes.

Molecular mechanisms for intestinal HCO3− secretion and its regulation by guanylin in seawater-acclimated eels

The intestine of marine teleosts secretes HCO3− into the lumen and precipitates Ca2+ and Mg2+ in the imbibed seawater as carbonates to decrease luminal fluid osmolality and facilitate water absorption. However, hormonal regulation of HCO3−secretion is largely unknown. Here, mucosally-added guanylin (GN) increased HCO3− secretion, measured by pH-stat, across isolated seawater-acclimated eel intestine bathed in saline at pH 7.4 (5% CO2). The effect of GN on HCO3− secretion was slower than that on the short-circuit current, and the time-course of the GN effect was similar to that of bumetanide. Mucosal bumetanide and serosal 4,4’-dinitrostilbene-2,2’-disulfonic acid (DNDS) inhibited the GN effect, suggesting an involvement of apical Na+-K+-2Cl− cotransporter (NKCC2) and basolateral Cl−/HCO3− exchanger (AE)/Na+-HCO3− cotransporter (NBC) in the GN effect. As mucosal DNDS failed to inhibit the GN effect, apical DNDS-sensitive AE may not be involved. To identify molecular species of transporters involved in the GN effect, we performed RNA-seq analyses followed by quantitative real-time PCR after transfer of eels to seawater. Among the genes upregulated after seawater transfer, AE genes, draa, b, and pat1a, c, on the apical membrane, and NBC genes, nbce1a, n1, n2a, and a AE gene, sat-1, on the basolateral membrane were candidates involved in HCO3− secretion. Judging from the slow effect of GN, we suggest that GN inhibits NKCC2b on the apical membrane and decreases cytosolic Cl− and Na+, which then activates apical DNDS-insensitive DRAs and basolateral DNDS-sensitive NBCs to enhance transcellular HCO3− flux across the intestinal epithelia of seawater-acclimated eels.

Prolactin controls Na+,Cl- cotransporter via Stat5 pathway in the teleost gill

In some freshwater fish species, the control of gill Na, Cl cotransporter (Ncc2b) by prolactin appears to be instrumental to ionic homeostasis. This study was carried out to examine the signaling pathways involved in prolactin-mediated salt retention using gill explants from Japanese medaka (Oryzias latipes). Ovine prolactin induced a concentration-dependent stimulation of ncc2b with significant effects of 10, 100 and 1000 ng of hormone per mL media (2-6 fold). To understand the molecular mechanisms mediating prolactin control of gill function, we analyzed effects on signaling pathways known to be involved in the hormones action in other systems, namely Stat5, Akt and Erk1/2. Their activation was examined in a time course and concentration response experiment. Prolactin (1 μg mL^-1) induced a rapid phosphorylation (stimulation) of Stat5 (10 min) that reached a plateau after 30 min and was maintained for at least 120 min. The effect of prolactin on Stat5 phosphorylation was concentration-dependent (4-12 fold). No activation of Akt or Erk1/2 was observed in either experiment. The Stat5 activation was further investigated in localization studies that demonstrated strong nuclear expression of phosphorylated Stat5 in prolactin-treated gill ionocytes. Using specific inhibitors, we analyzed the signalling pathways mediating prolactin induction of gill ncc2b. Co-incubation experiments showed that Stat5 inhibition blocked prolactin's stimulation of ncc2b expression, while PI3K-Akt and Mek1/2-Erk1/2 pathway inhibitors had no effect. These findings show that ncc2b expression is dependent on prolactin's downstream activation of Stat5 and its subsequent nuclear translocation within branchial ionocytes.

Regulation of Bicarbonate Secretion in Marine Fish Intestine by the Calcium-Sensing Receptor

In marine fish, high epithelial intestinal HCO₃− secretion generates luminal carbonate precipitates of divalent cations that play a key role in water and ion homeostasis. The present study was designed to expose the putative role for calcium and the calcium-sensing receptor (CaSR) in the regulation of HCO₃− secretion in the intestine of the sea bream (Sparus aurata L.). Effects on the expression of the CaSR in the intestine were evaluated by qPCR and an increase was observed in the anterior intestine in fed fish compared with unfed fish and with different regions of intestine. CaSR expression reflected intestinal fluid calcium concentration. In addition, anterior intestine tissue was mounted in Ussing chambers to test the putative regulation of HCO₃− secretion in vitro using the anterior intestine. HCO₃− secretion was sensitive to varying calcium levels in luminal saline and to calcimimetic compounds known to activate/block the CaSR i.e., R 568 and NPS-2143. Subsequent experiments were performed in intestinal sacs to measure water absorption and the sensitivity of water absorption to varying luminal levels of calcium and calcimimetics were exposed as well. It appears, that CaSR mediates HCO₃− secretion and water absorption in marine fish as shown by responsiveness to calcium levels and calcimimetic compounds.

Role of Prolactin in the Regulation of Bicarbonates Biodynamics in Female Rat Model of Cholestasis of Pregnancy

We studied possible involvement of prolactin in the regulation of bicarbonate biodynamics using female rat model of cholestasis of pregnancy induced by transplantation of the donor pituitary under the renal capsule of a recipient (hyperprolactinemia) and bile duct ligation (cholestasis). The concentration of bicarbonates in the bile and blood, their excretion, clearance, and reabsorption, as well as glomerular filtration rate and excretion of sodium ions were assessed. It was found that the main effect of prolactin was directed to the kidney-regulated pool of bicarbonates and consisted in stimulation of their clearance and inhibition of reabsorption, which led to a decrease in bicarbonate blood concentration. Parallel influence of prolactin on the clearance of bicarbonates and sodium ions was observed.

High rates of intestinal bicarbonate secretion in seawater tilapia (Oreochromis mossambicus)

Osmoregulation in fish is a complex process that requires the orchestrated cooperation of many tissues. In fish facing hyperosmotic environments, the intestinal absorption of some monovalent ions and the secretion of bicarbonate are key processes to favor water absorption. In the present study, we showed that bicarbonate levels in the intestinal fluid are several fold higher in seawater than in freshwater acclimated tilapia (Oreochromis mossambicus). In addition, we analyzed gene expression of the main molecular mechanisms involved in HCO₃^- movements i.e. slc26a6, slc26a3, slc4a4 and v-type H-ATPase sub C in the intestine of tilapia acclimated to both seawater and freshwater. Our results show an anterior/posterior functional regionalization of the intestine in tilapia in terms of expression patterns, which is affected by environmental salinity mostly in the anterior and mid intestine. Analysis of bicarbonate secretion using pH-Stat in tissues mounted in Ussing chambers reveals high rates of bicarbonate secretion in tilapia acclimated to seawater from anterior intestine to rectum ranging between ~900 and ~1700nmolHCO₃^-cm^-2h^-1. However, a relationship between the expression of slc26a6, slc26a3, slc4a4 and the rate of bicarbonate secretion seems to be compromised in the rectum. In this region, the low expression of the bicarbonate transporters could not explain the high bicarbonate secretion rates here described. However, we postulate that the elevated v-type H-ATPase mRNA expression in the rectum could be involved in this process.

A proteinaceous organic matrix regulates carbonate mineral production in the marine teleost intestine

Marine teleost fish produce CaCO₃ in their intestine as part of their osmoregulatory strategy. This precipitation is critical for rehydration and survival of the largest vertebrate group on earth, yet the molecular mechanisms that regulate this reaction are unknown. Here, we isolate and characterize an organic matrix associated with the intestinal precipitates produced by Gulf toadfish (Opsanus beta). Toadfish precipitates were purified using two different methods, and the associated organic matrix was extracted. Greater than 150 proteins were identified in the isolated matrix by mass spectrometry and subsequent database searching using an O. beta transcriptomic sequence library produced here. Many of the identified proteins were enriched in the matrix compared to the intestinal fluid, and three showed no substantial homology to any previously characterized protein in the NCBI database. To test the functionality of the isolated matrix, a micro-modified in vitro calcification assay was designed, which revealed that low concentrations of isolated matrix substantially promoted CaCO₃ production, where high concentrations showed an inhibitory effect. High concentrations of matrix also decreased the incorporation of magnesium into the forming mineral, potentially providing an explanation for the variability in magnesium content observed in precipitates produced by different fish species.

Hormonal regulation of aquaporin 3: opposing actions of prolactin and cortisol in tilapia gill

Aquaporins (Aqps) are expressed within key osmoregulatory tissues where they mediate the movement of water and selected solutes across cell membranes. We leveraged the functional plasticity of Mozambique tilapia (Oreochromis mossambicus) gill epithelium to examine how Aqp3, an aquaglyceroporin, is regulated in response to osmoregulatory demands. Particular attention was paid to the actions of critical osmoregulatory hormones, namely, prolactin (Prl), growth hormone and cortisol. Branchial aqp3 mRNA levels were modulated following changes in environmental salinity, with enhanced aqp3 mRNA expression upon transfer from seawater to freshwater (FW). Accordingly, extensive Aqp3 immunoreactivity was localized to cell membranes of branchial epithelium in FW-acclimated animals. Upon transferring hypophysectomized tilapia to FW, we identified that a pituitary factor(s) is required for Aqp3 expression in FW. Replacement with ovine Prl (oPrl) was sufficient to stimulate Aqp3 expression in hypophysectomized animals held in FW, an effect blocked by coinjection with cortisol. Both oPrl and native tilapia Prls (tPrl177 and tPrl188) stimulated aqp3 in incubated gill filaments in a concentration-related manner. Consistent with in vivo responses, coincubation with cortisol blocked oPrl-stimulated aqp3 expression in vitro Our data indicate that Prl and cortisol act directly upon branchial epithelium to regulate Aqp3 in tilapia. Thus, within the context of the diverse actions of Prl on hydromineral balance in vertebrates, we define a new role for Prl as a regulator of Aqp expression.

PTHrP regulates water absorption and aquaporin expression in the intestine of the marine sea bream (Sparus aurata, L.)

Water ingestion by drinking is fundamental for ion homeostasis in marine fish. However, the fluid ingested requires processing to allow net water absorption in the intestine. The formation of luminal carbonate aggregates impacts on calcium homeostasis and requires epithelial HCO3(-) secretion to enable water absorption. In light of its endocrine importance in calcium handling and the indication of involvement in HCO3(-) secretion the present study was designed to expose the role of the parathyroid hormone-related protein (PTHrP) in HCO3(-) secretion, water absorption and the regulation of aqp1 gene expression in the anterior intestine of the sea bream. HCO3(-) secretion rapidly decreased when PTHrP(1-34) was added to anterior intestine of the sea bream mounted in Ussing chambers. The effect achieved a maximum inhibition of 60% of basal secretion rates, showing a threshold effective dose of 0.1 ng ml(-1) compatible with reported plasma values of PTHrP. When applied in combination with the adenylate cyclase inhibitor (SQ 22.536, 100 μmol l(-1)) or the phospholipase C inhibitor (U73122, 10 μmol l(-1)) the effect of PTHrP(1-34) on HCO3(-) secretion was reduced by about 50% in both cases. In parallel, bulk water absorption measured in intestinal sacs was sensitive to inhibition by PTHrP. The inhibitory action conforms to a typical dose-response curve in the range of 0.1-1000 ng ml(-1), achieves a maximal effect of 60-65% inhibition from basal rates and shows threshold significant effects at hormone levels of 0.1 ng ml(-1). The action of PTHrP in water absorption was completely abolished in the presence of the adenylate cyclase inhibitor (SQ 22.536, 100 μmol l(-1)) and was insensitive to the phospholipase C inhibitor (U73122, 10 μmol l(-1)). In vivo injections of PTHrP(1-34) or the PTH/PTHrP receptor antagonist PTHrP(7-34) evoked respectively, a significant decrease or increase of aqp1ab, but not aqp1a. Overall the present results suggest that PTHrP acts as a key regulator of carbonate aggregate formation in the intestine of marine fish via its actions on water absorption, calcium regulation and HCO3(-) secretion.

Discovery of osmotic sensitive transcription factors in fish intestine via a transcriptomic approach

Background

Teleost intestine is crucial for seawater acclimation by sensing osmolality of imbibed seawater and regulating drinking and water/ion absorption. Regulatory genes for transforming intestinal function have not been identified. A transcriptomic approach was used to search for such genes in the intestine of euryhaline medaka.

Results

Quantitative RNA-seq by Illumina Hi-Seq Sequencing method was performed to analyze intestinal gene expression 0 h, 1 h, 3 h, 1 d, and 7 d after seawater transfer. Gene ontology (GO) enrichment results showed that cell adhesion, signal transduction, and protein phosphorylation gene categories were augmented soon after transfer, indicating a rapid reorganization of cellular components and functions. Among >50 transiently up-regulated transcription factors selected via co-expression correlation and GO selection, five transcription factors, including CEBPB and CEBPD, were confirmed by quantitative PCR to be specific to hyperosmotic stress, while others were also up-regulated after freshwater control transfer, including some well-known osmotic-stress transcription factors such as SGK1 and TSC22D3/Ostf1. Protein interaction networks suggest a high degree of overlapping among the signaling of transcription factors that respond to osmotic and general stresses, which sheds light on the interpretation of their roles during hyperosmotic stress and emergency.

Conclusions

Since cortisol is an important hormone for seawater acclimation as well as for general stress in teleosts, emergency and osmotic challenges could have been evolved in parallel and resulted in the overlapped signaling networks. Our results revealed important interactions among transcription factors and offer a multifactorial perspective of genes involved in seawater acclimation.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1134) contains supplementary material, which is available to authorized users.

Effects of salinity and prolactin on gene transcript levels of ion transporters, ion pumps and prolactin receptors in Mozambique tilapia intestine

Euryhaline teleosts are faced with significant challenges during changes in salinity. Osmoregulatory responses to salinity changes are mediated through the neuroendocrine system which directs osmoregulatory tissues to modulate ion transport. Prolactin (PRL) plays a major role in freshwater (FW) osmoregulation by promoting ion uptake in osmoregulatory tissues, including intestine. We measured mRNA expression of ion pumps, Na(+)/K(+)-ATPase α3-subunit (NKAα3) and vacuolar type H(+)-ATPase A-subunit (V-ATPase A-subunit); ion transporters/channels, Na(+)/K(+)/2Cl(-) co-transporter (NKCC2) and cystic fibrosis transmembrane conductance regulator (CFTR); and the two PRL receptors, PRLR1 and PRLR2 in eleven intestinal segments of Mozambique tilapia (Oreochromis mossambicus) acclimated to FW or seawater (SW). Gene expression levels of NKAα3, V-ATPase A-subunit, and NKCC2 were generally lower in middle segments of the intestine, whereas CFTR mRNA was most highly expressed in anterior intestine of FW-fish. In both FW- and SW-acclimated fish, PRLR1 was most highly expressed in the terminal segment of the intestine, whereas PRLR2 was generally most highly expressed in anterior intestinal segments. While NKCC2, NKAα3 and PRLR2 mRNA expression was higher in the intestinal segments of SW-acclimated fish, CFTR mRNA expression was higher in FW-fish; PRLR1 and V-ATPase A-subunit mRNA expression was similar between FW- and SW-acclimated fish. Next, we characterized the effects of hypophysectomy (Hx) and PRL replacement on the expression of intestinal transcripts. Hypophysectomy reduced both NKCC2 and CFTR expression in particular intestinal segments; however, only NKCC2 expression was restored by PRL replacement. Together, these findings describe how both acclimation salinity and PRL impact transcript levels of effectors of ion transport in tilapia intestine.

Endocrine regulation of carbonate precipitate formation in marine fish intestine by stanniocalcin and PTHrP

In marine fish, high epithelial bicarbonate secretion by the intestine generates luminal carbonate precipitates of divalent cations that play a key role in water and ion homeostasis. In vitro studies highlight the involvement of the calciotropic hormones PTHrP (parathyroid hormone-related protein) and stanniocalcin (STC) in the regulation of epithelial bicarbonate transport. The present study tested the hypothesis that calciotropic hormones have a regulatory role in carbonate precipitate formation in vivo. Sea bream (Sparus aurata) juveniles received single intraperitoneal injections of piscine PTHrP(1-34), the PTH/PTHrP receptor antagonist PTHrP(7-34) or purified sea bream STC, or were passively immunized with polyclonal rabbit antisera raised against sea bream STC (STC-Ab). Endocrine effects on the expression of the basolateral sodium bicarbonate co-transporter (Slc4a4.A), the apical anion exchangers Slc26a6.A and Slc26a3.B, and the V-type proton pump β-subunit (Atp6v1b) in the anterior intestine were evaluated. In keeping with their calciotropic nature, the hypocalcaemic factors PTHrP(7-34) and STC up-regulated gene expression of all transporters. In contrast, the hypercalcaemic factor PTHrP(1-34) and STC antibodies down-regulated transporters involved in the bicarbonate secretion cascade. Changes in intestine luminal precipitate contents provoked by calcaemic endocrine factors validated these results: 24 h post-injection either PTHrP(1-34) or immunization with STC-Ab reduced the carbonate precipitate content in the sea bream intestine. In contrast, the PTH/PTHrP receptor antagonist PTHrP(7-34) increased not only the precipitated fraction but also the concentration of HCO3(-) equivalents in the intestinal fluid. These results confirm the hypothesis that calciotropic hormones have a regulatory role in carbonate precipitate formation in vivo in the intestine of marine fish. Furthermore, they illustrate for the first time in fish the counteracting effect of PTHrP and STC, and reveal an unexpected contribution of calcaemic factors to acid-base balance.

AVT is involved in the regulation of ion transport in the intestine of the sea bream (Sparus aurata)

The intestine of marine fish plays a crucial role in ion homeostasis by selective processing of ingested fluid. Although arginine vasotocin (AVT) is suggested to play a role in ion regulation in fish, its action in the intestine has not been demonstrated. Thus, the present study investigated in vitro the putative role of AVT in intestinal ion transport in the sea bream (Sparus aurata). A cDNA encoding part of an AVT receptor was isolated and phylogenetic analysis revealed it clustered with the V1a2-type receptor clade. V1a2 transcripts were expressed throughout the gastrointestinal tract, from esophagus to rectum, and were most abundant in the rectum regardless of long-term exposure to external salinities of 12, 35 or 55p.p.t. Basolateral addition of AVT (10(-6)M) to the anterior intestine and rectum of sea bream adapted to 12, 35 or 55p.p.t. mounted in Ussing chambers produced rapid salinity and region dependent responses in short circuit current (Isc), always in the absorptive direction. In addition, AVT stimulation of absorptive Isc conformed to a dose-response curve, with significant effects achieved at 10(-8)M, which corresponds to physiological values of plasma AVT for this species. The effect of AVT on intestinal Isc was insensitive to the CFTR selective inhibitor NPPB (200μM) applied apically, but was completely abolished in the presence of apical bumetanide (200μM). We propose a role for AVT in the regulation of ion absorption in the intestine of the sea bream mediated by an absorptive bumetanide-sensitive mechanism, likely NKCC2.