Cl- absorption in European eel intestine and its regulation

Molecular mechanisms underlying active desalination and low water permeability in the esophagus of eels acclimated to seawater

Marine teleosts can absorb imbibed seawater (SW) to maintain water balance, with esophageal desalination playing an essential role. NaCl absorption from luminal SW was enhanced 10-fold in the esophagus of SW-acclimated eels, and removal of Na⁺ or Cl^- from luminal SW abolished the facilitated absorption, indicating coupled transport. Mucosal/serosal application of various blockers for Na⁺/Cl^- transporters profoundly decreased the absorption. Among the transporter genes expressed in eel esophagus detected by RNA-seq, dimethyl amiloride-sensitive Na⁺/H⁺ exchanger (NHE3) and 4,4'-diisothiocyano-2,2'-disulfonic acid-sensitive Cl^-/[Formula: see text] exchanger (AE) coupled by the scaffolding protein on the apical membrane of epithelial cells, and ouabain-sensitive Na⁺-K⁺-ATPases (NKA1α1c and NKA3α) and diphenylamine-2-carboxylic acid-sensitive Cl^- channel (CLCN2) on the basolateral membrane, may be responsible for enhanced transcellular NaCl transport because of their profound upregulation after SW acclimation. Upregulated carbonic anhydrase 2a (CA2a) supplies H⁺ and [Formula: see text] for activation of the coupled NHE and AE. Apical hydrochlorothiazide-sensitive Na⁺-Cl^- cotransporters and basolateral Na⁺-[Formula: see text] cotransporter (NBCe1) and AE1 are other possible candidates. Concerning the low water permeability that is typically seen in marine teleost esophagus, downregulated aquaporin genes (aqp1a and aqp3) and upregulated claudin gene (cldn15a) are candidates for transcellular/paracellular route. In situ hybridization showed that these upregulated transporters and tight-junction protein genes were expressed in the absorptive columnar epithelial cells of eel esophagus. These results allow us to provide a full picture of the molecular mechanism of active desalination and low water permeability that are characteristic to marine teleost esophagus and gain deeper insights into the role of gastrointestinal tracts in SW acclimation.

Identification of fxyd genes from the spotted scat (Scatophagus argus): molecular cloning, tissue-specific expression, and response to acute hyposaline stress

By interacting with Na(+), K(+)-ATPase (NKA), the FXYD domain-containing ion transport regulator (FXYD) is involved in teleost osmoregulation, but knowledge of FXYD in marine fish is limited. In the present study, fxyd11 and fxyd12 were identified from the spotted scat (Scatophagus argus), and the two members of the FXYD protein family were expressed in a tissue-specific manner. Fxyd11 mRNA was predominantly expressed in gills, whereas fxyd12 mRNA was mainly distributed in kidneys and intestines. Acute hyposaline stress altered the activity of NKA and the expression of fxyd11 and fxyd12 in gills, kidneys, and intestines. Branchial fxyd11 mRNA expression remained at a low level during freshwater acclimation, whereas NKA activity increased, showing a negative correlation that differed from previous reports. Similarly, renal expression of fxyd11 and fxyd12 mRNA was negatively correlated with NKA activity. Unlike in gills and kidneys, intestinal NKA activity and mRNA expression of fxyd11 and fxyd12 were comparably suppressed. Taken together, the salinity-dependent expression of fxyd11 and fxyd12, and correlation with NKA activity suggested that both fxyd11 and fxyd12 were involved in the response to acute hyposaline challenge in the spotted scat.

Cellular distribution of NKCC2 in the gastric mucosa and its response to short-term osmotic shock

The Na(+)-K(+)-2Cl(-) cotransporter-2 (NKCC2) has long been recognized as a "kidney-specific" transporter and is important in salt reabsorption. NKCC2 has been found in the gastric mucosa; however, its cellular distribution and function remain obscure. The present study characterized the distribution pattern of NKCC2 in mammalian gastric mucosa and investigated its response to osmotic challenge. Reverse transcription with the polymerase chain reaction, Western blot and immunofluorescence were used to determine NKCC2 expression and localization. The effect of osmotic shock on NKCC2 expression was studied in isolated gastric mucosa with variable osmolarity treatment. Results from all of the above studies were compared with those of NKCC1. Our data indicated that NKCC1 and NKCC2 were expressed in the gastric mucosa of rat, mouse and human. The mRNA transcripts and proteins for NKCC1 and NKCC2 were broadly expressed in the rat gastric mucosa. In rat and mouse, NKCC1 was largely confined to the lower part of the oxyntic and pyloric gland areas, whereas NKCC2 extended throughout the gastric glands. NKCC1 immunoreactivity was strongly expressed in the parietal and chief cells but was weaker in the mucous cells. NKCC2 was abundantly located in the parietal and mucous cells but faintly distributed in the chief cells. Hypertonic treatment increased the protein level of NKCC1 and caused evident membrane translocation. In contrast, NKCC2 was significantly downregulated and no obvious membrane translocation was observed. Thus, NKCC2 displayed a more ubiquitous distribution in the gastric mucosa and might work coordinately with NKCC1 to maintain cell volume homeostasis under hypertonic conditions.

Cellular localization of NKCC2 and its possible role in the Cl- absorption in the rat and human distal colonic epithelia

Recently, we demonstrated the expression of NKCC2, an absorptive isoform of NKCC specifically expressed in the kidney, in the rat gastrointestinal tract including the distal colonic mucosa. This study aims to investigate its localization in colonic epithelia and possible role in the colonic ion transport. Reverse transcription polymerase chain reaction (RT-PCR), Western blotting, and immunohistochemistry were used to investigate the expression and localization of NKCC2. The role of NKCC2 on the colonic ion transport was examined by mean of short-circuit current (I(SC)) monitoring. The results indicated that NKCC2 was expressed in the apical region of the epithelia in rat distal colon and human sigmoid colon. NKCC1, which is a secretive NKCC isoform, was localized predominantly in the basolateral membrane, which has been well documented. Serosal (basolateral) administration of bumetanide, an inhibitor of both NKCC1 and NKCC2, inhibited serosal forskolin-induced I(SC) increase by 66% but enhanced the luminal (apical) forskolin-induced I(SC) response by 63%. Furthermore, the blocking of epithelial Na(+) channels by apical addition of amiloride (10 μmol/L), K(+) channels by tetraethylammoniumion (TEA) (5 mmol/L), or glibenclimide (0.1 mmol/L) did not affect apical forskolin-induced I(SC) increase, excluding the involvement of cations, Na(+) and K(+), in the I(SC) response. The luminal forskolin-induced I(SC) increase was enhanced markedly by the apical pretreatment with bumetanide or the reduction of apical Cl(-) concentration by 114% and 198%, respectively, which were inhibited by apical addition of glibenclimide (1 mmol/L) by more than 60%. This finding suggests the involvement of an anion. Furthermore, the removal of basolateral HCO(3)(-) reduced apical forskolin-induced I(SC) by more than 75% indicated that the apical forskolin-induced I(SC) increase in rat distal colon was mediated by Cl(-) absorption and HCO(3)(-) secretion. In conclusion, NKCC2 is expressed widely in the colonic epithelium in rat distal colon and human sigmoid colon, especially in the apical membrane. It involves the process of colonic Cl(-) absorption coupled with HCO(3)(-) secretion.

Mechanosensitive signalling in fish gill and other ion transporting epithelia

Epithelia involved in vectorial salt transport respond to apical and basolateral changes in osmotic activity by moderating the transmural solute transport rate simultaneously with underlying volume regulatory mechanisms involved in regulatory volume increase (RVI) and decrease (RVD). This review examines rapid osmotic responses in salt secreting epithelia of marine and euryhaline teleost fish, with inclusion of recent results from other ion transporting epithelia that also respond rapidly to osmotic shock. Mitochondrion-rich chloride secreting cells of marine teleost fish gills and skin, when exposed to hypertonic shock, activate NaCl secretion via phosphorylation of Na(+), K(+), 2Cl(-) cotransporter (NKCC1) in the basolateral membrane and activation of anion channels in the apical membrane. Conversely, NaCl secretion is inhibited when chloride secreting cells are swollen osmotically. Mammalian airway epithelial cells also possess NKCC1 basally and apical anion channels [Cystic Fibrosis Transmembrane conductance Regulator (CFTR)]; with hypotonic shock, this epithelium releases ATP and NaCl secretion is stimulated via purinergic receptors, while hypertonic shock inhibits Na(+) uptake. In the eye, the ciliary epithelium activates Cl(-) channels in response to hypotonic shock as RVD, an effect that modulates transepithelial fluid transport rates. In the renal A6 cell line, K(+) and Cl(-) effluxes activate during RVD and RVI Na(+) transepithelial absorption. A common theme in these systems is ATP release in hypotonic shock with subsequent RVD-effective mechanisms such as NKCC1 inhibition and K(+) and Cl(-) efflux, but there are different effects of osmotic changes on transepithelial transport, apparently depending on the role of the epithelial system.

Expression of Na-K-Cl cotransporter and edema formation are age dependent after ischemic stroke

Age is the most important independent risk factor for stroke; however aging animals are rarely used in stroke studies. Previous work demonstrated that young male mice had more edema formation after an induced stroke than aging animals. An important contributor to cerebral edema formation is the Na-K-Cl cotransporter (NKCC). We examined the expression of NKCC in young (10-12 weeks) and aging (15-16 months) C57BL6 male mice after middle cerebral artery occlusion (MCAO) and investigated the effect of pharmacological inhibition of NKCC with Bumetanide on cerebral edema formation. Both immunofluorescent staining and Western blotting analysis showed that NKCC expression was significantly higher in the ischemic penumbra of young compared to aging mice after stroke. Edema formation was significantly more robust in young mice and was reduced with Bumetanide. Bumetanide had no effect on cerebral edema in aging mice after MCAO. This suggests that NKCC expression and edema formation are age dependent after ischemic stroke.

Expression of NKCC2 in the rat gastrointestinal tract

NKCC2, an isoform of Na+-K+-2Cl(-) cotransporter, is principally present in the kidney and plays a critical role in salt reabsorption. Expression of NKCC2 has been found in the apical membrane of intestinal epithelial cells in a number of marine fish, however, details for expression in the mammalian gastrointestinal tract are lacking. RT-PCR, Western blotting and immunohistochemistry were used to study the expression and localization of NKCC2 in the rat gastrointestinal tract. We found that mRNA transcripts, protein and immunoreactivity (IR) for NKCC2 were expressed in the stomach, small and large intestine of adult rats. NKCC2 IR was localized to the base of the gastric glands, intestinal epithelia, myenteric and submucosal plexuses. NKCC2 IR was expressed strongly in the apical membranes and weakly in the basolateral membranes of intestinal epithelial cells. In the enteric nervous system, NKCC2 IR was widely distributed and localized to enteric neurons with cholinergic, calretinin and nitrergic neuronal immunochemical codes in the myenteric plexus. It was localized to non-cholinergic secretomotor neurons in the submucosal plexus. In conclusion, this study for the first time clearly detected the expression of NKCC2 in the gastrointestinal tract of a mammalian species. Expression of NKCC2 in gastrointestinal epithelial cells suggested that this cation chloride cotransporter might be involved in gastrointestinal ion transport. Expression of NKCC2 in enteric neurons might contribute to the accumulation of Cl(-) and a more depolarized E(Cl)(-) in enteric neurons.

Physiological and molecular mechanisms of osmoregulatory plasticity in killifish after seawater transfer

We have explored the molecular and physiological responses of the euryhaline killifish Fundulus heteroclitus to transfer from brackish water (10% seawater) to 100% seawater for 12 h, 3 days or 7 days. Plasma [Na+] and [Cl-] were unchanged after transfer, and plasma cortisol underwent a transient increase. Na+/K+-ATPase activity increased 1.5-fold in the gills and opercular epithelium at 7 days (significant in gills only), responses that were preceded by three- to fourfold increases in Na+/K+-ATPase alpha(1a) mRNA expression. Expression of Na+/K+/2Cl- cotransporter 1, cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel, Na+/H+-exchanger 3 (significant in opercular epithelium only) and carbonic anhydrase II mRNA also increased two- to fourfold after transfer. Drinking rate increased over twofold after 12 h and remained elevated for at least 7 days. Surprisingly, net rates of water and ion absorption measured in vitro across isolated intestines decreased approximately 50%, possibly due to reduced salt demands from the diet in seawater, but water absorption capacity still exceeded the drinking rate. Changes in bulk water absorption were well correlated with net ion absorption, and indicated that slightly hyperosmotic solutions (>or=298 mmol l(-1)) were transported. There were no reductions in unidirectional influx of Na+ from luminal to serosal fluid or intestinal Na+/K+-ATPase activity after transfer. Overall, our results indicate that gill and opercular epithelia function similarly at a molecular level in seawater, in contrast to their divergent function in freshwater, and reveal unexpected changes in intestinal function. As such they provide further insight into the mechanisms of euryhalinity in killifish.

Regulation of ion transport in eel intestine by the homologous guanylin family of peptides

Since the gene expression of guanylin peptides and their receptors, guanylyl cyclase Cs, is enhanced in the intestine of seawater (SW)-adapted eels compared with fresh water (FW)-adapted fish, the guanylin family may play an important role in SW adaptation in eels. The present study analyzed the effect of three homologous guanylin peptides, guanylin, uroguanylin and renoguanylin, on ion movement through the eel intestine, and examined the target of guanylin action using Ussing chambers. The middle and posterior parts of the intestine, where water and ion absorption occurs actively in SW eels, exhibited serosa-negative transepithelial potential, while the anterior intestine was serosa-positive. Mucosal application of each guanylin in the middle or posterior intestine reduced the short-circuit current (Isc) dose dependently and reversed it at high doses, and reduced electric tissue resistance. The effects were greater in the middle intestine than in the posterior intestine. All three guanylins showed similar potency in the middle segment, but guanylin was more potent in the posterior segment. 8-bromo cGMP mimicked the effect of guanylins. The intestinal response to guanylin was smaller in FW eels. The mucosal presence of NPPB utilized as a CFTR blocker, but not of other inhibitors of the channels/transporters localized on the luminal surface in SW fish intestine, inhibited the guanylin-induced decrease in Isc. In eels, therefore, the guanylin family may be involved in osmoregulation by the intestine by binding to the receptors and activating CFTR-like channels on the mucosal side through cGMP production, perhaps resulting in Cl(-) and HCO3(-) secretion into the lumen.

The role of volume-sensitive ion transport systems in regulation of epithelial transport

This review focuses on using the knowledge on volume-sensitive transport systems in Ehrlich ascites tumour cells and NIH-3T3 cells to elucidate osmotic regulation of salt transport in epithelia. Using the intestine of the European eel (Anguilla anguilla) (an absorptive epithelium of the type described in the renal cortex thick ascending limb (cTAL)) we have focused on the role of swelling-activated K+- and anion-conductive pathways in response to hypotonicity, and on the role of the apical (luminal) Na+-K+-2Cl- cotransporter (NKCC2) in the response to hypertonicity. The shrinkage-induced activation of NKCC2 involves an interaction between the cytoskeleton and protein phosphorylation events via PKC and myosin light chain kinase (MLCK). Killifish (Fundulus heteroclitus) opercular epithelium is a Cl(-)-secreting epithelium of the type described in exocrine glands, having a CFTR channel on the apical side and the Na+/K+ ATPase, NKCC1 and a K+ channel on the basolateral side. Osmotic control of Cl- secretion across the operculum epithelium includes: (i) hyperosmotic shrinkage activation of NKCC1 via PKC, MLCK, p38, OSR1 and SPAK; (ii) deactivation of NKCC by hypotonic cell swelling and a protein phosphatase, and (iii) a protein tyrosine kinase acting on the focal adhesion kinase (FAK) to set levels of NKCC activity.

The intestinal guanylin system and seawater adaptation in eels

Guanylin and uroguanylin are principal intestinal hormones secreted into the lumen to regulate ion and water absorption via a specific receptor, guanylyl cyclase-C (GC-C). As the intestine is an essential organ for seawater (SW) adaptation in teleost fishes, the intestinal guanylin system may play a critical role in SW adaptation. Molecular biological studies identified multiple guanylins (guanylin, uroguanylin and renoguanylin) and their receptors (GC-C1 and GC-C2) in eels. The relative potency of the three ligands on cGMP production in transiently expressed receptors was uroguanylin > guanylin >or= renoguanylin for CG-C1 and guanylin >or= renoguanylin > uroguanylin for GC-C2. Eel guanylin and GC-C genes are expressed exclusively in the intestine and kidney, and the level of expression is greater in SW eels than in freshwater (FW) eels except for renoguanylin. Physiological studies using Ussing chambers showed that the middle and posterior intestine are major sites of action of guanylins, where they act on the mucosal side to decrease short circuit current (I(sc)) in a dose-dependent manner. The ID(50) of guanylins for transport inhibition was 50-fold greater than that of atrial natriuretic peptide that acts from the serosal side as an endocrine hormone. However, only guanylins reversed I(sc) to levels below zero. Pharmacological analyses using various blockers showed that among transporters and channels localized on the intestinal cells of SW teleost fish, the cystic fibrosis transmembrane conductance regulator Cl(-) channel (CFTR) on the apical membrane is the major target of guanylins. Collectively, guanylins are synthesized locally in the intestine and secreted into the lumen to act on the GC-Cs in the apical membrane of eel intestinal cells. Then, intracellular cGMP production after ligand-receptor interaction activates CFTR and probably induces Cl(-) and/or HCO3- secretion into the lumen as suggested in mammals. The physiological significance of the anion secretion induced by the luminal guanylin/GC-C system on SW adaptation may rival or exceed that of the serosally derived natriuretic peptides in the euryhaline eel.

Plasticity of osmoregulatory function in the killifish intestine: drinking rates, salt and water transport, and gene expression after freshwater transfer

We have explored intestinal function in the euryhaline killifish Fundulus heteroclitus after transfer from brackish water (10% seawater) to fresh water. Plasma Na+ and Cl- concentrations fell at 12 h post-transfer, but recovered by 7 days. Drinking rate decreased substantially at 12 h (32% of control value) and remained suppressed after 3 and 7 days in fresh water (34 and 43%). By contrast, there was a transient increase in the capacity for water absorption measured across isolated intestines in vitro (3.3- and 2.6-fold at 12 h and 3 days), which returned to baseline after 7 days. These changes in water absorption could be entirely accounted for by changes in net ion flux: there was an extremely strong correlation (R2=0.960) between water absorption and the sum of net Na+ and net Cl- fluxes (3.42+/-0.10 microl water micromol(-1) ion). However, enhanced ion transport across the intestine in fresh water would probably not increase water uptake in vivo, because the drinking rate was far less than the capacity for water absorption across the intestine. The increased intestinal ion absorption after freshwater transfer may instead serve to facilitate ion absorption from food when it is present in the gut. Modulation of net ion flux occurred without changes in mRNA levels of many ion transporters (Na+/K+-ATPase alpha(1a), carbonic anhydrase 2, CFTR Cl- channel, Na+/K+/2Cl- cotransporter 2, and the signalling protein 14-3-3a), and before a measured increase in Na+/K+-ATPase activity at 3 days, suggesting that there is some other mechanism responsible for increasing ion transport. Interestingly, net Cl- flux always exceeded net Na+ flux, possibly to help maintain Cl- balance and/or facilitate bicarbonate excretion. Our results suggest that intestinal NaCl absorption from food is important during the period of greatest ionic disturbance after transfer to fresh water, and provide further insight into the mechanisms of euryhalinity in killifish.

Digestive tract ontogeny of Dicentrarchus labrax: implication in osmoregulation

The ontogeny of the digestive tract (DT) and of Na(+)/K(+)-ATPase localization was investigated during the early postembryonic development (from yolk sac larva to juvenile) of the euryhaline teleost Dicentrarchus labrax reared at two salinities: seawater and diluted seawater. Histology, electron microscopy and immunocytochemistry were used to determine the presence and differentiation of ion transporting cells. At hatching, the DT is an undifferentiated straight tube over the yolk sac. At the mouth opening (day 5), it comprises six segments: buccopharynx, esophagus, stomach, anterior intestine, posterior intestine and rectum, well differentiated at the juvenile stage (day 72). The enterocytes displayed ultrastructural features similar to those of mitochondria-rich cells known to be involved in active ion transport. At hatching, ion transporting cells lining the intestine and the rectum exhibited a Na(+)/K(+)-ATPase activity which increased mainly after the larva/juvenile (20 mm) metamorphic transition. The immunofluorescence intensity was dependent upon the stage of development of the gut as well as on the histological configuration of the analyzed segment. The appearance and distribution of enteric ionocytes and the implication of the DT in osmoregulation are discussed.

The Na+-K+-2Cl- cotransporter and the osmotic stress response in a model salt transport epithelium

Epithelia are physiologically exposed to osmotic stress resulting in alteration of cell volume in several aspects of their functioning; therefore, the activation of 'emergency' systems of rapid cell volume regulation is fundamental in their physiology. In this review, the physiological response to osmotic stress, particularly hypertonic stress, was described in a salt-transporting epithelium, the intestine of the euryhaline teleost European eel. This epithelium is physiologically exposed to changes in extracellular osmolarity and represents a good physiological model for functional studies on cellular volume regulation, permitting the study of volume regulated ion transport mechanisms in a native tissue. An absorptive form of the cotransporter, homologue of the renal NKCC2, localized on the apical membrane, was found in the intestine of the euryhaline teleost European eel. This cotransporter accounts for the luminal uptake of Cl-; it operates in series with a basolateral Cl- conductance and presumably a basolateral electroneutral KCl cotransport and in parallel with a luminal K+ conductance. The ion transport model described for eel intestine, based on the operation of an absorptive luminal Na+-K+-2Cl-, is basically the same as the model that has been proposed for the thick ascending limb (cTAL) of the mammalian renal cortex. This paper focuses on the role of Na+-K+-2Cl- cotransport in the responses to hypertonic stress in the eel intestine and the role of cytoskeleton (either actin-based or tubulin based) is discussed.

Cell proliferation and apoptosis in the anterior intestine of an amphibious, euryhaline mudskipper (Periophthalmus modestus)

In order to replace the diffusive loss of water to the surrounding environment, seawater (SW)-acclimated euryhaline fishes have gastrointestinal tracts with higher ion/water flux in concert with greater permeability, and contrast that to freshwater (FW)-acclimated fish. To understand the cellular basis for these differences, we examined cell proliferation and apoptosis in the anterior intestine of mudskipper transferred from one-third SW to FW or to SW for 1 and 7 days, and those kept out of water for 1 day. The intestinal apoptosis (indicated by DNA laddering) increased during seawater acclimation. TUNEL staining detected numerous apoptotic cells over the epithelium of SW-acclimated fish. Cell proliferation ([3H]thymidine incorporation) in the FW fish was greater than those in SW 7 days after transfer. Labeling with a Proliferating cell nuclear antigen (PCNA) antibody indicated that proliferating cells were greater in number and randomly distributed in the epithelium of FW fish, whereas in SW fish they were almost entirely in the troughs of the intestinal folds. There were no changes in cell turnover in fish kept out of water. During acclimation to different salinities, modification of the cell turnover and abundance may play an important role in regulating the permeability (and transport capacity) of the gastrointestinal tract of fish.

Involvement of drinking and intestinal sodium absorption in hyponatremic effect of atrial natriuretic peptide in seawater eels

Atrial natriuretic peptide (ANP) decreases plasma Na+ concentration and promtes seawater (SW) adaptation in eels. The hyponatremia may most probably be caused by increased branchial extrusion of Na+, but the mechanism has not been determined yet. The present study examined initially the effects of ANP on branchial Na+ efflux in vivo using isotopic 22Na. However, the efflux rate was not altered by infusion of a hyponatremic dose of ANP (5 pmol.kg(-1).min(-1)). Therefore, we sought to examine whether the ANP-mediated hyponatremia is caused by a decrease in the uptake of Na+ from the environment. Since a decrease in drinking was highly correlated with a degree of hyponatremia, conscious SW eels were infused with dilute SW into the stomach at a normal drinking rate to offset the antidipsogenic effect of ANP. Under this regimen, the hyponatremic effect of ANP was abolished. Then, we examined the site of Na+ absorption in the alimentary tract by measuring the changes in ion composition of intraluminal fluid along the tract. Since Na+ was absorbed at the esophagus and anterior/middle intestine, a sac was prepared at each site and the effects of ANP were examined in situ in conscious SW eels. ANP infusion did not alter Na+ absorption at the esophagus, but it profoundly reduced the absorption at the intestine. Together with our previous finding that ANP does not alter renal Na+ excretion, we propose that ANP reduces plasma Na+ concentration in SW eels by inhibiting drinking and subsequent absorption of Na+ by the intestine.

Hypotonicity induced K+ and anion conductive pathways activation in eel intestinal epithelium

Control of cell volume is a fundamental and highly conserved physiological mechanism, essential for survival under varying environmental and metabolic conditions. Epithelia (such as intestine, renal tubule, gallbladder and gills) are tissues physiologically exposed to osmotic stress. Therefore, the activation of 'emergency' systems of rapid cell volume regulation is fundamental in their physiology. The aim of the present work was to study the physiological response to hypotonic stress in a salt-transporting epithelium, the intestine of the euryhaline teleost Anguilla anguilla. Eel intestinal epithelium, when symmetrically bathed with Ringer solution, develops a net Cl- current giving rise to a negative transepithelial potential at the basolateral side of the epithelium. The eel intestinal epithelium responded to a hypotonic challenge with a biphasic decrease in the transepithelial voltage (V(te)) and the short circuit current (I(sc)). This electrophysiological response correlated with a regulatory volume decrease (RVD) response, recorded by morphometrical measurement of the epithelium height. Changes in the transepithelial resistance were also observed following the hypotonicity exposure. The electrogenic V(te) and I(sc) responses to hypotonicity resulted from the activation of different K+ and anion conductive pathways on the apical and basolateral membranes of the epithelium: (a) iberiotoxin-sensitive K+ channels on the apical and basolateral membrane, (b) apamin-sensitive K+ channels mainly on the basolateral membrane, (c) DIDS-sensitive anion channels on the apical membrane. The functional integrity of the basal Cl- conductive pathway on the basolateral membrane is also required. The electrophysiological response to hypotonic stress was completely abolished by Ca2+ removal from the Ringer perfusing solution, but was not affected by depletion of intracellular Ca2+ stores by thapsigargin.