Control of epithelial Cl(−) secretion by basolateral osmolality in the euryhaline teleost Fundulus heteroclitus

Molecular targets of prolactin in mummichogs (Fundulus heteroclitus): Ion transporters/channels, aquaporins, and claudins

Prolactin (Prl) was identified over 60 years ago in mummichogs (Fundulus heteroclitus) as a "freshwater (FW)-adapting hormone", yet the cellular and molecular targets of Prl in this model teleost have remained unknown. Here, we conducted a phylogenetic analysis of two mummichog Prl receptors (Prlrs), designated Prlra and Prlrb, prior to describing the tissue- and salinity-dependent expression of their associated mRNAs. We then administered ovine Prl (oPrl) to mummichogs held in brackish water and characterized the expression of genes associated with FW- and seawater (SW)-type ionocytes. Within FW-type ionocytes, oPrl stimulated the expression of Na⁺/Cl^- cotransporter 2 (ncc2) and aquaporin 3 (aqp3). Alternatively, branchial Na⁺/H⁺ exchanger 2 and -3 (nhe2 and -3) expression did not respond to oPrl. Gene transcripts associated with SW-type ionocytes, including Na⁺/K⁺/2Cl^- cotransporter 1 (nkcc1), cystic fibrosis transmembrane regulator 1 (cftr1), and claudin 10f (cldn10f) were reduced by oPrl. Isolated gill filaments incubated with oPrl in vitro exhibited elevated ncc2 and prlra expression. Given the role of Aqps in supporting gastrointestinal fluid absorption, we assessed whether several intestinal aqp transcripts were responsive to oPrl and found that aqp1a and -8 levels were reduced by oPrl. Our collective data indicate that Prl promotes FW-acclimation in mummichogs by orchestrating the expression of solute transporters/channels, water channels, and tight-junction proteins across multiple osmoregulatory organs.

Blending physiology and RNAseq to provide new insights into regulation of epithelial transport: switching between ion secretion and reabsorption

This Review addresses the means by which epithelia change the direction of vectorial ion transport. Recent studies have revealed that insect Malpighian (renal) tubules can switch from secreting to reabsorbing K+. When the gut of larval lepidopterans is empty (during the moult cycle) or when the larvae are reared on K+-deficient diet, the distal ileac plexus segment of the tubule secretes K+ from the haemolymph into the tubule lumen. By contrast, in larvae reared on K+-rich diet, ions and fluid are reabsorbed from the rectal lumen into the perinephric space surrounding the cryptonephridial tubules of the rectal complex. Ions and fluid are then transported from the perinephric space into the lumen of the cryptonephridial tubules, thus supplying the free segments of the tubule downstream. Under these conditions, some of the K+ and water in the tubule lumen is reabsorbed across the cells of the distal ileac plexus, allowing for expansion of haemolymph volume in the rapidly growing larvae, as well as recycling of K+ and base equivalents. RNA sequencing data reveal large-scale changes in gene transcription that are associated with the switch between ion secretion and ion reabsorption by the distal ileac plexus. An unexpected finding is the presence of voltage-gated, ligand-gated and mechanosensitive ion channels, normally seen in excitable cells, in Malpighian tubules. Transcriptomic surveys indicate that these types of channels are also present in multiple other types of vertebrate and invertebrate epithelia, suggesting that they may play novel roles in epithelial cell signalling and regulation of epithelial ion transport.

Salinity-dependent expression of ncc2 in opercular epithelium and gill of mummichog (Fundulus heteroclitus)

Mummichogs (Fundulus heteroclitus) can tolerate abrupt changes in environmental salinity because of their ability to rapidly adjust the activities of ionocytes in branchial and opercular epithelia. In turn, the concerted expression of sub-cellular effectors of ion transport underlies adaptive responses to fluctuating salinities. Exposure to seawater (SW) stimulates the expression of Na⁺/K⁺/2Cl^- cotransporter 1 (nkcc1) and cystic fibrosis transmembrane regulator (cftr) mRNAs in support of ion extrusion by SW-type ionocytes. Given the incomplete understanding of how freshwater (FW)-type ionocytes actually operate in mummichogs, the transcriptional responses essential for ion absorption in FW environments remain unresolved. In a subset of species, a 'fish-specific' Na⁺/Cl^- cotransporter denoted Ncc2 (Slc12a10) is responsible for the uptake of Na⁺ and Cl^- across the apical surface of FW-type ionocytes. In the current study, we identified an ncc2 transcript that is highly expressed in gill filaments and opercular epithelium of FW-acclimated mummichogs. Within 1 day of transfer from SW to FW, ncc2 levels in both tissues increased in parallel with reductions in nkcc1 and cftr. Conversely, mummichogs transferred from FW to SW exhibited marked reductions in ncc2 concurrent with increases in nkcc1 and cftr. Immunohistochemical analyses employing a homologous antibody revealed apical Ncc2-immunoreactivity in Na⁺/K⁺-ATPase-immunoreactive ionocytes of FW-acclimated animals. Our combined observations suggest that Ncc2/ncc2-expressing ionocytes support the capacity of mummichogs to inhabit FW environments.

clc-2c is regulated by salinity, prolactin and extracellular osmolality in tilapia gill

Teleosts inhabiting fresh water (FW) depend upon ion-absorptive ionocytes to counteract diffusive ion losses to the external environment. A Clc Cl^- channel family member, Clc-2c, was identified as a conduit for basolateral Cl^- transport by Na⁺/Cl^- cotransporter 2 (Ncc2)-expressing ionocytes in stenohaline zebrafish (Danio rerio). It is unresolved whether Clc-2c/clc-2c is expressed in euryhaline species and how extrinsic and/or intrinsic factors modulate branchial clc-2c mRNA. Here, we investigated whether environmental salinity, prolactin (Prl) and osmotic conditions modulate clc-2c expression in euryhaline Mozambique tilapia (Oreochromis mossambicus). Branchial clc-2c and ncc2 mRNAs were enhanced in tilapia transferred from seawater (SW) to FW, whereas both mRNAs were attenuated upon transfer from FW to SW. Next, we injected hypophysectomized tilapia with ovine prolactin (oPrl) and observed a marked increase in clc-2c from saline-injected controls. To determine whether Prl regulates clc-2c in a gill-autonomous fashion, we incubated gill filaments in the presence of homologous tilapia Prls (tPrl₁₇₇ and tPrl₁₈₈). By 24 h, tPrl₁₈₈ stimulated clc-2c expression ~5-fold from controls. Finally, filaments incubated in media ranging from 280 to 450 mosmol/kg for 3 and 6 h revealed that extracellular osmolality exerts a local effect on clc-2c expression; clc-2c was diminished by hyperosmotic conditions (450 mosmol/kg) compared with isosmotic controls (330 mosmol/kg). Our collective results suggest that hormonal and osmotic control of branchial clc-2c contributes to the FW adaptability of Mozambique tilapia. Moreover, we identify for the first time a regulatory link between Prl and a Clc Cl^- channel in a vertebrate.

A Stenohaline Medaka, Oryzias woworae, Increases Expression of Gill Na(+), K(+)-ATPase and Na(+), K(+), 2Cl(-) Cotransporter 1 to Tolerate Osmotic Stress

The present study aimed to evaluate the osmoregulatory mechanism of Daisy's medaka, O. woworae,as well as demonstrate the major factors affecting the hypo-osmoregulatory characteristics of euryhaline and stenohaline medaka. The medaka phylogenetic tree indicates that Daisy's medaka belongs to the celebensis species group. The salinity tolerance of Daisy's medaka was assessed. Our findings revealed that 20‰ (hypertonic) saltwater (SW) was lethal to Daisy's medaka. However, 62.5% of individuals survived 10‰ (isotonic) SW with pre-acclimation to 5‰ SW for one week. This transfer regime, "Experimental (Exp.) 10‰ SW", was used in the following experiments. After 10‰ SW-transfer, the plasma osmolality of Daisy's medaka significantly increased. The protein abundance and distribution of branchial Na(+), K(+)-ATPase (NKA) and Na(+), K(+), 2Cl(-) cotransporter 1 (NKCC1) were also examined after transfer to 10‰ SW for one week. Gill NKA activity increased significantly after transfer to 10‰ SW. Meanwhile, elevation of gill NKA αα-subunit protein-abundance was found in the 10‰ SW-acclimated fish. In gill cross-sections, more and larger NKA-immunoreactive (NKA-IR) cells were observed in the Exp. 10‰ SW medaka. The relative abundance of branchial NKCC1 protein increased significantly after transfer to 10‰ SW. NKCC1 was distributed in the basolateral membrane of NKA-IR cells of the Exp. 10‰ SW group. Furthermore, a higher abundance of NKCC1 protein was found in the gill homogenates of the euryhaline medaka, O. dancena, than in that of the stenohaline medaka, O. woworae.

Nitric oxide inhibition of NaCl secretion in the opercular epithelium of seawater-acclimated killifish, Fundulus heteroclitus

Nitric oxide (NO) modulates epithelial ion transport pathways in mammals, but this remains largely unexamined in fish. We explored the involvement of NO in controlling NaCl secretion by the opercular epithelium of seawater killifish using an Ussing chamber approach. Pharmacological agents were used to explore the mechanism(s) triggering NO action. A modified Biotin-switch technique was used to investigate S-nitrosation of proteins. Stimulation of endogenous NO production via the nitric oxide synthase (NOS) substrate l-arginine (2.0 mmol l^-1), and addition of exogenous NO via the NO donor SNAP (10^-6 to 10^-4 mol l^-1), decreased the epithelial short-circuit current (I_sc). Inhibition of endogenous NO production by the NOS inhibitor l-NAME (10^-4 mol l^-1) increased I_sc and revealed a tonic control of ion transport by NO in unstimulated opercular epithelia. The NO scavenger PTIO (10^-5 mol l^-1) supressed the NO-mediated decrease in I_sc, and confirmed that the effect observed was elicited by release of NO. The effect of SNAP on I_sc was abolished by inhibitors of the soluble guanylyl cyclase (sGC), ODQ (10^-6 mol l^-1) and Methylene Blue (10^-4 mol l^-1), revealing NO signalling via the sGC/cGMP pathway. Incubation of opercular epithelium and gill tissues with SNAP (10^-4 mol l^-1) led to S-nitrosation of proteins, including Na⁺/K⁺-ATPase. Blocking of NOS with l-NAME (10^-6 mol l^-1) or scavenging of NO with PTIO during hypotonic shock suggested an involvement of NO in the hypotonic-mediated decrease in I_sc Yohimbine (10^-4 mol l^-1), an inhibitor of α₂-adrenoceptors, did not block NO effects, suggesting that NO is not involved in the α-adrenergic control of NaCl secretion.

Prolactin 177, prolactin 188, and extracellular osmolality independently regulate the gene expression of ion transport effectors in gill of Mozambique tilapia

This study characterized the local effects of extracellular osmolality and prolactin (PRL) on branchial ionoregulatory function of a euryhaline teleost, Mozambique tilapia (Oreochromis mossambicus). First, gill filaments were dissected from freshwater (FW)-acclimated tilapia and incubated in four different osmolalities, 280, 330, 380, and 450 mosmol/kg H2O. The mRNA expression of Na(+)/K(+)-ATPase α1a (NKA α1a) and Na(+)/Cl(-) cotransporter (NCC) showed higher expression with decreasing media osmolalities, while Na(+)/K(+)/2Cl(-) cotransporter 1a (NKCC1a) and PRL receptor 2 (PRLR2) mRNA levels were upregulated by increases in media osmolality. We then incubated gill filaments in media containing ovine PRL (oPRL) and native tilapia PRLs (tPRL177 and tPRL188). oPRL and the two native tPRLs showed concentration-dependent effects on NCC, NKAα1a, and PRLR1 expression; Na(+)/H(+) exchanger 3 (NHE3) expression was increased by 24 h of incubation with tPRLs. Immunohistochemical observation showed that oPRL and both tPRLs maintained a high density of NCC- and NKA-immunoreactive ionocytes in cultured filaments. Furthermore, we found that tPRL177 and tPRL188 differentially induce expression of these ion transporters, according to incubation time. Together, these results provide evidence that ionocytes of Mozambique tilapia may function as osmoreceptors, as well as directly respond to PRL to modulate branchial ionoregulatory functions.

In vivo and in vitro effects of high-K(+) stress on branchial expression of ROMKa in seawater-acclimated Mozambique tilapia

Recently, a teleost ortholog of renal outer medullary K(+) channel (ROMK) expressed in gill ionocytes (ROMKa) has emerged as a primary K(+)-excreting pathway in fish. However, the mechanisms by which ROMKa expression is regulated in response to perturbations of plasma K(+) levels are unknown. In this study, we aimed to identify potential links between the endocrine system and K(+) regulation in a euryhaline fish. We assessed time-course changes in multiple endocrine parameters, including plasma cortisol and gene expression of branchial glucocorticoid and mineralocorticoid receptors (GR1, GR2, and MR) and pituitary hormones, in seawater (SW)-acclimated Mozambique tilapia (Oreochromis mossambicus) exposed to high-K(+) (H-K) SW. Exposure to H-K SW elicited little effects on plasma cortisol or mRNA levels of GRs and pituitary hormones. Since plasma K(+) and branchial ROMKa expression was increased within 6h after H-K treatment in vivo, the effect of high K(+) was subsequently tested in a gill filament incubation experiment using media with differing K(+) concentrations. ROMKa mRNA levels were induced following incubation of filaments in H-K medium for 6h. The present study is the first to demonstrate that the expression of ROMKa in teleost ionocytes can respond to high K(+) conditions independent from systemic signaling.

The inner opercular membrane of the euryhaline teleost: a useful surrogate model for comparisons of different characteristics of ionocytes between seawater- and freshwater-acclimated medaka

The inner opercular membranes of the brackish medaka, Oryzias dancena, have numerous ionocytes, similar to the gill epithelia. By histological observation, this study demonstrated that it is possible to investigate the cellular morphology and function of ionocytes in the opercular membrane. The mitochondria-rich ionocytes in the opercular membranes were traced using rhodamine 123 and a cytochrome c oxidase IV antibody in vital and fixed situations, respectively. To validate different morphologies of seawater (SW)-type and freshwater (FW)-type ionocytes of the opercular membrane of euryhaline brackish medaka, a method of dual observation including immunofluorescence staining and subsequent scanning electron microscopy was used. The apical morphologies of SW- and FW-type ionocytes were hole and flat opening, respectively. In addition, the microvilli were found on the apical surface of the FW-type ionocytes. The SW-type ionocytes exhibited basolateral Na(+), K(+), 2Cl(-) cotransporter and the apical cystic fibrosis transmembrane conductance regulator. In contrast, in the apical region of FW-type ionocytes, Na(+), Cl(-) cotransporter and villin 1-like protein were expressed. In addition, histochemical staining of AgCl precipitation counterstained with a Na(+), K(+)-ATPase α-subunit antibody on the opercular membrane illustrated the role of Cl(-) secretion in the SW-type ionocytes of the brackish medaka. A combination of different observations in this study indicated that the opercular membrane could be a useful surrogate model for histological and functional studies on the epithelial ionocytes of fish gills.

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.

Functional genomics of physiological plasticity and local adaptation in killifish

Evolutionary solutions to the physiological challenges of life in highly variable habitats can span the continuum from evolution of a cosmopolitan plastic phenotype to the evolution of locally adapted phenotypes. Killifish (Fundulus sp.) have evolved both highly plastic and locally adapted phenotypes within different selective contexts, providing a comparative system in which to explore the genomic underpinnings of physiological plasticity and adaptive variation. Importantly, extensive variation exists among populations and species for tolerance to a variety of stressors, and we exploit this variation in comparative studies to yield insights into the genomic basis of evolved phenotypic variation. Notably, species of Fundulus occupy the continuum of osmotic habitats from freshwater to marine and populations within Fundulus heteroclitus span far greater variation in pollution tolerance than across all species of fish. Here, we explore how transcriptome regulation underpins extreme physiological plasticity on osmotic shock and how genomic and transcriptomic variation is associated with locally evolved pollution tolerance. We show that F. heteroclitus quickly acclimate to extreme osmotic shock by mounting a dramatic rapid transcriptomic response including an early crisis control phase followed by a tissue remodeling phase involving many regulatory pathways. We also show that convergent evolution of locally adapted pollution tolerance involves complex patterns of gene expression and genome sequence variation, which is confounded with body-weight dependence for some genes. Similarly, exploiting the natural phenotypic variation associated with other established and emerging model organisms is likely to greatly accelerate the pace of discovery of the genomic basis of phenotypic variation.

CFTR Cl- channel functional regulation by phosphorylation of focal adhesion kinase at tyrosine 407 in osmosensitive ion transporting mitochondria rich cells of euryhaline killifish

Cystic fibrosis transmembrane conductance regulator (CFTR) anion channels are the regulated exit pathway in Cl(-) secretion by teleost mitochondria rich salt secreting (MR) cells of the gill and opercular epithelia of euryhaline teleosts. By confocal light immunocytochemistry, immunogold transmission electron microscopy (TEM), and co-immunoprecipitation, using regular and phospho-antibodies directed against conserved sites, we found that killifish CFTR (kfCFTR) and the tyrosine kinase focal adhesion kinase (FAK) phosphorylated at Y407 (FAK pY407) are colocalized in the apical membrane and in subjacent membrane vesicles of MR cells. We showed previously that basolateral FAK pY407, unlike other FAK phosphorylation sites, is osmosensitive and dephosphorylates during hypotonic shock of epithelial cells (Marshall et al., 2008). In the present study, we found that hypotonic shock and the alpha(2)-adrenergic agonist clonidine (neither of which affects cAMP levels) rapidly and reversibly inhibit Cl(-) secretion by isolated opercular membranes, simultaneous with dephosphorylation of FAK pY407, located in the apical membrane. FAK pY407 is rephosphorylated and Cl(-) secretion rapidly restored by hypertonic shock as well as by forskolin and isoproterenol, which operate via cAMP and protein kinase A. We conclude that hormone mediated, cAMP dependent and osmotically mediated, cAMP independent pathways converge on a mechanism to activate CFTR and Cl(-) secretion, possibly through tyrosine phosphorylation of CFTR by FAK.

Swelling-activated transport of taurine in cultured gill cells of sea bass: physiological adaptation and pavement cell plasticity

We have investigated volume-activated taurine transport and ultrastructural swelling response of sea bass gill cells in culture, assuming that euryhaline fish may have developed particularly efficient mechanisms of salinity adaptation. In vivo, when sea basses were progressively transferred from seawater to freshwater, we noticed a decrease in blood osmotic pressure. When gill cells in culture were subjected to 30% hypotonic shock, we observed a five-fold stimulation of [3H]taurine efflux. This transport was reduced by various anion channel inhibitors with the following efficiency: 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) > niflumic acid > DIDS = diphenylamine-2-carboxylic acid. With polarized gill cells in culture, the hypotonic shock produced a five-fold stimulation of apical taurine transport, whereas basolateral exit was 25 times higher. Experiments using ionomycin, thapsigargin, BAPTA-AM, or removal of extracellular calcium suggested that taurine transport was regulated by external calcium. The inhibitory effects of lanthanum and streptomycin support Ca2+ entry through mechanosensitive Ca2+ channels. Branchial cells also showed hypotonically activated anionic currents sensitive to DIDS and NPPB. Similar pharmacology and time course suggested the potential existence of a common pathway for osmosensitive taurine and Cl− efflux through volume-sensitive organic osmolyte and anion channels. A three-dimensional structure study revealed that respiratory gill cells began to swell only 15 s after hypoosmotic shock. Apical microridges showed membrane outfoldings: the cell surface became smoother with a progressive disappearance of ridges. Therefore, osmotic swelling may not actually induce membrane stretch per se, inasmuch as the microridges may provide a reserve of surface area. This work demonstrates mechanisms of functional and morphological plasticity of branchial cells during osmotic stress.

Fish louseArgulus funduli(Crustacea: Branchiura) ectoparasites of the euryhaline teleost host,Fundulus heteroclitus, damage the ion-transport capacity of the opercular epithelium

Killifish ( Fundulus heteroclitus (L., 1766)) collected in the wild and kept in full-strength seawater were naturally parasitized by the ectoparasite Argulus funduli Krøyer, 1863, a copepod fish louse that creates inflamed skin lesions on the opercular epithelium and host gills. We assessed the damage done by lesions by counting the density of mitochondria-rich cells by fluorescence microscopy and by measuring Cl–secretion rate electrophysiologically using control (no lesions) and affected isolated opercular epithelia, often as paired left and right membranes from a single fish. Epithelia with lesions had a significantly reduced Cl–secretion rate, and in the lesions, the density of chloride cells was near zero. Contralateral membranes without lesions from infested fish had transport rates not significantly different from membranes taken from uninfected control animals, indicating no overcompensation on the contralateral membranes. Healthy control and infested animals were transferred to hypersaline conditions (twice seawater). Infested and control animals all survived transfer and had elevated plasma Na+and plasma osmolality. Infested animals failed to significantly elevate Imto the same level as healthy animals and there was a difference in hematocrit. Happily, the hypersaline challenge also resulted in detachment and death of adult A. funduli. We conclude that Argulus lesions impair salt transport in affected membranes but do not significantly affect survival on hypersaline challenge, and that hypersaline exposure is a successful treatment for A. funduli infestation in these strongly euryhaline teleosts.

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.

Unidirectional Na+ and Ca2+ fluxes in two euryhaline teleost fishes, Fundulus heteroclitus and Oncorhynchus mykiss, acutely submitted to a progressive salinity increase

Na(+) and Ca(2+) regulation were compared in two euryhaline species, killifish (normally estuarine-resident) and rainbow trout (normally freshwater-resident) during an incremental salinity increase. Whole-body unidirectional fluxes of Na(+) and Ca(2+), whole body Na(+) and Ca(2+), and plasma concentrations (trout only), were measured over 1-h periods throughout a total 6-h protocol of increasing salinity meant to simulate a natural tidal flow. Killifish exhibited significant increases in both Na(+) influx and efflux rates, with efflux slightly lagging behind efflux up to 60% SW, but net Na(+) balance was restored by the time killifish reached 100% SW. Whole body Na(+) did not change, in agreement with the capacity of this species to tolerate daily salinity fluctuations in its natural habitat. In contrast, rainbow trout experienced a dramatic increase in Na(+) influx (50-fold relative to FW values), but not Na(+) efflux between 40 and 60% SW, resulting in a large net loading of Na(+) at higher salinities (60-100% SW), and increases in plasma Na(+) and whole body Na(+) at 100% SW. Killifish were in negative Ca(2+) balance at all salinities, whereas trout were in positive Ca(2+) balance throughout. Ca(2+) influx rate increased two- to threefold in killifish at 80 and 100% SW, but there were no concomitant changes in Ca(2+) efflux. Ca(2+) flux rates were affected to a larger degree in trout, with twofold increases in Ca(2+) influx at 40% SW and sevenfold increases at 100% SW. Again, there was no change in Ca(2+) efflux with salinity, so plasma Ca(2+) concentration increased in 100% SW. As the killifish is regularly submitted to increased salinity in its natural environment, it is able to rapidly activate changes in unidirectional fluxes in order to ensure ionic homeostasis, in contrast to the trout.

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.

COX2 in a euryhaline teleost, Fundulus heteroclitus: primary sequence, distribution, localization, and potential function in gills during salinity acclimation

In the kidneys of mammals, cyclooxygenase type 2 (COX2) is expressed in medullary interstitial cells, the macula densa and epithelial cells of the cortical thick ascending limb where it generates prostaglandins that regulate hormone secretion, inhibit ion transport, and support cell survival during salt loading and dehydration. In teleosts, the gills are in direct contact with an aquatic environment and are the dominant site of osmoregulation. During transfers between salinities, specialized cells in the gills (chloride cells) rapidly regulate NaCl secretion for systemic osmoregulation while they simultaneously are exposed to acute osmotic shock. This study was conducted to determine if COX2 is expressed in the gills, and if so, to evaluate its function in cellular and systemic osmoregulation. Degenerate primers, reverse transcription–PCR and rapid amplification of cDNA ends were used to deduce the complete cDNA sequence of a putative COX2 enzyme from the gills of the euryhaline killifish (Fundulus heteroclitus). The 2738 base pair cDNA includes a coding region for a 610 amino acid protein that is over 70%identical to mammalian COX2. A purified antibody generated against a conserved region of mouse COX2 labeled chloride cells, suggesting that the enzyme may control NaCl secretion as an autocrine agent. Real-time PCR was then used to demonstrate that mRNA expression of the COX2 homologue was threefold greater in gills from chronic seawater killifish than in gills from chronic freshwater killifish. Expression of Na+/K+/2Cl–cotransporter and the cystic fibrosis transmembrane conductance regulator were also greater in seawater, suggesting that chronic COX2 expression in the gills is regulated in parallel to the key ion transporters that mediate NaCl secretion. Real-time PCR was also used to demonstrate that acute transfer from seawater to freshwater and from freshwater to seawater led to rapid, transient inductions of COX2 expression. Together with previous physiological evidence,the present molecular and immunological data suggest that constitutive branchial COX2 expression is enhanced in seawater, where prostaglandins can regulate NaCl secretion in chloride cells. Our data also suggest that branchial COX2 expression may play a role in cell survival during acute osmotic shock.

The Na+, K+, 2Cl- cotransporter of estuarine pufferfishes (Sphoeroides testudineus and S. greeleyi) in hypo- and hyper-regulation of plasma osmolality

The pufferfishes Sphoeroides testudineus and Sphoeroides greeleyi are estuarine species that osmoregulate efficiently, but S. testudineus tolerates seawater dilution to a much higher degree than S. greeleyi. This study aimed at testing whether NKCC is involved with their differential tolerance of seawater dilution, through the analysis of in vivo furosemide (NKCC inhibitor) injection both on hypo-regulation (in 35 per thousand salinity) and hyper-regulation (in 5 per thousand salinity). After exposure for 6 h or 5 days to both salinities, blood samples were obtained for determination of plasma osmolality, chloride, sodium and hematocrit, and muscle samples for determination of water content. Furosemide injection led to increased plasma osmolality and sodium in 35 per thousand and decreased osmolality and chloride in 5 per thousand, when compared to saline-injected controls. Furosemide injection led to hematocrit reduction in both salinities, and muscle water content increase in 5 per thousand and decrease in 35 per thousand in S. testudineus. The results are compatible with NKCC working in branchial NaCl secretion in 35 per thousand, in both species, and a higher role in cell volume regulation in blood and muscle cells of S. testudineus, in both salinities, which could partially explain the stronger capacity of S. testudineus to tolerate seawater dilution during low tide.

Energetics of osmoregulation: I. oxygen consumption byFundulus heteroclitus

We have developed a flow-through method for measuring oxygen consumption in fish which allows continuous monitoring over periods of days with good accuracy. Our goal was to determine the changes in basal metabolic rate in estuarine fish as a function of salinity. We show that in Fundulus heteroclitus, the oxygen consumption drops by 50% during the first 12 hr in the respirometer, as the fish cease exploratory movements. We have determined the influence of temperature and body size on resting respiratory rate, but failed to find any circadian or tidal rhythm in aerobic respiration. With these variables controlled, we determined that changing from 10 to 30 ppt water had no demonstrable effect on oxygen uptake. Since there must be a large change in osmotic flux due to this change in salinity, it appears that the fish might be diverting energy from other uses rather than increasing aerobic energy production to meet the increased osmoregulatory work load.

The effect of seawater composition and osmolality on hemolymph levels of methyl farnesoate in the green crab Carcinus maenas

Green crabs, Carcinus maenas, exposed to dilute seawater (e.g., 5 ppt salinity, approximately 150 mOsm/kg) have hemolymph levels of methyl farnesoate (MF) that are up to 10-fold higher than animals in isosmotic seawater (27 ppt, approximately 800 mOsm/kg). In this paper, we examine aspects of osmotic and ionic stress to identify factors involved in elevating MF levels. MF levels did not rise after exposure to concentrated seawater, so only hypoosmotic stress elevates MF. MF levels rose in animals exposed to dilute seawater containing mannitol to make it isosmotic, indicating that the hypoosmotic rise in MF is due to decreased ion concentrations. Individual ions were investigated by exposing crabs either to isosmotic seawater with low concentrations of an ion or to dilute seawater with high concentrations of an ion. Ca(2+) and Mg(2+) in combination affected MF levels. Finally, we found that the increase in MF levels was accelerated when hemolymph osmolality was precociously lowered by partially replacing hemolymph with deionized water prior to transferring animals to dilute seawater. Thus, the 6-8 h delay between exposing crabs to dilute sea water and observing an increase in MF appears to reflect the time needed for specific hemolymph ions to decrease below a threshold concentration.

Aquaporin-3 expressed in the basolateral membrane of gill chloride cells in Mozambique tilapia Oreochromis mossambicus adapted to freshwater and seawater

We have cloned a homologue of mammalian aquaporin-3 (AQP3) from gills of Mozambique tilapia using a reverse transcription-polymerase chain reaction (RT-PCR). The deduced amino acid sequence shared 64-75% homology with other vertebrate AQP3 homologues. RT-PCR revealed that tilapia AQP3 was expressed in the brain, pituitary, kidney, spleen, intestine, skin, eye and gill in tilapia adapted to freshwater (FW) and seawater (SW). We also examined functional characteristics of tilapia AQP3 using Xenopus oocytes as an in vitro transcribed cRNA expression system. Osmotic water permeability (Pf) of Xenopus oocytes expressing tilapia AQP3 was about 30-fold higher than that of control oocytes, and was 80% inhibited by treatment with 0.3 mmol l(-1) HgCl2. Light-microscopic immunocytochemistry of branchial epithelia revealed that tilapia AQP3 was expressed in gill chloride cells of FW- and SW-adapted tilapia. Electron-microscopic immunocytochemistry further demonstrated that tilapia AQP3 was localized in the basolateral membrane of gill chloride cells. Basolateral localization of AQP3 in gill chloride cells suggests that AQP3 is involved in regulatory volume changes and osmoreception, which could trigger functional differentiation of chloride cells.

Gene expression after freshwater transfer in gills and opercular epithelia of killifish: insight into divergent mechanisms of ion transport

We have explored the molecular basis for differences in physiological function between the gills and opercular epithelium of the euryhaline killifish Fundulus heteroclitus. These tissues are functionally similar in seawater, but in freshwater the gills actively absorb Na+ but not Cl-, whereas the opercular epithelium actively absorbs Cl- but not Na+. These differences in freshwater physiology are likely due to differences in absolute levels of gene expression (measured using real-time PCR), as several proteins important for Na+ transport, namely Na+,H+-exchanger 2 (NHE2), carbonic anhydrase 2 (CA2), Na+,HCO3- cotransporter 1, and V-type H+-ATPase, were expressed at 3- to over 30-fold higher absolute levels in the gills. In gills, transfer from 10% seawater to freshwater increased the activity of Na+,K+-ATPase by twofold (from 12 h to 7 days), increased the expression of NHE2 (at 12 h) and CA2 (from 12 h to 7 days), and decreased the expression of NHE3 (from 12 h to 3 days). In opercular epithelium, NHE2 was not expressed; furthermore, Na+,K+-ATPase activity was unchanged after transfer to freshwater, CA2 mRNA levels decreased, and NHE3 levels increased. Consistent with their functional similarities in seawater, killifish gills and opercular epithelium expressed Na+,K+-ATPase alpha 1a, Na+,K+,2Cl- cotransporter 1 (NKCC1), cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel and the signalling protein 14-3-3a at similar absolute levels. Furthermore, NKCC1 and CFTR were suppressed equally in each tissue after freshwater transfer, and 14-3-3a mRNA increased in both. These results provide insight into the mechanisms of ion transport by killifish gills and opercular epithelia, and demonstrate a potential molecular basis for the differences in physiological function between these two organs.

Modulation of tyramine signaling by osmolality in an insect secretory epithelium

The control of water balance in multicellular organisms depends on absorptive and secretory processes across epithelia. This study concerns the effects of osmolality on the function of the Malpighian tubules (MTs), a major component of the insect excretory system. Previous work has shown that the biogenic amine tyramine increases transepithelial chloride conductance and urine secretion in Drosophila MTs. This study demonstrates that the response of MTs to tyramine, as measured by the depolarization of the transepithelial potential (TEP), is modulated by the osmolality of the surrounding medium. An increase in osmolality caused decreased tyramine sensitivity, whereas a decrease in osmolality resulted in increased tyramine sensitivity; changes in osmolality of +/-20% resulted in a nearly 10-fold modulation of the response to 10 nM tyramine. The activity of another diuretic agent, leucokinin, was similarly sensitive to osmolality, suggesting that the modulation occurs downstream of the tyramine receptor. In response to continuous tyramine signaling, as likely occurs in vivo, the TEP oscillates, and an increase in osmolality lengthened the period of these oscillations. Increased osmolality also caused a decrease in the rate of urine production; this decrease was attenuated by the tyraminergic antagonist yohimbine. A model is proposed in which this modulation of tyramine signaling enhances the conservation of body water during dehydration stress. The modulation of ligand signaling is a novel effect of osmolality and may be a widespread mechanism through which epithelia respond to changes in their environment.

Intraspecific variation in gene expression after seawater transfer in gills of the euryhaline killifish Fundulus heteroclitus

Previous research has suggested that northern populations of the euryhaline killifish (Fundulus heteroclitus) are better adapted to freshwater environments than their southern counterparts. In this study, we examined whether this adaptation has come at an ionoregulatory cost in seawater, by comparing published data for northern killifish to newly acquired data on the molecular responses of southern killifish to seawater transfer. After abrupt transfer from brackish water (10 per thousand) to seawater, Na,K-ATPase activity, Na,K-ATPase alpha(1a) mRNA expression, and NKCC1 mRNA expression increased 1 and 4 days after transfer in the gills of southern fish (by 2-3-fold), but increased at 1 day and not 4 days after transfer in northern fish. Small increases in mRNA expression were observed in both populations at 14 days. CFTR expression also increased in southern and northern fish at 1 and 4 days into seawater, and was also elevated at 14 days in northern fish. Because fish from both southern and northern populations maintained plasma Na(+) and Cl(-) balance after seawater transfer, the differences in activity and expression could not have been caused by differences in plasma ion levels. Instead, some other regulatory factor may account for the differences in expression between populations. This study shows that freshwater adaptation in northern populations of killifish has not necessarily come at a significant ionoregulatory cost in seawater, but has altered the molecular responses of their gills to seawater transfer compared to southern killifish.

Hypotonic shock mediation by p38 MAPK, JNK, PKC, FAK, OSR1 and SPAK in osmosensing chloride secreting cells of killifish opercular epithelium

Hypotonic shock rapidly inhibits Cl(-) secretion by chloride cells, an effect that is osmotic and not produced by NaCl-depleted isosmotic solutions, yet the mechanism for the inhibition and its recovery are not known. We exposed isolated opercular epithelia, mounted in Ussing chambers, to hypotonic shock in the presence of a variety of chemicals: a general protein kinase C (PKC) inhibitor chelerythrine, Gö6976 that selectively blocks PKC alpha and beta subtypes, H-89 that blocks PKA, SB203580 that blocks p38 mitogen-activated protein kinase (MAPK), as well as serine/threonine protein phosphatase (PP1 and 2A) inhibitor okadaic acid, and finally tamoxifen, a blocker of volume-activated anion channels (VSOAC). Chelerythrine has no effect on hypotonic inhibition but blocked the recovery, indicating PKC involvement in stimulation. Gö6976 had little effect, suggesting that PKC alpha and PKC beta subtypes are not involved. H-89 did not block hypotonic inhibition but decreased the recovery, indicating PKA may be involved in the recovery and overshoot (after restoration of isotonic conditions). SB203580 significantly enhanced the decrease in current by hypotonic shock, suggesting an inhibitory role of p38 MAPK in the hypotonic inhibition. Okadaic acid increased the steady state current, slowed the hypotonic inhibition but made the decrease in current larger; also the recovery and overshoot were completely blocked. Hypotonic stress rapidly and transiently increased phosphorylated p38 MAPK (pp38) MAPK (measured by western analysis) by eightfold at 5 min, then more slowly again to sevenfold at 60 min. Hypertonic shock slowly increased p38 by sevenfold at 60 min. Phosphorylated JNK kinase was increased by 40-50% by both hypotonic and hypertonic shock and was still elevated at 30 min in hypertonic medium. By immunoblot analysis it was found that the stress protein kinase (SPAK) and oxidation stress response kinase 1 (OSR1) were present in salt and freshwater acclimated fish with higher expression in freshwater. By immunocytochemistry, SPAK, OSR1 and phosphorylated focal adhesion kinase (pFAK) were colocalized with NKCC at the basolateral membrane. The protein tyrosine kinase inhibitor genistein (100 micromol l(-1)) inhibited Cl(-) secretion that was high, increased Cl(-) secretion that was low and reduced immunocytochemical staining for phosphorylated FAK. We present a model for rapid control of CFTR and NKCC in chloride cells that includes: (1) activation of NKCC and CFTR via cAMP/PKA, (2) activation of NKCC by PKC, myosin light chain kinase (MLCK), p38, OSR1 and SPAK, (3) deactivation of NKCC by hypotonic cell swelling, Ca(2+) and an as yet unidentified protein phosphatase and (4) involvement of protein tyrosine kinase (PTK) acting on FAK to set levels of NKCC activity.

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

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

Intraspecific divergence of ionoregulatory physiology in the euryhaline teleostFundulus heteroclitus: possible mechanisms of freshwater adaptation

We examined intraspecific variation in ionoregulatory physiology within euryhaline killifish, Fundulus heteroclitus, to understand possible mechanisms of freshwater adaptation in fish. Pronounced differences in freshwater tolerance existed between northern (2% mortality) and southern (19%mortality) killifish populations after transfer from brackish water (10 g l-1) to freshwater. Differences in Na+ regulation between each population might partially account for this difference in tolerance, because plasma Na+ was decreased for a longer period in southern survivors than in northerns. Furthermore, northern fish increased Na+/K+-ATPase mRNA expression and activity in their gills to a greater extent 1-14 days after transfer than did southerns, which preceded higher whole-body net flux and unidirectional influx of Na+ at 14 days. All observed differences in Na+regulation were small, however, and probably cannot account for the large differences in mortality. Differences in Cl- regulation also existed between populations. Plasma Cl- was maintained in northern fish, but in southerns, plasma Cl- decreased rapidly and remained low for the duration of the experiment. Correspondingly, net Cl-loss from southern fish remained high after transfer, while northerns eliminated Cl- loss altogether. Elevated Cl- loss from southern fish in freshwater was possibly due to a persistence of seawater gill morphology, as paracellular permeability (indicated by extrarenal clearance rate of PEG-4000) and apical crypt density in the gills (detected using scanning electron microscopy) were both higher than in northern fish. These large differences in the regulation of Cl- balance probably contributed to the marked differences in mortality after freshwater transfer. Glomerular filtration rate and urination frequency were also lower in southerns. Taken together, these data suggest that northern killifish are better adapted to freshwater environments and that minimizing Cl-imbalance appears to be the key physiological difference accounting for their greater freshwater tolerance.

Changes in gene expression in gills of the euryhaline killifish Fundulus heteroclitus after abrupt salinity transfer

Maintenance of ion balance requires that ionoregulatory epithelia modulate ion flux in response to internal or environmental osmotic challenges. We have explored the basis of this functional plasticity in the gills of the euryhaline killifish Fundulus heteroclitus. The expression patterns of several genes encoding ion transport proteins were quantified after transfer from near-isosmotic brackish water [10 parts/thousand (ppt)] to either freshwater (FW) or seawater (SW). Many changes in response to SW transfer were transient. Increased mRNA expression occurred 1 day after transfer for Na(+)-K(+)-ATPase-alpha(1a) (3-fold), Na(+)-K(+)-2Cl(-)-cotransporter 1 (NKCC1) (3-fold), and glucocorticoid receptor (1.3-fold) and was paralleled by elevated Na(+)-K(+)-ATPase activity (2-fold). The transient increase in NKCC1 mRNA expression was followed by a later 2-fold rise in NKCC protein abundance. In contrast to the other genes studied in the present work, mRNA expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel generally remained elevated (2-fold) in SW. No change in protein abundance was detected, however, suggesting posttranscriptional regulation. The responses to FW transfer were quite different from those to SW transfer. In particular, FW transfer increased Na(+)-K(+)-ATPase-alpha(1a) mRNA expression and Na(+)-K(+)-ATPase activity to a greater extent than did SW transfer but had no effect on V-type H(+)-ATPase expression, supporting the current suggestion that killifish gills transport Na(+) via Na(+)/H(+) exchange. These findings demonstrate unique patterns of ion transporter expression in killifish gills after salinity transfer and illustrate important mechanisms of functional plasticity in ion-transporting epithelia.

Rapid regulation of NaCl secretion by estuarine teleost fish: coping strategies for short-duration freshwater exposures

This review summarizes the mechanism of Cl(-) active secretion and its regulation in estuarine teleost fish. Small estuarine fish such as the killifish, Fundulus heteroclitus, forage in shallow water following advancing tides and are exposed regularly to very dilute microenvironments. Using the killifish opercular epithelium and related teleost membranes containing mitochondria-rich cells, the regulation includes a reduction of active Cl(-) secretion and passive diffusive ion loss in a three-stage process spanning approximately 30 min. There is a combination of sympathetic neural reflex mediated by alpha(2)-adrenoceptors operating via intracellular inositol tris phosphate and intracellular Ca(2+) and a cellular hypotonic shock response, followed by covering over of ion-secreting cells by pavement cells. This effectively minimizes salt loss in dilute media. The upregulation of salt secretion on return to full strength seawater may be via hormones (arginine vasotocin and urotensin I) and neurotransmitter (vasoactive intestinal polypeptide) in combination with hypertonic shock. A hypothetical model includes involvement of protein kinase A and C and protein phosphatases 1 and 2A in regulation of the NKCC1 cotransporter on the basolateral side and protein kinase A regulation of the CFTR-like apical anion channel.

Na+ versus Cl- transport in the intact killifish after rapid salinity transfer

Much of the early research elucidating the general mechanisms of euryhalinity was performed on the common killifish. More recently, its opercular epithelium with abundant mitochondria-rich cells has proven to be a powerful model for analyzing the mechanisms of active NaCl transport under Ussing conditions in vitro (i.e., with isotonic saline on both surfaces, at short-circuit). However, it is unclear whether this preparation duplicates the gill under real world conditions-i.e., at open-circuit, with real seawater (SW) or freshwater (FW) on the mucosal surface. There have been only limited studies, mostly about 35 years ago, on ion transport in the intact killifish. Therefore, using radioisotopes (22Na, 36Cl), we developed and evaluated methods for the independent measurement of unidirectional Na(+) and Cl(-) influx and efflux rates and internal pools in intact killifish acclimated to 10% SW and abruptly transferred to either 100% SW or FW. Internal Na(+) pools were disturbed less than internal Cl(-) pools by transfer, and were corrected after 3 days in 100% SW or 7 days in FW. Influx and efflux rates in 10% SW were about 3000 micromol kg(-1) h(-1) and increased to 15,000-18,000 micromol kg(-1) h(-1) after transfer to 100% SW, remaining approximately equal and equimolar for Na(+) and Cl(-), and stable from 0.5 to 7 days post-transfer. After transfer to FW, Na(+) influx and efflux rates dropped to 1000-1500 micromol kg(-1) h(-1), with efflux slightly exceeding influx, and remained approximately stable from 0.5 to 7 days. However, while Cl(-) efflux responded similarly, Cl(-) influx rate dropped immediately to negligible values (20-50 micromol kg(-1) h(-1)) without recovery through 7 days. These results differ from early ion transport data in 100% SW, and demonstrate that fluxes stabilize quickly after salinity transfer. They also show that the intact animal responds more quickly than the epithelium, provide qualitative but not quantitative support for the opercular epithelium as a model for the gill under real world SW conditions, and no support for its use as a gill model under real world FW conditions, where branchial Cl(-) uptake is negligible.

NaCl transport across the opercular epithelium ofFundulus heteroclitusis inhibited by an endothelin to NO, superoxide, and prostanoid signaling axis

Recent evidence suggests that paracrine signaling agents, such as endothelin (ET), nitric oxide (NO), superoxide (O2-), and prostanoids can modulate mammalian renal function by affecting both hemodynamic and epithelial ionic transport pathways. Since these signaling pathways have been described in fish blood vessels, we hypothesized that they may control salt transport across the gill epithelium—the primary site of ion excretion in marine teleost fishes. We found that ET, the NO donors sodium nitroprusside and spermine NONOate, and the prostanoid PGE2each can produce a concentration-dependent reduction in the short circuit current ( Isc) across the isolated opercular epithelium of the killifish ( Fundulus heteroclitus), the generally accepted model for the marine teleost gill epithelium. Sarafotoxin S6c was equipotent to ET-1, suggesting that ETBreceptors are involved. Incubation with NG-nitro-l-arginine methyl ester (l-NAME) or indomethacin reduced the effect of subsequent addition of SRXS6c by 17 and 89%, respectively, suggesting the presence of an ET to NO and PGE axis. The effects of l-NAME and indomethacin were not additive, but the superoxide dismutase mimetic 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL) reduced the effect of SRXS6c by 34% and preincubation with l-NAME, indomethacin, and TEMPOL reduced the SRXS6c response to zero. This suggests a direct role for O2-in this axis. COX-2 appears to be the major enzyme involved in this axis because the specific COX-2 inhibitor NS-398 was twice as effective as the COX-1 inhibitor SC560 in inhibiting the SRXS6c effect. The Iscwas stimulated by the EP2agonist butaprost and inhibited by the EP1,3agonist sulprostone, suggesting both stimulatory and inhibitory PGE receptors in this tissue. Carbaprostacyclin (PGI2analog), thromboxane A2, PGF2α, and PGD2did not affect the Isc. Our data are the first to suggest the importance of an ET-stimulated and NO-, O2--, and PGE2-mediated signaling axis that can modify active extrusion of NaCl across the killifish opercular epithelium and, by inference, the marine teleost gill epithelium.

Ion transport in the intestine of Gobius niger in both isotonic and hypotonic conditions

Ion transport in the intestine of Gobius niger, a euryhaline teleost, was studied in both isotonic and hypotonic conditions. Isolated tissues, mounted in Ussing chambers and bilaterally perfused with isotonic Ringer solution, developed a serosa negative transepithelial voltage and a short circuit current indicating a net negative current in absorptive direction. Bilateral removal of Cl- and Na+ from the bathing solutions as well as the luminal removal of K+in the presence of Ba2+(10(-3) M) almost abolished both Vt and Isc. Similar results were obtained by adding bumetanide (10(-5)M) to the luminal bath while other inhibitors of Cl- transport mechanisms were ineffective. These observations suggest that salt absorption begins with a coupled entry of Na+, Cl-, and K+ across the apical membrane; a Ba2+inhibitable K+ conductance, demonstrated also by micropuncture experiments, recycles the ion into the lumen. Salt entry into the cell is driven by the operation of the basolateral Na+/K(+)-ATPase since serosal ouabain (10(-4)M) completely abolished both Vt and Isc; this pump also completes the Na(+) absorption. The inhibitory effect of both serosal bumetanide (10(-4)M) and SITS (5 x 10(-4)M) suggests that Cl- would leave the cell via the KCl cotransport, the Cl/HCO3- antiport and/or conductive pathways. Bilateral exposure of tissues to hypotonic media produced a reduction of both the transepithelial voltage and the short circuit current probably due to the activation of homeostatic ionic fluxes involved in cell volume regulation. The results of experiments with both isolated enterocytes and intestine exposed to hypotonic solution suggested that the recovery of cell volume, after the initial cell swelling, involves a parallel opening of K+ and Cl- channels to facilitate net solute and water effluxes from the cell. J. Exp. Zool. 301A:49-62, 2004.

The bioelectric field of the catfish Ictalurus nebulosus

The variability of the bioelectric field of the electrosensitive catfish, Ictalurus nebulosus, was investigated by recording the potential variation occurring when the fish passed a stationary electrode, and by recording the field of a stationary fish by a 15-electrode array. A good first order approximation of the recorded field of a 20 cm long fish is a dipole dc source with the source and sink about 7 cm apart, carrying a current of about 1 microA in water with a specific resistivity of 3.3 kohm cm. At 5 cm distance from the dipole axis such a source generates an electric potential swing in the order of 50 microV in free space, head negative, tail positive. Superimposed on the basic component are respiration related fluctuations, and fluctuations related to the activity of the alimentary canal, gills, and skin. Novel stimuli, or stressors like investigators approaching the aquarium, evoke sudden increases in field strength. which last about 15 min. Demineralization of the aquarium water causes changes in field strength and reversal of field polarity. The administration of food causes field variations in the vicinity of the anal opening. The bioelectric field shows diurnal fluctuations of 100 microV. The peak is at about 04:00, the dip at 14:00. The fluctuations of the bioelectric field are sufficiently strong and specilic to serve as electrical stimuli to other electrosensitive catfish. It is suggested that the field changes allow a simple form of electrocommunication. i.e. inform conspecifics about some physiological properties of the field source. The cellular mechanisms underlying the fluctuations of the bioelectric field are homeostatic processes mediated by ion pumps and ion channels.

Control of Cl- transport in the operculum epithelium of Fundulus heteroclitus: long- and short-term salinity adaptation

The eurohaline fish, Fundulus heteroclitus, adapts rapidly to enhanced salinity by increasing the ion secretion by gill chloride cells. An increase of approximately 70 mOsm in plasma osmolarity was previously found during the transition. To mimic this in vitro, isolated opercular epithelia of seawater-adapted Fundulus mounted in a modified Ussing chamber were exposed to an increase in NaCl and/or osmolarity on the basolateral side, which immediately increased I(SC). Various Cl(-) channel blockers as well as the K(+) channel blocker Ba(2+) added to the basolateral side all inhibited the steady-state as well as the hypertonic stimulation of I(SC). The exists -agonist isoproterenol stimulates I(SC) in standard Ringer solutions. In contrast, when cell volume was kept at the larger value by simultaneous addition of water, the stimulation with isoproterenol was abolished, suggesting that the key process for activation of the Na(+), K(+), 2Cl(-) cotransporter is cell shrinkage. The protein kinase C (PKC) inhibitor chelerythrine and the myosin light chain kinase (MLCK) inhibitor ML-7 had strong inhibitory effects on the mannitol activation of I(SC), thus both MLCK and PKC are involved. The two specific protein kinase A (PKA) inhibitors H-89 and KT 5720 had no effect after mannitol addition whereas isoproterenol stimulation was completely blocked by H-89. This indicates that PKA is involved in the activation of the apical Cl(-) channel via c-AMP whereas the shrinkage activation of the Na(+), K(+), 2Cl(-) cotransporter is independent of PKA activation. The steady-state Cl(-) secretion was stimulated by an inhibitor of serine/threonine phosphatases of the PP-1 and PP-2A type and inhibited by a PKC inhibitor but not by a PKA inhibitor. Thus, it seems to be determined by continuous phosphorylation and dephosphorylation involving PKC but not PKA. The steady-state Cl(-) secretion and the maximal obtainable Cl(-) secretion were measured in freshwater-adapted fish and in fish retransferred to saltwater. No I(SC) could be measured in freshwater-adapted fish or in the fish within the first 18 h after transfer to saltwater. As evidenced from Western blot analysis using antiserine-antibodies, a heavily serine phosphorylated protein of about 190 kDa was consistently observed in the saltwater-acclimated fish, but was only weakly present in freshwater-acclimated fish. This observation indicates that acclimatization to saltwater stimulates the expression of this 190-kDa protein and/or a serine/threonine kinase, which subsequently phosphorylates the protein.

Cystic fibrosis transmembrane conductance regulator in teleost fish

The gills and intestinal epithelia of teleost fish express cystic fibrosis transmembrane conductance regulator (CFTR), and utilize this low conductance anion channel in the apical membrane for ion secretion in seawater gill and in the basolateral membrane for ion absorption in freshwater gill. Similarly, in the intestine CFTR is present in the basolateral membrane for intestinal absorption and also in the apical membrane of secreting intestine. The expression of CFTR and the directed trafficking of the protein to the apical or basolateral membrane is salinity-dependent. The CFTR gene has been cloned and sequenced from several teleost species and although all the major elements in the human gene are present, including two nucleotide binding domains that are common to all ATP binding cassette (ABC) transporters, the sequences are divergent compared to shark or human. In euryhaline fish adapting to seawater, CFTR, localized immunocytochemically, redistributes slowly from a basolateral location to the apical membrane while ion secretory capacity increases. The facility with which teleosts regulate CFTR expression and activation during salinity adaptation make this system an appealing model for the expression and trafficking operation of this labile gene product.

Na(+), Cl(-), Ca(2+) and Zn(2+) transport by fish gills: retrospective review and prospective synthesis

The secondary active Cl(-) secretion in seawater (SW) teleost fish gills and elasmobranch rectal gland involves basolateral Na(+),K(+)-ATPase and NKCC, apical membrane CFTR anion channels, and a paracellular Na(+)-selective conductance. In freshwater (FW) teleost gill, the mechanism of NaCl uptake is more controversial and involves apical V-type H(+)-ATPase linked to an apical Na(+) channel, apical Cl(-)-HCO-3 exchange and basolateral Na(+),K(+)-ATPase. Ca(2+) uptake (in FW and SW) is via Ca(2+) channels in the apical membrane and Ca(2+)-ATPase in the basolateral membrane. Mainly this transport occurs in mitochondria rich (MR) chloride cells, but there is a role for the pavement cells also. Future research will likely expand in two major directions, molded by methodology: first in physiological genomics of all the transporters, including their expression, trafficking, operation, and regulation at the molecular level, and second in biotelemetry to examine multivariable components in behavioral physiological ecology, thus widening the integration of physiology from the molecular to the environmental levels while deepening understanding at all levels.

In Vitro Effects of Environmental Salinity and Cortisol on Chloride Cell Differentiation in Embryos of Mozambique Tilapia, OREOCHROMIS MOSSAMBICUS, Measured Using a Newly Developed ‘Yolk-Ball’ Incubation System

To examine the functional differentiation of chloride cells in the yolk-sac membrane of tilapia (Oreochromis mossambicus) embryos, we developed a ‘yolk-ball’ incubation system in which the yolk sac was separated from the embryonic body and subjected to incubation in vitro. The yolk-ball preparation consists of the yolk and the covering yolk-sac membrane, which contains a rich population of chloride cells. After appropriate cutting, the incision on the yolk ball healed during incubation in balanced salt solution for 3h, so that the yolk-sac membrane completely enclosed the yolk. Yolk balls prepared from freshwater-acclimated embryos were transferred either to fresh water or to sea water and incubated for 48 and 96h to elucidate the morphological changes in the chloride cells in response to environmental salinity. The chloride cells in the yolk-sac membrane were larger in sea water than in fresh water. In yolk balls transferred to sea water, chloride cells often formed multicellular complexes characteristic of seawater-type chloride cells. In those transferred to fresh water, however, the cells were small and rarely formed such complexes. These responses of chloride cells were identical to those observed in intact embryos. Thus, chloride cells in the yolk-sac membrane could differentiate into the seawater type independent of the embryonic body. To examine the possible effects of exogenous cortisol on chloride cell differentiation, the yolk balls were incubated for 48h in fresh water or sea water containing different doses of cortisol (0.1–10μgml−1). Although chloride cells were consistently larger in sea water than in fresh water in all experimental groups, cortisol administration had no effect on chloride cell surface area in either medium. These findings indicate that the chloride cells in the yolk-sac membrane are equipped with an autonomous mechanism of functional differentiation that is independent of the embryonic endocrine and nervous systems. The yolk-ball incubation system established here is an excellent experimental model for further studies on chloride cell differentiation and function.

Dynamics of Pavement Cell–Chloride Cell Interactions During Abrupt Salinity Change in FUNDULUS HETEROCLITUS

Freshwater-adapted killifish (Fundulus heteroclitus) opercular epithelia were dissected and subjected to blood-side hypertonic bathing solution in Ussing-style chambers to simulate the increase in blood osmolality during migration to sea water. Conversely, seawater-acclimated killifish opercular epithelia were subjected to hypotonic bathing solutions to simulate the initial stages of migration to fresh water. Freshwater-acclimation (hypertonic stress) induced a rapid (approximately 30min) increase in membrane conductance (Gt) from 3.10±0.56 to 7.52±1.15mScm−2 (P<0.01, N=27), whereas seawater-acclimation (hypotonic stress) induced a rapid decrease in Gt from 8.22±1.15 to 4.41±1.00mScm−2 (P<0.01, N=27; means ± s.e.m.). Control seawater-acclimated membranes had a density of apical crypts (where chloride cells are exposed to the environment; detected by scanning electron microscopy) of 1133±96.4cryptsmm−2 (N=12), whereas the hypotonically shocked specimens had a lower crypt density of 870±36.7cryptsmm−2 (P<0.01 N=10; means ± s.e.m.). Hypertonic shock of freshwater membranes increased crypt density from 383.3±73.9 (N=12) to 630±102.9cryptsmm−2 (P<0.05; N=11; means ± s.e.m.). There was no change in density of chloride cells, as detected by fluorescence microscopy; hence, osmotic stress changes the degree of exposure, not the number of chloride cells. Cytochalasin D (5.0μmoll−1) completely blocked the conductance response to hypotonic shock and the reduction in apical crypt density measured by scanning electron microscopy, while phalloidin (33μmoll−1), colchicine (3×10−4moll−1) and griseofulvin (1.0μmoll−1) were ineffective. Actin imaging by phalloidin staining and confocal microscopy revealed extensive actin cords in pavement cell microridges and a ring of actin at the apex of chloride cells. We conclude that the actin cytoskeleton of chloride cells is required to maintain crypt opening and that osmotic shock causes chloride cells to adjust their apical crypt size.