Permeability and morphology of a cultured branchial epithelium from the rainbow trout during prolonged apical exposure to fresh water

Investigations to extend viability of a rainbow trout primary gill cell culture

The primary culture of fish gill cells can provide functional, cell diverse, model in vitro platforms able to tolerate an aqueous exposure analogous to in vivo tissues. The utility of such models could be extended to a variety of longer term exposure scenarios if a method could be established to extend culture viability when exposed to water for longer periods. Here we report findings of a series of experiments to establish increased longevity, as monitored by culture transepithelial electrical resistance (TEER) and concurrent histological developments. Experimental cultures improved TEER during apical freshwater exposure for a mean of twelve days, compared to previous viabilities of up to 3 days. Cultures with larger surface areas and the use of trout serum rather than foetal bovine serum (FBS) contributed to the improvement, while perfusion of the intact gill prior to cell harvest resulted in a significantly faster preparation. Detailed scanning electron microscopy analysis of cultures revealed diverse surface structures that changed with culture age. Cultures grown on membranes with an increased porosity, collagen coating or 3D structure were of no benefit compared to standard membranes. Increased culture longevity, achieved in this study and reported for the first time, is a significant breakthrough and opens up a variety of future experimentation that has previously not been possible. The extended viability facilitates exploration of in vitro chronic or pulse-exposure test paradigms, longer term physiological and environmental monitoring studies and the potential for interactive co-culture with other organoid micro-tissues.

Gill cell culture systems as models for aquatic environmental monitoring

A vast number of chemicals require environmental safety assessments for market authorisation. To ensure acceptable water quality, effluents and natural waters are monitored for their potential harmful effects. Tests for market authorisation and environmental monitoring usually involve the use of large numbers of organisms and, for ethical, cost and logistic reasons, there is a drive to develop alternative methods that can predict toxicity to fish without the need to expose any animals. There is therefore a great interest in the potential to use cultured fish cells in chemical toxicity testing. This review summarises the advances made in the area and focuses in particular on a system of cultured fish gill cells grown into an epithelium that permits direct treatment with water samples.

Claudin 28b and F-actin are involved in rainbow trout gill pavement cell tight junction remodeling under osmotic stress

Permeability of rainbow trout gill pavement cells cultured on permeable supports (single seeded inserts) changes upon exposure to freshwater or treatment with cortisol. The molecular components of this change are largely unknown, but tight junctions that regulate the paracellular pathway are prime candidates in this adaptational process. Using differential display polymerase chain reaction we found a set of 17 differentially regulated genes in trout pavement cells that had been exposed to freshwater apically for 24 h. Five genes were related to the cell-cell contact. One of these genes was isolated and identified as encoding claudin 28b, an integral component of the tight junction. Immunohistochemical reactivity to claudin 28b protein was concentrated in a circumferential ring colocalized to the cortical F-actin ring. To study the contribution of this isoform to changes in transepithelial resistance and Phenol Red diffusion under apical hypo-or hyperosmotic exposure we quantified the fluorescence signal of this claudin isoform in immunohistochemical stainings together with the fluorescence of phalloidin-probed F-actin. Upon hypo-osmotic stress claudin 28b fluorescence and epithelial tightness remained stable. Under hyperosmotic stress, the presence of claudin 28b at the junction significantly decreased, and epithelial tightness was severely reduced. Cortical F-actin fluorescence increased upon hypo-osmotic stress, whereas hyperosmotic stress led to a separation of cortical F-actin rings and the number of apical crypt-like pores increased. Addition of cortisol to the basolateral medium attenuated cortical F-actin separation and pore formation during hyperosmotic stress and reduced claudin 28b in junctions except after recovery of cells from exposure to freshwater. Our results showed that short-term salinity stress response in cultured trout gill cells was dependent on a dynamic remodeling of tight junctions, which involves claudin 28b and the supporting F-actin ring.

Ammonia transport in cultured gill epithelium of freshwater rainbow trout: the importance of Rhesus glycoproteins and the presence of an apical Na+/NH4+ exchange complex

The mechanisms of ammonia excretion at fish gills have been studied for decades but details remain unclear, with continuing debate on the relative importance of non-ionic NH(3) or ionic NH(4)(+) permeation by various mechanisms. The presence of an apical Na(+)/NH(4)(+) exchanger has also been controversial. The present study utilized an in vitro cultured gill epithelium (double seeded insert, DSI) of freshwater rainbow trout as a model to investigate these issues. The relationship between basolateral ammonia concentration and efflux to apical freshwater was curvilinear, indicative of a saturable carrier-mediated component (K(m)=66 micromol l(-1)) superimposed on a large diffusive linear component. Pre-exposure to elevated ammonia (2000 micromol l(-1)) and cortisol (1000 ng ml(-1)) had synergistic effects on the ammonia permeability of DSI, with significantly increased Na(+) influx and positive correlations between ammonia efflux and Na(+) uptake. This increase in ammonia permeability was bidirectional. It could not be explained by changes in paracellular permeability as measured by [(3)H]PEG-4000 flux. The mRNA expressions of Rhbg, Rhcg2, H(+)-ATPase and Na(+)/H(+) exchanger-2 (NHE-2) were up-regulated in DSI pre-exposed to ammonia and cortisol, CA-2 mRNA was down-regulated, and transepithelial potential became more negative. Bafilomycin (1 micromol l(-1)), phenamil (10 micromol l(-1)) and 5-(N,N-hexamethylene)amiloride (HMA, 10 micromol l(-1)) applied to the apical solution significantly inhibited ammonia efflux, indicating that H(+)-ATPase, Na(+) channel and NHE-2 pathways on the apical surface were involved in ammonia excretion. Apical amiloride (100 micromol l(-1)) was similarly effective, while basolateral HMA was ineffective. Pre-treatment with apical freshwater low in [Na(+)] caused increases in both Rhcg2 mRNA expression and ammonia efflux without change in paracellular permeability. These data suggest that Rhesus glycoproteins are important for ammonia transport in the freshwater trout gill, and may help to explain in vivo data where plasma ammonia stabilized at 50% below water levels during exposure to high environmental ammonia ( approximately 2300 micromol l(-1)). We propose an apical ;Na(+)/NH(4)(+) exchange complex' consisting of several membrane transporters, while affirming the importance of non-ionic NH(3) diffusion in ammonia excretion across freshwater fish gills.

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.

Apolipoprotein AI could be a significant determinant of epithelial integrity in rainbow trout gill cell cultures: A study in functional proteomics

The freshwater fish gill forms a barrier against an external hypotonic environment. By culturing rainbow trout gill cells on permeable supports, as intact epithelia, this study investigates barrier property mechanisms. Under symmetrical conditions the apical and basolateral epithelial surfaces contact cell culture media. Replacing apical media with water, to generate asymmetrical conditions (i.e. the situation encountered by the freshwater gill), rapidly increases transepithelial resistance (TER). Proteomic analysis revealed that this is associated with enhanced expression of pre-apolipoprotein AI (pre-apoAI). To test the physiological relevance, gill cells were treated with a dose of 50 microg ml(-1) human apolipoprotein (apoAI). This was found to elevate TER in those epithelia which displayed a lower TER prior to apoAI treatment. These results demonstrate the action of apoAI and provide evidence that the rainbow trout gill may be a site of apoAI synthesis. TER does not differentiate between the trans-cellular (via the cell membrane) and para-cellular (via intercellular tight junctions) pathways. However, despite the apoAI-induced changes in TER, para-cellular permeability (measured by polyethylene glycol efflux) remained unaltered suggesting apoAI specifically reduces trans-cellular permeability. This investigation combines proteomics with functional measurements to show how a proteome change may be associated with freshwater gill function.

Effects of cortisol and prolactin on Na+ and Cl- transport in cultured branchial epithelia from FW rainbow trout

The electrophysiological and ion-transporting properties of cultured gill epithelia from freshwater (FW) rainbow trout were examined in the presence of cortisol and prolactin as media supplements. Epithelia were of the double-seeded insert (DSI) type containing both pavement cells (PVCs) and mitochondria-rich cells (MRCs) and were grown in Leibovitz's L15 media on filters allowing exposure to different apical media conditions. Experiments were carried out in two series after 7-9 days symmetrical (L15 apical-L15 basolateral) culture. In both series, 100% L15 was maintained as the basolateral medium throughout and supplemented with physiologically relevant doses of either prolactin (50 ng/ml), cortisol (500 ng/ml), or cortisol + prolactin (500 + 50 ng/ml, respectively). In series 1, epithelia were exposed to progressively diluted apical media (100, 75, 50, 25, 12.5% L15, and FW) at 24-h intervals. The preparations retained integrity [high transepithelial resistance (TER); low ion efflux rates] during this prolonged dilution protocol. Cortisol, or cortisol + prolactin, resulted in a greater TER and reduced ion efflux rates during dilution, likely an effect on junctional permeability of PVCs, but did not promote active Na+ and Cl- uptake from apical FW. In series 2, epithelia were directly exposed to apical FW and ion fluxes measured over the first 6 h. Under these conditions, cortisol or cortisol + prolactin promoted active uptake of both Na+ and Cl- simultaneously from apical FW, probably attributable to actions on the MRCs. However, Na+-K+-ATPase activities were not significantly altered by any of the treatments in either series. Overall, prolactin alone did not appear to promote FW adaptation but exhibited synergism with cortisol. These results provide further support for the cultured DSI epithelium as an in vitro model for ion transport in FW fish.

Dilute culture media as an environmental or physiological simulant in cultured gill epithelia from freshwater rainbow trout

The electrophysiological and ion-transporting properties of cultured gill epithelia from freshwater (FW) rainbow trout were examined in the presence of dilute cell culture media as an environmental or physiological simulant. Gill epithelia were cultured on cell culture inserts under symmetrical conditions (L15 apical-L15 basolateral) for 6-7 d. The following experiments were then conducted. (1) To mimic a gradual lowering of environmental salinity, apical L15 medium was progressively diluted with FW (first to 2/3 L15 for 8 h and then to 1/3 L15 for 6 h) before the introduction of apical FW (FW apical-L15 basolateral, analogous to a fish in a natural FW environment). Dilute apical media had no significant effect on the electrophysiological properties of preparations compared with symmetrical culture conditions, and no evidence for active Na(+) or Cl(-) transport was observed. Preparations subsequently exposed to apical FW exhibited a negative transepithelial potential and evidence of active Cl(-) uptake and slight Na(+) extrusion. (2) To mimic the extracellular fluid dilution that occurs in euryhaline fish after abrupt transfer from saline to FW, the osmolality or ionic strength (or both) of basolateral media was reduced by 20-40% (using either FW or FW + mannitol) while simultaneously replacing apical media with FW. Under these conditions, Na(+) and Cl(-) influx rates were low compared with efflux rates, while the Ussing flux ratio analysis generally indicated active Cl(-) uptake and Na(+) extrusion. The Na(+)-K(+) adenosine triphosphatase activity was not affected by alterations in basolateral osmolality. Our studies indicate that cultured trout gill epithelia are tolerant of media dilution from both the apical and the basolateral direction; however, neither treatment alone appeared to increase ion influx rates or stimulate active Na(+) uptake in cultured trout gill epithelia.

Studies on lipid metabolism in trout (Oncorhynchus mykiss) branchial cultures

Cultured branchial cell epithelia from freshwater rainbow trout were incubated with ((32)P)phosphate and ((14)C)acetate as lipid precursors under both symmetrical (L15 media apical/L15 media basolateral) and asymmetrical (freshwater apical/L15 media basolateral) culture conditions. Epithelia composed of pavement cells alone, or containing both pavement cells and chloride cells, were examined. Lipids (labeled with (32)P and (14)C) were isolated and assayed by thinlayer chromatography, and fatty acids (labeled with (14)C) were isolated and assayed by paper chromatography. The main goal was to see whether the loss of a major incorporation into ((32)P)phosphatidylethanolamine [((32)P)PE], previously seen in eel gills in vivo when the fish were transferred from an osmotic steady state to more dilute media, was the result of a hormonal regulation, i.e., did it only apply to gill tissue in vivo or could it also be seen in the absence of hormonal modulation after incorporation of ((32)P)phosphate in vitro? We likewise wished to see whether a major incorporation into ((32)P)PE was dependent upon the presence of chloride cells. Results show that it is possible to obtain a ((32)P)PE dominated incorporation pattern, even in the pavement cells alone, provided that ((32)P)phosphate is added specifically to freshwater on the apical side of epithelia bathed asymmetrically (freshwater/L15). This is identical to the pattern seen in vivo in trout adapted to freshwater. However, this pattern is not seen under symmetrical conditions (L15/L15) or when ((32)P)phosphate is added to the basolateral media. The shift from symmetrical (L15/L15) to asymmetrical (freshwater/L15) culture conditions thus leads to the establishment of a major incorporation into ((32)P)PE and not to the equivalent loss as seen in vivo in more dilute apical media. We conclude that hormonal control is not needed to change the pattern of short-term lipid formation but, nevertheless, the responses are not altogether the same in vitro and in vivo. Furthermore, cultured trout gill epithelia, in contrast to gills in vivo, do not exhibit a marked incorporation of ((14)C)acetate into palmitoleic acid.

Cultured gill epithelia as models for the freshwater fish gill

We review recent progress in the development of models for the freshwater teleost gill based on reconstructed flat epithelia grown on permeable filter supports in primary culture. Methods are available for single-seeded insert (SSI) preparations consisting of pavement cells (PVCs) only from trout and tilapia, and double-seeded insert (DSI) preparations from trout, containing both PVCs (85%) and mitochondria-rich cells (MRCs, 15%), as in the intact gill. While there are some quantitative differences, both SSI and DSI epithelia manifest electrical and passive permeability characteristics typical of intact gills and representative of very tight epithelia. Both preparations withstand apical freshwater exposure, exhibiting large increases in transepithelial resistance (TER), negative transepithelial potential (TEP), and low rates of ion loss, but there is only a small active apical-to-basolateral "influx" of Cl(-) (and not of Na(+)). Responses to various hormonal treatments are described (thyroid hormone T3, prolactin, and cortisol). Cortisol has the most marked effects, stimulating Na(+),K(+)-ATPase activity and promoting active Na(+) and Cl(-) influxes in DSI preparations, and raising TER and reducing passive ion effluxes in both epithelia via reductions in paracellular permeability. Experiments using DSI epithelia lacking Na(+) uptake demonstrate that both NH(3) and NH(4)(+) diffusion occur, but are not large enough to account for normal rates of branchial ammonia excretion, suggesting that Na(+)-linked carrier-mediated processes are important for ammonia excretion in vivo. Future research goals are suggested.

Mechanisms of acid secretion in pseudobranch cells of rainbow trout(Oncorhynchus mykiss)

Cell suspensions of rainbow trout Oncorhynchus mykisspseudobranch, prepared by Ca2+ depletion and mechanical maceration,contained a distinct population of cells that always kept their relatively cuboidal shape and did not round up in suspension or proliferate after adhering to the surface of cell culture dishes. Phasecontrast microscopy revealed an extensive system of basal membrane invaginations, and Na+-K+-ATPase- and anionexchanger-like immunoreactivity could be localized in cell membranes. The cells were characterized by a high mitochondrial density. Using specific antibodies, V-ATPase subunit B was localized in the plasma membrane. Using a cytosensor microphysiometer, the rate of acid secretion of these cells was measured and compared with the activity of a gill cell preparation. Incubation of pseudobranch cells with bafilomycin A1 (10-6 moll-1), a specific inhibitor of V-ATPase, reduced the rate of acid secretion by about 10% under control conditions, while no effect of bafilomycin on the rate of acid secretion of gill cells was observed. Application of amiloride (5×10-5moll-1) reduced the rate of acid secretion in cells of both organs,pseudobranch and gills. Incubation of pseudobranch cells with DIDS(10-3 moll-1) resulted in a minor increase in the rate of proton secretion, but in cells prepared from the gills of rainbow trout acid secretion was reduced by about 30-40%. It is concluded that pseudobranch cells are equipped with various pathways to secrete protons, and that the anion exchange activity especially of pseudobranch cells appears to be different from that in gills.

Prolactin effects on cultured pavement cell epithelia and pavement cell plus mitochondria-rich cell epithelia from freshwater rainbow trout gills

The physiological effects of ovine prolactin (oPRL) and recombinant rainbow trout prolactin (rbtPRL) on cultured gill epithelia derived from freshwater rainbow trout were assessed. Epithelia composed of either pavement cells only (single seeded inserts, SSI) or both pavement and mitochondria-rich cells (double seeded inserts, DSI) were cultured in media, supplemented with doses of oPRL ranging from 10 to 100 ng/ml. Under symmetrical culture conditions (L15 media apical/L15 media basolateral), oPRL had no effect on transepithelial resistance, paracellular permeability (assessed with PEG-4000), or Na(+) and Cl(-) transport across both preparations of cultured gill epithelia. Under asymmetrical conditions (freshwater apical/L15 media basolateral), SSI epithelia treated with oPRL (10 and 50 ng/ml), in comparison to comparably treated epithelia receiving no oPRL, exhibited a greater increase in the transepithelial resistance, particularly during the first 12h of freshwater exposure, no difference in paracellular permeability and Na(+)-K(+)-ATPase activity, and lowered net Na(+) flux rates (i.e., reduced basolateral to apical loss rates). These reflected reduced unidirectional efflux rates. The PRL effect appeared to be mainly a reduction in transcellular permeability. SSI epithelia treated with rbtPRL (10 ng/ml) exhibited similar patterns of response to those treated with oPRL. Na(+)-K(+)-ATPase activity increased in DSI epithelia treated with oPRL; however, oPRL did not stimulate ion uptake across either SSI or DSI epithelial preparations. The data demonstrated that, as the sole hormone supplement for cultured gill epithelia, PRL did not promote active ion uptake. However, the observed PRL-induced alterations in cultured gill epithelial physiology were consistent with the in vivo actions of PRL on the gills of freshwater teleost fish.

Swimbladder gas gland cells cultured on permeable supports regain their characteristic polarity

A cell culture system has been developed in which swimbladder gas gland cells from the European eel (Anguilla anguilla) were cultured on a permeable support. Cells seeded on Anodisc 13 (Whatman) or Costar Transwell 13 mm membranes form a confluent cell layer within the first 2 or 3 days of culture but, on the basis of measurements of transepithelial resistance, it is a ‘leaky’ cell layer. In a superfusion system, the apical and basal sides of the cells were superfused asymmetrically, with saline on the apical side and a glucose-containing cell culture medium on the basal side. Under these conditions, the cells continuously produced lactic acid, and approximately 60–70 % of this lactate was released at the basal side. To mimic the in vivo situation, the saline solution supplied to the apical side was replaced by humidified air in an additional series of experiments. Cells cultured in an air/liquid system produced even more lactate, and this lactate was only released to the basal side; there was no leakage of fluid to the apical side. After 4 or 5 days in the superfusion system, the cells were fixed for histological examination. The cells were columnar, similar to gas gland cells in vivo, and showed a clear polarity, with some small microvilli at the apical membrane and extensive membrane foldings at lateral and basal membranes. Immunohistochemical localization of Na+/K+-ATPase revealed that this ATPase was present mainly in the lateral membranes; it was never found in the apical membranes. Cells cultured in the air/liquid system showed a similar structure and polarity.

Effect of cortisol on the physiology of cultured pavement cell epithelia from freshwater trout gills

Cortisol had dose-dependent effects on the electrophysiological, permeability, and ion-transporting properties of cultured pavement cell epithelia derived from freshwater rainbow trout gills and grown on cell culture filter supports. Under both symmetrical (L15 media apical/L15 media basolateral) and asymmetrical (freshwater apical/L15 media basolateral) culture conditions, cortisol treatment elevated transepithelial resistance, whereas permeability of epithelia to a paracellular permeability marker (polyethylene glycol-4000) decreased. Cortisol did not alter the Na(+)-K(+)-ATPase activity or the total protein content of the cultured preparations. During 24-h exposure to asymmetrical conditions, the net loss rates of both Na(+) and Cl(-) to the water decreased with increasing cortisol dose, an important adaptation to dilute media. Unidirectional Na(+) and Cl(-) flux measurements and the application of the Ussing flux-ratio criterion revealed cortisol-induced active uptake of both Na(+) and Cl(-) under symmetrical culture conditions together with an increase in transepithelial potential (positive on the basolateral side). Under asymmetrical conditions, cortisol did not promote active ion transport across the epithelium. These experiments provide evidence for the direct action of cortisol on cultured pavement cell epithelia and, in particular, emphasize the importance of cortisol for limiting epithelial permeability.

The physiological effects of 3,5',3'-triiodo-L-thyronine alone or combined with cortisol on cultured pavement cell epithelia from freshwater rainbow trout gills

The effects of 3,5',3'-triiodo-L-thyronine (T3; 10 or 100 ng ml(-1)), alone or combined with cortisol (500 ng ml(-1)), on the physiological properties of cultured pavement cell epithelia from freshwater rainbow trout gills were assessed. T3 had dose-dependent effects on electrophysiological, biochemical, and ion transporting properties of cultured epithelia in both the absence and the presence of cortisol. These included reduced transepithelial resistance (TER), increased net Na(+) and Cl(-) movement (basolateral to apical) under asymmetrical culture conditions (freshwater apical/L15 media basolateral), and elevated Na(+)-K(+)-ATPase activity. However, paracellular permeability was elevated only in high-dose T3-treated preparations. In T3 + cortisol-treated epithelia, similar T3-induced alterations in TER, net Na(+) and Cl(-) movement, and paracellular permeability were observed, whereas the activity of Na(+)-K(+)-ATPase was further elevated. Under symmetrical culture conditions (L15 medium apical/L15 medium basolateral), T3 had no effect on transepithelial Na(+) and Cl(-) transport, which was passive. However, T3 + cortisol treatment resulted in active Na(+) extrusion (basolateral to apical). Under asymmetrical conditions, hormone treatment did not change the pattern of ion movement (active Na(+) extrusion, active Cl(-) uptake). These experiments demonstrate that cultured pavement cell epithelia from freshwater rainbow trout are T3-responsive and provide evidence for the direct action of T3 and the interaction of T3 and cortisol on the physiology of this preparation.

7-Ethoxyresorufin O-deethylase induction in rainbow trout gill epithelium cultured on permeable supports: asymmetrical distribution of substrate metabolites

The induction of 7-ethoxyresorufin O-deethylase (EROD) has been measured in cultured epithelia from rainbow trout gills. Epithelia incubated with water on the apical side and culture media at the basolateral side were exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), beta-naphthoflavone (betaNF), benzo[k]fluoranthene (B(k)F), and 3,3',4,4',5-pentachlorobiphenyl (PCB#126) from the water. EROD activity was measured as the formation of resorufin from 7-ethoxyresorufin over time in intact epithelia. The EC(50) values obtained after 24 h of exposure (mean+/-S.D.) were for TCDD (n=9) 4.1+/-3.2x10(-11) M, for betaNF (n=6) 1.6+/-3.8x10(-9) M, for B(k)F (n=4) 5.4+/-3.0x10(-9) M and for PCB#126 (n=4) 6.15+/-10.1x10(-9) M. When assaying for EROD activity, it was found that the resorufin concentrations differed between the apical and the basolateral compartments, indicating an asymmetrical distribution of the enzymatically formed resorufin molecules. Generally, the resorufin concentration was highest in the basolateral compartment, but there were differences between epithelia obtained from different fish individuals. Of a total of 13 preparations 10 had the highest resorufin concentration in the basolateral compartment, while in three preparations, the resorufin was uniformly distributed or slightly higher in the apical compartment. The reasons for this asymmetrical distribution of substrate metabolites are not known, and the addition of multidrug resistance inhibitors (verapamil and cyclosporin A) did not alter the asymmetrical pattern. The transepithelial electrical resistance (TER) was also measured to diagnose the tightness of the epithelia. The change from culture media to experimental water (containing TCDD, betaNF, or DMSO as control) in the apical compartment resulted in a large increase in TER, followed by a decline, measured after 24 h. The cytochrome P450 1A (CYP1A) inducers had no effect on the TER and were judged, therefore, not to affect the tightness of the epithelia.

Biological functions of trout pavement-like gill cells in primary culture on solid support: pH(i) regulation, cell volume regulation and xenobiotic biotransformation

This review presents results obtained on rainbow trout gill cells in primary culture on solid support. Ultrastructural analysis showed that cultured gill cells displayed features of pavement cells in situ. Several biological functions have been investigated on these cultured cells. First, it was shown that their intracellular pH at rest and after acidosis is regulated by a Na+/H+ exchanger. Second, gill cells in primary culture can regulate their volume after a cell swelling. Intracellular calcium appears to be involved in this regulation. The effects of different xenobiotics on the capacity of gill cells to regulate their volume are presented. Third, cultured pavement cells contain biotransformation enzymes to metabolize xenobiotics. All these results demonstrate that gill cells in primary culture on solid support represent a promising in vitro model for the study of pavement cells physiology. In conclusion, applications of this culture are discussed and compared with the permeable filter method, together with the limitations and prospects of this in vitro model on solid support.

Transport properties of cultured branchial epithelia from freshwater rainbow trout: a novel preparation with mitochondria-rich cells

A new double-seeded insert (DSI) technique is described for culture of branchial epithelial preparations from freshwater rainbow trout on filter supports. DSI epithelia contain both pavement cells and mitochondria-rich (MR) cells (15.7+/−2.5 % of total cell numbers). MR cells occur singly or in clusters, are voluminous, open apically to the ‘external environment’ and exhibit ultrastructural characteristics similar to those found in the ‘chloride cells’ of freshwater fish gills. After 6–9 days in culture with Leibovitz's L-15 medium on both surfaces (symmetrical conditions), transepithelial resistance (TER) stabilized at values as high as 34 k capomega cm(2), indicative of electrically ‘tight’ epithelia. The density of MR cells, the surface area of their clusters and transepithelial potential (TEP; up to +8 mV basolateral positive, mean +1.9+/−0.2 mV) were all positively correlated with TER. In contrast, preparations cultured using an earlier single-seeded insert (SSI) technique contained only pavement cells and exhibited a negligible TEP under symmetrical conditions. Na(+)/K(+)-ATPase activities of DSI preparations were comparable with those in gill filaments, but did not differ from those of SSI epithelia. Replacement of the apical medium with fresh water to mimic the in vivo situation (asymmetrical conditions) induced a negative TEP (−6 to −15 mV) and increased permeability to the paracellular marker PEG-4000. Under symmetrical conditions, unidirectional Na(+) and Cl(−) fluxes were in balance, and there was no active transport by the Ussing flux ratio criterion. Under asymmetrical conditions, there were large effluxes, small influxes and evidence for active Cl(−) uptake and Na(+) extrusion. Unidirectional Ca(2+) fluxes were only 0.5-1.0 % of Na(+) and Cl(−) fluxes; active net Ca(2+) uptake occurred under symmetrical conditions and active net extrusion under asymmetrical conditions. Thus, DSI epithelia exhibit some of the features of the intact gill, but improvements in culture conditions are needed before the MR cells will function as true freshwater ‘chloride cells’.