Toad urinary bladder epithelial cells in culture: maintenance of epithelial structure, sodium transport, and response to hormones

The amphibian invitrome: Past, present, and future contributions to our understanding of amphibian immunity

Many amphibian populations are declining worldwide, and infectious diseases are a leading cause. Given the eminent threat infectious diseases pose to amphibian populations, there is a need to understand the host-pathogen-environment interactions that govern amphibian susceptibility to disease and mortality events. However, using animals in research raises an ethical dilemma, which is magnified by the alarming rates at which many amphibian populations are declining. Thus, in vitro study systems such as cell lines represent valuable tools for furthering our understanding of amphibian immune systems. In this review, we curate a list of the amphibian cell lines established to date (the amphibian invitrome), highlight how research using amphibian cell lines has advanced our understanding of the amphibian immune system, anti-ranaviral defence mechanisms, and Batrachochytrium dendrobatidis replication in host cells, and offer our perspective on how future use of amphibian cell lines can advance the field of amphibian immunology.

Epithelial Sodium Channel Inhibition by Amiloride Addressed with THz Spectroscopy and Molecular Modeling

THz spectroscopy is important for the study of ion channels because it directly addresses the low frequency collective motions relevant for their function. Here we used THz spectroscopy to investigate the inhibition of the epithelial sodium channel (ENaC) by its specific blocker, amiloride. Experiments were performed on A6 cells' suspensions, which are cells overexpressing ENaC derived from Xenopus laevis kidney. THz spectra were investigated with or without amiloride. When ENaC was inhibited by amiloride, a substantial increase in THz absorption was noticed. Molecular modeling methods were used to explain the observed spectroscopic differences. THz spectra were simulated using the structural models of ENaC and ENaC-amiloride complexes built here. The agreement between the experiment and the simulations allowed us to validate the structural models and to describe the amiloride dynamics inside the channel pore. The amiloride binding site validated using THz spectroscopy agrees with previous mutagenesis studies. Altogether, our results show that THz spectroscopy can be successfully used to discriminate between native and inhibited ENaC channels and to characterize the dynamics of channels in the presence of their specific antagonist.

Establishing Cell Lines from Fresh or Cryopreserved Tissue from the Great Crested Newt (Triturus cristatus):A Preliminary Protocol

This study describes a successful protocol for establishing cell lines from the threatened Triturus cristatus in terms of collection, preparing, establishing, cryopreserving, thawing and quality checking. Different parameters such as media, media change, fresh vs. cryopreserved tissue and seeding density were tested to optimize culture conditions for this species. With fresh tissue, no considerable differences in the use of two different media were found, but with cryopreserved tissue, a combination of ITS (insulin/transferrin/selenite) and 2-mercaptoethanol had a positive effect on growth. Real-time measurements on the cell lines were used, for the first time in amphibian cells, to investigate the effect of different treatments such as media change with or without washing. Media change had a positive impact on the cells, whereas the effect was negative when combined with washing. It is concluded that establishment of cell lines is possible from the great crested newt, especially when using fresh tissue, but much more challenging if the tissue has been cryopreserved. Real-time measurement during cell culture is a useful tool to visualize the sensitivity of amphibian cells during different culture treatments.

Glucocorticoid and mineralocorticoid receptors regulate paracellular permeability in a primary cultured gill epithelium

The role of corticosteroid receptors (CRs) in the regulation of gill permeability was examined using a primary cultured trout gill epithelium. The epithelium expressed both glucocorticoid receptors (GR1 and GR2) and a mineralocorticoid receptor (MR), and cortisol treatment significantly increased transepithelial resistance (TER) and decreased paracellular [(3)H]PEG-4000 flux. Epithelial permeability was unaffected by deoxycorticosterone or aldosterone. The GR antagonist RU486 as well as MR antagonists spironolactone and RU26752 significantly reduced, but did not completely block, the effects of cortisol. The MR antagonist eplerenone was without effect. Only RU486 + spironolactone or RU486 + RU26752 treatment completely suppressed the effects of cortisol. On its own, RU486 had cortisol-like effects which could be blocked by spironolactone, suggesting that although RU486 is a GR antagonist, in this system it may also have agonistic properties that are mediated through the MR. The GR agonist dexamethasone increased TER and reduced [(3)H]PEG-4000 flux across cultured epithelia and was unaffected by MR antagonists. Therefore, alterations in transcript abundance of select tight junction (TJ) proteins were examined in response to cortisol, dexamethasone (a GR agonist) and RU486 (as a MR agonist). Occludin and claudin-7, -8d, -12 and -31 mRNA were significantly elevated in response to cortisol, dexamethasone or RU486 treatment. Claudin-3a mRNA was significantly elevated in response to cortisol or dexamethasone only, and claudin-28b and -30 mRNA were significantly altered following cortisol or RU486 treatment only. The data support a role for the GRs and MR in regulating gill permeability and suggest that TJ proteins are responsive to cortisol through both or individual CR types.

Biogenesis of epithelial cell plasma membranes

Polarized monolayers of cultured epithelial cells, such as the kidney-derived MDCK cell line, when infected with enveloped viruses, provide a convenient model system for study of the intracellular routes followed by newly synthesized glycoproteins to reach specific domains of the plasma membrane. The polarized nature of the monolayers is reflected in the asymmetric assembly of enveloped viruses, some of which, such as influenza and simian virus 5 (SV5), bud from the apical surfaces of the cells, while others, such as vesicular stomatitis virus (VSV), emerge from the basolateral surfaces. MDCK cells can sustain double infection with viruses of different budding polarity, and within such cells the envelope glycoproteins of the two viruses are synthesized simultaneously and assembled into virions at different sites. Immunoelectron microscopic observations of doubly infected cells show that glycoproteins of influenza and VSV traverse the same Golgi apparatus. This indicates that critical sorting steps must take place during or after passage of the glycoproteins through the organelle. Following passage through the Golgi, the HA glycoprotein accumulates almost exclusively at the apical surface, where the influenza virions assemble. Significant amounts of the G protein, however, are detected on both plasma membranes in singly and doubly infected cells, although VSV virion assembly is limited to basolateral domains. These observations indicate that the site of VSV budding is not exclusively determined by the presence of G polypeptides on a given cell-surface domain. It is possible that other cellular or viral components are responsible for the selection of the appropriate budding domain or that the G protein found on the apical surface must be transferred to the basolateral domain before it becomes competent for assembly.

Aldosterone and tight junctions: modulation of claudin-4 phosphorylation in renal collecting duct cells

Aldosterone classically modulates Na transport in tight epithelia such as the renal collecting duct (CD) through the transcellular route, but it is not known whether the hormone could also affect paracellular permeability. Such permeability is controlled by tight junctions (TJ) that form a size- and charge-selective barrier. Among TJ proteins, claudin-4 has been highlighted as a key element to control paracellular charge selectivity. In RCCD2 CD cells grown on filters, we have identified novel early aldosterone effects on TJ. Endogenous claudin-4 abundance and cellular localization were unaltered by aldosterone. However, the hormone promoted rapid (within 15-20 min) and transient phosphorylation of endogenous claudin-4 on threonine residues, without affecting tyrosine or serine; this event was fully developed at 10 nM aldosterone and appeared specific for aldosterone (because it is not observed after dexamethasone treatment and it depends on mineralocorticoid receptor occupancy). Within the same delay, aldosterone also promoted an increased apical-to-basal passage of 125I (a substitute for 36Cl), whereas 22Na passage was unaffected; paracellular permeability to [3H]mannitol was also reduced. Later on (45 min), a fall in transepithelial resistance was observed. These data indicate that aldosterone modulates TJ properties in renal epithelial cells.

MineralocorticoidversusGlucocorticoid Receptor Occupancy Mediating Aldosterone-Stimulated Sodium Transport in a Novel Renal Cell Line

Aldosterone controls sodium balance by regulating an epithelial sodium channel (ENaC)-mediated sodium transport along the aldosterone-sensitive distal nephron, which expresses both mineralocorticoid (MR) and glucocorticoid receptors (GR). Mineralocorticoid specificity is ensured by 11beta-hydroxysteroid dehydrogenase type 2, which metabolizes cortisol or corticosterone into inactive metabolites that are unable to bind MR and/or GR. The fractional occupancy of MR and GR by aldosterone mediating the sodium transport response in the aldosterone-sensitive distal nephron cannot be studied in vivo. For answering this question, a novel mouse cortical collecting duct cell line (mCCD(cl1)), which expresses significant levels of MR and GR and a robust aldosterone sodium transport response, was used. Aldosterone elicited a biphasic response: Low doses (K(1/2) = approximately 0.5 nM) induced a transient and early increase of sodium transport (peaking at 3 h), whereas high doses (K(1/2) = approximately 90 nM) entailed an approximately threefold larger, long-lasting response. At 3 h, the corticosterone dose-response curve was shifted to the right compared with that of aldosterone by more than two log concentrations, an effect that was fully reverted in the presence of the 11beta-hydroxysteroid dehydrogenase type 2 inhibitor carbenoxolone. Low doses of dexamethasone (0.1 to 1 nM) failed to induce an early response, but high doses elicited a long-lasting response (K(1/2) = approximately 8 nM), similar to that observed for high aldosterone concentrations. Equilibrium binding assays showed that both aldosterone and corticosterone bind to a high-affinity, low-capacity site, whereas dexamethasone binds to one site. Within the physiologic range of aldosterone concentrations, sodium transport is predicted to be controlled by MR occupancy during circadian cycles and by MR and GR occupancy during salt restriction or acute stress.

Control of apical membrane chloride permeability in the renal A6 cell line by nucleotides

1. The effect of extracellular nucleotides applied on the apical side of polarised A6 cells grown on permeant filters was investigated by measuring the changes in (i) the 36Cl efflux through the apical membranes, (ii) the intracellular chloride concentrations (aCli, measured with N-(6-methoxyquinolyl) acetoethyl ester, MQAE), (iii) ICl, the short-circuit current in the absence of Na+ transport and (iv) the characteristics of the apical chloride channels using a patch-clamp approach. 2. ATP or UTP (0.1-500 microM) transiently stimulated ICl. The sequence of purinergic agonist potencies was UTP = ATP > ADP >> the P2X-selective agonist beta,gamma-methylene ATP = the P2Y-selective agonist 2-methylthioATP. Suramin (100 microM) as the P2Y antagonist Reactive Blue 2 (10 microM) had no effect on the UTP (or ATP)-stimulated current. These findings are consistent with the presence of P2Y2-like receptors located on the apical membranes of A6 cells. Apical application of adenosine also transiently increased ICl. This effect was blocked by theophylline while the UTP-stimulated ICl was not. The existence of a second receptor, of the P1 type is proposed. 3. ATP (or UTP)-stimulated ICl was blocked by apical application of 200 microM N-phenylanthranilic acid (DPC) or 100 microM niflumic acid while 100 microM glibenclamide was ineffective. 4. Ionomycin and thapsigargin both transiently stimulated ICl; the nucleotide stimulation of ICl was not suppressed by pre-treatment with these agents. Chlorpromazin (50 microM), a Ca2+-calmodulin inhibitor strongly inhibited the stimulation of ICl induced either by apical UTP or by ionomycin application. BAPTA-AM pre-treatment of A6 cells blocked the UTP-stimulated ICl. Niflumic acid also blocked the ionomycin stimulated ICl. 5. A fourfold increase in 36Cl effluxes through the apical membranes was observed after ATP or UTP application. These increases of the apical chloride permeability could also be observed when following aCli changes. Apical application of DPC (1 mM) or 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB; 500 microM) produced an incomplete inhibition of 36Cl effluxes through the apical membranes in ATP-stimulated and in untreated monolayers. 6. In single channel patch-clamp experiments, an apical chloride channel with a unitary single channel conductance of 7.3 +/- 0.6 pS (n = 12) was usually observed. ATP application induced the activation of one or more of these channels within a few minutes. 7. These results indicate that multiple purinergic receptor subtypes are present in the apical membranes of A6 cells and that nucleotides can act as modulators of Cl- secretion in renal cells.

Epithelial sodium channel regulatory proteins identified by functional expression cloning

We describe here our current strategy for identifying and cloning proteins involved in the regulation of the epithelial sodium channel (ENaC). We have set up a complementation functional assay in the Xenopus laevis oocyte expression system. Using this assay, we have been able to identify a channel-activating protease (CAP-1) that can increase ENaC activity threefold. We propose a novel extracellular signal transduction pathway controlling ionic channels of the ENaC gene family that include genes involved in mechanotransduction (degenerins), in peptide-gated channels involved in neurotransmission (FaNaCh), in proton-gated channels involved in pH sensing (ASIC) or pain sensation (DRASIC).

An epithelial serine protease activates the amiloride-sensitive sodium channel

Sodium balance, and ultimately blood pressure and extracellular fluid volume, is maintained by precise regulation of the activity of the epithelial sodium channel (ENaC). In a Xenopus kidney epithelial cell line (A6), exposure of the apical membrane to the protease inhibitor aprotinin reduces transepithelial sodium transport. Sodium-channel activity can be restored by subsequent exposure to the nonspecific protease trypsin. Using A6 cells and a functional complementation assay to detect increases in ENaC activity, we have cloned a 329-residue protein belonging to the serine protease family. We show that coexpression of this protein with ENaC in Xenopus oocytes increases the activity of the sodium channel by two- to threefold. This channel-activating protease (CAP1) is expressed in kidney, gut, lung, skin and ovary. Sequence analysis predicts that CAP1 is a secreted and/or glycosylphosphatidylinositol-anchored protein: ENaC activity would thus be regulated by the activity of a protease expressed at the surface of the same cell. This previously undiscovered mechanism for autocrine regulation may apply to other ion channels, in particular to members of the ENaC family that are present in neurons and epithelial cells.

Chloride channels in primary cultures of seawater fish (Dicentrarchus labrax) gill

Patch-clamp experiments were undertaken on primary cultures of respiratory cells originating from sea bass (Dicentrarchus labrax) gills. A small-conductance Cl- channel of 8 pS was characterized in cell-attached configuration with 140 mM N-methyl-D-glucamine-Cl in the pipette and bath solutions. No activity was recorded below a membrane holding potential of +20 mV (-Vp, referenced to the pipette solution), and the channel showed an inward rectification. In the inside-out configuration the Cl- channel was active at all membrane holding potentials. Its open probability strongly increased with membrane depolarization. The channel activity could be increased by the application of protein kinase A+ATP. This channel was inhibited by 5-nitro-2-(3-phenylpropylamino)-benzoic acid, diphenylamino-2-carboxylic acid, and I- and was insensitive to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. The presence of Cl- channels in the apical membrane of respiratory cells provides additional evidence for an important role of this cell type in the control of ion homeostasis of seawater fish.

The molecular basis of focal cyst formation in human autosomal dominant polycystic kidney disease type I

Autosomal dominant polycystic kidney disease (ADPKD) is a common disease and an important cause of renal failure. It is characterized by considerable intrafamilial phenotypic variation and focal cyst formation. To elucidate the molecular basis for these observations, we have developed a novel method for isolating renal cystic epithelia from single cysts and have used it to show that individual renal cysts in ADPKD are monoclonal. Loss of heterozygosity was discovered within a subset of cysts for two closely linked polymorphic markers located within the PKD1 gene. Genetic analysis revealed that it was the normal haplotype that was lost. This study provides a molecular explanation for the focal nature of cyst formation and a probable mechanism whereby mutations cause disease. The high rate at which "second hits" must occur to account for the large number of cysts observed suggests that unique structural features of the PKD1 gene may be responsible for its mutability.

Characteristics of a rat cortical collecting duct cell line that maintains high transepithelial resistance

This study describes the establishment of a rat kidney cortical collecting duct (CCD) clonal cell line (RCCD1 cells) that maintains high transepithelial resistance and specific hormonal sensitivities. Immortalized cells were obtained by infection of primary cultured CCD cells with the wild-type simian virus 40. Grown on Petri dishes, RCCD1 cells are organized as monolayers of cuboid cells separated by tight junctions and form domes. Grown on permeable filters, confluent RCCD1 cells exhibit high transepithelial resistance (Rt: 2390 +/- 140 omega. cm2), transepithelial potential difference (PD) of -10.5 +/- 1.2 mV lumen negative, an associated short-circuit current (Isc) of 4.3 +/- 0.5 microA/cm2, and generated significant Na+, K+, H+ and HCO3- gradients, reflecting Na+ and H+ reabsorption and K+ and HCO3- secretion. RCCD1 cells exhibit features of both principal (PC) and intercalated (IC) cells. Consistent with PC phenotype, about 50% of the cells were positively stained by a PC-specific agglutinin. In situ hybridization studies revealed the presence of alpha, beta and gamma subunit mRNAs of the amiloride-sensitive epithelial Na+ channel and alpha 1 and beta 1 subunits of Na(+)-K(+)-ATPase. Moreover, Na(+)-K(+)-ATPase was immunolocalized at the basolateral side of the cells. Arginine vasopressin (AVP) induced a significant increase in both cellular cAMP content and Isc. Amiloride decreased in a dose-dependent manner Isc from untreated and AVP-treated RCCD1 cells. In addition, a barium-sensitive K+ conductance was evidenced in the apical side of the cells. Consistent with IC phenotype, isoproterenol (ISO) provoked a large increase in cellular cAMP and stimulated Isc. The effect of ISO on Isc was blocked by 5 x 10(-3) M DPC, a chloride channel blocker. Finally, AVP plus ISO had additive effect on Isc. Taken together, these results provide evidence that the RCCD1 cell line has maintained many of the original properties of rat CCD from which they were derived.

Basolateral membrane chloride permeability of A6 cells: implication in cell volume regulation

The permeability to Cl- of the basolateral membrane (blm) was investigated in renal (A6) epithelial cells, assessing their role in transepithelial ion transport under steady-state conditions (isoosmotic) and following a hypoosmotic shock (i.e. in a regulatory volume decrease, RVD). Three different complementary studies were made by measuring: (1) the Cl- transport rates (delta F/Fo s-1 (x10(-3))), where F is the fluorescence of N-(6-methoxyquinoyl) acetoethyl ester, MQAE, and Fo the maximal fluorescence (x10(-3)) of both membranes by following the intracellular Cl- activities (ai Cl-, measured with MQAE) after extracellular Cl- substitution (2) the blm 86Rb and 36Cl uptakes and (3) the cellular potential and Cl- current using the whole-cell patch-clamp technique to differentiate between the different Cl- transport mechanisms. The permeability of the blm to Cl- was found to be much greater than that of the apical membranes under resting conditions: aiCl- changes were 5.3 +/- 0.7 mM and 25.5 +/- 1.05 mM (n = 79) when Cl- was substituted by NO3(-) in the media bathing apical and basolateral membranes. The Cl- transport rate of the blm was blocked by bumetanide (100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 50 microM) but not by N-phenylanthranilic acid (DPC, 100 microM). 86Rb and 36Cl uptake experiments confirmed the presence of a bumetanide- and a NPPB-sensitive Cl- pathway, the latter being approximately three times more important than the former (Na/K/2Cl cotransporter). Appli-cation of a hypoosmotic medium to the serosal side of the cell increased delta F/Fo s-1 (x10(-3)) after extracellular Cl- substitution (1.03 +/- 0.10 and 2.45 +/- 0.17 arbitrary fluorescent units s-1 for isoosmotic and hypoosmotic conditions respectively, n = 11); this delta F/Fo s-1 (x10(-3)) increase was totally blocked by serosal NPPB application; on the other hand, cotransporter activity was decreased by the hypoosmotic shock. Cellular Ca2+ depletion had no effect on delta F/Fo s-1 (x10(-3)) under isoosmotic conditions, but blocked the delta F/Fo s-1 (x10(-3)) increase induced by a hypoosmotic stress. Under isotonic conditions the measured cellular potential at rest was -37.2 +/- 4.0 mV but reached a maximal and transient depolarization of -25.1 +/- 3.7 mV (n = 9) under hypoosmotic conditions. The cellular current at a patch-clamping cellular potential of -85 mV (close to the Nernst equilibrium potential for K+) was blocked by NPPB and transiently increased by hypoosmotic shock (≈50% maximum increase). This study demonstrates that the major component of Cl- transport through the blm of the A6 monolayer is a conductive pathway (NPPB-sensitive Cl- channels) and not a Na/K/2Cl cotransporter. These channels could play a role in transepithelial Cl- absorption and cell volume regulation. The increase in the blm Cl- conductance, inducing a depolarization of these membranes, is proposed as one of the early events responsible for the stimulation of the 86Rb efflux involved in cell volume regulation.

Na/Ca exchange in the basolateral membrane of the A6 cell monolayer: role in Cai homeostasis

The presence of a Na/Ca exchanger in A6 cells was investigated by measuring intracellular calcium (Cai) fluctuations and the 45Ca fluxes through the basolateral membranes (blm) of the cell monolayer. Removal of Na+ from the medium produced a transient increase in Cai followed by a regulatory phase returning Cai to control levels in 3-4 min, this phase being greatly accelerated (< 60 s) by NaCl addition (apparent Km of approximately 5 mM Na+). The Cai increase was only found with the Na(+)-free medium on the basolateral side of the cell monolayer. A twofold increase in the 45Ca influx was observed under these conditions. In Ca(2+)- depleted cells, the initial Cai increase after Ca2+ addition to the medium was greater when the putative Na/Ca exchanger was not functioning (i.e. in a Na(+)-free medium). 45Ca effluxes through the blm of the monolayer were greatly and transiently increased by a Na(+)-free medium on the serosal side and blocked by orthovanadate (1 mM). The Cai increased induced by a hypo-osmotic shock was greater in cells bathed in a Na(+)-medium, conditions expected to block the activity of the Na/Ca exchanger. These findings support the hypothesis that a Na/Ca exchanger is present on the blm of A6 cells and affirm its role in Cai homeostasis in steady-state conditions and following osmotic shock. In addition, a Ca2+ pump also located on the blm and Ca2+ stores sensitive to inositol 1,4,5-trisphosphate were found to be implicated in Cai homeostasis.

Aldosterone modulates sodium kinetics of Na,K-ATPase containing an alpha 1 subunit in A6 kidney cell epithelia

Short-term aldosterone (10(-6) M, 2.5 h) induces in A6-C1 cell epithelia an increase in Na transport, which is due to the in situ activation of the apical Na channel and, presumably, the basolateral Na pump (Na,K-ATPase). We have now directly measured the effect of aldosterone on the transport activity of endogenous Na pumps and hybrid Na pumps containing an exogenous alpha 1 subunit by measuring the pump current (Ip) across epithelia apically permeabilized with amphotericin B. Aldosterone (2.5 h) had no significant early effect on the maximal Ip, nor on the Na concentration required for half-maximal activation. In contrast, it increased the Ip at physiological intracellular Na concentrations (1.7-fold at 5 mM Na). This effect was blocked by the protein synthesis inhibitor cycloheximide. Hybrid pumps containing the transfected cardiotonic steroid-resistant alpha 1 subunit of Bufo marinus were also stimulated by aldosterone (2.5 h). A long aldosterone treatment (4 days) increased the maximal Ip produced by the endogenous pumps 1.5 to 2.1-fold. In conclusion, aldosterone acts on Na pumps containing an alpha 1 subunit in two ways. During its early phase of action it stimulates their transport activity by increasing their apparent Na affinity at physiological intracellular Na concentrations. In the long term it produces an increase in the maximal transport capacity, which corresponds to the known increase in the number of Na pumps.

Characterization and localization of epithelial Na+ channels in toad urinary bladder

The toad urinary bladder and epithelial cell lines derived from the urinary bladder, including TBM, serve as model systems for the study of transepithelial Na+ transport. We examined biochemical characteristics of epithelial Na+ channels in toad urinary bladder and TBM cells and their cellular localization in the urinary bladder. The radiolabeled amiloride analogue [3H]benzamil bound to a single class of high-affinity binding sites in membrane vesicles from toad urinary bladder with a dissociation constant (Kd) of 10 nM. Photoactive benzamil analogues specifically labeled a 135,000-Da polypeptide in toad urinary bladder and TBM cells. A monoclonal anti-Na+ channel antibody directed against the amiloride-binding component of the channel specifically recognized a 135,000-Da polypeptide in TBM cells. Polyclonal anti-Na+ channel antibodies generated against purified bovine epithelial Na+ channel specifically recognized a 235,000-Da polypeptide in toad urinary bladder and localized Na+ channels to the apical plasma membrane of urinary bladder epithelial cells. The biochemical characteristics and the cellular localization of epithelial Na+ channels in toad urinary bladder are similar to those previously described in mammalian kidney and in the A6 cell line.

Basolateral potassium membrane permeability of A6 cells and cell volume regulation

The K+ permeabilities (86Rb(K) transport) of the basolateral membranes (JbK) of a renal cell line (A6) were compared under isosmotic and hypo-osmotic conditions (serosal side) to identify the various components involved in cell volume regulation. Changing the serosal solution to a hypo-osmotic one (165 mOsm) induced a fast transient increase in Cai (max < 1 min) and cell swelling (max at 3-5 min) followed by a regulatory volume decrease (5-30 min) and rise in the SCC (stabilization at 30 min). In isosmotic conditions (247 mOsm), the 86Rb(K) transport and the SCC were partially blocked by Ba2+, quinidine, TEA and glibenclamide, the latter being the least effective. Changing the osmolarity from isosmotic to hypo-osmotic resulted in an immediate (within the first 3-6 min) stimulation of the 86Rb(K) transport followed by a progressive decline to a stable value higher than that found in isosmotic conditions. A serosal Ca(2+)-free media or quinidine addition did not affect the initial osmotic stimulation of JbK but prevented its "secondary regulation", whereas TEA, glibenclamide and DIDS completely blocked the initial JbK increase. Under hypo-osmotic conditions, the initial JbK increase was enhanced by the presence of 1 mM of barium and delayed with higher concentrations (5 mM). In addition, cell volume regulation was fully blocked by quinidine, DIDS, NPPB and glibenclamide, while partly inhibited by TEA and calcium-free media. We propose that a TEA- and glibenclamide-sensitive but quinidine-insensitive increase in K+ permeability is involved in the very first phase of volume regulation of A6 cells submitted to hypo-osmotic media. In achieving cell volume regulation, it would play a complementary role to the quinidine-sensitive K+ permeability mediated by the observed calcium rise.

Tolbutamide-sensitive potassium conductance in the basolateral membrane of A6 cells

K+ channels sensitive to intracellular ATP (KATP channels) have been described in a number of cell types and are selectively inhibited by sulfonylurea drugs. To look for the presence of this type of K+ channel in the basolateral membrane of tight epithelia, we have used an amphibian renal cell line, the A6 cells, grown on filters. After the selective permeabilization of the apical membrane with amphotericin B, the basolateral conductance was studied under voltage-clamp conditions. Tolbutamide inhibited 65.8 +/- 6.3% of the barium-sensitive current. The tolbutamide-sensitive conductance had an equilibrium potential of -83 +/- 1 mV and was inward rectifying in spite of the outwardly directed K+ gradient. Similar results were obtained with glibenclamide. The half-inhibition constants were 25.7 +/- 3.0 microM and 0.114 +/- 0.018 microM for tolbutamide and glibenclamide, respectively. To study the relation between cellular ATP and the activity of this conductance, A6 cells were treated with glucose (5 mM) and insulin (250 microU/ml). This maneuver significantly increased the cellular ATP and abolished the tolbutamide-sensitive conductance. A sulfonylurea-sensitive K+ conductance is present and active in the basolateral membrane of A6 cells. This conductance appears to be modulated by physiological changes of intracellular ATP.

Virus infection of polarized epithelial cells

This chapter focuses on the interaction of viruses with epithelial cells. The role of specific pathways of virus entry and release in the pathogenesis of viral infection is examined together with the mechanisms utilized by viruses to circumvent the epithelial barrier. Polarized epithelial cells in culture, which can be grown on permeable supports, provide excellent systems for investigating the events in virus entry and release at the cellular level, and much information is being obtained using such systems. Much remains to be learned about the precise routes by which many viruses traverse the epithelial barrier to initiate their natural infection processes, although important information has been obtained in some systems. Another area of great interest for future investigation is the process of virus entry and release from other polarized cell types, including neuronal cells.

Na+/H(+)-exchange in A6 cells: polarity and vasopressin regulation

We have analyzed the mechanism of Na(+)-dependent pHi recovery from an acid load in A6 cells (an amphibian distal nephron cell line) by using the intracellular pH indicator 2'7'-bis(2-carboxyethyl)5,6 carboxyfluorescein (BCECF) and single cell microspectrofluorometry. A6 cells were found to express Na+/H(+)-exchange activity only on the basolateral membrane: Na+/H(+)-exchange activity follows simple saturation kinetics with an apparent Km for Na+ of approximately 11 mM; it is inhibited in a competitive manner by ethylisopropylamiloride (EIPA). This Na+/H(+)-exchange activity is inhibited by pharmacological activation of protein kinase A (PKA) as well as of protein kinase C (PKC). Addition of arginine vasopressin (AVP) either at low (subnanomolar) or at high (micromolar) concentrations inhibits Na+/H(+)-exchange activity; AVP stimulates IP3 production at low concentrations, whereas much higher concentrations are required to stimulate cAMP formation. These findings suggest that in A6 cells (i) Na+/H(+)-exchange is located in the basolateral membrane and (ii) PKC activation (heralded by IP3 turnover) is likely to be the mediator of AVP action at low AVP concentrations.

Early effects of aldosterone on apical and basolateral membrane conductances of TBM cells

The early effects (1-4 h) of aldosterone on membrane conductance were studied in the TBM cells, an epithelial cell line derived from the toad urinary bladder. Aldosterone stimulated Na+ transport (measured as short-circuit current) and induced parallel increases of the amiloride-sensitive apical membrane Na+ conductance and the basolateral membrane conductance. In another set of experiments, we prevented the aldosterone-induced increase of the Na+ transport by adding amiloride to the apical solution in concentrations sufficient to reduce short-circuit current to values similar to that of matched control. The effect of aldosterone on the basolateral conductance was not inhibited. This effect was essentially due to a change of the barium-sensitive K(+)-specific conductance. We also observed that aldosterone prevented a time-dependent decline of the paracellular resistance observed in control preparations. Aldosterone induces an early increase of the basolateral membrane conductance, an effect that is not secondary to the increase of the Na+ load to the cell.

Sodium-independent effect of aldosterone on initial rate of ouabain binding in A6 cells

To estimate the early effect of aldosterone on the number of active Na(+)-K+ pumps at the basolateral membrane of amphibian tight epithelia, we have measured the initial rate (at 4 min) of [3H]ouabain binding to the basolateral membrane of intact monolayers of A6 cells grown on permeable supports. Within 3 h, aldosterone induced a threefold increase of the Na transport and, simultaneously, a twofold increase of the binding rate of ouabain. Because the affinity of ouabain, estimated either by equilibrium binding studies or inhibition kinetics, was not modified by aldosterone, the effect on the initial rate of ouabain binding was due to an increase in the number of binding sites. This effect on ouabain binding was not prevented by 10 microM amiloride, which reduced the transepithelial sodium transport below control level. By contrast, the effect of aldosterone on ouabain binding was abolished by cycloheximide (5 micrograms/ml) or actinomycin D (2 micrograms/ml), doses which inhibited the aldosterone-dependent sodium transport response. These data suggest that aldosterone elicits an early, sodium-independent, protein synthesis-dependent increase in the expression of active Na(+)-K(+)-ATPase molecules.

Basolateral membrane potassium conductance of A6 cells

To study the properties of the basolateral membrane conductance of an amphibian epithelial cell line, we have adapted the technique of apical membrane selective permeabilization (Wills, N.K., Lewis, S.A., Eaton, D.C. 1979b, J. Membrane Biol. 45:81-108). Monolayers of A6 cells cultured on permeable supports were exposed to amphotericin B. The apical membrane was effectively permeabilized, while the high electrical resistance of the tight junctions and the ionic selectivity of the basolateral membrane were preserved. Thus the transepithelial current-voltage relation reflected mostly the properties of the basolateral membrane. Under "basal" conditions, the basolateral membrane conductance was inward rectifying, highly sensitive to barium but not to quinidine. After the induction of cell swelling either by adding chloride to the apical solution or by lowering the osmolarity of the basolateral solution, a large outward-rectifying K+ conductance was observed, and addition of barium or quinidine to the basolateral side inhibited, respectively, 82.4 +/- 1.9% and 90.9 +/- 1.0% of the transepithelial current at 0 mV. Barium block was voltage dependent; the half-inhibition constant (Ki) varied from 1499 +/- 97 microM at 0 mV to 5.7 +/- 0.5 microM at -120 mV. Cell swelling induces a large quinidine-sensitive K+ conductance, changing the inward-rectifying basolateral membrane conductance observed under "basal" conditions into a conductance with outward-rectifying properties.

Combination of culture on collagen gels and glucose starvation for cloning human colon cancer cells. Obtention of clones exhibiting different patterns of enterocytic differentiation

Glucose starvation has been widely used to select differentiated subpopulations from the heterogenous human colon cancer cell line HT29. We observed that the important cell loss elicited by culturing these cells in glucose-free medium could be limited when type I collagen gel was used as substratum instead of conventional plastic support. We took advantage of this property to develop a new protocol, which combined glucose starvation and culture on collagen gels, for cloning HT29 cells. Using this procedure we have isolated four clones that were characterized on the basis of morphological (optical and transmission electron microscopy), electrophysiological (determination of transepithelial electrical parameters) and biochemical (detection of villin, sucrase-isomaltase and carcinoembryonic antigen) criteria. These four clones expressed different patterns of enterocytic differentiation regarding to these criteria. These results confirmed the heterogeneity of the HT29 cell line. One of these clones, HT29-A7, which displayed numerous intercellular cysts that disappeared at confluency, appears as a complementary model in the study of epithelial biogenesis.

Apical and basolateral membrane conductances in the TBM cell line

Cultured cell lines present several advantages over whole organ or ex vivo isolated epithelium for the physiological and biochemical study of epithelial transport. We have developed a new technique allowing for simultaneous intracellular and transepithelial electrophysiological measurements in the epithelium formed by a cultured cell line grown on thin collagen membranes. This technique was applied to the TBM 18/23 (toad bladder origin) cell line. The transepithelial and basolateral membrane potentials were -30 +/- 11 and -72 +/- 8 (SD) mV (n = 36), respectively. With the use of the effect of amiloride, which partially blocked the apical membrane conductance, and circuit analysis, the apical and basolateral membrane conductances were estimated to 0.7 +/- 0.1 and 2.8 +/- 0.4 mS/cm2, respectively. A sodium-selective conductive pathway was demonstrated in the apical membrane, and a barium-sensitive K(+)-selective conductance was shown to be present in the basolateral membrane. The basolateral membrane conductance was not modified by sudden inhibition of sodium transport by amiloride, but it was significantly reduced after a long-term decrease of Na+ transport. The cultured TBM cell line appears to be a convenient model to investigate the regulation of membrane ionic conductances in tight epithelia.

Characterization of a mouse cortical collecting duct cell line

A cortical collecting duct (CCD) cell line has been developed from a mouse transgenic for the early region of simian virus 40, Tg(SV40E)Bri/7. CCDs were microdissected and placed on collagen gels. Monolayers were subsequently subcultured onto permeable collagen membranes and maintained in serum-supplemented medium. One line, designated M-1, retained many characteristics of the CCD, including a typical epithelial appearance and CCD-specific antigens. M-1 cells, when grown in monolayers on permeable supports, exhibited a high transepithelial resistance (885.7 +/- 109.6 ohms/cm2) and developed a lumen negative transepithelial potential difference (PD) of -45.7 +/- 3.5 mV. The associated short-circuit current (SCC) averaged 71.8 +/- 10.3 microA/cm2, and was reduced by 95% by luminal application of amiloride. The cultured cells responded to arginine vasopressin (AVP) with a significant increase in SCC. M-1 cells generated significant transepithelial solute gradients. After 24 hours incubation, the composition of the luminal (L) and basolateral (B) media (in mM) was: [Na+], L = 106.7 +/- 0.9 and B = 127.4 +/- 0.4; [K+], L = 8.6 +/- 0.6 and B = 2.1 +/- 0.3; [Cl], L = 68.6 +/- 5.8 and B = 101.8 +/- 6.6; [HCO3], L = 15.5 +/- 1.5 and B = 8.6 +/- 1.2; while pH was 7.16 +/- 0.03 at the luminal and 6.94 +/- 0.03 at the basolateral side. The formation of these concentration gradients indicates that the CCD cultures absorb Na+ and Cl- and secrete K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Video microscopy of intracellular pH in primary cultures of rabbit proximal and early distal tubules

The purpose of this study was to investigate intracytoplasmic pH (pHi) regulation in primary cultures of proximal (PCT) and distal bright (DCTb) convoluted tubules. PCT and DCTb segments were microdissected from rabbit kidney cortex and cultured in a hormonally defined medium. The cultured epithelia were grown on semi-transparent permeable supports. The pHi was determined by video microscopy and digital image processing using 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) and measuring the ratio of BCECF fluorescence excited by two successive wavelengths (490 nm and 450 nm). Resting pHi values, determined in bicarbonate-free medium (extracellular pH: 7.40), were 7.25 +/- 0.02 (n = 23) and 7.17 +/- 0.04 (n = 30) for cultured PCT and DCTb respectively. After the acid-loading procedure, cultured proximal cells recovered their pHi by means of the classic Na+/H+ antiporter, sensitive to amiloride and located in the apical membrane only. In cultured DCTb part of the pHi recovery was mediated by a Na+/H+ exchange present in the basolateral side. Moreover, at physiological initial pHi values, chloride removal from the apical solution caused the pHi to increase in the presence of bicarbonate. In acidified cultured DCTb cells, a partial pHi recovery was induced in sodium-free media by 15 mM HCO(-3) in the presence of an outward chloride gradient. This pHi change was completely abolished by 4,4'-diisothiocyanostilbene 2,2'-disulfonic acid (1 mM). These data suggest that DCTb cells possess in apical anion/base exchanger that resembles the Na(+)-independent Cl-/HCO(-3) exchanger.

Aldosterone receptors in A6 cells: physicochemical characterization and autoradiographic study

The A6 cell line is derived from the kidney of Xenopus laevis. Aldosterone increases sodium transport across A6 cell epithelia. In the present study, aldosterone binding characteristics were studied in A6 cell cytosol. Both type I (mineralocorticoid) and type II (glucocorticoid) receptors are present in the cytosolic fraction of these cells. Aldosterone and corticosterone had a high affinity for type I sites (Kd = 1.25 and 0.16 nM, respectively) and a lower affinity for type II sites (Kd = 39 and 10 nM, respectively). Testosterone and estradiol did not compete for aldosterone binding. RU 26988, a highly specific glucocorticoid agonist, competed with aldosterone for type II but not for type I sites. Hydrodynamic parameters of both type I and type II corticosterone receptor complexes were identical. Their Stokes radius was approximately 6 nm, as estimated by high-performance size-exclusion chromatography, and their sedimentation coefficient determined by ultracentrifugation on glycerol gradients was approximately 9s. The molecular mass calculated from these parameters was approximately 200 kDa, a value that is very close to the value estimated for nontransformed mineralocorticoid and glucocorticoid receptors of other species. The [3H]aldosterone labeling of intact A6 cells was examined by autohistoradiography. At every concentration tested (2, 20, and 50 nM), all cells were found to be specifically labeled in both cytoplasm and nucleus. At 20 nM, in the presence of an excess of RU 26988, labeling was also detected. At every concentration the labeling data was compatible with a Gaussian distribution, indicating that A6 cells correspond to a homogeneous population with regard to aldosterone binding and that probably both type I and type II sites are present in the same cells.

Inhibition of N-glycosylation affects transepithelial Na+ but not Na+-K+-ATPase transport

Tunicamycin (TM) was used in toad urinary bladder (TBM) cells to study the role of N-glycosylation of the beta-subunit of Na+-K+-ATPase. Inhibition of the beta-subunit core glycosylation was dose dependent and coincided with a specific 70% decrease in newly synthesized beta- and alpha-subunits. Na+-K+-ATPase activity paralleled the decrease in the cellular content of the alpha-subunit, although the cellular and cell surface-expressed Na+-K+-ATPase pool was progressively filled up with nonglycosylated beta-subunits. In addition, the decrease in maximal Na+ transport capacity of the Na+-K+-ATPase as assessed by short-circuit current (SCC) measurements in the presence of amphotericin B correlated with the decrease in the total cell surface-expressed beta-subunit population despite the fact that it was composed of 47% nonglycosylated beta-subunits after 42 h of TM treatment. These results are consistent with the interpretation that beta-subunit glycosylation is not important either for the enzyme's intracellular sorting to the plasma membrane or its hydrolytic and transport properties. Finally, TM produced effects on basal SCC and electrical resistance that differed in their times of onset and time periods needed for recovery. Thus, in addition to the Na+-K+-ATPase, other glycoproteins in the apical membrane and the tight junctions must be implicated in the maintenance of transepithelial Na+ transport.

Human sweat duct cells in primary culture. Basic bioelectric properties of cultures derived from normals and patients with cystic fibrosis

Human sweat duct cells from the coiled reabsorptive segment have been cultured successfully, free from fibroblasts, in a low serum, hormone-supplemented medium, Ham's F12. The cultured cells exhibited a typical epithelial cobblestone pattern, and microvilli-covered luminal cells were seen joined together with typical junctional complexes. In cultures derived from normals and patients with cystic fibrosis (CF), growth and morphologic characteristics were indistinguishable. When grown on a membranous support, and mounted in an Ussing chamber, vectorial electroconductive ion-transport could be identified. The epithelial preparations produced active mucosa to serosa-directed sodium flux via amiloride-sensitive, apical sodium channels and ouabain-sensitive sodium pumps located in the basolateral membrane, which also contained a potassium shunt. These findings are consistent with a polarized epithelium with properties similar to the intact organ. High transepithelial resistance and increased amiloride sensitivity were typical for cells derived from CF, indicating that principal normal as well as pathologic properties of the sweat duct are preserved in culture.

Antigenic expression of aminopeptidase M, dipeptidyl-peptidase IV and endopeptidase by primary cultures from rabbit kidney proximal tubule

Techniques using microdissected tubules from rabbit kidney allow the isolation of well defined segments which can be cultured to obtain pure renal cell epithelia. From microdissected proximal tubules, we obtained epithelia the cells of which exhibit some of the antigenic expressions of the initial proximal cells. For this purpose, we used three monoclonal antibodies raised against apical brush border membranes of the proximal tubules. We determined with precision the identity and some of the molecular characteristics of the antigens bound by these three antibodies and found that they correspond to three hydrolases present in the brush borders of proximal renal cells (amino-peptidase, dipeptidyl-peptidase IV and endopeptidase). These apical markers are expressed by the growing cells of primary cultures from proximal tubules, suggesting strongly that they are effectively proximal cells and that no appreciable dedifferentiation occurred during the growth process. We have also shown that apical expression of these hydrolases on the plasma membrane of the epithelium occurred only after several days of culture and determined the complete polarization of the cells. Electron microscopy studies confirmed the degree of polarization of the cultured cells by the presence of numerous microvilli on their apical face.

Dome formation of keratin-containing agranular cells from rat anterior pituitary gland in vitro

A certain kind of cell in the pituitary gland exhibited immunoreactive keratin and dome formations in vitro. We obtained epithelial cells, which were able to subculture, from the outgrowth of anterior pituitary organ cultures. These cells lacked hormone secretory granules and exhibited immunoreactive keratin. Furthermore, they produced dome formations or cystic structures in monolayer culture and under three-dimensional culture condition using type I collagen gel. Dome formation was stimulated by dibutyryl cyclic AMP (dbcAMP, 10(-3) to 10(-5) M). Their responsiveness to dbcAMP is similar to that of several other epithelial cells that possess transport functions in vivo and in vitro. Although the origin of our cultured cells is unknown, these cells formed dome formations that possessed transport function and were related to cystic structures in the pituitary gland in vivo.

Patch clamp study on primary culture of isolated proximal convoluted tubules

Primary cultures were obtained from microdissected rabbit proximal tubules (S1 segments). The growing epithelia were maintained in culture for up to 30 days. Electron microscopy study revealed that the cells formed a monolayer and showed a morphological polarity with apical microvilli and tight junctions. An immunofluorescence technique using two monoclonal antibodies raised against two apical brush border enzymes of the proximal tubule (LAP, DPP IV) revealed that these enzymes were expressed in the cultured cells. Membrane associated and cytosolic enzyme activities were measured on 12, 20 and 30-day-old cultures. Cultured epithelia exhibited leucine aminopeptidase, gamma glutamyl transferase and fructose 1-6 biphosphatase activities that remained constant for up to 30 days, whereas alkaline phosphatase activity decreased in the oldest cultures. Hexokinase activity on the other hand, increased after 12 days of culture. Cyclic AMP synthesis was stimulated by parathyroid hormone at 12, 20 and 30 days of culture and was insensitive to arginine vasopressin. After 20 days of culture the epithelia grown on permeable supports developed a transepithelial potential of -0.13 mV (apical negative) and a transepithelial resistance of 37 omega cm2 that increased to -1.13 mV and 60 omega cm2 respectively in 30-day-old cultures. The patch clamp technique was applied to the apical membrane of 12-15-day-old cultures. In the whole cell recording configuration, a cellular potential of -61.5 mV was measured, which was mainly due to K+ diffusion. A non-selective cationic channel was present in the apical membrane of the cultured cells. In cell-attached patches the channel carried an inward current and had a conductance of 13 pS. On excised patches the channel discriminated poorly between Na+ and K+ and was impermeant to Cl- and its conductance ranged between 20 and 28 pS. The channel activity was not voltage dependent but required a high calcium concentration (1 mM Ca2+) on the cytoplasmic face.

Role of the Na,K-ATPase beta-subunit in the cellular accumulation and maturation of the enzyme as assessed by glycosylation inhibitors

No functional role could yet be established for the glycosylated beta-subunit of the Na,K-ATPase. In this study, we describe the intracellular processing of the beta-subunit as a glycoprotein in toad bladder cells and the consequences of its structural perturbation with glycosylation inhibitors on the cellular expression of the alpha- and beta-subunits and on the structural and functional maturation of the enzyme. Controlled trypsinolysis of homogenates from pulse-labeled cells reveals that the beta-subunit is subjected to glycosylation-dependent structural rearrangements during its intracellular routing. Inhibition of correct terminal glycosylation of the beta-subunit with deoxynojirimycin or swainsonine has no effect on the trypsin sensitivity of the alpha-subunit, its ability to perform cation-dependent conformation changes or the cellular Na,K-ATPase activity. Acquisition of core-sugars is sufficient for the enzyme to assume its catalytic functions. On the other hand, complete inhibition of glycosylation with tunicamycin leads to a destabilization of both the beta- and the alpha-subunits as judged by their higher trypsin sensitivity. In addition, tunicamycin treatment results in a decrease of the amount of newly synthesized beta- and alpha-subunit indicating that a glycoprotein, possibly the beta-subunit itself, plays a role in the efficient accumulation of the alpha-subunit in the endoplasmic reticulum.

Experimental pitfalls in evaluating vectorial protein secretion in vitro; Sertoli cell secretion of androgen-binding protein and transferrin in two-compartment culture chambers

We examined the influence of various Millipore filter pretreatments on the amounts of androgen-binding protein (ABP) and transferrin (Trf) found in the outer (OC) and inner (IC) compartment of two-compartment Sertoli cell (Sc) cultures. When Sc were cultured on untreated Millipore filters, less than 10% of ABP was found in OC during 3 initial culture days compared to similar cultures on pretreated filters. Most of the glycoprotein was shown to be bound by the filter. Pretreatment of Millipore filters with 5% bovine serum albumin (BSA) or 2% fetal bovine serum (FBS) maximally saturated the nonspecific protein-binding sites resulting in OC:IC ratio of ABP similar to that found in cultures on polycarbonate membranes, which exhibit very low protein-binding capacity. In contrast to ABP, about 40% of Trf was bound by the Millipore filter on Day 1, with only trace amounts bound thereafter. These differences were due to much higher secretion rate of Trf than ABP, resulting in a relatively smaller fraction of Trf bound to the filter. Again, the nonspecific binding of Trf was greatly reduced by filter pretreatment with 5% BSA or 2% FBS. It is concluded that complete saturation of protein-binding sites of cellulose ester supports is necessary for reliable evaluation of vectorial protein secretion by Sc and other polarized epithelial cells maintained in this type of culture. The implications of partial saturation of protein-binding sites of culture support in interpreting experimental results are discussed.

Comparison of ion transport by cultured secretory and absorptive canine airway epithelia

The use of primary cell culture techniques to predict the function of native respiratory epithelia was tested in studies of dog airway epithelia. Epithelial cells from Cl- secretory (tracheal) and Na+ absorptive (bronchial) airway regions were isolated by enzymatic digestion, plated on collagen matrices, and maintained in serum-free, hormone-supplemented media. Transepithelial and intracellular studies showed that both the tracheal and bronchial culture preparations exhibited bioelectric parameters quantitatively similar to those of intact tissues. Similar to the native tissue, the tracheal preparation exhibited an equivalent short-circuit circuit (Ieq) that was sensitive to inhibitors of Cl- transport (bumetanide, diphenylamine carboxylic acid) but was insensitive to an inhibitor of Na+ transport, amiloride. In contrast, the bronchial preparation, like the native tissue, exhibited an Ieq sensitive to amiloride but insensitive to bumetanide. As compared with the trachea, the bronchial (absorptive) epithelium is characterized by 1) a large amiloride-sensitive cellular conductance and 2) a relatively depolarized basolateral membrane. We conclude that this primary cell culture technique provides epithelial preparations comparable to the native tissue and suitable for quantitative studies of regional differences in ion transport function.

Purification of PRL receptors from toad kidney: comparisons with rabbit mammary PRL receptors

The binding characteristics of the prolactin (PRL) receptors present in toad (Bufo marinus) kidneys were investigated and compared to those of PRL receptors present in rabbit mammary glands. The molecular characteristics of the Triton X-100 solubilized renal and mammary PRL receptors were assessed by gel filtration and by migration analysis on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) after affinity labeling of the binding sites with 125I-human growth hormone. Similar results were obtained for both receptors. Partial purification of the toad PRL receptor could be achieved by affinity chromatography. The molecular weight of this purified receptor could be determined by analysis on SDS-PAGE. With the use of a polyclonal antiserum raised against a purified preparation of rabbit mammary PRL receptor, one or several antigenic epitope(s) could be identified on the core of the toad renal PRL receptor. In conclusion, although the structure and the biological role(s) of PRL have substantially changed during evolution, the receptor for this hormone has retained many of its structural features as could be assessed between an amphibian and a mammalian species on functionally different target tissues.

Maturation of the catalytic alpha-subunit of Na,K-ATPase during intracellular transport

The protease sensitivity of the catalytic alpha-subunit of Na,K-ATPase during intracellular transport along the exocytic pathway has been investigated in two amphibian epithelial cell lines. Controlled trypsinolysis followed by immunoprecipitation of cell homogenates or microsomal fractions from [35S]methionine pulse-chased A6 kidney cells revealed distinct cleavage patterns by SDS-PAGE. Shortly after synthesis (7-min pulse), the 98-kD alpha-subunit is fully sensitive to trypsin digestion and is cleaved into a 35-kD membrane-bound and a 27.5-kD soluble peptide. With a 15-min pulse, 10% of the newly synthesized polypeptide becomes resistant to trypsin digestion. With longer chase time, the proportion of protease-resistant alpha-subunit further increases. Concomitantly, the alpha-subunit acquires the ability to undergo cation-dependent conformational transitions, as reflected by distinct tryptic digest patterns in the presence of Na+ or K+. Similar results were obtained in TBM cells, a toad bladder cell line. Our data indicate that the catalytic subunit of Na,K-ATPase is structurally rearranged during intracellular transport from its site of synthesis to its site of action at the cell surface, a modification which might mark the functional maturation of the enzyme.

Spontaneous reversal of polarity of the voltage across LLC-PK1 renal epithelial cell sheets

While sterilely monitoring transepithelial voltage (potential difference) across LLC-PK cell sheets over a 24-hr period, we noted that the apical-negative, transepithelial voltage, a key property of the LLC-PK1 renal epithelial cell line, reverses polarity to become apical-positive. This spontaneous change of polarity of electrical potential difference (PD) across LLC-PK1 cell sheets cultured on permeable filters was observed to occur approximately 12 hr after refeeding. Unlike the apical (luminal)-negative PD, the apical-positive PD was insensitive to phlorizin and ouabain. Both were insensitive to the diuretics amiloride, furosemide, and 4-acetamido-4-isothiocynato-stilbene-2,2-disulfonic acid (SITS). A pH gradient existed across apical-positive cell sheets (apical medium more acidic by 0.3 units) but an osmotic gradient did not. Unlike the temperature-sensitive apical-negative PD, the apical positive PD was unaffected by brief exposure to 4 degrees C temperature. Junctional disruptive agents such as the tumor promotor, TPA, dissipated both types of PD with similar time courses. The formation of the apical-positive PD correlated in time with apical glucose levels falling below the reported Km of the Na+-sugar contransporter. A high glycolytic rate per se may not be essential for this PD polarity reversal since the reversal could occur in glucose-free medium with a normal time course and magnitude. The lysis with time of floating cells with consequent release of KCl into the apical compartment was also considered as a possible cause of the polarity reversal, but the turnover of even 2 X 10(6) cells in 12 hr was found not to raise apical KCl sufficiently to produce the polarity shift. Although a significant K+ gradient did not exist across cell sheets with apical-positive PD values, a sizable gradient of Cl- did exist, directed apical to basolateral. This gradient, coupled with anion-selective tight junctions, should contribute to the observed apical positive voltage. The voltage polarity shift seen in these cell cultures with time is not unlike the polarity shift occurring in the renal proximal convoluted tubule, with distance from the glomerulus.

The effect of neutrophil migration on epithelial permeability

To reach an inflammatory lesion, neutrophils must frequently traverse the epithelium of an infected organ. Whether the actual migration of neutrophils alters the epithelial permeability is unknown. Through the use of an in vitro model system it was possible to directly determine the effect of neutrophil emigration on the transepithelial electrical resistance of the monolayer. Human neutrophils (5 X 10(6) cells/ml) were placed in the upper compartment of a combined chemotaxis/resistance chamber and stimulated for 40 min by a gradient of 10(-7) M n-formyl-methionyl-leucyl-phenylalanine to traverse a confluent monolayer of canine kidney epithelial cells grown on micropore filters. Neither the chemoattractant alone (10(-5)-10(-9) M) nor the accumulation of an average of eight neutrophils per millimeter of epithelium lowered the transepithelial electrical resistance. However, under certain conditions the migration of neutrophils temporarily increased the permeability of the monolayer. The resistance fell approximately 48% within 5 min if the migratory cells were stimulated to reverse their migration across the same monolayer. As re-migration continued, the resistance returned to its initial levels within 60 min. Doubling the initial neutrophil concentration to 10 X 10(6) cells/ml resulted in the accumulation of an average of 66 neutrophils per millimeter of epithelium and an average fall in resistance of 46% (r = 0.98; P less than 0.001) in 40 min. If the resistance had fallen less than 45%, removal of the neutrophils remaining in the upper compartment resulted in a return of the transepithelial electrical resistance to its initial level within 65 min. However, when the fall was greater than 45%, the resistance only recovered to 23.5% of its initial levels within the same time frame. Thus, these results suggest that the integrity of an epithelium can, under certain conditions, be affected by the emigration of neutrophils, but that this effect is either completely or partially reversible within 65 min.

Increased chloride permeability of amphibian epithelia treated with aldosterone

The transepithelial flux of chloride was increased by aldosterone treatment of amphibian skin and bladder and this was reflected by increased "shunt" conductance. The hormonal effect depended on the presence of chloride on the epithelial side of the preparation. These changes in tissue conductance and chloride permeability appear to be a direct effect of aldosterone as they did not occur when sodium transport was stimulated with vasopressin or hypotonicity. Chloride efflux was reduced in magnitude by indacrinone and DIDS, as well as after removal of chloride from the solution on the epithelial side of the preparations. These results suggest that, rather than merely diffusing along (a) paracellular pathway(s), chloride flows through (a) cellular structure(s), notably mitochondria-rich cells. These cells can therefore be considered as targets for aldosterone.