Cell transformation induces a cytoplasmic Ca2+ oscillator in Madin-Darby canine kidney cells

Alkaline stress transforms Madin-Darby canine kidney (MDCK) cells as indicated by loss of epithelial structure, multilayer cell growth and formation of foci. In the present study we report that transformed MDCK cells (MDCK-F cells) exhibit spontaneous and lasting oscillations of intracellular Ca2+ concentration ([Ca2+]i), which are absent in non-transformed cells. Oscillations, as revealed by Fura-2 video imaging, were due to the activity of an inositol 1,4,5-trisphosphate-(InsP3)-sensitive Ca2+ store since their frequency was dependent on bradykinin concentration and they were abolished by the phosphoinositidase C inhibitor U73122. Moreover, blockers of the cytoplasmic Ca(2+)-ATPase, thapsigargin and 2,5-di-(tetr-butyl)-1,4-benzohydroquinone inhibited oscillatory activity. In contrast, neither injection of ruthenium red, ryanodine nor caffeine had any effect on oscillations. Analysis of the spatial distribution of [Ca2+]i showed that Ca2+ transients originated from an initiation site constant for a given cell and spread through the cell as an advancing Ca2+ wave. Oscillations started in a random manner from single cells and spread over neighbouring cells, suggesting a kind of intercellular communication. We conclude that MDCK-F cells have acquired the ability for endogenous Ca2+ release through transformation. Oscillations are primarily due to the activity of an InsP3-sensitive cytosolic Ca2+ oscillator.

Cytoplasmic Ca2+ determines the rate of Ca2+ entry into Mardin-Darby canine kidney-focus (MDCK-F) cells

Transformed Mardin-Darby canine kidney-focus (MDCK-F) cells exhibit spontaneous Ca2+ oscillations from an inositol 1,4,5-triphosphate-sensitive cytoplasmic Ca2+ store. In this study, Ca2+ entry from the extracellular space and its role in generation of oscillations were investigated by means of Ca2+ video imaging and the Fura-2/Mn2+ quenching technique. Oscillations were dependent on extracellular Ca2+ concentration and were inhibited by extracellularly applied La3+, Co2+ and Ni2+. Depolarization of the cell membrane with high K+ concentrations and the L-type Ca2+ channel blocker nifedipine had no effect on oscillations, indicating the lack of involvement of voltage-gated Ca2+ channels. Mn2+ quenching experiments disclosed significant Ca2+ influx into MDCK-F cells. The rate of this influx was constant between Ca2+ spikes, but markedly increased during the spontaneous Ca2+ spikes. Similar transient increases in Ca2+ entry could be mimicked by agents triggering intracellular Ca2+ release such as bradykinin and thapsigargin. We conclude that the plasma membrane of MDCK-F cells exhibits a marked voltage-independent Ca2+ permeability permitting Ca2+ entry into the cytoplasm. The rate of Ca2+ entry which determines the frequency of oscillations is most likely to be regulated by the cytoplasmic Ca2+ concentration.

Imaging the lamellipodium of migrating epithelial cells in vivo by atomic force microscopy

Cell locomotion originates at a specific region of the cell surface, the leading edge of a migrating cell. Various factors have been proposed to contribute to the propulsion of a cell over the substratum. Rapid turnover processes of cytoskeletal elements inside the cell and insertion of new plasma membrane at the leading edge of the cell permit the extension of a cell in a given direction. Our goal was to image in vivo plasma membrane turnover by means of atomic force microscopy (AFM) and to resolve dynamic processes at the nanometer level. As an experimental model we used migrating kidney cells derived from the Madin-Darby canine kidney (MDCK) cell line that was transformed by alkaline stress. These so-called MDCK-F cells exhibit spontaneous calcium-dependent oscillatory activity of plasma membrane potential associated with cell locomotion. We imaged cells during migration and observed dynamic invagination processes in the cell surface close to the leading edge, indicating internalization of plasma membrane. Invaginations were prevented by removal of calcium from the perfusate. During calcium reduction plasma membrane uncoupled from the underlying cytoskeleton and lipidic pores with diameters of about 30 nm could be disclosed and imaged. This study demonstrates that the AFM can readily trace dynamic physiological processes in vivo, emphasizing the potential role of calcium in maintaining plasma membrane integrity and function.

Ochratoxin A impairs "postproximal" nephron function in vivo and blocks plasma membrane anion conductance in Madin-Darby canine kidney cells in vitro

Ochratoxin A (OTA) is a widespread nephrotoxin which causes porcine nephropathy and is supposed to have caused the human Balkan endemic nephropathy. We performed experiments in vivo and in vitro to elucidate the mechanism of OTA action in renal epithelium. Application of OTA to male Wistar rats [1.25 mumol/(kg.day)] for 6 days led to a reduction of glomerular filtration rate (to 63% of control), an increased fractional water (194% of control), Na+ (199% of control), K+ (147% of control) and Cl- (270% of control) excretion and an increased dependence of the osmole clearance on urine flow. Acute application of OTA to rats (3 mumol/kg) increased urinary pH from 6.0 +/- 0.2 to 6.6 +/- 0.1 and urinary NaCl excretion, but decreased titratable acid excretion to 47% of control. As these in vivo findings may be the result of an action of OTA beyond the proximal tubule ("postproximal") we investigated the effect of OTA on cultured Madin-Darby canine kidney (MDCK) cells, regarded as a model of collecting duct epithelium. In confluent monolayers formed by MDCK cells OTA reduced the number of domes in a dose-dependent manner and impaired the formation of a transepithelial Cl- gradient. Electrophysiological measurements in giant MDCK cells revealed that OTA blocks fractional anion conductance of the plasma membrane with an IC50 value of 30 +/- 5 nmol/l, unmasking OTA as a naturally occurring anion conductance blocker about 20-times more effective than the most potent synthetic blocker 5-nitro-2-(3-phenylpropyl-amino) benzoic acid (NPPB) (IC50 = 600 +/- 50 nmol/l).(ABSTRACT TRUNCATED AT 250 WORDS)

20-OH-ecdysone swells nuclear volume by alkalinization in salivary glands of Drosophila melanogaster

Ecdysteroids play an important role in the larval moulting process of insects. Ecdysone-induced stimulation causes specific "puffs" in polytene chromosomes of salivary gland cells resulting in nuclear swelling. During this process, changes of intracellular ion composition are thought to act as an early regulatory mechanism of gene activation. By use of video-imaging analysis and electrophysiological techniques, we examined ecdysone-induced nuclear swelling in Drosophila salivary glands in situ and its dependence on pH and calcium. Isolated glands of the third larval stage were superfused with a solution mimicking the haemolymph. Addition of 5 x 10(-6) mol/1 20-OH-ecdysone led, after a lag period of 50 min, to a sustained Ca(2+)-dependent increase of nuclear volume by 23.0 +/- 2.3%. Amiloride, a blocker of plasma membrane Na+/H+ exchange, prevented 20-OH-ecdysone-induced nuclear swelling. Decreasing pH in the superfusate from 7.15 to 6.8 led to nuclear shrinkage by 16.9 +/- 3.9%. Measurements of pH in salivary gland cells with ion-sensitive microelectrodes disclosed an alkalinization of 0.23 +/- 0.05 pH units after stimulation with 20-OH-ecdysone. We postulate that 20-OH-ecdysone activates the amiloride-sensitive plasma membrane Na+/H+ exchanger. This leads to intracellular alkalinization and concomitant decondensation of the nuclear chromatin visible as nuclear swelling. Thus, cell alkalinization could be a potentially important stimulatory mechanism in mediating ecdysteroid-induced activation of the cell nucleus.

Spontaneously oscillating K+ channel activity in transformed Madin-Darby canine kidney cells

Intracellular alkalinization is known to be associated with tumorigenic transformation. Besides phenotypical alterations alkali-transformed Madin-Darby canine kidney (MDCK) cells exhibit a spontaneously oscillating cell membrane potential (PD). Using single-channel patch clamp techniques, it was the aim of this study to identify the ion channel underlying the rhythmic hyperpolarizations of the PD. In the cell-attached patch configuration, we found that channel activity was oscillating. The frequency of channel oscillations is 1.1 +/- 0.1 min-1. At the peak of oscillatory channel activity, single-channel current was -2.7 +/- 0.05 pA, and in the resting state it was -1.95 +/- 0.05 pA. Given the single-channel conductance of 53 +/- 3 pS for inward (and of 27 +/- 5 pS for outward) current the difference of single-channel current amplitude corresponded to a hyperpolarization of approximately 14 mV. The channel is selective for K+ over Na+. Channel kinetics are characterized by one open and by three closed time constants. The channel is Ca2+ sensitive. Half maximal activation in the inside-out patch mode is achieved at a Ca2+ concentration of 10 mumol/liter. In addition, we also found a 13-pS K+ channel that shows no oscillatory activity in the cell-attached patch configuration and that was not Ca2+ sensitive. We conclude that the Ca(2+)-sensitive 53-pS K+ channel is underlying spontaneous oscillations of the PD. It has virtually identical biophysical properties as a Ca(2+)-sensitive K+ channel in nontransformed parent MDCK cells. Hence, alkali-induced transformation of MDCK cells did not affect the channel protein itself but its regulators thereby causing spontaneous fluctuations of the PD.

Role of H+ ions in volume and voltage of epithelial cell nuclei

Condensation of chromatin depends upon the ion composition in the cell nucleus. We tested in isolated nuclei of Madin-Darby canine kidney cells the influence of various ions on nuclear volume (i. e. DNA packing) and intranuclear voltage. After isolation, nuclei were superfused with cytosolic solutions in which Na+, K+, Ca2+ and H+ ions were varied. With video-imaging and microelectrode techniques nuclear volume and intranuclear potential were measured in response to the various ions. In control cytosolic solution, isolated nuclei exhibited an intranuclear electrical potential of -6.5 +/- 0.5 mV (relative to a reference electrode in the cytosolic solution) corresponding to a nuclear volume of 250 +/- 10 fl (n = 104). Changing the Na+, K+ or free Ca2+ concentration in the superfusate in the physiological range resulted in minor changes of volume and intranuclear potential whereas pH altered both parameters dramatically. Nuclear swelling and intranuclear negative voltage increased with alkalinization and decreased when pH was reduced. An intact nuclear envelope was found to be no prerequisite for maintaining intranuclear negativity, indicating that the composition and functional state of nuclear chromatin rather than specific ion permeabilities of the nuclear envelope determine nuclear electrical potential. We present a model that explains nuclear volume and voltage on the basis of interaction between negatively charged DNA and positively charged histones of the nuclear chromatin.

Mechanism of activation of K+ channels by minoxidil-sulfate in Madin-Darby canine kidney cells

We studied the mechanism of K+ channel activation by minoxidil-sulfate (MxSO4) in fused Madin-Darby canine kidney (MDCK) cells. Patch-clamp techniques were used to assess single channel activity, and fluorescent dye techniques to monitor cell calcium. A Ca(2+)-dependent inward-rectifying K+ channel with slope conductances of 53 +/- 3 (negative potential range) and 20 +/- 3 pS (positive potential range) was identified. Channel activity is minimal in cell-attached patches. MxSO4 initiated both transient channel activation and an increase of intracellular Ca2+ (from 94.2 +/- 9.1 to 475 +/- 12.6 nmol/liter). The observation that K+ channel activity of excised inside-out patches was detected only at Ca2+ concentrations in excess of 10 mumol/liter suggests the involvement of additional mechanisms during channel activation by MxSO4. Transient K+ channel activity was also induced in cell-attached patches by 10 mumol/liter of the protein kinase C activator 1-oleoyl-2-acetyl-glycerol (OAG). OAG (10 mumol/liter in the presence of 1.6 mmol/liter ATP) increased the Ca2+ sensitivity of the K+ channel in inside-out patches significantly by lowering the Km for Ca2+ from 100 mumol/liter to 100 nmol/liter. The channel activation by OAG was reversed by the protein kinase inhibitor H8. Staurosporine, a PKC inhibitor, blocked the effect of MxSO4 on K+ channel activation. We conclude that MxSO4-induced K+ channel activity is mediated by the synergistic effects of an increase in intracellular Ca2+ and a PKC-mediated enhancement of the K+ channel's sensitivity to Ca2+.

Oscillations: a key event in transformed renal epithelial cells

Intracellular pH (pHi) plays a critical role in the entry of cells into the DNA-synthesis phase of the cell cycle. Alterations in pHi may contribute to abnormal proliferative responses such as those seen in tumorigenic cells. We observed that alkaline stress leads to genomic transformation of Madin-Darby canine kidney (MDCK) cells. Transformed cells (F cells) form "foci" in culture, lack contact inhibition, and are able to migrate, typical characteristics of dedifferentiated tumorigenic cells. F cells exhibit spontaneous biorhythmicity. Rhythmic transmembrane Ca2+ flux activates plasma membrane K+ channels and Na+/H+ exchange. This leads to periodic changes of membrane voltage and pHi at about one cycle per minute. We conclude that endogenous oscillatory activity could be a trigger mechanism for DNA synthesis, proliferation, and abnormal growth of renal epithelial cells in culture.

Renal potassium bicarbonate release in humans exposed to an acute volume load

Cells of the renal medulla regulate their volume by transmembrane ion movements when exposed to large changes in osmolality. Since renal cells in culture release KHCO3 in response to hypotonic stress [11], we investigated the effect of an acute water load on urinary KHCO3 excretion in 5 healthy individuals. Water diuresis was induced by the ingestion of 1.5 l hypoosmolal fluid (22 mosm/kg H2O) over 15 min. The rate of urinary volume excretion increased from an initial value of 1.4 ml/min to 9.3 ml/min after 75 min. Urinary osmolality dropped from an initial value of 940 +/- 32 mosm/kg H2O to 74 +/- 4 mosm/kg H2O (n = 5). The decrease of osmolality was accompanied by the transient release of potassium and bicarbonate. Peak values of KHCO3 excretion were observed between 30 and 45 min after the onset of the experiment corresponding to the drop of urinary osmolality. The magnitude of renal potassium release correlated significantly (r = 0.93; P less than 0.05) with endogenous plasma aldosterone concentrations measured prior to the experiment in the 5 volunteers. We conclude that medullary epithelial cells release KHCO3 when exposed to hypotonic stress. The volume regulatory response is upregulated by aldosterone.

Spontaneous membrane potential oscillations in Madin-Darby canine kidney cells transformed by alkaline stress

High pH is known to be associated with normal cell growth and neoplastic transformation. We observed that Madin-Darby canine kidney (MDCK) cells grown under sustained alkaline stress (pH 7.7) develop "foci" composed of spindle-shaped cells lacking contact inhibition and exhibiting only poor adhesion to the culture support. Foci-developing (F) cells were cloned and grown in control medium (pH 7.4), where they maintained their neoplastic features indicating a stable pH-induced genetic transformation. After F cells had been fused to giant cells with polyethylene glycol, the cell membrane potential (Vm) was measured by means of microelectrodes. In contrast to non-transformed MDCK cells, Vm of F cells showed spontaneous biorhythmicity caused by periodic opening of Ca2(+)-activated K+ channels. Spiking activity was blunted by the Ca2+ channel blocker nifedipine, by the K+ channel blocker Ba2+, by the Na+/H+ exchange blocker amiloride and its analogue ethylisopropylamiloride, and by an extracellular pH of 7.6 and 6.8. We conclude that MDCK cells transformed by sustained alkaline stress have lost their stable plasma membrane potential but, instead, exhibit endogenous Ca2(+)- and pH-sensitive oscillations.

Endothelin-1 blunts transepithelial transport and differentiation of Madin-Darby canine kidney cells

We investigated the effects of endothelin-1 (ET-1) on Madin-Darby canine kidney (MDCK) cells, a cell line originating from the renal collecting duct. The activity of transepithelial transport was assessed as the rate of dome formation in monolayers grown on solid support. The pH value of the dome fluid (dome pH) was measured by means of pH-selective microelectrodes. Differentiation of monolayer cells was estimated as the peanut-lectin(PNA)-binding capacity of the apical membrane. Confluent monolayers were incubated for 12-72 h in serum-free medium at various concentrations of ET-1. Exposure to 1 nmol/l ET-1 reduced dome formation by a maximum of 41 +/- 8% (n = 4; P less than 0.02) after 24 h. ET-1 (10 nmol/l; 24 h) decreased dome pH from 7.52 +/- 0.02 (n = 53) to 7.36 +/- 0.03 (n = 51; P less than 0.02). Apical application of amiloride (1 mmol/l) reduced dome pH in both ET-1-treated and non-treated domes to essentially the same level, 7.25 +/- 0.03 (n = 19) and 7.23 +/- 0.03 (n = 17) respectively. ET-1 (10 nmol/l; 24 h) reduced PNA-binding capacity by 19 +/- 3% (n = 5; P less than 0.02). Moreover, ET-1 prevented the increase in PNA binding (+ 53 +/- 7%; n = 5) induced by 0.1 mumol/l aldosterone. We conclude that ET-1 inhibits transepithelial transport and PNA binding via inhibition of apical Na+/H+ exchange, thus antagonizing aldosterone action in MDCK cells.

Patchy accumulation of apical Na+ transporters allows cross talk between extracellular space and cell nucleus

Intracellular Na+ activities and local current densities were measured in fused Madin-Darby canine kidney cells using Na+ and voltage-sensing microelectrodes. Na+ that enters the cell across the apical plasma membrane accumulates initially in the nucleoplasm, several seconds ahead of its appearance in the cell cytoplasm. The spatial distribution of Na+ currents, produced by a local superfusion of the cell surface, indicates a nonuniform, patchy accumulation of apical Na+ transporters in the vicinity of the nucleus. Such pathways for direct Na+ flux between extracellular space and cell nucleus could be potentially important for gene activation.

Alkaline stress transforms Madin-Darby canine kidney cells

Similar to growth factors aldosterone stimulates Na+/H+ exchange in renal target cells leading to cytoplasmic alkalinization. An alkaline intracellular pH reduces the H+ bonds between repressor proteins and DNA leading to the destabilization of the nuclear chromatin. We observed that sustained alkaline stress "per se" can lead to malignant transformation of Madin-Darby canine kidney (MDCK) cells. Cells grown for two weeks in alkaline culture medium (pH 7.8) developed multiple "foci" composed of spindle-shaped pleomorphic cells lacking contact inhibition and exhibiting poor adhesion to the culture support, typical characteristics of dedifferentiated tumor cells. "Focus" cells were cloned and grown in standard medium (pH 7.4). Cells maintained their abnormal growth pattern, indicating stable pH-induced genetic transformation. Cells were fused with polyethylene glycol to giant cells and impaled with microelectrodes. In contrast to non-transformed giant MDCK cells the plasma membrane potential showed spontaneous oscillations that could be virtually abolished by the omission of extracellular Ca2+ or by the addition of the K+ channel blocker Ba2+. We conclude that sustained alkaline stress can induce malignant transformation in MDCK cells indicated by an abnormal growth pattern and by membrane potential oscillations most likely due to Ca2+ activated K+ channels in the plasma membrane.

Hypertonicity in fused Madin-Darby canine kidney cells: transient rise in NaHCO3 followed by sustained KCl accumulation

We investigated mechanisms of regulatory volume increase in fused Madin-Darby canine kidney (MDCK) cells, a cell line originally derived from renal collecting duct. The intracellular ion concentrations as well as the concentration of the volume marker tetramethylammonium+ were measured by means of ion-selective microelectrodes. Application of hypertonic Ringer bicarbonate solution (+150 mmol/l mannitol) resulted in cell shrinkage to 84 +/- 2% of the initial cell volume (shrinkage expected for an ideal osmometer = 66%), indicating a significant regulatory volume increase. During the first 90 s of the hypertonic stress, a transient increase in intracellular Na+ and HCO3- concentrations was observed. It was followed by a sustained increase in intracellular K+ and Cl- concentrations. Ouabain (0.1 mmol/l) as well as amiloride (1 mmol/l) reduced K+ accumulation significantly, whereas the H+/K(+)-ATPase inhibitor SCH 28080 had no effect. Hypertonic stress hyperpolarized the cell membrane potential by 19 +/- 2 mV, owing to the decrease of the ratio of Cl- conductance to K+ conductance of the cell membrane. We conclude: (a) acute hypertonic stress activates Na+/H+ exchange in MDCK cells; (b) transient alteration of intracellular Na+ and pH stimulates Na+/K(+)-ATPase and Cl-/HCO3- exchange, exchange, both leading to the sustained intracellular accumulation of KCl; (c) a high intracellular KCl concentration is maintained by the partial reversion of the Cl-/K+ conductance ratio of the plasma membrane.

Epithelial Cell Fusion: New Tool for Cellular and Molecular Physiology

Osmotic shock leads to the formation of giant epithelial cells. Grown in culture, they provide a sensitive assay system for the identification of mRNA and serve as a model system in localizing ion transporters in the plasma membrane.

Hypotonic stress-induced release of KHCO3 in fused renal epitheloid (MDCK) cells

Mechanisms of cell volume regulation induced by the reduction of the osmolality of the Ringer solution by one-third were studied in fused Madin-Darby canine kidney (MDCK) cells. Intracellular HCO3-, K+ and Cl- concentrations [ion]i in parallel with cell membrane potential (PD), cell membrane conductance (Gm) and conductances of individual ions (Gmion) were evaluated with microelectrode techniques. Fused cells regulate their cell volume by about 50%. Gm increased from 0.43 +/- 0.03 mS/cm2 in isotonic Ringer solution to 4.3 +/-0.3 mS/cm2 in the steady state phase of cell swelling. GmCl was 0.31 +/- 0.03 mS/cm2 in isotonic Ringer solution and thus was the dominant individual ion conductance. In the initial phase of cell swelling GmK increased transiently 64-fold to 0.32 +/- 0.03 mS/cm2, and consequently PD hyperpolarized. At peak hyperpolarization GmCl transiently decreased by 15%. Cell swelling increased GmCl 11-fold and GmHCO3 28-fold to 0.95 +/- 0.1 mS/cm2 in the steady state phase of cell swelling. In this phase GmCl and GmHCO3 were dominating, whereas GmK was only slightly increased compared to isotonic conditions. The hyperpolarization of PD was paralleled by cytoplasmic acidification. At peak acidification [HCO3-]i decreased by 6.4 mmol/kg H2O. Cl- extrusion was not detectable in the initial phase of cell swelling. In isotonic Ringer solution [K+]i was 125 +/- 5 mmol/kg H2O. During the initial phase of cell swelling 23 +/- 5 mmol/kg H2O K+ was extruded, indicating that yet unknown anions participated in cell volume regulation in this phase of cell swelling. In the steady state phase of cell swelling [pH]i was normalized by replenishing [HCO3-]i, whereas Cl- was extruded. We conclude that fused renal epitheloid cells acutely release KHCO3 in response to hypotonicity, but then regain pH homeostasis in the steady state phase of cell swelling.

Aldosterone-regulated ion transporters in the kidney

Madin-Darby canine kidney (MDCK) cells resemble intercalated cells of the renal collecting duct. In these cultured epithelial cells aldosterone activates apical Na+/H+ exchange, initiating a cascade of intracellular events such as cell growth, epithelial cell polarity, and stimulation of transepithelial ion transport. Transepithelial K+ secretion is triggered by the insertion of new ion channels and the activation of previously quiescent channels with increasing cytoplasmic pH. Aldosterone supplies the cell with ion transporters necessary for adequate function of the renal collecting duct when the organism is metabolically challenged.

Madin-Darby canine kidney cells. II. Aldosterone stimulates Na+/H+ and Cl-/HCO3- exchange

Experiments in dome epithelium of Madin-Darby canine kidney (MDCK) cells were performed to elucidate aldosterone action on acid-base transport. By means of pH-sensitive microelectrodes the pH of the dome fluid was measured while the apical plasma membrane was superfused. In the absence of HCO3- the dome fluid (facing the basolateral cell membrane) alkalinized in response to 10(-7) mol/l aldosterone. Amiloride (10(-3) mol/l) inhibited dome formation and pH recovery of the dome fluid from an extracellular acid load. In the presence of HCO3- dome fluid acidified in response to aldosterone. The stilbene derivative diisothiocyanate-stilbene-2,2'-disulphonic acid (DIDS) or removal of Cl- from the apical perfusate inhibited this dome acidification. In aldosterone-depleted MDCK monolayers HCO3- was actively accumulated in the dome fluid in contrast to aldosterone-supplemented cells. The results indicate that aldosterone stimulates both amiloride-sensitive Na+/H+ exchange and DIDS-sensitive Cl-/HCO3- exchange in the apical cell membrane of MDCK cells. In the absence of aldosterone the HCO3- extrusion process is localized in the basolateral membrane in series with apical Na+/H+ exchange, while in the presence of aldosterone Cl-/HCO3- is mainly localized in the apical membrane in parallel with Na+/H+ exchange. Cl- exits the cell through apical Cl- channels and is absorbed via the paracellular route.

Madin-Darby canine kidney cells. I. Aldosterone-induced domes and their evaluation as a model system

Vectorial transport of salt and water in the Madin-Darby canine kidney (MDCK) cell line is indicated by the formation of domes when a monolayer is grown on an impermeable support. We investigated aldosterone-induced dome formation and evaluated the dome as an experimental model. Transepithelial dome resistance was about 80 omega cm2 and constant when dome size exceeded 2.10(-4) cm2. The relative ion conductances (expressed as transference numbers) across the dome epithelium were tNa:tCl:tk = 0.64:0.24:0.06. They reflect the permeability properties of the paracellular shunt pathway tested at physiological concentrations of the individual ions. Aldosterone accelerated dome formation in serum-deprived MDCK monolayers. Prostaglandin E1 and transferrin were supportive but not essential for aldosterone-induced dome formation. After 72 h dome density was equal in monolayers cultured in serum-supplemented medium either in the presence or absence of mineralocorticoids. We conclude that aldosterone induces cell polarization in MDCK monolayers, leading to the formation of domes. The dome epithelium appears to be electrically isolated from the adjacent monolayer and can be studied by microelectrode techniques.

Madin-Darby canine kidney cells. III. Aldosterone stimulates an apical H+/K+ pump

Functionally and morphologically, Madin-Darby canine kidney (MDCK) cells resemble intercalated cells of urinary epithelia. Experiments were performed on domes of confluent MDCK monolayers to test for apical H+ secretion. Apical application of 10(-3) mol/l amiloride or of Na(+)-free solution significantly reduced the limiting pH gradient across the dome epithelium (delta pHd) consistent with inhibition of apical Na+/H+ exchange. Short-circuit current (SCC) measurements disclosed an acetazolamide-sensitive, (basolateral to apical) positive transepithelial current stimulated by 10(-7) mol/l aldosterone and inhibited by acidification of apical medium to pH = 4.5. Histochemical evaluation of carbonic anhydrase (CA) activity revealed cytoplasmic and apical-membrane-bound CA particularly in dome-forming cells. Apical substitution of Na+ by K+ increased delta pHd, whereas a reduction of K+ concentration to 0.5 mmol/l or addition of barium or omeprazole (10(-5) mol/l) to the apical superfusate reduced delta pHd by at least 75%. Aldosterone-stimulated SCC was completely abolished by the apical application of barium. We conclude that besides Na+/H+ exchange MDCK cells can express an apically located H(+)-K+ pump stimulated by aldosterone and inhibited directly by the anti-ulcer agent omeprazole or indirectly, either by blocking apical K+ recycling or by interfering with the CA-dependent intracellular formation of H+ ions.