Fusion of cultured dog kidney (MDCK) cells: I. Technique, fate of plasma membranes and of cell nuclei

Electrical approach to study rhodopsin activation in single cells with early receptor current assay

The ERC is a conformation-dependent charge motion similar to the gating currents of ionic channels. Both the waveforms and bandwidth of ERCs and ionic channel gating currents are similar, providing support to the initial suggestion that the ERP was a kind of gating current. In ionic channels the electrostatic field promotes motion of alpha-helical elements that stimulate large-scale molecular events that promote opening of the ionic pore. In ionic channels gating currents of expressed channel mutants has contributed significantly to understanding the mechanism of activation. Given the known role of electrical processes to rhodopsin activation, the ERC approach applied to mutant and wild-type visual pigments is likely to lead to a fuller understanding of the mechanism of conformational activation. This method is currently well suited to investigate the later phases of rhodopsin activation that are thought to be electrostatic in nature. We anticipate that ERC studies will make significant contributions to understanding how the breakdown of the electrostatic interaction between the PSB and its counterion is initiated and propagated to induce the proton uptake on the cytoplasmic surface of the pigment and the shaping of the transducin docking domain. We encourage collaboration to apply the ERC methodology to interesting mutant pigments and retinal analogs. We expect that the ERC methodology can soon be applied to understand rapid charge displacements associated with photochemistry (i.e., R1), the effects of transduction proteins on R2, and the measurement of electrical processes during cone visual pigment activation.

Normal and mutant rhodopsin activation measured with the early receptor current in a unicellular expression system

The early receptor current (ERC) represents molecular charge movement during rhodopsin conformational dynamics. To determine whether this time-resolved assay can probe various aspects of structure-function relationships in rhodopsin, we first measured properties of expressed normal human rhodopsin with ERC recordings. These studies were conducted in single fused giant cells containing on the order of a picogram of regenerated pigment. The action spectrum of the ERC of normal human opsin regenerated with 11-cis-retinal was fit by the human rhodopsin absorbance spectrum. Successive flashes extinguished ERC signals consistent with bleaching of a rhodopsin photopigment with a normal range of photosensitivity. ERC signals followed the univariance principle since millisecond-order relaxation kinetics were independent of the wavelength of the flash stimulus. After signal extinction, dark adaptation without added 11-cis-retinal resulted in spontaneous pigment regeneration from an intracellular store of chromophore remaining from earlier loading. After the ERC was extinguished, 350-nm flashes overlapping metarhodopsin-II absorption promoted immediate recovery of ERC charge motions identified by subsequent 500-nm flashes. Small inverted R(2) signals were seen in response to some 350-nm flashes. These results indicate that the ERC can be photoregenerated from the metarhodopsin-II state. Regeneration with 9-cis-retinal permits recording of ERC signals consistent with flash activation of isorhodopsin. We initiated structure-function studies by measuring ERC signals in cells expressing the D83N and E134Q mutant human rhodopsin pigments. D83N ERCs were simplified in comparison with normal rhodopsin, while E134Q ERCs had only the early phase of charge motion. This study demonstrates that properties of normal rhodopsin can be accurately measured with the ERC assay and that a structure-function investigation of rapid activation processes in analogue and mutant visual pigments is feasible in a live unicellular environment.

Time-resolved rhodopsin activation currents in a unicellular expression system

The early receptor current (ERC) is the charge redistribution occurring in plasma membrane rhodopsin during light activation of photoreceptors. Both the molecular mechanism of the ERC and its relationship to rhodopsin conformational activation are unknown. To investigate whether the ERC could be a time-resolved assay of rhodopsin structure-function relationships, the distinct sensitivity of modern electrophysiological tools was employed to test for flash-activated ERC signals in cells stably expressing normal human rod opsin after regeneration with 11-cis-retinal. ERCs are similar in waveform and kinetics to those found in photoreceptors. The action spectrum of the major R(2) charge motion is consistent with a rhodopsin photopigment. The R(1) phase is not kinetically resolvable and the R(2) phase, which overlaps metarhodopsin-II formation, has a rapid risetime and complex multiexponential decay. These experiments demonstrate, for the first time, kinetically resolved electrical state transitions during activation of expressed visual pigment in a unicellular environment (single or fused giant cells) containing only 6 x 10(6)-8 x 10(7) molecules of rhodopsin. This method improves measurement sensitivity 7 to 8 orders of magnitude compared to other time-resolved techniques applied to rhodopsin to study the role particular amino acids play in conformational activation and the forces that govern those transitions.

Nephrotoxicity testing in vitro--what we know and what we need to know

The kidney is affected by many chemicals. Some of the chemicals may even contribute to end-stage renal disease and thus contribute considerably to health care costs. Because of the large functional reserve of the kidney, which masks signs of dysfunction, early diagnosis of renal disease is often difficult. Although numerous studies aimed at understanding the mechanisms underlying chemicals and drugs that target various renal cell types have delivered enough understanding for a reasonable risk assessment, there is still an urgent need to better understand the mechanisms leading to renal cell injury and organ dysfunction. The increasing use of in vitro techniques using isolated renal cells, nephron fragments, or cell cultures derived from specific renal cell types has improved our insight into the molecular mechanisms involved in nephrotoxicity. A short overview is given on the various in vitro systems currently used to clarify mechanistic aspects leading to sublethal or lethal injury of the functionally most important nephron epithelial cells derived from various species. Whereas freshly isolated cells and nephron fragments appear to represent a sufficient basis to study acute effects (hours) of nephrotoxins, e.g., on cell metabolism, primary cultures of these cells are more appropriate to study long-term effects. In contrast to isolated cells and fragments, however, primary cultures tend to first lose several of their in vivo metabolic properties during culture, and second to have only a limited life span (days to weeks). Moreover, establishing such primary cultures is a time-consuming and laborious procedure. For that reason many studies have been carried out on renal cell lines, which are easy to cultivate in large quantities and which have an unlimited life span. Unfortunately, none of the lines display a state of differentiation comparable to that of freshly isolated cells or their primary cultures. Most often they lack expression of key functions (e.g., gluconeogenesis or organic anion transport) of their in vivo correspondents. Therefore, the use of cell lines for assessment of nephrotoxic mechanisms will be limited to those functions the lines express. Upcoming molecular biology approaches such as the transduction of immortalizing genes into primary cultures and the utilization of cells from transgenic animals may in the near future result in the availability of highly differentiated renal cells with markedly extended life spans and near in vivo characteristics that may facilitate the use of renal cell culture for routine screening of nephrotoxins.

Optimization of a mammalian expression system for the measurement of sodium channel gating currents

We describe a new mammalian expression system that optimizes conditions for the measurement of Na channel gating currents (Ig). The small magnitude of Ig limits their study to preparations with high numbers of Na channels to improve signal-to-noise ratios. To increase Na channel Ig signals, single tsA201 cells (approximately 20 microns in diameter) were fused into large, multinucleated cells by treatment with polyethylene glycol. After being placed in cell culture for 48-72 h, fused tsA201 cells develop a spherical geometry with diameters up to 200 microns. Because of the large plasma membrane surface area, fused tsA201 cells are able to express high levels of Na channels after transient transfection with Na channel cDNAs using Lipofectamine. Typically, 5 days after transfection, fused tsA201 cells that are 60-100 microns in diameter are selected for voltage clamp with a large suction pipette (a pore size of 20-30 microns) that allows for both a low series resistance and internal perfusion. Approximately two-thirds of transfected fused tsA201 cells express Na current, with nearly one-third of transfected cells expressing sufficient numbers of Na channels to allow for the ready measurement of Ig. In addition to fused tsA201 cells being a preparation well suited for the study of Ig, they should also be useful for measurement of electrical signals from other voltage-gated channels and transporters that generate small electrical signals.

Phenotypically and karyotypically distinct Madin-Darby canine kidney cell clones respond differently to alkaline stress

We isolated two cell clones from the wild-type Madin-Darby canine kidney cell line (MDCK) that resembles renal collecting duct epithelium. Morphology and karyotypes of the two cell clones were evaluated. The MDCK-C7 cell clone morphologically resembles principal cells (polygonal cell shape, flat), while the MDCK-C11 clone resembles intercalated cells (cuboidal cell shape, high). The diploid chromosome number of MDCK-C7 cells is 83.1 +/- 0.2 (n = 139); that for MDCK-C11 cells is 78.8 +/- 0.1 (n = 128). Culture of MDCK-C7 cells in alkaline medium (pH 7.7) induced irreversible phenotypical and genotypical alterations. Transformed MDCK-C7F cells are characterized by two abnormal (biarmed) chromosomes. In contrast, MDCK-C11 cells are not phenotypically altered by alkaline stress. In order to elucidate the role of intracellular pH (pHi) in the transformation process, we measured pHi under control conditions (pH 7.4), after 5 min exposure to alkaline stress ("acute experiment," pH 7.7) and after incubation of the cells in alkaline medium for two weeks ("chronic experiment," pH 7.7). Under control conditions, MDCK-C7 cells maintained pHi at 7.14 +/- 0.01 (n = 154) and MDCK-C11 cells at 7.01 +/- 0.01 (n = 147). Acute alkaline stress increased pHi of both cell types to similar steady-state values. Under chronic alkaline stress, MDCK-C7 cells were unable to maintain intracellular pH within normal limits exhibiting sustained alkalinization, whereas MDCK-C11 cells could successfully regulate pHi. We conclude that wild-type MDCK cells consist of two genetically distinct subpopulations with different morphology and function. Only the MDCK-C7 clone that resembles the principle cell type of renal collecting duct can be transformed by alkaline stress while the MDCK-C11 clone resists this treatment, due to efficient pHi control mechanisms.

Characterization of two distinct Cl- conductances in fused human respiratory epithelial cells. II. Relation to cystic fibrosis gene product

The present microelectrode experiments on fused respiratory epithelial cells of cystic fibrosis (CF) origin and non-CF origin aim at characterizing the molecular basis of the Cl- conductances regulated by cyclic adenosine monophosphate (cAMP) or respectively Ca2+, as described in the preceding publication. Cell membrane potential (Vm) and resistance (Rm) were recorded as well as their response to substitution of 90% of bath Cl- by isethionate (delta Vm,ISE), by I- (delta Vm,I), or by other halide anions. Fused CF cells had significantly (P < 0.05) higher control Vm values (-18.0 +/- 9.4 mV, +/- SD, n = 68) than fused non-CF cells (-12.5 +/- 6.6 mV, n = 69) and responded to the Ca2+ ionophore A23187 with an increase in the Vm response to Cl- substitution, but did not respond to forskolin. This indicates that CF cells express only the Ca(2+)-stimulated Cl- conductance. Injection of the antibody M3A7 against a fusion protein containing amino acids 1195 to 1480 of the CF gene product into young, forskolin-stimulated or old non-CF cells decreased delta Vm,ISE and delta Vm,I within 15 min to values observed in CF cells. This indicates inhibition of the cAMP-stimulated Cl- conductance and supports the molecular identity of this conductance with the CF gene product. However, the slow onset of inhibition does not allow secondary effects to be excluded and a slight fall in Rm remains unexplained. Stimulation of the Ca(2+)-regulated Cl- conductance was not impaired. Injection of M3A7 into CF cells or of a control antibody in non-CF cells had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of two distinct Cl- conductances in fused human respiratory epithelial cells. I. Anion selectivities, stimulation and intermeshing signal transduction pathways

With the aim of further elucidating the role of the epithelial Cl- conductance and its defect in cystic fibrosis (CF) patients we studied the properties and regulation of the Cl- conductance in primary cultures of human nasal polyp epithelia. To facilitate microelectrode punctures and to gain access to the cytoplasmic compartment for injection of antibodies, we prepared giant cells using a polyethylene-glycol fusion technique. The membrane potential (Vm) and resistance (Rm) and their responses to ionic substitutions in the bath were measured under control conditions and in the presence of different secretagogues. In non-CF cells Vm averaged-12.5 mV (SD +/- 6.6 mV, n = 69) and was independent of time after fusion, while Rm dropped from 12.4 +/- 7.3 M omega (n = 51) to 3.5 +/- 5.5 M omega (n = 24) in the 2nd week after fusion. The low Vm values reflected a vanishing K+ conductance in the presence of a dominating Cl- conductance that increased with time. In young cells, a Cl- conductance prevailed which could be stimulated by application of the Ca2+ ionophore, A23187, or of carbachol. As determined in CF cells, it had an outwardly rectifying current/voltage (ilV) relationship and exhibited the selectivity sequence I- > Br- > Cl- > F- > isethionate (ISE-) both in Vm and Rm measurements. With increasing age after fusion, a Cl- conductance prevailed in non-CF cells which could be stimulated by cyclic adenosine monophosphate (cAMP) or forskolin and which was downregulated by A23187. It had a linear ilV relationship and exhibited the selectivity sequence Br- > Cl- > I- > F- > ISE- if determined from Vm measurements, but a sequence of Cl- > Br- > F- = ISE- > I- if determined from Rm measurements. This points to multiple-ion pore behaviour of the respective Cl- channel. In agreement with observations described in the following publication, the results suggest that the cAMP-regulated Cl- conductance corresponds to the CF-gene product while the molecular nature of the Ca(2+)-regulated Cl conductance is not yet known.

Subtypes of Madin-Darby canine kidney (MDCK) cells defined by immunocytochemistry: further evidence for properties of renal collecting duct cells

The Madin-Darby canine kidney (MDCK) cell line has been proposed as a model for studying intercalated (IC) cells of the renal cortical collecting duct. The IC cells are characterized by peanut lectin (PNA) binding capacity, carbonic anhydrase (CA) activity and Cl(-)-HCO3- exchange mediated by a band 3-related protein. It has been suggested that these properties are also expressed in MDCK cells. So far however, the nature of the specific protein involved in Cl(-)-HCO3- exchange, the type of CA isozyme and the relationship between these two characteristics and PNA binding, have not been investigated in MDCK cells by immunocytochemical methods. Using two antibodies raised against human erythrocyte band 3 protein and two against human erythrocyte CA I and II isozymes, our study provides evidence that a protein related to band 3 is expressed in about 5% of cultured MDCK cells; these band 3-positive cells do not bind PNA and are not reactive for CAI or CAII. About 30% of the MDCK cells bind PNA, two-thirds of which are also CAII-positive. A majority (about 65%) of MDCK cells is not reactive for the three markers used; their density is increased after incubation with aldosterone. These data indicate (i) that the Cl(-)-HCO3- exchange of the MDCK cells could be related to human erythrocyte band 3, (ii) that the CA activity of the MDCK cell line bears antigenic identity with the erythrocyte CA II isozyme and (iii) that the latter is always co-localized with PNA binding.(ABSTRACT TRUNCATED AT 250 WORDS)

The mycotoxin ochratoxin A deranges pH homeostasis in Madin-Darby canine kidney cells

Ochratoxin A (OTA) is a nephrotoxin which blocks plasma membrane anion conductance in Madin-Darby canine kidney (MDCK) cells. Added to the culture medium, OTA transforms MDCK cells in a manner similar to exposure to alkaline stress. By means of video-imaging and microelectrode techniques, we investigated whether OTA (1 mumol/liter) affects intracellular pH (pHi), Cl- (Cl-i) or cell volume of MDCK cells acutely exposed to normal (pHnorm = 7.4) and alkaline (pHalk = 7.7) conditions. At pHnorm, OTA increased Cl-i by 2.6 +/- 0.4 mmol/liter (n = 14, P < 0.05) but had no effect on pHi. At pHalk, application of OTA increased Cl-i by 8.6 +/- 2.6 mmol/liter (n = 10, P < 0.05) and raised pHi by 0.11 +/- 0.03 (n = 8, P < 0.05). The Cl-/HCO3- exchange inhibitor DNDS (4,4'-dinitro-stilbene-2,2'-disulfonate; 10 mumol/liter) eliminated the OTA-induced changes of pHi and Cl-i. OTA did not affect cell volume under both pHnorm and pHalk conditions. We conclude that the OTA-induced blockade of plasma membrane anion conductance increases Cl-i without changing cell volume. The driving force of plasma membrane Cl-/HCO3- exchange dissipates, leading to a rise of pHi when cells are exposed to an acute alkaline load. Thus, OTA interferes with pHi and Cl-i homeostasis leading to morphological and functional alterations in MDCK cells.

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.

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)

Aldosterone actions on basolateral Na+/H+ exchange in Madin-Darby canine kidney cells

In recent studies, there has been a re-evaluation of the polarity of Na+/H+ exchange in Madin-Darby canine kidney (MDCK) cells. This study was designed to examine aldosterone actions on basolaterally located Na+/H+ exchange of MDCK cell monolayers grown on permeant filter supports; pHi was analysed in the absence of bicarbonate by using the pH-sensitive fluorescent probe 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. Pre-exposure of MDCK cells to aldosterone led within 10-20 min to an alkalization of pHi (approximately 0.3 pH unit); this effect is prevented by an addition of dimethylamiloride to the basolateral superfusate. Addition of aldosterone led to stimulation of the basolaterally located Na+/H+ exchange activity (Na(+)-dependent recovery from an acid load); this effect required preincubation (more then 3 min) and was observed at 0.1 nM aldosterone. Pre-exposure (15 min) of MDCK monolayers to phorbol 12-myristate 13-acetate also led to an activation of Na+/H+ exchange; pre-exposure to 8-bromo-cAMP led to inhibition of Na+/H+ exchange activity. An inhibitory effect of aldosterone was observed if Na+/H+ exchange activity was analysed in the presence of aldosterone; the highest inhibitory effects (20%-30%) occurred at concentrations of 5 nM and higher. Aldosterone-dependent inhibition does not require preincubation and is fully reversible; it was only observed at low (20 mM) but not at high Na+ concentrations (130 mM). The data suggest that aldosterone has an instantaneous inhibitory effect on basolaterally located Na+/H+ exchange activity under conditions of low Na+, but stimulates the rate of transport activity upon preincubation under conditions of physiological Na+ concentrations.

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.

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.

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.

Fusion of cultured dog kidney (MDCK) cells: II. Relationship between cell pH and K+ conductance in response to aldosterone

We have chosen the MDCK cell line to investigate aldosterone action on H+ transport and its role in regulating cell membrane K+ conductance (GKm). Cells grown in a monolayer respond to aldosterone indicated by the dose-dependent formation of domes and by the alkalinization of the dome fluid. The pH sensitivity of the plasma membrane K+ channels was tested in "giant cells" fused from individual MDCK cells. Cytoplasmic pH (pHi) and GKm were measured simultaneously while the cell interior was acidified gradually by an extracellular acid load. We found a steep sigmoidal relationship between pHi and GKm (Hill coefficient 4.4 +/- 0.4), indicating multiple H+ binding sites at a single K+ channel. Application of aldosterone increased pHi within 120 min from 7.22 +/- 0.04 to 7.45 +/- 0.02 and from 7.15 +/- 0.03 to 7.28 +/- 0.02 in the absence and presence of the CO2/HCO-3 buffer system, respectively. We conclude that the hormone-induced cytoplasmic alkalinization in the presence of CO2/HCO-3 is limited by the increased activity of a pHi-regulating HCO-3 extrusion system. Since GKm is stimulated half-maximally at the pHi of 7.18 +/- 0.04, internal H+ ions could serve as an effective intracellular signal for the regulation of transepithelial K+ flux.