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

Non-genomic action of the mineralocorticoid aldosterone on cytosolic sodium in cultured kidney cells

1. The mineralocorticoid aldosterone is essential for the regulation of electrolyte homeostasis, extracellular volume and blood pressure. As a steroid hormone the classical way of action is genomic. Previously we reported a non-genomic action of aldosterone on cytosolic Ca2+ and pH in renal epithelial (MDCK) cells. In parallel, aldosterone induces Zn2+-sensitive cytosolic acidification when extracellular Na+ is absent. 2. We now show that aldosterone (EC50, 7 x 10-11 mol l-1) induces a non-genomic increase in cytosolic sodium in MDCK cells. The membrane-impermeable aldosterone-bovine serum albumin (BSA) conjugate exerted the same effect. The effect of aldosterone was completely abolished by inhibition of Na+-H+ exchange with ethyl-isopropanol amiloride (EIPA). Aldosterone-induced Na+ influx exceeded H+ efflux more than 10-fold. 3. Omission of extracellular Ca2+, inhibition of protein kinase C or pretreatment with pertussis toxin reduced the effect of aldosterone significantly. Zn2+ (IC50, 3.3 x 10-6 mol l-1), but not ouabain, abolished the increase in Na+ almost completely. 4. The aldosterone-induced increase in cytosolic sodium was accompanied by an EIPA- and Zn2+-sensitive cell swelling. 5. Thus, physiological concentrations of aldosterone induce a non-genomic increase in cytosolic sodium concentration by activation of Na+-H+ exchange. Aldosterone exerts its effect, at least in part, at the plasma membrane via interaction with a G-protein-coupled mechanism. 6. The simultaneous activation of the acidification mechanism and Na+-H+ exchange by aldosterone allows a dramatic sodium influx without excessive changes in cytosolic pH and leads to changes in cell volume.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Properties and regulation of ion channels in MDCK cells

The MDCK cell has proven to be a useful model cell line for the study of properties and regulation of renal epithelial ion channels. Patch clamp studies disclosed the existence of several K+ channels and of a Cl- channel, and their regulation by hormones, cell volume, trace elements and drugs. Most hormones affect K+ channels at least in part by increasing cytosolic Ca2+. However, indirect evidence points to additional mechanisms contributing to K+ channel activation. Cell swelling activates both K+ channels and unselective anion channels. ICln, a protein cloned from MDCK cells, is either a Cl- channel or a regulator of thereof. ICln is up-regulated by cellular acidification and is crucial for rapid regulatory cell volume decrease.

Polarized ion transport during migration of transformed Madin-Darby canine kidney cells

Epithelial cells lose their usual polarization during carcinogenesis. Although most malignant tumours are of epithelial origin little is known about ion channels in carcinoma cells. Previously, we observed that migration of transformed Madin-Darby canine kidney (MDCK-F) cells depended on oscillating K+ channel activity. In the present study we examined whether periodic K+ channel activity may cause changes of cell volume, and whether K+ channel activity is distributed in a uniform way in MDCK-F cells. After determining the average volume of MDCK-F cells (2013+/-270 microm3; n=8) by means of atomic force microscopy we deduced volume changes by calculating the K+ efflux during bursts of K+ channel activity. Therefore, we measured the membrane conductance of MDCK-F cells which periodically rose by 22.3+/-2.5 nS from a resting level of 6.5+/-1.4 nS (n=12), and we measured the membrane potential which hyperpolarized in parallel from -35.4+/-1.2 mV to -71.6+/-1.8 mV (n=11). The distribution of K+ channel activity was assessed by locally superfusing the front or rear end of migrating MDCK-F cells with the K+ channel blocker charybdotoxin (CTX). Only exposure of the rear end to CTX inhibited migration providing evidence for "horizontal" polarization of K+ channel activity in transformed MDCK-F cells. This is in contrast to the "vertical" polarization in parent MDCK cells. We propose that the asymmetrical distribution of K+ channel activity is a prerequisite for migration of MDCK-F cells.

Molecular mechanisms for passive and active transport of water

Water crosses cell membranes by passive transport and by secondary active cotransport along with ions. While the first concept is well established, the second is new. The two modes of transport allow cellular H2O homeostasis to be viewed as a balance between H2O leaks and H2O pumps. Consequently, cells can be hyperosmolar relative to their surroundings during steady states. Under physiological conditions, cells from leaky epithelia may be hyperosmolar by roughly 5 mosm liter-1, under dilute conditions, hyperosmolarities up to 40 mosm liter-1 have been recorded. Most intracellular H2O is free to serve as solvent for small inorganic ions. The mechanism of transport across the membrane depends on how H2O interacts with the proteinaceous or lipoid pathways. Osmotic transport of H2O through specific H2O channels such as CHIP 28 is hydraulic if the pore is impermeable to the solute and diffusive if the pore is permeable. Cotransport of ions and H2O can be a result of conformational changes in proteins, which in addition to ion transport also translocate H2O bound to or occlude in the protein. A cellular model of a leaky epithelium based on H2O leaks and H2O pumps quantitatively predicts a number of so-far unexplained observations of H2O transport.

Characterization of two MDCK-cell subtypes as a model system to study principal cell and intercalated cell properties

Madin-Darby canine kidney (MDCK) cells originate from the renal collecting duct and consist of different cell subtypes. We cloned two MDCK cell subtypes denominated as C7 and C11 with different morphology and different function. The two clones maintained their functional differences after cloning. C7 monolayers exhibit a high transepithelial resistance (Rte = 5648 +/- 206 omega.cm2, n = 20) and secrete K+ (delta K+ = 1.31 +/- 0.08 mmol/l, n = 10) into the apical medium. C11 monolayers display a low Rte (330 +/- 52 omega.cm2, n = 20) and secrete Cl- (delta Cl- = 16.9 +/- 1.8 mmol/l, n = 10) into the apical medium. Aldosterone (1 mumol/l) stimulates K+ secretion (delta K+ of 3.58 +/- 0.11 mmol/l, n = 7) in C7 cells and H+ secretion in C11 cells (delta pH = 0.060 +/- 0.007, n = 10). Aldosterone-induced stimulation of K+ secretion is inhibited by apical application of amiloride (1 mumol/l). cAMP stimulates H+ secretion in C11 cells (delta pH = -0.068 +/- 0.004, n = 10). Furthermore, C7 cells are peanut-lectin(PNA)-negative and exhibit an intracellular pH of 7.39 +/- 0.05 (n = 7), whereas C11 cells maintain intracellular pH at 7.16 +/- 0.05 (n = 8) and a major fraction of cells is PNA positive. We conclude that we have cloned two subtypes of MDCK cells which stably express different functional characteristics. The C7 subtype resembles principal cells (PC) of the renal collecting duct, whereas the C11 subtype resembles intercalated cells (ICC) of the renal collecting duct.(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.

Oscillating activity of a Ca(2+)-sensitive K+ channel. A prerequisite for migration of transformed Madin-Darby canine kidney focus cells

Migration plays an important role in the formation of tumor metastases. Nonetheless, little is known about electrophysiological phenomena accompanying or underlying migration. Previously, we had shown that in migrating alkali-transformed Madin-Darby canine kidney focus (MDCK-F) cells a Ca(2+)-sensitive 53-pS K+ channel underlies oscillations of the cell membrane potential. The present study defines the role this channel plays in migration of MDCK-F cells. We monitored migration of individual MDCK-F cells by video imaging techniques. Under control conditions, MDCK-F cells migrated at a rate of 0.90 +/- 0.03 microns/min (n = 201). Application of K+ channel blockers (1 and 5 mmol/liter Ba2+, 5 mmol/liter tetraethylammonium, 100 mumol/liter 4-aminopyridine, 5 nmol/liter charybdotoxin) caused marked inhibition of migration, pointing to the importance of K+ channels in migration. Using patch-clamp techniques, we demonstrated the sensitivity of the Ca(2+)-sensitive 53-pS K+ channel to these blockers. Blockade of this K+ channel and inhibition of migration were closely correlated, indicating the necessity of oscillating K+ channel activity for migration. Migration of MDCK-F cells was also inhibited by furosemide or bumetanide, blockers of the Na+/K+/2Cl- cotransporter. We present a model for migration in which oscillations of cell volume play a central role. Whenever they are impaired, migration is inhibited.

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.

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.

Effect of bicarbonate on intracellular potential of rabbit ciliary epithelium

Extracellular HCO3- hyperpolarizes the intracellular potential and makes the aqueous medium negative with respect to the stromal surface of the rabbit ciliary epithelial syncytium. The bases for these observations have been unclear. We have been studying the bicarbonate-induced hyperpolarization (BIH) with sustained intracellular recordings for periods as long as 1-2 hrs. The BIH was observed [6.0 +/- 0.4 mV (mean +/- SE, N = 22)] even when the external pH was clamped constant by appropriately changing the CO2 tension. External HCO3- was required since aeration with CO2 at low external pH did not replicate the BIH. DIDS [4,4'-diisothiocyano-2,2'-disulfonic acid] did not abolish the effect. The hyperpolarization is unlikely to reflect the pH dependence of K+ channels alone, since the effect was not reduced by either 2 mM Ba2+ alone or 2 mM Ba2+ together with 50-100 microM quinidine. The BIH depends directly or indirectly on external Na+, since the sign of the polarization response was reversed either by replacing Na+ with N-methyl-D-glucamine or by blocking the Na+,K(+)-exchange pump with 50-100 microM ouabain. Replacement of external Cl- with NO3- or application of the Cl(-)channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate] depolarized the membrane and reversed the sign of the BIH. The response of the ciliary epithelium to HCO3- is complex and may arise from several mechanisms. We suggest that one important element is an anion channel whose conductance is reduced by bicarbonate and whose reversal potential is indirectly dependent on the operations of the Na+,K(+)-pump and a Cl(-)-linked symport.

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.

Efflux and accumulation of amino nitrogen in relation to the volume of rat renal inner medullary cells exposed to media of variable osmolality

The rate of efflux of 2-amino[14C]isobutyric acid (AIB) from pre-loaded slices of rat renal inner medulla has been followed during incubation in media whose osmolality was varied between 350 and 2500 mosmol/kg H2O by adjustment of NaCl and urea concentrations. Efflux was biphasic, and it was assumed that the second, slower phase represented mainly cellular loss of AIB. As a function of cell volume (water content) the mean net rate of 2nd phase efflux declined far more abruptly (-36%) during an increase in external osmolality from 350 to 720 mosmol/kg than during further increase to 2500 mosmol/kg, over which range the rate of efflux fell by only a further 12%. Conversely, relative decrements of steady-state cell water contents during these two transitions were -17% and -37%, respectively. It is probable that in strongly hyperosmolal media (above 720 mosmol/kg) reduction in the rate of amino acid catabolism, with resultant cellular accumulation, becomes more important than passive efflux as a determinant of cell amino nitrogen content, and that this is caused by the enzyme-destabilizing effect of high intracellular concentrations of permeant urea. This interpretation is supported by the finding in the present study that trimethylamine N-oxide, which is known to counteract the destabilizing effect of urea, completely inhibited the accumulation of amino nitrogen (ninhydrin-positive substances) in media stronger than 720 mosmol/kg, as well as leading to further reduction of steady-state cell water contents, but was without effect on either variable in more dilute media. It is proposed that, under the conditions of this investigation, amino acids contribute to cell volume maintenance mainly by efflux and by metabolic accumulation under mildly and strongly hyperosmolal conditions, respectively, and that this interpretation is consistent with recent findings on the fluctuations in medullary levels of Na+, urea and total amino nitrogen in the intact kidneys of rats during acute water diuresis and oliguria (Law, R.O. (1991) Pflügers Arch. 418, 442-446).

The electrolytes in hyponatremia

It is commonly taught that retention of free water is the dominant factor reducing the serum sodium concentration in hyponatremia. To determine whether the concentrations of other electrolytes are similarly diluted, we identified 51 patients with hyponatremia (Na = 121 +/- 1 mmol/L [mEq/L]) and compared electrolyte and laboratory values at the time of hyponatremia with values at a time when serum sodium was in the normal range (138 +/- 1 mmol/L). The medium interval between these measurements was 12 days. At the time of hyponatremia, serum sodium and chloride were substantially and significantly reduced by 12% to 15%. Although many hyponatremic patients had overtly increased or decreased concentrations of the other measured electrolytes, there were no significant changes in the mean concentration for any of these at the time of hyponatremia. Unchanged mean values were found for the plasma concentration of bicarbonate (26.1 +/- 0.6 normal v 25.2 +/- 0.8 mmol/L at the time of hyponatremia), potassium (4.31 +/- 0.10 v 4.33 +/- 0.15 mmol/L), albumin, phosphate, and creatinine. The stability of these laboratory values was observed both in patients with clinically normal extracellular fluid (ECF) volume and in those with true or effective ECF depletion. The urinary sodium (UNa) concentration was found to be a reliable predictor of the ECF volume status, whereas the fractional sodium excretion (FENa) was not. Electrolyte derangements are common in patients with hyponatremia, but are usually confined to patients on diuretics or who have an abnormal ECF volume. In the absence of these complicating situations, the plasma electrolytes are typically normal and are not reduced by dilution to the same extent as Na and CI. Based on a review of both the classic and recent knowledge concerning electrolyte regulation in hyponatremia, we propose that two factors explain these observations. First, the degree of dilution is overestimated because of Na losses in urine and perhaps Na shift into cells. Second, both renal and extrarenal adaptive mechanisms are activated by hyponatremia that stabilizes the concentration of other ions. One of these mechanisms is cell swelling, which triggers a volume-regulatory response leading to the release of ions and water into the ECF. Other adaptive mechanisms are mediated by antidiuretic hormone (ADH) per se, and by atrial natriuretic peptide (ANP).

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.