Volume-induced increase of K+ and Cl- permeabilities in Ehrlich ascites tumor cells. Role of internal Ca2+

Na+ + K+ + 2Cl- cotransport in animal cells--its role in volume regulation

Cell membranes of various vertebrate cells catalyze a Na+ + K+ + 2Cl- cotransport specifically inhibitable by furosemide and other high ceiling diuretics. The energetics of this process is not elucidated unequivocally. It was clearly shown that cotransport is no ATP-consuming process. We assume that transport is secondary active functionally coupled to the operation of the electrogenic Na+-K+ pump. The role of this transport system in transepithelial ion movement is that it serves as flux amplifier, doubling from 6 to 12 the number of osmotically active particles transported per ATP hydrolyzed. In concert with Na+-K+ pump, cotransport provokes net uptake of KCl into the cell and therefore cellular swelling. This process is regulated by a feedback control system for cell volume; if actual volume reaches reference value, cotransport is switched off to prevent further swelling. How cell volume is measured is not known, nor is the nature of the signal generated to switch cotransport from the operating to the nonoperating state or vice versa. cAMP-level or intracellular Ca2+ play no role as signals or as part of the volume-sensoring mechanism. Theophylline, other alkylxanthines, and some purine ribosides influence cotransport indirectly by reducing reference volume. The role of cytoskeleton in volume regulation is obscure. While high Concentrations of cytochalasin B and of colchicin do not influence cell volume, it is reduced by vinblastine and also by lectins, for example concanavalin A. Volume reduction is accompanied by reduction in cellular KCl content. The observation that during hypertonic incubation protein synthesis is inhibited can be traced back to a correlation between cell volume and protein synthesis and not to elevation of osmolarity per se. Reduction in cell volume under isotonic conditions by varying K+ and/or Cl- concentration or by furosemide inhibition of cotransport is strongly correlated to inhibition of protein synthesis. The reason for this correlation is not yet clarified. Not all cells showing furosemide-sensitive cotransport are able to regulate it, for example lymphocytes. For mammalian erythrocytes drastic species differences exist; while cells from man, rabbit, rat, and mouse all show cotransport, only cells from rat (and mouse?) are able to regulate cotransport.

Electrophysiological studies of sodium cotransport in epithelia: toward a cellular model

During the past two decades, microelectrophysiological studies of small intestine and renal proximal tubule employing conventional as well as ion-selective microelectrodes have contributed significantly to our understanding of the nature of Na-coupled entry processes at the apical membrane as well as the overall workings of the simple model illustrated in FIGURE 1. These studies have unequivocally established the rheogenic and conductive nature of the Na-coupled sugar and amino-acid entry processes across the apical membrane of small intestine (and renal proximal tubule) and have, in addition, disclosed that the properties of the basolateral membrane respond to an increase in Na-coupled solute entry with an increase in the ability of the Na-K pump to extrude Na with little or no change in (Na)c32 and a parallel increase in the conductance of that barrier to K. Although these responses may be "triggered" by cell swelling, it is unclear how a cell "recognizes" minimal swelling and how this recognition, in turn, culminates in the observed changes in basolateral membrane pump-leak properties. Clearly, these findings have brought us to the interfaces between cell physiology and cell and molecular biology and have raised a number of intriguing questions that focus on the more global question: How do epithelial cells work?

K+ and Cl- conductances in the apical membrane from secreting oxyntic cells are concurrently inhibited by divalent cations

This study concerns the properties of rapid K+ and Cl transport pathways that are present in the (H+ + K+)-ATPase membrane from stimulated, and secreting, gastric oxyntic cells. Ion permeabilities in the isolated stimulation-associated vesicles were monitored via the rates of H+ efflux under conditions of exclusive H+/K+ counterflux or H+ - Cl co-efflux, as well as by comparison of equilibration rates for 86Rb and 36Cl under conditions of equilibrium exchange and unidirectional salt flux. These latter studies suggest that Rb+ and Cl pathways are conductive and independent. In spite of the functional independence of the ion pathways, several divalent cations inhibit Rb+ and Cl isotopic exchange as well as the H+ efflux that is dependent on either K+ or anion (Cl, SCN, NO2) fluxes. Zn2+ is the more potent inhibitor, reducing by 50% the sensitive component of K+, Cl, and NO2 fluxes at about 20 microM; Mn2+ has a similar effect at 200 microM. Ni2+ and Co2+ were roughly equipotent to Mn2+ while Mg2+ and Ca2+ had no inhibitory effect. These results suggest that the stimulation-induced permeabilities, while functioning independently, may be physically linked, i.e., residing within a single entity. In similar studies carried out in (H+ + K+)-ATPase vesicles obtained from nonstimulated cells, no vestiges of sensitivity to the inhibitory divalent cations could be detected. The implications of these findings for the physiology of the oxyntic cell in the context of a model for membrane fusion are discussed.

Effects of osmotic shocks on the ultrastructure of different tissues and cell types

This study deals with the effects of hyper- and hypo-osmotic media on the ultrastructure of four different types of cells and tissues: rat pheochromocytoma cells of line PC12, mouse Ehrlich ascites tumor cells, rat kidney cortex and intestine. Application of hyper-osmotic conditions induces in the nuclear compartment of the tested cell types a condensation of chromatin, a ruffling of the nuclear envelope with loosening of condensed chromatin from the lamina, and an apparent loss of nucleolar fibrillar component which disappears in a background of diffuse granular material. In hypo-osmotic media, there is a marked decondensation of chromatin and a fragmentation of the granular material of the nucleolus. As far as the cytoplasmic compartment is concerned, the electron density of the cytosol is markedly increasing when going from hypo- to hyper-osmotic conditions and there is no vacuolization in hypo-osmotic media. In kidney cortex slices, application of hypo-osmotic shocks further results in a marked reduction of the extracellular space delimited by the infoldings of the tubular cells plasma membranes. These modifications are discussed in relation to the volume regulation process and the changes in ion concentration that occur in cells submitted to anisosmotic media.

Passive K+-Cl- fluxes in low-K+ sheep erythrocytes: modulation by A23187 and bivalent cations

A fraction of the ouabain-resistant (OR) K+ flux of low-K+ (LK) sheep erythrocytes is Cl- dependent (K+-Cl- transport) and is activated reversibly by cell swelling or irreversibly by treatment with N-ethylmaleimide (NEM). The effect of the ionophore A23187 plus bivalent cations (Me2+) or ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) was studied on K+-Cl- transport in control or NEM-treated LK cells. The following observations were made. 1) A23187 (6 microM), at a hematocrit of 10% (vol/vol) and in the presence of 1 mM EGTA, activated severalfold OR K+-Cl- transport in shrunken or swollen cells but failed to stimulate further K+-Cl- flux in NEM-treated cells. 2) In the absence of EGTA, but at very low external Ca2+ concentrations [( Ca2+]o = 10(-7) M), A23187 stimulated OR K+-Cl- flux in controls less than with EGTA and inhibited it slightly in NEM-treated cells. 3) When [Ca2+]o was raised to 10(-3) M, an almost complete inhibition of OR K+-Cl- fluxes occurred in shrunken, swollen, or NEM-treated cells. 4) Other Me2+ inhibited OR K+-Cl- flux in the presence of A23187 in the following order of decreasing potency: Mn2+ much greater than Ca2+ greater than Mg2+ greater than Sr2+ much much greater than Ba2+. 5) Stimulation of OR K+-Cl- flux by A23187 +/- EGTA and inhibition by A23187 + Ca2+ were reversible and did not alter significantly cellular ATP. 6) The stimulatory effect of A23187 plus EGTA, perhaps by Me2+ removal, on K+-Cl- flux and its inhibition by Ca2+ were reversibly abolished in metabolically depleted cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-dependent ion cotransport in steady-state Ehrlich ascites tumor cells

The Ehrlich tumor cell possesses an anion-cation cotransport system which operates as a bidirectional exchanger during the physiological steady state. This cotransport system, like that associated with the volume regulatory mechanism (i.e. coupled net uptake of Cl- + Na+ and/or K+), is Cl- -selective and furosemide-sensitive, suggesting the same mechanism operating in two different modes. Since Na+ has an important function in the volume regulatory response, its role in steady-state cotransport was investigated. In the absence of Na+, ouabain-insensitive K+ and DIDS-insensitive Cl- transport (KCl cotransport) are low and equivalent to that found in 150 mM Na+ medium containing furosemide. Increasing the [Na+] results in parallel increases in K+ and Cl- transport. The maximum rate of each (18 to 20 meq/(kg dry wt) . min) is reached at about 20 mM Na+ and is maintained up to 55 mM. Thus, over the range 1 to 55 mM Na+ the stoichiometry of KCl cotransport is 1:1. In contrast to K+ and Cl-, furosemide-sensitive Na+ transport is undetectable until the [Na+] exceeds 50 mM. From 50 to 150 mM Na+, it progressively rises to 7 meq/(kg dry wt) . min, while K+ and Cl- transport decrease to 9 and 16 meq/(kg dry wt) . min, respectively. Thus, at 150 mM Na+ the stoichiometric relationship between Cl-, Na+ and K+ is 2:1:1. These results are consistent with the proposal that the Cl- -dependent cation cotransport system when operating during the steady state mediates the exchange of KCl for KCl or NaCl for NaCl; the relative proportion of each determined by the extracellular [Na+].

Potassium movements associated with amino acid and sugar transport in enterocytes isolated from rabbit jejunum

Active transport of amino acids and the sugar, alpha-methyl-D-glucoside (alpha-MG) caused an increase in the rate of K efflux from isolated rabbit enterocytes. These effects were inhibited by apamin (5 X 10(-7) M), quinidine (10(-3) M), Ba (5 X 10(-3) M) and trifluoperazine (5 X 10(-5) M) but not by the loop diuretic furosemide (10(-4) M). None of these drugs affected the basal rate of K efflux. The stimulatory effects of amino acids or alpha-MG on K efflux are too great to be explained in terms of an increase in the electrical driving force across the plasma membrane of these cells and a change in membrane permeability is envisaged. An apparent Ca-dependent K permeability in isolated enterocytes can be demonstrated using the Ca ionophore A23187. The effect of the ionophore on K efflux is abolished by apamin or Ba. It is proposed that Ca-dependent K channels mediate the sugar and amino acid induced increases of K efflux. Under control conditions there is a decrease in intracellular K concentration during accumulation of alanine or alpha-MG. Ba by itself does not alter K concentration but it did produce a marked increase when used in conjunction with alanine or alpha-MG. The accumulation of alpha-MG was inhibited in the presence of Ba. This is consistent with an interference with the driving force for sugar accumulation. It is suggested that the increase in K permeability described has a role in both maintaining ion homoeostasis during Na-coupled transport and contributing to the driving force for sugar and amino acid absorption.

The effect of phenylalanine on the electrical properties of proximal tubule cells in the frog kidney

The present study was designed to elucidate the effects of sodium-coupled transport on the electrical properties of proximal tubule cells in the isolated perfused frog kidney. Cable analysis techniques have been employed to determine the resistance of the luminal and peritubular cell membranes in parallel (Rm) and the apparent ratio of the luminal over the peritubular cell membrane resistance (VDR). Furthermore, the sensitivity of the potential difference across the peritubular cell membrane (PDpt) to 6-fold increases of peritubular potassium concentration (delta PDk) was taken as a measure of the relative potassium conductance of this membrane. In the absence of luminal phenylalanine, PDpt amounts to -60 +/- 1 mV (n = 90), Rm to 36 +/- 3 k omega cm (n = 22), VDR to 1.81 +/- 0.14 (n = 20), and delta PDk to 15.0 +/- 0.9 mV (n = 25). The application of 10 mmol/l phenylalanine replacing 10 mmol/l raffinose leads to a rapid (within 30 s) depolarisation of PDpt to 50 +/- 5% of its control value and to a delayed (within 12 min) recovery to 95 +/- 5% of control. The rapid depolarisation is associated with a decline of Rm and VDR, indicating a decrease mainly of the luminal cell membrane resistance. During recovery of PDpt there is a parallel increase of VDR and a further decline of Rm pointing to a decline of the basolateral cell membrane resistance. Delta PDk is decreased during rapid depolarisation but increases again during the recovery phase. Thus, phenylalanine initially decreases but then increases above control the apparent potassium conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of A23187 and Ca2+ on volume- and thiol-stimulated, ouabain-resistant K+C1- fluxes in low K+ fluxes in low K+ sheep erythrocytes

Ouabain-resistant (OR), C1- -dependent K+ (K+C1-) transport measured by Rb+ influx in isosmotic and anisosmotic media was stimulated by the Ca2+ ionophore A23187 and EGTA (ethylene-glycol-tetracetic acid) in low K+ (LK) but not in high K+ (HK) sheep red cells. Increasing external Ca2+ concentrations, [Ca2+]o, from about 10(-7) to 10(-3)M in presence of A23187 and in absence of EGTA inhibited OR Rb+ influx, in LK red cells osmotically shrunken or swollen as well as treated with the thiol reagent N-ethylmaleimide (NEM). Hence the volume- and the NEM-stimulated K+C1- transport system in LK cells can be experimentally modulated by cellular Ca2+ or other Me2+, which may interact with sites on the K+C1- transporter under the control of membrane sulfhydryl (SH) groups.

Volume regulation of Chinese hamster ovary cells in anisoosmotic media

Chinese hamster ovary (CHO) cells when suspended in anisoosmotic media regulate their volumes by the activation of specific ion transport pathways. In hypoosmotic media the cells first swell and then return to their isoosmotic volumes by the loss of cellular KCl and osmotically obliged water. This regulatory volume decrease (RVD) is insensitive to ouabain or bumetanide but is blocked by quinine, cetiedil and oligomycin C. Based on cell volume and membrane potential measurements under various experimental conditions, we conclude that hypoosmotic shock activates independent, conductive transport pathways for K+ and for Cl-, respectively. The anion pathway can also transport NO3- and SCN- but not gluconate- anions. Osmotic shrinkage of CHO cells does not produce a regulatory volume increase (RVI) unless the cells have previously undergone a cycle of RVD. RVI is a Na+-dependent, amiloride-sensitive, but ouabain- and oligomycin-insensitive process, probably involving a Na+-H+ exchange system. Internal acidification of isoosmotic cells by addition of a permeable weak acid also activates an amiloride-sensitive Na+-H+ exchange, producing a volume increase. Both RVD and RVI in CHO cells seem to involve molecular mechanisms similar to those described for the volume regulation of lymphocytes, indicating the prevalence of these phenomena in nucleated mammalian cells. Cultured CHO cell lines may provide a basis for a genetic characterization of the volume-regulatory transport pathways.

Volume-regulatory K+ efflux during concentrative uptake of alanine in isolated rat hepatocytes

Changes in cell volume and 42K+ efflux associated with concentrative alanine uptake were studied in isolated rat hepatocytes suspended in Krebs-Ringer bicarbonate buffer. After addition of 10 mM-alanine, cellular water volume increased by 15% and the rate constant of 42K+ efflux by 250%. Alanine-induced 42K+ efflux was abolished by quinine and was strongly decreased when the cell-volume increase was counteracted by sucrose. The results suggest that K+ efflux during alanine uptake is implicated in a volume-regulatory response.

Na+,Cl- cotransport in Ehrlich ascites tumor cells activated during volume regulation (regulatory volume increase)

Ehrlich ascites cells were preincubated in hypotonic medium with subsequent restoration of tonicity. After the initial osmotic shrinkage the cells recovered their volume within 5 min with an associated KCl uptake. The volume recovery was inhibited when NO-3 was substituted for Cl-, and when Na+ was replaced by K+, or by choline (at 5 mM external K+). The volume recovery was strongly inhibited by furosemide and bumetanide, but essentially unaffected by DIDS. The net uptake of Cl- was much larger than the value predicted from the conductive Cl- permeability. The undirectional 36Cl flux, which was insensitive to bumetanide under steady-state conditions, was substantially increased during regulatory volume increase, and showed a large bumetanide-sensitive component. During volume recovery the Cl- flux ratio (influx/efflux) for the bumetanide-sensitive component was estimated at 1.85, compatible with a coupled uptake of Na+ and Cl-, or with an uptake via a K+,Na+,2Cl- cotransport system. The latter possibility is unlikely, however, because a net uptake of KCl was found even at low external K+, and because no K+ uptake was found in ouabain-poisoned cells. In the presence of ouabain a bumetanide-sensitive uptake during volume recovery of Na+ and Cl- in nearly equimolar amounts was demonstrated. It is proposed that the primary process during the regulatory volume increase is an activation of an otherwise quiescent, bumetanide-sensitive Na+,Cl- cotransport system with subsequent replacement of Na+ by K+ via the Na+/K+ pump, stimulated by the Na+ influx through the Na+,Cl- cotransport system.