Mechanism and role of furosemide-sensitive K+ transport in L cells: a genetic approach

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Potassium cotransport at the rat choroid plexus

The choroid plexuses are involved in cerebrospinal fluid (CSF) secretion and CSF K homeostasis. We examined K transport mechanisms present in the isolated rat choroid plexus that may be involved in these functions, predominantly using 86Rb as a marker for K. The study demonstrates that there are two primary uptake mechanisms. Ouabain-sensitive Na-K-adenosinetriphosphatase and bumetanide-sensitive cotransport, probably of the Na-K-2Cl form, account for 48 and 46% of uptake, respectively. Efflux studies demonstrate that the primary K efflux mechanism is also bumetanide-sensitive cotransport with the other major component probably being by K channels as it is inhibitable by barium or quinidine. Efflux via the cotransporter was not inhibited by R(+)-butylindazone, a KCl cotransport inhibitor, but it was enhanced in the presence of ouabain (P < 0.001) or increased extracellular Na concentration (P < 0.01). Furthermore, Na efflux was bumetanide sensitive (P < 0.05). In all, these data suggest that the efflux cotransporter is also of the Na-K-Cl form and that it is the same transporter as the influx mechanism operating in both directions. The evidence presented leads us to hypothesize that this cotransporter is on the apical membrane of the choroid plexus and that it may have a central role in CSF secretion and perhaps CSF K homeostasis.

Blockers of platelet-derived growth factor-activated nonselective cation channel inhibit cell proliferation

In serum-deprived G(o)-arrested cells, the addition of serum or growth factors initiates a cascade of events that culminates in DNA synthesis and mitosis. Recently, we showed that in mouse L-M(TK-) fibroblasts a 28-pS nonselective cation channel (NS channel) becomes quiescent at G(o) arrest and rapidly active within seconds of platelet-derived growth factor (PDGF) or serum addition, placing this response very early in the postreceptor signaling cascade. However, lack of specific channel blockers hindered determination of whether channel activation was necessary for mitogenesis. Derivatives of N-phenylanthranilic acid (DCA) have been reported to block a pancreatic nonselective channel. Therefore, using single-channel analysis, we examined the effect of these agents on the L-M(TK-) NS channel. Flufenamic acid and mefenamic acid rapidly produced reversible channel block with an inhibitory constant (Ki) approximately 10 microM. Furthermore, the component of the macroscopic K+ efflux shown to be mediated by the NS channel was blocked with a similar Ki value. DCA effects on cell proliferation were tested by measuring cloning efficiency and growth rate. Both were inhibited over the range of concentration that affected channel activity, and a 50% inhibitory dose of 50-100 microM was determined. This observation further substantiates the hypothesis that NS channel activation forms a necessary component in the transduction of the mitogenic signal from the PDGF receptor.

A simple method for evaluation of Rb+ transport and Na(+)-K+ pump stoichiometry in adherent cells

This report describes a method based on flame photometry for the evaluation of transmembrane Rb+ transport and Na(+)-K+ pump stoichiometry in adherent cells. In monolayers of cultured fibroblasts, the rates of 86Rb+, an isotope widely used as a K+ congener in transport studies, and nonradioactive Rb+ influx were equivalent when measured in the absence and presence of the transport inhibitors ouabain and bumetanide. Ouabain- and bumetanide-sensitive Rb+ fluxes were also equal with the two methods. Flame photometry allowed the simultaneous determination of intracellular [Na+] in the same sample in which Rb was measured. The incubation of human fibroblasts with ouabain for 5 min promoted a significant increase in intracellular [Na+]. Under appropriate experimental conditions, the ratio between the rate of ouabain-promoted increase in intracellular [Na+] and ouabain-sensitive Rb+ influx was 1.4, close to the theoretical value of 1.5 corresponding to a Na(+)-K+ pump stoichiometry of 3 Na+ extruded from the cell in exchange for 2 K+.

Channel-mediated and carrier-mediated uptake of K+ into cultured ovine oligodendrocytes

Uptake of radioactive K+ by mature ovine oligodendrocytes (OLGs) maintained in primary culture was measured under steady-state conditions, i.e., in cells maintained in a normal tissue culture medium (5.4 mM K+), and in cells after depletion of intracellular K+ to less than 15% of its normal value by pre-incubation in K(+)-free medium. The latter value is dominated by an active, carrier-mediated uptake (although it may include some diffusional uptake), whereas the former, in addition to active uptake, also reflects passive K+ diffusion through ion selective channels and possible self-exchange between extracellular and intracellular K+, which may be carrier-mediated. The total uptake rate was 144 +/- 10 nmol/min/mg protein, and the uptake after K+ depletion was 60 +/- 2 nmol/min/mg protein, much lower rates than previously observed in astrocytes. The uptake into K(+)-depleted cells was inhibited by about 80% in the presence of ouabain (1 mM) and about 30% in the presence of furosemide (2 mM). Activators of protein kinase C (phorbol esters) and cAMP-dependent protein kinase (forskolin) have been shown to alter the myelinogenic metabolism as well as outward K+ current in cultured OLGs. The present study demonstrates that K+ homeostasis in OLGs is modulated through similar second messenger pathways. Active uptake was inhibited by about 60% in the presence of active phorbol esters (100 nM) but was not affected by forskolin (100 nM). Forskolin likewise had no effect on total uptake, whereas phorbol esters caused a much larger inhibition than expected from their effect on carrier-mediated uptake alone, suggesting that channel-mediated uptake was also reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

The Na+,K+,2Cl- -cotransport system in HeLa cells and HeLa cell mutants exhibiting an altered efflux pathway

We have investigated the characteristics of a transport system in HeLa cells, which turned out to be very similar to a previously described Na+, K+, 2Cl- -cotransport system. For further understanding about the physiological role of the cotransporter, we have mutagenized HeLa cells and selected progeny cells for growth in low potassium (0.2 mM) medium. The selected HeLa cells (LK1) exhibited alterations in the Na+,K+,2Cl- -cotransport system. LK1 cells showed a remarkable reduction of 86Rb+ efflux via the cotransporter when compared to the parental HeLa cells. In contrast, bumetanide-sensitive potassium influx, measured by 86Rb+ uptake, was increased in the LK1 cells (increase in Vmax). Km values of the cotransporter in HeLa cells and LK1 mutants revealed similar properties for 86Rb+ and 22Na+ uptake. In addition, (3H)-bumetanide binding studies were carried out on intact HeLa cells; 1.7 pmol/mg protein (3H)-bumetanide was specifically bound to HeLa parental cells, which could be calculated to a number of 103,000 binding sites/cell. LK1 cells present, 1.44 pmol/mg protein, specifically bound (3H)-bumetanide and, respectively, 137,000 binding sites/cell. The LK1 cells also exhibited an increase in the number of (3H)-ouabain binding sites as well as an increase in the activity of the Na+,K+-ATPase, expressed as a function of ouabain-sensitive 86Rb+ uptake. Furthermore, LK1 cells were different in the concentrations of intracellular Na+ (increases) and K+ (decreases) when compared to the HeLa parental cells. When grown in low K+ medium (0.2 mM K+), protein content and cell volume were increased in the LK1 cells, while the DNA content was not significantly different between both cell lines.

Activation of single-channel currents in mouse fibroblasts by platelet-derived growth factor

A nonselective cation channel that we characterized in the mouse L-cell membrane becomes quiescent with serum deprivation (arrested cell growth) and rapidly active upon readdition of serum or, specifically, platelet-derived growth factor (PDGF). Using the patch-clamp technique, we find that the predominant channel in the LMTK- cell line is a bursting nonselective cation channel (the NS channel). In cell-attached and inside-out patches, the channel has a conductance of 28 pS; equal selectivity for Na+, K+, and Cs+; and no anion or divalent cation permeability. The channel open probability is voltage insensitive and in inside-out patches does not correlate with intracellular calcium (0.5 nM to 50 microM). When cultures are rendered quiescent by incubation in serum-free medium, channel open probability is virtually 0 as compared to 0.26 (+/- 0.17) in exponentially growing cultures. If mitogenesis is initiated by readdition of serum to quiescent cells while maintaining cell-attached recording, there is a rapid (15-30 s) activation of the channel (n = 12). The open probability of the patch increases (greater than 0.75) for 2-3 min and then decreases. We have attempted applications of several growth factors (fibroblast-derived growth factor, epidermal growth factor, insulin, bombesin, alpha-thrombin, and vasopressin, individually or in combination) but find that only PDGF (5-100 ng/ml; n = 9) produces channel activation. This activation should provide a Na+ entry pathway parallel to that of the Na/H exchanger.

Purification of proteins of the Na/Cl cotransporter from membranes of Ehrlich ascites cells using a bumetanide-sepharose affinity column

Bumetanide-binding proteins were isolated from membranes of Ehrlich ascites tumor cells by affinity chromatography. An affinity column was constructed with the active moiety of bumetanide as a ligand using 4'-azidobumetanide, a photoactive analogue which inhibits Na/Cl cotransport in Ehrlich cells with high specificity. Covalent binding of the 4'-azidobumetanide with Sepharose was promoted by photolysis. Membranes isolated from Ehrlich cells were solubilized with n-octylglucoside. Solubilized proteins retarded by the affinity column were readily eluted by bumetanide. In reducing gels the major proteins eluted by bumetanide were approximately 76 kDa and 38-39 kDa. There were also two proteins of 32 to 35 kDa eluted in lesser amounts. No proteins retarded by the affinity column were eluted with extensive washing without bumetanide. Furthermore, bumetanide eluted no proteins from a "control" column lacking the specific ligand. Upon rechromatography with bumetanide in solution, bumetanide-eluted proteins were not retarded, but their purity was increased by the retardation of contaminating proteins. Bumetanide-binding protein purified in this manner were characterized further by electrophoresis in nonreducing, nondenaturing gels.

Selectable mutations altering two mechanisms of mammalian K+ transport are dominant

Somatic cell mutants with altered K+ transport have previously been isolated from mutagenzied LMTK- cells for their ability to grow at subthreshold low-potassium concentrations (0.2 mM). These mutants fall into two classes: one class, LTK-5, possesses a functionally altered furosemide-sensitive Na+-K+-Cl- cotransport system and the other, LTK-1, an altered K+-conducting channel. Somatic cell hybrids have been formed between each of these cell lines and a wild-type L-cell line, making use of complementing selectable marker mutations carried by these parents, to establish the dominance of the K+ transport mutations. Hybrids were isolated and studied in two ways: clonal hybrid cell lines were selected in a manner unbiased toward their K+ transport phenotype, which was later assayed; and the number of independent hybrids arising in this single-selective condition was compared with the number arising in a condition which is double selective for the mutant phenotype as well. By both assays, hybrids formed with LTK-1 or LTK-5 as a parent uniformly exhibited the mutant phenotype by growth and cloning, whereas control hybrids with LMTK- as parent never did. This demonstrates both transport mutations to be dominant and thus potentially isolatable.

Na+/K+/Cl- cotransport in cultured vascular smooth muscle cells: stimulation by angiotensin II and calcium ionophores, inhibition by cyclic AMP and calmodulin antagonists

The specific activity of the Na+/K+/Cl cotransporter was assayed by measuring the initial rates of furosemide-inhibitable 86Rb+ influx and efflux. The presence of all three ions in the external medium was essential for cotransport activity. In cultured smooth muscle cells furosemide and bumetanide inhibited influx by 50% at 5 and 0.2 microM, respectively. The dependence of furosemide-inhibitable 86Rb+ influx on external Na+ and K+ was hyperbolic with apparent Km values of 46 and 4 mM, respectively. The dependence on Cl was sigmoidal. Assuming a stoichiometry of 1:1:2 for Na+/K+/Cl-, a Km of 78 mM was obtained for Cl. In quiescent smooth muscle cells cotransport activity was approximately equal to Na+ pump activity with each pathway accounting for 30% of total 86Rb+ influx. Growing muscle cells had approximately 3 times higher cotransport activity than quiescent ones. Na+ pump activity was not significantly different in the growing and quiescent cultures. Angiotensin II (ANG) stimulated cotransport activity as did two calcium-transporting ionophores. A23187 and ionomycin. The removal of external Ca2+ prevented A23187, but not ANG, from stimulating the cotransporter. Calmodulin antagonists selectively inhibited 86Rb+ influx via the cotransporter. Beta-adrenoreceptor stimulation with isoproterenol, like other treatments which increase cAMP, inhibited cotransport activity. Cultured porcine endothelial cells had 3 times higher cotransport activity than growing muscle cells. Calmodulin antagonists inhibited cotransport activity, but agents which increase cAMP or calcium had no effect on cotransport activity in the endothelial cells.

Anion transport systems in the plasma membrane of vertebrate cells

In the case of the red blood cell, anion transport is a highly specific one-for-one exchange catalyzed by a major membrane protein known as band 3 or as capnophorin. This red cell anion-exchange system mediates the Cl-(-)HCO3- exchange responsible for most of the bicarbonate transport capacity of the blood. The rapidly expanding knowledge of the molecular biology and the transport kinetics of this specialized transport system is very briefly reviewed in Section III. Exchange diffusion mechanisms for anions are found in many cells other than erythrocytes. The exchange diffusion system in Ehrlich cells has several similarities to that in red cells. In several cell types (subsection IV-B), there is evidence that intracellular pH regulation depends on Cl-(-)HCO3- exchange processes. Anion exchange in other single cells is described in Section IV, and its role in pH regulation is described in Section VII. Anion exchange mechanism operating in parallel with, and only functionally linked to Na+-H+ or K+-H+ exchange mechanisms can also play a role in cell volume regulation as described in Section VII. In the Ehrlich ascites cell and other vertebrate cells, electroneutral anion transfer has been found to occur also by a cotransport system for cations and chloride operating in parallel with the exchange diffusion system. The cotransport system is capable of mediating secondary active chloride influx. In avian red cells, the cotransport system has been shown to be activated by adrenergic agonists and by cyclic AMP, suggesting that the cotransport is involved in regulatory processes (see subsection V-A.). In several cell types, cotransport systems are activated and play a role during volume regulation, as described in Section V and in Section VII. It is also likely that this secondary active cotransport of chloride plays a significant role for the apparently active extrusion of acid equivalents from certain cells. If a continuous influx of chloride against an electrochemical gradient is maintained by a cotransport system, the chloride disequilibrium can drive an influx of bicarbonate through the anion exchange mechanism, as described in Section VII. Finally, even the electrodiffusion of anions is shown to be regulated, and in Ehrlich cells and human lymphocytes an activation of the anion diffusion pathway plays a major role in cell volume regulation as described in Section VI and subsection VII-B.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic mechanism of Na+, K+, Cl--cotransport as studied by Rb+ influx into HeLa cells: effects of extracellular monovalent ions

Ouabain-insensitive, furosemide-sensitive Rb+ influx (JRb) into HeLa cells was examined as functions of the extracellular Rb+, Na+ and Cl- concentrations. Rate equations and kinetic parameters, including the apparent maximum JRb, the apparent values of Km for the three ions and the apparent Ki for K+, were derived. Results suggested that one unit molecule of this transport system has one Na+, one K+ and two Cl- sites with different affinities, one of the Cl- sites related with binding of Na+, and the other with binding of K+(Rb+). A 1:1 stoichiometry was demonstrated between ouabain-insensitive, furosemide-sensitive influxes of 22Na+ and Rb+, and a 1:2 stoichiometry between those of Rb+ and 36Cl-. The influx of either one of these ions was inhibited in the absence of any one of the other two ions. Monovalent anions such as nitrate, acetate, thiocyanate and lactate as substitutes for Cl- inhibited ouabain-insensitive Rb+ influx, whereas sulfamate and probably also gluconate did not inhibit JRb. From the present results, a general model and a specialized cotransport model were proposed: In HeLa cells, one Na+ and one Cl- bind concurrently to their sites and then one K+(Rb+) and another Cl- bind concurrently. After completion of ion bindings Na+, K+(Rb+) and Cl- in a ratio of 1:1:2 show synchronous transmembrane movements.

Phorbol esters and gene expression: the role of rapid changes in K+ transport in the induction of ornithine decarboxylase by 12-O-tetradecanoylphorbol-13-acetate in BALB/c 3T3 cells and a mutant cell line defective in Na+K+Cl- cotransport

A BALB/c 3T3 preadipose cell line defective in Na+K+Cl- cotransport (3T3-E12a cells) has been used to study the relationship between phorbol ester-induced rapid changes in cation fluxes and changes in expression of a gene known to be modulated by this agent. In contrast to its effect on parental 3T3 cells, 12-O-tetradecanoylphorbol-13-acetate (TPA) did not inhibit either furosemide-sensitive 86Rb+ influx or the rate of 86Rb+ efflux from preloaded mutant cells. TPA-induced changes in intracellular K+ content were diminished in 3T3-E12a cells as compared with parental cells. Thus, mutation of the Na+K+Cl- cotransport system renders overall potassium transport in mutant cells largely insensitive to modulation by TPA. The morphological and functional responses of 3T3 and 3T3-E12a cells to TPA were also compared. In contrast to the extensive and long-lasting changes in morphology of 3T3 cells after 0.16 microM TPA addition, only slight and shorter-lived morphological effects of TPA were observed in 3T3-E12a cells. The transport properties of mutant cells were not totally unresponsive to TPA since hexose transport (2-deoxyglucose uptake) could be stimulated in both cell types. To establish a possible link between early changes in cation fluxes and activation of gene expression by TPA, the induction of the enzyme ornithine decarboxylase (ODC) was studied in detail. Addition of fresh medium containing serum or exposure to hypoosmotic conditions resulted in the induction of ODC in both 3T3 and 3T3-E12a cells. However, TPA failed to cause an increase in ODC activity in mutant cells, although a substantial induction of the enzyme was seen in parental cells. These results suggest that rapid changes in ion fluxes mediated by the Na+K+Cl- cotransport system are necessary for at least one of the phorbol ester-induced changes in gene expression in responsive cells.

Characterization of a BALB/c 3T3 preadipose cell mutant with altered Na+K+Cl- cotransport activity

A BALB/c 3T3 cell mutant (3T3-E12) was isolated by its ability to survive at a low extracellular K+ concentration (0.14 mM). The growth rate of mutant cells was less dependent on external K+ than parental cells. Analysis of potassium transport revealed that 3T3-E12 cells have a decreased activity of the furosemide-sensitive Na+K+Cl- cotransport system, both in the efflux and influx modes. This is shown to be a result of a decrease in the apparent affinity of the transport system for K+ and Na+, but not Cl-. Upon exposure to the phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate (TPA), BALB/c 3T3 cells exhibited a maximal volume decrease of 20%, while mutant cells shrunk by only 7%, suggesting that regulation of cell volume, at least four exposure to a tumor promoter, is impaired in mutant cells compared to parental 3T3 cells.

Calcium-induced heterogeneous changes in membrane potential detected by flow cytofluorimetry

Ionophore-induced changes in the cell-associated fluorescence of samples of approx. 50000 individual murine L1210 leukemia cells which had been incubated with the voltage-sensitive dye 3,3'-dihexyloctacarbocyanine iodide (DiOC6(3] were monitored by flow cytometry. The K+ ionophore valinomycin (1 microM) produced homogeneous changes in the fluorescence of the entire population, the magnitude of which was dependent upon the concentration of extracellular K+. These changes allowed the estimation of the potassium equilibrium potential of the cells, by the null-point method, to be -11.9 mV. The Ca2+ ionophore A23187 (500 nM) produced heterogeneous changes in fluorescence, with populations of both hyperpolarised and depolarised cells. In addition, the depolarised population underwent an apparent size change, with a reduction in cell volume. This heterogeneity of response resulted in a minimal change in the median fluorescence value for the whole population, which suggests that it would not have been detectable by methods dependent upon net population-averaged changes in fluorescence. Removal of extracellular Na+ or preincubation of cells with amiloride (500 microM) effectively eliminated the depolarised population. Removal of extracellular K+ increased the hyperpolarised population. These findings provide evidence for the presence of Ca2+-induced Na+ exchange and Ca2+-induced K+ efflux mechanisms in these cells which may be expressed simultaneously in the cell population.

Rapid changes in bidirectional K+ fluxes preceding DMSO-induced granulocytic differentiation of HL-60 human leukemic cells

When grown in medium containing 5 mM potassium and 140 mM sodium, HL-60, a human promyelocytic cell line, maintained a steady-state intracellular K+ concentration of 145 mmol/L cells and a steady-state intracellular Na+ concentration of 30 mmol/L cells. Nearly 90% of the unidirectional 42K+ influx could be inhibited by the cardiac glycoside ouabain with a Ki of 5 X 10(-8) M. This ouabain-sensitive component of influx rose as a saturating function of the extracellular K+ concentration with a K1/2 of 0.85 mM. The component of 42K+ influx resistant to ouabain inhibition was a linear function of the extracellular K+ concentration and was insensitive to inhibition by the diuretic furosemide. Unidirectional K+ efflux followed first order kinetics with a half-time of 55 min. Addition of 1.5% dimethyl sulfoxide (DMSO) to a culture of HL-60 cells allowed two population doublings followed by the cessation of growth without an impairment of cell viability. Beginning 2 to 3 days after DMSO addition, the cells underwent a dramatic reduction in volume (from 925 microns 3 to 500 microns 3) and began to take on the morphological features of mature granulocytes. Throughout this process of differentiation there was no change in the intracellular sodium or potassium concentration. However, immediately following the addition of DMSO to a culture of cells, there began an immediate, coordinated reduction in bidirectional K+ flux. The initial rate of the ouabain-sensitive component of K+ influx fell with a half-time of 11 h to a final rate, at 6 days induction, equal to one ninth that of the uninduced control, and over the same period, the rate constant for K+ efflux fell with a half-time of 14 h to a final value one fourth that of the uninduced control. The rapidity with which these flux changes occur raises the possibility that they play some role in the control of subsequent events in the process of differentiation.

Furosemide-sensitive potassium efflux in cultured mouse fibroblasts

Transfer of LM(TK-) cells from normal growth medium to medium lacking K+ leads to a rapid loss of intracellular K+, which is 50-70% inhibited by furosemide or bumetanide. The diuretic-sensitive component of K+ efflux requires both Na+ and Cl-, and is presumably mediated by a K+, Na+, Cl- cotransport system of the kind described in avian erythrocytes and Ehrlich ascites cells. It can be calculated that such a system should be near equilibrium under normal growth conditions but should mediate net efflux (as observed) when the driving force is altered by reducing extracellular K+. The diuretic-sensitive component of net K+ efflux is also sensitive to amiloride. This effect is probably indirect, however, with amiloride acting to block the Na+ influx that supplies Na+ to the cotransport system. At the low extracellular K+ concentrations employed in these studies, the diuretic-sensitive system is a physiologically important pathway of K+ loss. The rate of growth in low-K+ medium can be increased (or the rate of cell lysis decreased) by adding diuretic or by reducing external Na+ or Cl-.

Isolation and characterization of a CHO amino acid transport mutant resistant to melphalan (L-phenylalanine mustard)

A mutant of Chinese hamster ovary cells, CHY-2, was isolated on the basis of its reduced ability to grow on a limiting concentration of leucine and was found to be defective in uptake of leucine via the sodium-independent L system. Consistent with published reports that the L system can mediate melphalan uptake, the D10 of the mutant for melphalan was increased threefold under conditions designed to limit drug uptake to the L system (brief exposure in sodium-free medium). Unlike a previously described melphalan-resistant CHO mutant (CHr), CHY-2 displays no cross-resistance to colchicine or puromycin. It differs from a second melphalan-resistant CHO mutant, melr, in its sensitivity to melphalan in the presence of high Na+, and from a melphalan-resistant mouse leukemic cell in possessing normal levels of intracellular glutathione. Thus, CHY-2 represents a new melphalan-resistant mutant class. The effect of the CHY-2 mutation is pleiotropic, involving significant reductions in amino acid uptake via the L, A and Ly+ (but not ASC) systems. The primary defect is unknown; however, the mutant possesses normal intracellular concentrations of Na+ and K+ and normal membrane fluidity. The growth rate of the mutant in standard medium is greatly reduced (generation time of 60 h vs. 24 h), although it can be improved by the addition of a supplement containing high concentrations of leucine, proline, and peptides.

Thiol-dependent passive K/Cl transport in sheep red cells: IV. Furosemide inhibition as a function of external Rb+, Na+, and Cl-

The effect of the loop diuretic furosemide (4-chloro-N-furfuryl-5-sulfamoyl-anthranilic acid) on the thiol-dependent, ouabain-insensitive K(Rb)/Cl transport in low K+ sheep red cells was studied at various concentrations of extracellular Rb+, Na+ and Cl-. In Rb+-free NaCl media, 2 X 10(-3) M furosemide inhibited only one-half of thiol-dependent K+ efflux. In the presence of 23 mM RbCl, however, the concentration of furosemide to produce 50% K+ efflux inhibition (IC50) was 5 X 10(-5) M. In Rb+ containing NaCl media, the inhibitory effect of 10(-3) M furosemide was equal to that caused by NO-3 replacement of Cl- in the medium. The apparent synergistic action of furosemide and external Rb+ on K+ efflux was also seen in the ouabain-insensitive Rb+ influx. A preliminary kinetic analysis suggests that furosemide binding alters both maximal K+(Rb+) transport and apparent external Rb+ affinity. In the presence of external Rb+, Na+ (as compared to choline) exerted a small but significant augmentation of the furosemide inhibition of K+(Rb+) fluxes. There was no effect of Cl- on the IC50 value of furosemide. As there is no evidence for coupled Na+K+ cotransport in low K+ sheep red cells, furosemide may modify thiol-dependent K+(Rb+)/Cl flux or Rb+ (and to a slight degree Na+) modulate the effect of furosemide.

Phorbol ester inhibits furosemide-sensitive potassium transport in BALB/c 3T3 preadipose cells

The tumor promoter phorbol 12-myristate 13-acetate (PMA) rapidly decreased the rate of 86Rb+ uptake into BALB/c 3T3 preadipose cells. The component of total 86Rb+ influx affected by PMA is insensitive to ouabain but sensitive to the diuretic furosemide. Experiments designed to investigate the characteristics of the K+ transport system sensitive to PMA revealed that: (i) 86Rb+ uptake is highly dependent on external Na+, (ii) 86Rb+ uptake is highly dependent on external Cl-, (iii) 22Na+ uptake is dependent on external K+, and (iv) a major component of 86Rb+ efflux that is sensitive to PMA and furosemide is not dependent on extracellular K+. These features strongly implicate a Na+K+/Cl- cotransport system as the target of PMA and furosemide in these experiments. PMA caused a net intracellular accumulation of K+ within 15 min in these cells, presumably via its inhibitory effect on furosemide-sensitive K+ transport. Within 30 min after PMA treatment, the mean cell volume was significantly reduced in treated compared to control cells, with a maximum decrease of 21% attained at 4 hr after PMA. The significance of these findings for biologic changes induced by PMA is discussed.

Increase in K+ and alpha-AIB active transport in CHO cells after low [K+] treatment

We have studied the effects of prolonged incubation in low [K+] medium (approximately 0.3 mM) on both K+ and amino acid transport in Chinese hamster ovary (CHO) cells. When incubated in low [K+] medium, CHO cells redressed partially the loss of intracellular K+ after 12 h. After 24 h of incubation, both the activity of Na+-K+-ATPase in crude homogenates, and the transport capacity (Vmax) for ouabain-sensitive (i.e., active) K+ influx approximately doubled. The magnitude of the ouabain-insensitive (i.e., passive) K+ influx decreased by 50%. Thus the regulatory response involves an apparent increase in Na+-K+ pump and a decrease in K+ leak. The transport capacity for the nonmetabolized amino acid, alpha-aminoisobutyric acid (alpha-AIB), also increased after 24 h in low [K+] medium. The Vmax for the Na+-dependent (i.e., active) alpha-AIB influx increased by about 150%, and the magnitude of the Na+-independent influx increased by 20-40%. These changes in alpha-AIB transport result in a twofold greater capacity to accumulate this amino acid. Thus the regulation of K+ and alpha-AIB transport systems appears to be linked and possible mechanisms of this linkage are discussed.

Effects of ouabain and osmolarity on bumetanide-sensitive potassium transport in simian virus-transformed 3T3 cells

Unidirectional potassium influx in simian virus-transformed 3T3 cells was dissected into a ouabain-inhibitable "pump' component, a bumetanide-sensitive and chloride-dependent "cotransport' component, and a residual "leak' flux. The bumetanide-sensitive component was stimulated 2-3-fold by a 60-min preincubation with ouabain. Subsequent washing of the cells and incubation in ouabain-free saline reversed both the inhibition of the Na+ pump and the stimulation of bumetanide-sensitive flux. Bumetanide-sensitive potassium influx was also stimulated by hypertonic cell shrinkage (induced by 0.1 M or 0.2 M sorbitol). This latter observation suggests that the bumetanide-sensitive system may play a role in cellular volume regulation.

Reduction of K+ efflux in cultured mouse fibroblasts, by mutation or by diuretics, permits growth in K+-deficient medium

The mouse fibroblastic cell line LM(TK-) is unable to grow at external K+ concentrations below a threshold value of 0.4 mM. At subthreshold K+ concentrations, LM(TK-) cells rapidly lose intracellular K+ and eventually lyse. We have analyzed the pathway primarily responsible for K+ efflux under these experimental conditions and reports its specific inhibition by two diuretics, furosemide and bumetanide. Bumetanide, an analog of furosemide, was a more potent inhibitor (by several orders of magnitude) than was furosemide itself. The effects of ouabain and bumetanide were additive, suggesting independence of diuretic-sensitive K+ efflux from Na+/K+ pump-mediated fluxes. Characterization of K+ efflux in LTK-5, a mutant derived from LM(TK-) and selected for its ability to grow at 0.2 mM K+ indicated that the mutant had lost the diuretic-sensitive K+ efflux pathway. Net cation fluxes, steady-state intracellular cation concentrations, and growth at reduced K+ concentrations were comparable for LM(TK-) cells maximally inhibited by diuretics and for the LTK-5 mutant grown either in the presence or absence of diuretics. Thus, reduction in K+ efflux, either by diuretic addition diuretics. Thus, reduction in K+ efflux, either by diuretic addition or by genetic alteration, can permit the cell to maintain normal cation gradients and to grow at otherwise subthreshold external K+ concentrations.