Characterization of a potassium carrier in rabbit reticulocyte cell membrane

K(86Rb) transport heterogeneity in the low-density fraction of sickle cell anemia red blood cells

Previous studies have suggested ion transport heterogeneity among sickle cell anemia (SS) reticulocytes that could influence their dehydration susceptibility. We examined Ca2(+)-independent K transport in the lowest density (F1), reticulocyte-rich SS cells, measuring the effects of acidification, ouabain, and bumetanide on their unidirectional K(86Rb) fluxes. Unlike those of normal red blood cells and SS discocytes, the SS-F1 K(86Rb) fluxes were highly nonlinear, with large 5-min flux components (previously unobserved) and a more gradual decline over 60 min. Analysis revealed two distinct K pools: a rapid-turnover pool in a small fraction of cells, whose major ouabain-resistant K(86Rb) transport path showed distinctive properties including inhibition by high concentrations of bumetanide (> or = 1 mM) and stimulation at pH 7.0, and another heterogeneous, relatively slow-turnover pool, in most of the F1 cells, whose main ouabain-resistant K(86Rb) path was insensitive to bumetanide but was stimulated at pH 7.0, which is consistent with heterogeneous expression of the acid-sensitive K-Cl cotransport and with both rapid and slower generation of dehydrated SS cells.

Effect of cell age and phenylhydrazine on the cation transport properties of rabbit erythrocytes

We studied the effect of cell age on the cation transport systems of rabbit erythrocytes by increasing the proportion of circulating young erythrocytes with either repeated bleeding or with phenylhydrazine (PHZ) treatment. We found that when the reticulocyte content of rabbit blood is increased by bleeding (from 1 to 40-50% of the circulating red cells), the response of the various transport pathways differs. The largest increase (fivefold) was found in the activity of K-Cl cotransport, which peaked 3 days after the last bleeding. The Na-K pump activity peaked at a similar time, but the % increase was twofold less than the K-Cl cotransport. There was a very small increase in the activity of the Na-Li exchange, whereas the Na-H exchange reached peak values 10 days after the last bleeding (twofold increase), when activities of K-Cl cotransport and Na-K pump had returned to almost normal levels. In vivo PHZ treatment resulted in anemia and marked reticulocytosis (80-90% of circulating cells). Transport rates were markedly increased (Na-K pump 9.6-fold, Na-H exchange 6.8-fold, Na-Li exchange 2.75-fold; K-Cl cotransport: 10-20-fold). When blood from PHZ-treated rabbits was incubated in vitro for 24-48 hours, red cell volume and K content decreased. This process was associated with a 70% reduction in the activity of the K-Cl cotransport after 24 hours and a 90% reduction after 48 hours. The activity of the other systems also declined and approached baseline values after 48 hours. Loss of transport activity was not affected by 10 microM E-64, whereas 10 mM methylamine reduced the inactivation of the Na-H exchange and of the Na-Li exchange. PHZ treatment of rabbit red cells in vitro resulted in marked increase of the K-Cl cotransport and inhibition of Na-K pump, Na-H exchange, and Na-Li exchange. These effects were abolished by DTT, with the exception of the Na-K pump inhibition, which was DTT insensitive. Thus both cell age and oxidative damage are important determinants of cation transport in rabbit red cells.

A mathematical model of the volume, pH, and ion content regulation in reticulocytes. Application to the pathophysiology of sickle cell dehydration

We developed a mathematical model of the reticulocyte, seeking to explain how a cell with similar volume but much higher ionic traffic than the mature red cell (RBC) regulates its volume, pH, and ion content in physiological and abnormal conditions. Analysis of the fluxbalance required by reticulocytes to conserve volume and composition predicted the existence of previously unsuspected Na(+)-dependent Cl- entry mechanisms. Unlike mature RBCs, reticulocytes did not tend to return to their original state after brief perturbations. The model predicted hysteresis and drift in cell pH, volume, and ion contents after transient alterations in membrane permeability or medium composition; irreversible cell dehydration could thus occur by brief K+ permeabilization, transient medium acidification, or the replacement of external Na+ with an impermeant cation. Both the hysteresis and drift after perturbations were shown to depend on the pHi dependence of the K:Cl cotransport, a major reticulocyte transporter. This behavior suggested a novel mechanism for the generation of irreversibly sickled cells directly from reticulocytes, rather than in a stepwise, progressive manner from discocytes. Experimental tests of the model's predictions and the hypothesis are described in the following paper.

Role of the Na+/K+/Cl- transporter in the positive inotropic effect of ouabain in cardiac myocytes

In this study we have characterized the bumetanide-sensitive K+/Na+/Cl- cotransport in cultured rat cardiac myocytes. 1) It carries about 10% of the total K+ influx. 2) It is sensitive to furosemide (Ki0.5 = 10(-6)M) and bumetanide (Ki0.5 = 10(-7)M). 3) It is strongly dependent on the extracellular concentrations of Na+ and Cl-. 4) It carries out influx of both ions, K+ and Na+. A therapeutic concentration of ouabain (10(-7) M) stimulated the bumetanide-sensitive K+ influx (as measured by 86Rb+), in the cultured myocytes, with no effect on the bumetanide-resistant K+ influx, which was mediated mostly by the Na+/K+ pump. Stimulation of the bumetanide-sensitive Rb+ influx by a low ouabain concentration was strongly dependent on Na+ and Cl- in the extracellular medium. A low concentration of ouabain (10(-7) M) was found to increase the steady-state level of cytosolic Na+ by 15%. This increase was abolished by the addition of bumetanide or furosemide. These findings suggest that ouabain, at a low (10(-7) M) concentration, induced its positive inotropic effect in rat cardiac myocytes by increasing Na+ influx into the cells through the bumetanide-sensitive Na+/K+/Cl- cotransporter. In order to examine this hypothesis, we measured the effect of bumetanide on the increased amplitude of systolic cell motion induced by ouabain. Bumetanide or furosemide, added to cultured cardiac myocytes, inhibited the increased amplitude of systolic cell motion induced by ouabain. Neither bumetanide nor furosemide alone has any significant effect on the basal amplitude of systolic cell motion. We propose that stimulation of bumetanide-sensitive Na+ influx plays an essential role in the positive inotropic effect in rat cardiac myocytes induced by low concentration of ouabain.

Reduced Na-K-Cl cotransport in vascular smooth muscle cells from spontaneously hypertensive rats

We have previously demonstrated the presence of a prominent, cyclic nucleotide-sensitive Na-K-Cl cotransport in vascular smooth muscle cells (VSMC). Others have observed that Na-K-Cl cotransport levels are reduced in erythrocytes of patients with essential hypertension and have proposed that a defect in this Na transport system may play a role in the pathogenesis of the disease. However, such a defect has not been demonstrated in the putative target tissue for essential hypertension, i.e., the VSMC. In the present study, we compared Na-K-Cl cotransport of VSMC from spontaneously hypertensive rats (SHR) with Na-K-Cl cotransport of VSMC from normotensive Wistar-Kyoto rats (WKY). We found that Na-K-Cl cotransport of SHR VSMC is significantly reduced relative to that of WKY VSMC (3.09 vs. 4.39 mumol K.g protein-1.min-1). The apparent ion affinities for Na-K-Cl cotransport of SHR VSMC did not differ from those determined for WKY VSMC. Furthermore, cyclic nucleotide regulation of cotransport also appeared to be the same for the two types of VSMC. In contrast, maximal saturable binding of [3H]bumetanide observed in SHR VSMC was markedly reduced compared with that of WKY VSMC, but the Kd values were similar. Our data suggest that the reduction in cotransport observed in SHR VSMC is the result of a decrease in the number of available cotransport sites.

Interactions of cardiac glycosides with cells and membranes. IV. Effects of ouabain and bumetanide on 86Rb+ influx in cultured cardiac myocytes from neonatal rats

Ouabain at nanomolar concentrations stimulates total Rb+ influx by 20 +/- 2% in monolayer cultures of myocytes which were either in physiologic ionic steady-state conditions ('control') or 'loaded with Na+' following exposure to K+-free medium. The ouabain-stimulated Rb+ influx was completely abolished by 0.1 mM bumetanide both in 'control' and in 'Na+-loaded' myocytes. Thus, addition of nanomolar concentrations of ouabain to myocytes markedly stimulate the bumetanide-sensitive Rb+ influx. This influx was increased up to 3- and 4-fold in 'control' and 'Na+-loaded' myocytes, respectively. Ouabain at nanomolar concentrations had no significant effect on the component of 86Rb+ influx which is inhibited by millimolar concentrations of ouabain (the so called 'ouabain-sensitive' or 'pump-mediated' Rb+ influx) in 'control' and 'Na+-loaded' cells. It is proposed that the increased rates of bumetanide-sensitive Rb+ influx are accompanied by an increased bumetanide-sensitive Na+ influx through the Na+/K+ cotransporter and thus to a transient increase in intracellular Na+ concentrations [Na+]i. The increase in [Na+]i, subsequently causes a transient elevation in [Ca2+]i via the Na+/Ca2+ exchanger and may be involved in the regulation of cardiac cells' contractility.

Irreversible reduction in potassium fluxes accompanies terminal differentiation of human myoblasts to myotubes

Potassium and sodium fluxes believed to be important in the cellular response to serum and growth factors have not been widely investigated in cells which have undergone terminal differentiation. In this study we have analyzed two main K+ transport systems--the ouabain-sensitive Na+/K+ pump and the bumetanide-sensitive transporter--in human muscle in vitro at two developmental stages: proliferating myoblasts and differentiated myotubes. Myoblast differentiation to myotubes was accompanied by a marked decrease in both the ouabain-sensitive and the bumetanide-sensitive K+ (Rb+) influxes. The addition of serum to the terminally differentiated myotubes had no effect on these K+ transporters. However, serum addition to serum-deprived, undifferentiated myoblasts produced a marked stimulation of these K+ fluxes. The bumetanide-sensitive K+ transporter in human myoblasts and myotubes has the following properties: (1) It carries 30% and 40% of the total K+ influx in myoblasts and myotubes, respectively. (2) It performs net efflux of K+ in the undifferentiated myoblasts and zero net flux (self-exchange) in terminally differentiated myotubes. (3) It is dependent on extracellular Na+ and Cl- in addition to K+. (4) In myoblasts, the Km value for K+ is 1.36 mM, similar to the Km for K+ of the Na+/K+ pump. (5) It is resistant to ouabain (up to 2 mM) and sensitive to furosemide (K0.5 = 5 X 10(-6) M) and bumetanide (K0.5 = 10(-7) M). These data indicate that following terminal differentiation of proliferating myoblasts to mitotically inactive myotubes there is an irreversible reduction of K+ fluxes with a change in the net flux of K+ carried by the bumetanide-sensitive transporter.

Cell volume, K transport, and cell density in human erythrocytes

We report here studies on the regulation of cell volume and K transport in human erythrocytes separated according to density. When cell volume was increased (isosmotic swelling, nystatin technique), erythrocytes of the least dense but not of the densest fraction shrunk back toward their original volume. This process was due to a ouabain (0.1 mM) and bumetanide (0.01 mM) (OB)-resistant K loss. OB-resistant K+ efflux from the least dense fraction was stimulated by hypotonic swelling and had a bell-shaped dependence on pH (pH optimum 6.75-7.0). These pH and volume effects were not evident in the densest fraction. The swelling-induced K+ efflux from the least dense fraction was inhibited when chloride was substituted by nitrate, thiocyanate, and acetate, whereas it was stimulated by bromide. Increasing cell Mg2+ content also markedly inhibited K+ efflux from isosmotically swollen cells. N-ethylmaleimide (NEM, 1 mM) greatly increased OB-resistant K+ efflux from the least dense fraction but not from the densest fraction. These data reveal the presence, in the lease dense fraction of normal human erythrocytes, of a pathway for K+ transport that is dependent on volume, pH, and chloride, is inhibited by internal Mg2+, and possibly plays a role in determining the erythrocyte water and cation content.

Fibroblast growth factor induces a transient net K+ influx carried by the bumetanide-sensitive transporter in quiescent BALB/c 3T3 fibroblasts

The bumetanide-sensitive transport system performed a net efflux of K+ in serum deprived quiescent cells. The addition of partially purified fibroblast growth factor (FGF) to G0/G1 phase 3T3 fibroblasts induced a transient net influx of K+, carried out by the bumetanide-sensitive transport system for 2-6 minutes. The stimulation of the bumetanide-sensitive K+ influx by FGF was followed by stimulation of the ouabain-sensitive K+ influx. In addition, both the bumetanide-sensitive and the ouabain-sensitive K+ influxes were found to be similarly stimulated when the G0/G1 3T3 cells were treated with insulin. These results suggest that growth factors such as FGF and insulin induce a change in the action of the bumetanide-sensitive transporter from performing net K+ efflux along its concentration gradient to an uphill transport pumping of K+ into the cell. We propose, therefore, that the bumetanide-sensitive transporter contributes to the increase in the intracellular K+ (and probable Na+) stimulated by growth factors such as FGF and insulin in early G1 phase of the cell cycle.

Amiloride-sensitive sodium-hydrogen exchange in osmotically shrunken rabbit red blood cells

As part of a detailed study of cell volume regulation in high-potassium mammalian erythrocytes, we have characterized ouabain-insensitive sodium transport in normal and osmotically shrunken rabbit red cells. In cells of normal volume and physiological pH, there is no amiloride-inhibited component of the sodium efflux (into either sodium-containing or sodium-free media). Osmotic shrinkage activates an amiloride-sensitive (50% inhibitory concentration = 10(-5) M) sodium transport system that can catalyze net sodium movement in either direction. This system appears to be distinct from the sodium-sodium (sodium-lithium) counter-transporter that operates in cells of normal volume. Replacement of chloride with acetate does not inhibit the sodium flux, but replacement with either nitrate or thiocyanate is inhibitory. An inward sodium gradient in shrunken cells induces a net uphill efflux of acid equivalents, indicating that the sodium transport is a sodium-hydrogen exchange. However, a sevenfold inward gradient of hydrogen ions (pHo = 6.4; pHi = 7.2) does not stimulate net sodium efflux in shrunken cells. This suggests that the extracellular affinity of the transport site for hydrogen ions is high, and that there is an extracellular noncompetitive inhibitory site for proton binding. Bilateral pH reduction stimulates an amiloride-inhibitable sodium flux in cells of normal volume; this indicates that, as has been found in kidney, brain, and lymphocytes, there is an intracellular protonation site that can activate the transport. Shrinkage of the cells shifts the pH dependence of the transport, suggesting that part of the signal for the osmotic activation of the transport is a shift in the pKa of this modifier site.

Basic characterization of an ouabain-resistant, bumetanide-sensitive K+ carrier-mediated transport system in J774.2 mouse macrophage-like cell line and in variants deficient in adenylate cyclase and cAMP-dependent protein kinase activities

86Rb(K+) transport across the plasma membrane of macrophage-like cells was studied. The cells used were the wild-type J774.2 and its two variants, CT2 cells, deficient in adenylate cyclase, and J7H1 cells, deficient in cAMP-dependent protein kinase. In the three cell lines about 15% of the total 86Rb(K+) influx is transported by the K+ carrier-mediated transport system. The 86Rb(K+) efflux carried by the same transporter is negligible when measured in the absence of ouabain in the medium. Therefore this carrier conducts a net inward flux of K+ under the experimental conditions used. The transporter is sensitive to extracellular Na+ and inhibited by 'loop' diuretics; bumetanide inhibits ouabain-resistant 86Rb(K+) influx with IC50 of 0.1, 5.0, and 0.05 microM for J774.2, CT2 and J7H1 macrophages, respectively. The membrane potential of the three cells was measured, using the distribution of [3H]tetraphenylphosphonium [( 3H]TPP+) across the plasma membrane, and found to be -80.1, -108.5 and -105.1 mV for J774.2, CT2 and J7H1 cells, respectively. The addition of bumetanide to the cell medium does not alter [3H]TPP+ uptake indicating that the transporter is electrically silent. It is concluded that despite the differences in cAMP metabolism by the three macrophages, the basic characteristics of K+ carrier-mediated transport system of the three cells are very similar.

Serum-induced net K+ influx performed by the diuretic-sensitive transport system in quiescent NIH 3T3 mouse fibroblasts

In serum deprived NIH 3T3 mouse cells the diuretic-sensitive transport system performs K+ self-exchange. The addition of serum which stimulates cell proliferation induces a net influx of K+, carried out by the diuretic-sensitive transport system. Thus, serum growth factors appear to induce a change in the mechanism of action of the diuretic-sensitive transporter from K+ self-exchange to an uphill transport pumping K+ into the cell. I propose here that this uphill uptake of K+ contributes to the increase of intracellular K+ content, found in the early G1 phase of the cell cycle.

Pig reticulocytes. V. Development of Rb+ influx during in vitro maturation

Influx of the K+ analogue Rb+ was measured through the ouabain-sensitive Na+/K+ pump and the ouabain-insensitive "leak" pathways in Cl- or NO3- in mature red cells from adult pigs and in reticulocytes naturally occurring in 7-day-old piglets. In reticulocytes, Rb+ influxes by the two pathways were of about equal magnitude in Cl- (13 and 10 mmoles/liter cells X hr) and at least 25-fold larger than in mature red cells (0.5 and 0.4 mmoles/liter cells X hr). In Na+ media, a portion of the ouabain-insensitive "leak" flux of Rb+ was Cl(-)-dependent (Rb+Cl- transport) as NO3- replacement reduced Rb+ influx by 90% in reticulocytes and by 40% in mature red cells. The sulfhydryl reagent N-ethylmaleimide (NEM) stimulated Rb+Cl- transport about twofold in reticulocytes and up to 13-fold in mature red cells. When reticulocytes matured to erythrocytes during in vitro incubation, about 90% of both ouabain-sensitive Rb+ pump and ouabain-insensitive Rb+Cl- influx were lost. In contrast, the NEM-stimulated Rb+Cl- transport changed much less throughout this period, suggesting an entity operationally but not necessarily structurally distinct from the basal Rb+Cl- transport. Although the experimental variability precluded a full assessment of significant changes in the small Na+/K+ (Rb+) pump and Rb+Cl- fluxes in mature pig red cells kept for the same time period in vitro, Rb+ flux changes in reticulocytes appear to be maturational in nature, reflecting parallel activity transitions of Na+/K+ pump and Cl(-)-dependent K+ fluxes in vivo.

Ouabain-resistant Na+, K+ transport system in mouse NIH 3T3 cells

It is shown that the ouabain-resistant (OR) furosemide-sensitive K+(Rb+) transport system performs a net efflux of K+ in growing mouse 3T3 cells. This conclusion is based on the finding that under the same assay conditions the furosemide-sensitive K+(Rb+) efflux was found to be two- to threefold higher than the ouabain-resistant furosemide-sensitive K+(Rb+) influx. The ouabain-resistant furosemide-sensitive influxes of both 22Na and 86Rb appear to be Cl- dependent, and the data are consistent with coupled unidirectional furosemide-sensitive influxes of Na+, K+ and Cl- with a ratio of 1:1:2. However, the net efflux of K+ performed by this transport system cannot be coupled to a ouabain-resistant net efflux of Na+ since the unidirectional ouabain-resistant efflux of Na+ was found to be negligible under physiological conditions. This latter conclusion was based on the fact that practically all the Na+ efflux appears to be ouabain-sensitive and sufficient to balance the Na+ influx under such steady-state conditions. Therefore, it is suggested that the ouabain-resistant furosemide-sensitive transport system in growing cells performs a facilitated diffusion of K+ and Na+, driven by their respective concentration gradients: a net K+ efflux and a net Na+ influx.

Rb+ influxes differentiate between growth arrest of cells by different agents

The effect of cell cycle on Rb+ (K+) fluxes was studied in NIH 3T3 mouse fibroblasts. Serum starvation or isoleucine deprivation resulted in cell arrest at an early G1/G0 phase, accompanied by a marked decrease in both ouabain-sensitive and ouabain-resistant Rb+ influx. On the other hand, cells arrested at late G1/G0 phase by hydroxyurea treatment have high ouabain-sensitive and ouabain-resistant Rb+ influx. Butyric acid treatment resulted in cell arrest at an early G1/G0 phase, but in contrast to serum or isoleucine starvation did not decrease Rb+ influxes. It is thus shown that quiescent cells may have Rb+ influx rates as high as that of logarithmically growing cells. The results are consistent with the hypothesis that an increased ion permeability of the cell is initiated at a critical stage in G1/G0 phase, and that butyric acid may arrest the cell beyond that stage.

Thiol-dependent passive K/Cl transport in sheep red cells: II. Loss of Cl- and N-ethylmaleimide sensitivity in maturing high K+ cells

A fraction of the passive, ouabain-insensitive K+ fluxes in mature low Kr(LK) but not in high K+ (HK) sheep red cells requires the presence of Cl- anions and can be stimulated by volume expansion (Dunham, P.B., Ellory, J.C., J. Physiol (London) 318:511-530, 1981) or treatment with 2 mM N-ethylmaleimide (NEM) (Lauf, P.K., Theg., B.E., Biochem. Biophys. Res. Commun. 92:1422-1428, 1980). In the present study it is shown that reticulocytes of both anemic LK and HK sheep possess the Cl- -dependent K+ transport system which subsequently remains functional in mature LK but not in HK red cells. Kinetically, Cl- -mediated K+ fluxes of reticulocytes of LK sheep are different from mature red cells only in their Vmax values as measured in Na+ or choline+ media, while there is an apparently much lower affinity for external K+ ions in reticulocytes of HK sheep. N-ethylmaleimide stimulated K+ transport in the reticulocytes of both sheep genotypes suspended in Cl- but failed to do so in NO3- media. The data are interpreted in terms of their biochemical, physiologic, and genetic implications for the HK/LK transition in sheep red cells.

Na+ K+ pump and passive K+ transport in large and small red cell populations of anemic high and low K+ sheep

Reticulocytes, isolated by centrifugal elutriation from massively bled sheep and identified by cytometric techniques, were analyzed with respect to their cation transport properties. In sheep with genetically high K+ (HK) or low K+ (LK) red cells, two reticulocyte types were distinguished by conventional or fluorescence-staining techniques 5-6 days after hemorrhage: Large reticulocytes as part of a newly formed macrocytic (M) erythrocyte population, and small reticulocytes present among the adult red cell population (volume population III of normal sheep blood, Valet et al., 1978). Although cellular reticulin disappeared within a few days, the M-cell population persisted throughout weeks in the peripheral circulation permitting a transport study of in vivo maturation. At all times, M cells of LK sheep had lower K+ and higher Na+ contents than M cells of HK sheep. Regardless of the sheep genotypes, M cells apparently reduced their volume during their first days in circulation; however, throughout the observation period, they did not attain that characteristic for adult red cells. Both ouabain-sensitive K+ pump and ouabain-insensitive K+ leak fluxes were elevated in M cells of both HK and LK sheep. The increased K+ pump flux was mainly due to higher K+ pump turnover rather than to the modestly increased number of pumps as measured by [3H]ouabain binding. In contrast, small reticulocytes enriched from separated volume population III cells by a Percoll-density gradient exhibited transport parameters close to their prospective mature HK or LK red cells. The data support the concept that the M cells derived from emergency reticulocytes while the small reticulocytes represented precursors of normal red cell maturation. The Na+ and K+ composition found in M cells of HK and LK sheep, respectively, suggest development of the LK steady state at or prior to the reticulocyte state, a finding consistent with that of Lee and Kirk (1982) on low K+ dog red cells.

Differentiation between serum stimulation of ouabain-resistant and sensitive Rb influx in quiescent NIH 3T3 cells

The addition of serum to quiescent NIH 3T3 mouse cell cultures resulted in a 10- to 20-fold increase of Rb influx which was resistant to ouabain, and only a three- to fourfold activation of ouabain-sensitive Rb influx. Stimulation of the ouabain-resistant Rb influx following serum addition reached its maximum within 2 min. The stimulation of ouabain-resistant Rb influx was a result of Vm increase while the Km for Rb was unchanged. Ouabain-resistant Rb influx, after serum addition, was resistant to amiloride and sensitive to ethacrynic acid. Replacing chloride in the medium by NO3-, CO3- and CH3COO- resulted in a drastic decrease in the ouabain-resistant Rb influx. It appeared, therefore, that the ouabain-resistant Rb influx in NIH 3T3 cells was Cl--dependent.