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

Oxygen-sensitive cation transport in sickle cells

There are many examples of O2-sensitive solute transport in vertebrate red cells. The response is selective, specific, and conserved across the entire vertebrate spectrum. A number of possible physiological roles have been proposed, but abnormal responses to O2 may also be important pathologically. Significant alterations in O2 dependence of red cell cation transport are observed in sickle cell disease (and also following exposure to oxidants) and probably contribute to its pathophysiology. In this paper, we review some of the features of O2-sensitive solute transporters in red cells and possible reasons for the abnormal response in sickle cells. Our aim is to identify specific, novel pharmacological inhibitors of these abnormal pathways and thereby ameliorate the disease.

Differential oxygen sensitivity of the K+-Cl- cotransporter in normal and sickle human red blood cells

1. K+ influx and efflux were measured in normal (HbA) and sickle (HbS) red blood cells to investigate the interaction of swelling, H+ ions and urea with O2 (0 to 150 mmHg O2) in the presence of ouabain and bumetanide (both 100 microM). 2. In HbA cells, K+-C1- cotransport was O2 dependent. At low oxygen tensions (PO2s) the transporter was inactive and refractory to low pH, swelling or urea. 3. C1--independent K+ influxes in sickle cells were elevated at low PO2s, as previously reported. C1--dependent K+ influxes were large at both high and low PO2s, whether stimulated by swelling, H+ ions or urea. In the absence of O2, C1--dependent K+ influxes were similar in magnitude to those measured at high PO2s. The minimum for C1--dependent K+ influx was observed at PO2s of about 40-70 mmHg. 4. K+ efflux from HbS cells was stimulated by the addition of urea (500 mM). The rate constants were of similar magnitude whether measured at high PO2 or in the absence of O2, and were predominantly C1- dependent under both conditions. 5. In HbS red blood cells, reduction of extracellular Ca2+, addition of 1 mM Mg2+ or nitrendipine (10 microM) to the saline had no effect. Inhibitors of K+-C1- cotransport, [(dihydroindenyl)oxy] alkanoic acid (DIOA; 100 microM) or calyculin A (0.1 microM), inhibited influxes by a similar magnitude to C1- substitution. 6. Results are significant for the pathophysiology of sickle cell disease. Low pH and urea are able to stimulate KC1 loss from sickle cells, leading to cellular dehydration, even in regions of low PO2.

Deficiency of Src family kinases Fgr and Hck results in activation of erythrocyte K/Cl cotransport

Src-family kinases play a central role in regulation of hematopoietic cell functions. We found that mouse erythrocytes express the Src-family kinases Fgr and Hck, as well as Lyn. To directly test whether Fgr and Hck play any role in erythrocyte function, we analyzed red cells isolated from fgr-/-, hck-/-, and fgr-/- hck-/- knock-out mice. Mean corpuscular hemoglobin concentration and median density are increased, while K content is decreased, in fgr-/- hck-/- double-mutant erythrocytes compared with wild-type, fgr-/-, or hck-/- erythrocytes. Na/K pump and Na/K/Cl cotransport were not altered, but K/Cl cotransport activity was significantly and substantially higher (approximately three-fold) in fgr-/- hck-/- double-mutant erythrocytes. This enhanced K/Cl cotransport activity did not depend on cell age. In fact, in response to bleeding, K/Cl cotransport activity increased in parallel with reticulocytosis in wild-type erythrocytes, while abnormal K/Cl cotransport did not change as a consequence of reticulocytosis in fgr-/- hck-/- double-mutant erythrocytes. Okadaic acid, an inhibitor of a phosphatase that has been implicated in activation of the K/Cl cotransporter, inhibited K/Cl cotransport in wild-type and fgr-/- hck-/- double-mutant erythrocytes to a comparable extent. In contrast, staurosporine, an inhibitor of a kinase that has been suggested to negatively regulate this same phosphatase enhanced K/Cl cotransport in wild-type but not in fgr-/- hck-/- double-mutant erythrocytes. On the basis of these findings, we propose that Fgr and Hck are the kinases involved in the negative regulation of the K/Cl cotransporter-activating phosphatase. Abnormality of erythrocyte K/Cl cotransport in fgr-/- hck-/- double-mutant animals represents the first demonstration that Src-family kinases may be involved in regulation of membrane transport.

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.

Expression of GLUT-2 cDNA in human B lymphocytes: analysis of glucose transport using flow cytometry

The molecular characterization of transport proteins is often limited by transient functional expression or the need for a simple method to select functional cDNA clones. We used a mammalian expression system to obtain long-term expression of GLUT-2, an isoform of glucose permease. Rat GLUT-2 cDNA was ligated into an EBV vector (pLPP) and transfected into B lymphocytes which lack GLUT-2. Northern and Western analyses confirmed expression of GLUT-2 protein in membranes of transfected cells. Two functional assays using flow cytometry were developed to distinguish GLUT-2 transfectants from control/pLPP transfectants. Uptake of NBD-glucosamine, a fluorescent analogue of glucose, was increased in GLUT-2 transfectants. In addition, when exposed to hypertonic glucose medium, GLUT-2 transfectants and control/pLPP transfectants exhibited a difference in forward-angle light scatter (FALS), an index of cell volume, indicating a difference in glucose permeability. Independent measurements of glucose uptake (isotopic) and cell volume (video microscopy) confirmed the flow cytometry observations. This expression system used in combination with flow cytometry is useful for studying the functional properties of glucose and other solute transporters.

Decrease in Na(+)-K(+)-ATPase associated with maturation of sheep reticulocytes

Na(+)-K(+)-ATPase of immature and mature sheep red blood cells of both the high-K+ and low-K+ genotype and of immature cells matured in vitro was detected using polyclonal antiserum to purified sheep kidney Na(+)-K(+)-ATPase. This antiserum detects both alpha (alpha 1)- and alpha + (alpha 2 and/or alpha 3)-isoforms of the catalytic subunit as well as the beta-subunit of brain and kidney Na(+)-K(+)-ATPase. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting, a single major immunologically reactive component corresponding to the alpha-subunit was detected in membranes of immature and mature cells from sheep of both genotypes. Cells separated according to increasing density showed a corresponding decrease in ouabain binding sites on the cells and Na(+)-activated ATP hydrolysis of membranes isolated from the cells. A progressive decrease in immunologically reactive alpha-subunit was also observed. After in vitro culture of reticulocytes, reduction in ouabain binding to the cells was also associated with loss in alpha-subunit. As well, appearance of immunologically reactive alpha-subunit was detected in membranous material shed into the incubation medium, accounting for a fraction (less than or equal to 30%) of the material lost from the cells. Proteolytic sensitivity of the alpha-subunit indicates that, in this material, the cytoplasmic surface of the enzyme is exposed to the medium. The shed material was largely devoid of function as evidenced in little, if any, Na(+)-dependent phosphorylation of Na(+)-K(+)-ATPase. The existence in reticulocytes of an intracellular pool of ouabain binding sites was indicated by the transient appearance on the cell surface of ouabain binding sites after rapid ATP depletion and also after addition of chloroquine to cells during culture. Taken together, these findings indicate that the maturation-associated loss of sodium pump protein involves, at least partly, energy-dependent endocytosis and, presumably, processing whereby inactivation of function occurs as well as release of pump protein into the extracellular milieu.

Voltage-activated cation permeability in high-potassium but not low-potassium red blood cells

We have recently reported that voltage-activated fluxes of Na, K, and Ca occur in human red blood cells [J.A. Halperin, C. Brugnara, M. Tosteson, T. Van Ha, and D. C. Tosteson. Am. J. Physiol. 257 (Cell Physiol. 26): C986-C996, 1989]. The cation permeability increases progressively as the membrane potential becomes more inside positive above +20 mV. In this paper we show that this effect also occurs in high-potassium (HK), but not in low-potassium (LK), sheep and dog red blood cells. This result suggests that the voltage-activated cation transport pathway is not the result of nonspecific dielectric breakdown of the lipid bilayer but, rather, relates to some membrane component, presumably a protein, that is expressed in HK human and sheep but not in LK sheep and dog red blood cells.

Direct evidence for chloride-dependent volume reduction in macrocytic sheep reticulocytes

Cytometric analysis of the volume-distribution of macrocytic reticulocytes from 6-8 days acutely anemic sheep of both high and low potassium erythrocyte type revealed hyposmotically induced cell volume reduction in K-free NaCl but not in Na-methane sulfonate (CH3SO3Na) media. Furthermore N-ethylmaleimide, known to stimulate K:Cl efflux in these cells, and low extracellular pH caused cell shrinkage in isosmotic NaCl but not in CH3SO3Na. These data suggest that cell volume reduction, physiologically occurring during reticulocyte maturation, is a Cl-dependent process most likely involving electro-neutral K:Cl transport known to exist in reticulocytes of both sheep cation genotypes.

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.

Inactivation of regulatory volume decrease in human peripheral blood lymphocytes by N-ethylmaleimide

The sulfhydryl group reagent N-ethylmaleimide was found to inhibit in a dose dependent manner regulatory volume decrease of human peripheral lymphocytes swollen in buffered hyposmotic NaCl media. In hyposmotic KCl media NEM treated lymphocytes prevented an additional secondary swelling seen in control lymphocytes. The data suggest that N-ethylmaleimide acts on ion transport mechanisms involved in volume regulatory changes. This effect contrasts with the stimulation by N-ethylmaleimide of apparently volume sensitive K/Cl fluxes in certain mammalian red cells.