Cytodifferentiation and membrane transport properties in LK sheep red cells

Decline in number of Na-K pumps on low-K+ sheep reticulocytes during maturation is modulated by Lp antigen

The number of the Na-K pumps on sheep red blood cells declines markedly during cell maturation. In addition, in red blood cells of the low-K+ (LK) phenotype, there is an increase during maturation in the affinity of the pumps for intracellular K+. This increase does not occur in cells of the high-K+ (HK) phenotype. This HK/LK polymorphism is associated with the M/L blood group antigen system. The Lp antigen, which is on only LK cells, promotes the increase in affinity for K+ [Am. J. Physiol. 265 (Cell Physiol. 34): C99-C105, 1993]. Mature LK cells have fewer pumps than mature HK cells. The present study shows that the Lp antigen also promotes the loss of pumps in LK cells. The evidence was that modification of the Lp antigen of immature LK red blood cells either with anti-Lp antibody or by trypsinization diminished the loss of pumps during culture in vitro (numbers determined from [3H]ouabain binding). Confirmation came from demonstration of the decline during maturation of the amount of the alpha-subunit of the Na-K pump (measured by immunoblotting), which was also retarded by pretreatment with anti-Lp or trypsin. Comparisons of the relative amounts of Lp antigen on immature and mature LK cells showed that there is little decline in number of antigens during maturation. Therefore there is an increase in the antigen-to-pump ratio during maturation even though an association between pumps and antigens is necessary for the loss of pumps.

K-Cl cotransport, pH, and role of Mg in volume-clamped low-K sheep erythrocytes: three equilibrium states

Ouabain-resistant K efflux and Rb influx in Cl and NO3 media were studied in volume-clamped low-K (LK) sheep red blood cells (SRBC) with normal and experimentally reduced cytoplasmic Mg (Mgi) levels as function of pH and at 37 degrees C. Sucrose was added to solutions with constant ionic strength and variable pH to maintain normal cell volume. Cl-dependent ouabain-resistant K(Rb) fluxes (K-Cl cotransport) at unity relative cell volume exhibited a maximum at pH approximately 7 in normal-Mgi LK cells consistent with the apparent acid pH activation reported for human erythrocytes. However, in LK SRBC with Mgi lowered by A-23187 and an external Mg chelator, K(Rb)-Cl cotransport was reversibly activated as the pH was raised from 6.5 to 9. The alkaline pH effect on Cl-dependent Rb influx in low-Mgi LK SRBC was due to a 10-fold rise in the maximum velocity values without a major change in the Km values. The pH dependence of the experimental flux reversal point, i.e., the extracellular Rb concentration at which no net K-Cl cotransport occurs, approximately paralleled that of the flux reversal point predicted from the ratio of the ion products, in both control and low-Mgi LK cells, albeit with a small displacement to higher extracellular Rb concentration at all pH values. The kinetic data can be explained by a general minimum three-state equilibrium in which deprotonation recruits transporters from a resting R state into the active A state modified by Mgi to an inactive I state.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Energy depletion retards the loss of membrane transport during reticulocyte maturation

The effect of metabolic depletion on the maturation-associated loss of membrane functions has been studied by using sheep reticulocytes incubated in vitro at 37 degrees C for periods up to 41 hr. ATP was either maintained with glucose, adenosine plus inosine, or depleted with 2-deoxyglucose plus arsenate. Two membrane transport systems were studied: Na+-dependent glycine transport activity and the sodium pump, estimated from measurements of the number of [3H]ouabain binding sites per cell. Both transport systems were decreased during maturation. However, the decrease was much less in ATP-depleted cells compared to ATP-replete cells. It is concluded that the loss of certain functions during reticulocyte maturation is retarded by metabolic depletion.

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.

Study of maturation of membrane transport function in red blood cells by X-ray microanalysis

Red blood cells of certain species of animals, such as dogs and cats contain low potassium and high sodium, whereas the erythropoietic stem cells giving rise to these cells are of high potassium type. This paper examines the sequence of membrane transport changes during erythropoiesis by analyzing the K, Na and Fe in single bone marrow cells, reticulocytes and mature red blood cells with X-ray microanalysis. The relationship between K/Na ratios and Fe/(K + Na), which is analogous to hemoglobin concentration, gives an index of maturation stage. The relationships between K/Na and Fe/(K + Na) in the marrow cells of normal adult dog and those of a phenylhydrazine-injected dog with accelerated erythropoiesis show that the modification of cation composition occurs after the initiation of hemoglobin synthesis but before its completion. Similar relationships in the reticulocytes obtained from phenylhydrazine-injected dogs as well as from newborn dogs show a consistent decrease in K/Na with increased Hb, indicating a drastic change in cation composition during the maturation of the reticulocytes. Therefore the modification in membrane transport function must have occurred before or during the formation of reticulocytes.

Potassium fluxes in the rat reticulocyte. Ouabain sensitivity and changes during maturation

K+ turnover is markedly enhanced in the rat reticulocyte, both influx and efflux rates being increased by factors of approximately 3 over the corresponding rates in adult cells. These accelerated fluxes are observed despite the absence of any appreciable change in intracellular K+ concentration during the course of maturation. Qualitative characteristics of the active transport process for K+ influx appear to be identical in reticulocytes and mature erythrocytes with regard both to K+ sensitivity, and to ouabain sensitivity as a function of external K+ concentration. The number of ouabain binding sites per unit volume of cells, however, is increased by a factor of approximately three in the reticulocyte and thus correlates well with the observed degree of enhancement of active K+ influx in these cells. Half-maximal rates of ouabain-sensitive K+ influx are observed at external K+ concentrations well below 1 mM for both reticulocytes and mature erythrocytes. It is concluded that the enhanced rate of K+ accumulation in the reticulocyte can be quantitatively attributed to an increased number of pump units which are qualitatively identical to those in the mature cell, and which function at a near-maximal rate at the ambient K+ concentration present in normal rat plasma.

LK sheep reticulocytosis: effect of anti-L on K influx and in vitro maturation

After massive hemorrhage, adult sheep with genotypically low potassium (LK) red cells temporarily produce high potassium (HK) cells with ouabain-sensitive K+ pump fluxes equivalent to mature HK red cells. In light of recent reports of different red cell volume populations accompanying the HK-LK transition also occurring in newborn LK sheep and the unresolved controversy over the effect of anti-L on K+ transport in these immature red cells, we have reexamined the K+ transport changes and the effect of anti-L in the newly formed HK cells at various times after anemic stress and under in vitro conditions. We found that approximately 7 d after bleeding, maximum reticulocytosis occurred in the peripheral blood. After separation by density centrifugation, the top 10% cell fraction contained 100% reticulocytes, with a mean cell volume 2.5 times larger than that of mature erythrocytes. These immature red cells were of HK type, and their K+ pump and leak fluxes were 30 and 10 times higher, respectively, than those found in mature LK cells. The new cells may possess HK- and LK-type pumps because K+ pump influx was significantly stimulated by anti-L. When separated by density centrifugation on days 9, 17, and 23 after bleeding, some of the cells apparently maintained their large size while gaining higher density. Large cells from day 9, kept in vitro for 22 h, showed anti-L-sensitive K+ pump and leak fluxes that declined within hours, paralleling the behavior of these cells in vivo, whereas cellular K+ levels changed much less. It is concluded that the newly formed red cells may belong to a stress-induced macrocytic cell population that does not acquire all of the characteristics of adult LK cells.

Electron probe microanalysis of red blood cells. II. Cation changes during maturation

To understand the sequence of maturation of membrane transport and hemoglobin production during erythropoiesis, we have measured the K, Na, and Fe content in single mature red blood cells and bone marrow cells of dog using electron probe microanalysis (EPMA). Mature red blood cells of dog are low in potassium (LK) and high in sodium. These cells are derived from erythroblastic stem cells, which are high in potassium (HK) and low in sodium. This change from HK stem cells to LK red cells occurs in the marrow. The ratio of K/Na was found to be less than 0.2 independent of Fe/(K + Na) in circulating red cells. However, a significant number of marrow cells had both low K/Na and low Fe/(K + Na). We conclude that the changes in cation transport properties responsible for the conversion of HK to LK cells occur before the synthesis of hemoglobin in at least some marrow cells.

Changes in amino acid transport during red cell maturation

We studied amino acid transport in sheep red blood cells (RBCs) as a function of cell maturation. Transport of amino acids is decreased strikingly in the mature mammalian RBC compared to the immature reticulocyte. Blood obtained 5-6 days after massive bleeding was fractionated on dextran gradients. In the mature erythrocyte amino acids are taken up only slowly, and in the normal experimental interval (60 min) the concentration in the cell does not reach that of the medium. In contrast, the reticulocyte-rich (top) fraction (50-90% reticulocyte) accumulates certain amino acids, particularly histidine, methionine, and leucine. The underlying process is ATP-independent and Na+-insensitive, and has properties consistent with exchange diffusion, i.e., accelerated uptake or efflux when unlabeled solute is present on the trans side. The process is apparent not only in intact cells but also in resealed ghosts. The decrease in activity of amino acid transport is a function of red cell maturation. Thus it can be shown that (a) separation of cells according to their density 1, 2, and 3 weeks after bleeding leads to progressively lower amino acid transport activity with increasing cell density; and (b) during in vitro long-term incubation at 37 degrees C of reticulocyte-rich, unfractionated blood (5-10% reticulocytes), amino acid transport decreases while red cell integrity is maintained, as evidenced by the retention of a normal K+ gradient and the absence of hemolysis. The progressive loss is seen with resealed ghosts as well as with intact cells. Not all the amino acids examined participate in this exchange process. The most actively exchanged are histidine, leucine, methionine, and phenylalanine. Glycine, proline, arginine, and a-amino isobutyric acid do not participate in the exchange process.

Ouabain binding and potassium transport in young and old populations of human red cells

Human red blood cells were separated according to density by centrifugation through mixtures of phthalate esters. The densest 20% of the erythrocyte population (old cells) had reduced volume and water content compared to the lightest 20% of the cells (young cells). Corpuscular hemoglobin content was unchanged. Young cells had 50% more potassium (K+) than old cells, but their total intracellular concentration was only slightly higher, old cells had a small increase in sodium (Na+) concentration. Active K+ transport of young cells was 37% higher than that of old cells. [3H] + Ouabain binding revealed that this difference was the result of more K+ pump sites on young cells, which bound 530 ouabain molecules per cell at 100% K+ pump inhibition, as compared to 400 for old cells; unseparated cells bound 450-500 molecules. The relative rates of ouabain binding were identical for the two cell types. Old cells exhibited a greater passive permeability to K+, having a rate coefficient for ouabain-insensitive K+ influx 1.8 times that of young cells. There is evidence to suggest that in the face of reduced pump activity this augmented K+ "leak" might enhance the osmotic stability of the old cells and function to lengthen their life span.

Active potassium transport in reticulocytes of high-K+ and low-K+ sheep

The kinetics of active K+ transport were studied in immature red blood cells cells from high-K+ and low-K+ sheep particulary with respect to the effects of varying intracellular K+ concentration, [K]i. Comparison was made with active transport, or pump, activity in mature high-K+ and low-K+ red cells. Reticulocytes from both types of sheep had much higher maximal active K+ influxes than did mature cells. In both types of reticulocytes, and in mature high-K+ cells as well, the pump was relatively insensitive to increasing [K]i. In contrast, intracellular K+ markedly inhibited the pump in mature low-K+ cells. Active K+ transport in low-K+ reticulocytes, however, as in mature low-K+ cells, is stimulated by specific isoimmune anti-L serum. Therefore the K+ pumps of high-K+ and low-K+ reticulocytes have similar kinetic properties. Maturation of the red cells, involving inactivation of most of the pump activity in both cell types, results in mature high-K+ and low-K+ cells with K+ pumps of very different kinetic characteristics.

Membrane transport during development in animals

This brief and necessarily incomplete survey of available evidence on the development of transport systems in animal cells reveals a primitive state of knowledge full of interesting possibilities for future development. The assembly of membrane-bound transport systems during embryonic development provides unique opportunities for approaching questions relating to gene expression, the synthesis and insertion of membrane proteins into phospholipid layers, the composition and structure of transport systems and the conditions required for their functioning. It seems plausible to assume that the growth and differentiation of animal cells is regulated, in part at least, by the rate of transport of metabolites and ions across the cell membranes. Therefore the sequence of the expression of transport systems is likely to have a profound effect on subsequent stages of growth and differentiation. Feedback regulation of the synthesis of transport proteins by changes in the intracellular or extracellular concentrations of the transported metabolites or ions [52, 53, 85-87] may be a key element in the regulation of the rate of transport processes during development.

Changes from high potassium (hk) to low potassium (lk) in bovine red cells

Red cells of newborn calves contain 105-110 mmole K(+) and 1-5 mmole Na(+) per liter of cells. As the animals age the K(+) content decreases to a value of 25-30 mmole/liter of cells after about 60 days. At approximately the same time, the sodium content reaches a value of 60-70 mmole/liter. The time required for half change (t((1/2))) is 35-37 days for both Na(+) and K(+). The activity of (Na + K)-adenosine triphosphatase (ATPase) and the influx of K(42) and Rb(86) into the red cells are high at birth and are reduced to 5 and 15% of their original values, respectively, in mature animals. t((1/2)) for both is of the order of 30-35 days. The membrane Mg-ATPase activity is also high at birth and is reduced with a t((1/2)) of 28-32 days to a final value of about 20% of its activity at birth. Separation of red cells according to their age showed that, in animals at the age of transition, newly formed red cells contain a higher K/Na ratio and a higher active transport capacity than older red cells of the same animal. It is suggested that the changes observed are a reflection of the average age of the red cell population as the animal grows.

Active cation transport and ouabain binding in high potassium and low potassium red blood cells of sheep

Red cells from high K sheep contained 82 mM K/liter cells and had a pump flux of 0.86 mM K/liter cells x hr; similarly, LK cells had 16.5 mM K/liter cells and a pump flux of 0.12 mM K/liter cells x hr. Using [(3)H]-ouabain, the relation between the number of ouabain molecules bound per cell and the concomitant per cent inhibition of the pump was found to be approximately linear for both HK and LK cells. The number of glycoside molecules necessary for 100 % inhibition of the pump was 42 for HK cells and 7.6 for LK cells, after correction for six nonspecific binding sites for each type of cell. The ratio of ouabain molecules/cell at 100 % inhibition was 5.5, HK to LK, and the ratio of the normal K pump fluxes was 7.2, HK to LK. The similarity of these ratios suggests that an important difference between HK and LK cells, determining the difference in pump fluxes, is the number of pump sites. The turnover times (ions/site x min) are 6000 and 4800 for HK and LK cells, respectively. The results also indicate a high specificity of binding of ouabain to pump sites.