Furosemide-sensitive Na and K fluxes in human red cells. Net uphill Na extrusion and equilibrium properties

Cryo-EM structures of DrNKCC1 and hKCC1: a new milestone in the physiology of cation-chloride cotransporters

New milestones have been reached in the field of cation-Cl- cotransporters with the recently released cryo-electron microscopy (EM) structures of the Danio rerio (zebrafish) Na+-K+-2Cl- cotransporter (DrNKCC1) and the human K+-Cl- cotransporter (hKCC1). In this review we provide a brief timeline that identifies the multiple breakthroughs in the field of solute carrier 12 transporters that led to the structure resolution of two of its key members. While cation-Cl- cotransporters share the overall architecture of carriers belonging to the amino acid-polyamine-organocation (APC) superfamily and some of their substrate binding sites, several new insights are gained from the two individual structures. A first major feature relates to the largest extracellular domain between transmembrane domain (TMD) 5 and TMD6 of KCC1, which stabilizes the dimer and forms a cap that likely participates in extracellular gating. A second feature is the conservation of the K+ and Cl- binding sites in both structures and evidence of an unexpected second Cl- coordination site in the KCC1 structure. Structural data are discussed in the context of previously published studies that examined the basic and kinetics properties of these cotransport mechanisms. A third characteristic is the evidence of an extracellular gate formed by conserved salt bridges between charged residues located toward the end of TMD3 and TMD4 in both transporters and the existence of an additional neighboring bridge in the hKCC1 structure. A fourth feature of these newly solved structures relates to the multiple points of contacts between the monomer forming the cotransporter homodimer units. These involve the TMDs, the COOH-terminal domains, and the large extracellular loop for hKCC1.

Effect of complete protein 4.1R deficiency on ion transport properties of murine erythrocytes

Moderate hemolytic anemia, abnormal erythrocyte morphology (spherocytosis), and decreased membrane stability are observed in mice with complete deficiency of all erythroid protein 4.1 protein isoforms (4.1(-/-); Shi TS et al. J Clin Invest 103: 331, 1999). We have examined the effects of erythroid protein 4.1 (4.1R) deficiency on erythrocyte cation transport and volume regulation. 4.1(-/-) mice exhibited erythrocyte dehydration that was associated with reduced cellular K and increased Na content. Increased Na permeability was observed in these mice, mostly mediated by Na/H exchange with normal Na-K pump and Na-K-2Cl cotransport activities. The Na/H exchange of 4.1(-/-) erythrocytes was markedly activated by exposure to hypertonic conditions (18.2 +/- 3.2 in 4.1(-/-) vs. 9.8 +/- 1.3 mmol/10(13) cell x h in control mice), with an abnormal dependence on osmolality (EC(50) = 417 +/- 42 in 4.1(-/-) vs. 460 +/- 35 mosmol/kgH(2)O in control mice), suggestive of an upregulated functional state. While the affinity for internal protons was not altered (K(0.5) = 489.7 +/- 0.7 vs. 537.0 +/- 0.56 nM in control mice), the V(max) of the H-induced Na/H exchange activity was markedly elevated in 4.1(-/-) erythrocytes (V(max) 91.47 +/- 7.2 compared with 46.52 +/- 5.4 mmol/10(13) cell x h in control mice). Na/H exchange activation by okadaic acid was absent in 4.1(-/-) erythrocytes. Altogether, these results suggest that erythroid protein 4.1 plays a major role in volume regulation and physiologically downregulates Na/H exchange in mouse erythrocytes. Upregulation of the Na/H exchange is an important contributor to the elevated cell Na content of 4.1(-/-) erythrocytes.

Isoosmotic shrinkage by self-stimulated outward Na-K-Cl cotransport in quail erythrocytes

In mammalian erythrocytes, outward fluxes by the Na-K-Cl cotransporter NKCC have been clearly characterized, but NKCC fluxes are small and their physiological role, if any, is poorly understood. Avian erythrocytes are nucleated cells, in which a physiologically relevant NKCC acts as a cell volume regulator. Therefore, we further investigated outward cotransport and its relation to cell volume by using quail erythrocytes. Unlike human or rat erythrocytes, quail erythrocytes exhibit outward cotransport fluxes: (1) of high magnitude [maximal rate of bumetanide-sensitive Li+ efflux=12.3+/-1.1 mmol (l cells x h)(-1), mean +/-SEM, n=23] and (2) strongly stimulated by hyperosmotic media (by 100-200% in 500 mosmol/l media). Na+- or Li+-loaded quail erythrocytes exhibited rapid cell shrinkage when incubated in K+-free media. Thus, cell volume remained stationary up to 5-10 min and then started to shrink. Shrinkage was first slow, but progressively accelerated, finally reaching a new stationary state where cell volume had decreased by about 20%. Such rapid cell shrinkage was fully inhibited by bumetanide and was associated with outward cotransport stimulation (self-stimulated or an auto-catalytic process, i.e. a reaction stimulated by its product). External K+ reduced all these phenomena, but significant cell shrinkage was still observed at an external K+ concentration of 2.8 mM. K+ removal failed to stimulate outward cotransport in hypotonic media (250 mosmol/l). Finally, reincubation of shrunken erythrocytes in physiological saline revealed that inward cotransport was stimulated more than outward cotransport. In conclusion, isoosmotic hypokalaemia drives a rapid shrinkage of quail erythrocytes, due to auto-catalytic net outward cotransport stimulation. Whether this is an experimental curiosity or indicates that outward cotransport can have some physiological role deserves further investigation.

Potassium fluxes in contracting human skeletal muscle and red blood cells

The present study examined K+ fluxes in red blood cells and muscle during muscle contractions. Seven subjects performed two-legged submaximal knee-extensor exercise for 30 min. After 10 min of leg exercise (L1), intense arm exercise was also performed for 10 min (L2+A). Plasma epinephrine and norepinephrine concentrations were higher (P < 0.05) in L2+A compared with L1. Arterial plasma K+ at the end of L2+A was higher than in L1 (5.6 vs. 4.4 mM, P < 0.05) and returned to the L1 level on cessation of arm exercise. A net K+ release of 0.16 mmol/min from the active legs during L1 was turned to a net K+ uptake of 0.79 mmol/min during L2+A. Both arterial and venous red blood cell K+-to-hemoglobin ratios were constant during exercise. The present data suggest that contracting muscle can take up K+ probably by a combination of K+ and hormone activation of the Na+-K+ pump. Furthermore, changes in red blood cell K+ concentrations during muscle activity appear to be due to water movements and not transmembrane fluxes of K+.

Regulatory interaction of ATP Na+ and Cl- in the turnover cycle of the NaK2Cl cotransporter

To probe the mechanism by which intracellular ATP, Na+, and Cl- influence the activity of the NaK2Cl cotransporter, we measured bumetanide-sensitive (BS) 86Rb fluxes in the osteosarcoma cell line UMR-106-01. Under physiological gradients of Na+, K+, and Cl-, depleting cellular ATP by incubation with deoxyglucose and antimycin A (DOG/AA) for 20 min at 37 degrees C reduced BS 86Rb uptake from 6 to 1 nmol/mg protein per min. Similar incubation with 0.5 mM ouabain to inhibit the Na+ pump had no effect on the uptake, excluding the possibility that DOG/AA inhibited the uptake by modifying the cellular Na+ and K+ gradients. Loading the cells with Na+ and depleting them of K+ by a 2-3-h incubation with ouabain or DOG/AA increased the rate of BS 86Rb uptake to approximately 12 nmol/mg protein per min. The unidirectional BS 86Rb influx into control cells was approximately 10 times faster than the unidirectional BS 86Rb efflux. On the other hand, at steady state the unidirectional BS 86Rb influx and efflux in ouabain-treated cells were similar, suggesting that most of the BS 86Rb uptake into the ouabain-treated cells is due to K+/K+ exchange. The entire BS 86Rb uptake into ouabain-treated cells was insensitive to depletion of cellular ATP. However, the influx could be converted to ATP-sensitive influx by reducing cellular Cl- and/or Na+ in ouabain-treated cells to impose conditions for net uptake of the ions. The BS 86Rb uptake in ouabain-treated cells required the presence of Na+, K+, and Cl- in the extracellular medium. Thus, loading the cells with Na+ induced rapid 86Rb (K+) influx and efflux which, unlike net uptake, were insensitive to cellular ATP. Therefore, we suggest that ATP regulates a step in the turnover cycle of the cotransporter that is required for net but not K+/K+ exchange fluxes. Depleting control cells of Cl- increased BS 86Rb uptake from medium-containing physiological Na+ and K+ concentrations from 6 to approximately 15 nmol/mg protein per min. The uptake was blocked by depletion of cellular ATP with DOG/AA and required the presence of all three ions in the external medium. Thus, intracellular Cl- appears to influence net uptake by the cotransporter. Depletion of intracellular Na+ was as effective as depletion of Cl- in stimulating BS 86Rb uptake.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetic study on the effects of intracellular K+ and Na+ on Na+, K+, Cl- cotransport of HeLa cells by Rb+ influx determination

The effects of intracellular K+ and Na+ (K+c, Na+c) on the Na+, K+, Cl- cotransport pathway of HeLa cells were studied by measuring ouabain-insensitive, furosemide-sensitive Rb+ influx (JRb) at various intracellular concentrations of K+ and Na+ ([K+]c, [Na+]c). When [K+]c was increased and [Na+]c was decreased, keeping the sums of their concentrations almost constant, JRb as a function of the extracellular Rb+ or Na+ concentration ([Rb+]e, [Na+]e) was stimulated. However, the apparent K0.5 for Rb+e or Na+e remained unchanged and the ratio of the apparent K0.5 for K+c and the apparent Ki for Na+c was larger than 1. When JRb was increased by hypertonicity by addition of 200 mM mannitol, the apparent maximum JRb increased without change in the apparent K0.5 for Rb+e. These results show that K+c stimulates and Na+c inhibits JRb, without change in the affinities of the pathway for Rb+e and Na+e. The affinity for K+c is slightly lower than that for Na+c. Hypertonicity enhances JRb without any change in the affinity for Rb+e. We derived a kinetic equation for JRb with respect to K+c and Na+c and proposed a general and a special model of the pathway. The special model suggests that, in HeLa cells, JRb takes place when Rb+e binds to the external K+ binding site of the pathway after the binding of K+c to the internal regulatory site.

Cation transport and volume regulation in sickle red blood cells

Cellular dehydration is one of several pathological features of the sickle cell. Cation depletion is quite severe in certain populations of sickle cells and contributes to the rheological dysfunction that is the root cause of vascular occlusion in this disease. The mechanism of dehydration of sickle cells in vivo has not been ascertained, but three transport pathways may play important roles in this process. These include the deoxygenation-induced pathway that permits passive K+ loss and entry of Na+ and Ca2+; the K(+)-Cl- cotransport pathway, activated by acidification or cell swelling; and the Ca(2+)-activated K+ channel, or Gardos pathway, presumably activated by deoxygenation-induced Ca2+ influx. Recent evidence suggests that these pathways may interact in vivo. Heterogeneity exists among sickle cells as to the rate at which they become dense, suggesting that other factors may affect the activity or interactions of these pathways. Understanding the mechanism of dehydration of sickle cells may provide opportunities for pharmacological manipulation of cell volume to mitigate some of the symptoms of sickle cell disease.

Internal magnesium, 2,3-diphosphoglycerate, and the regulation of the steady-state volume of human red blood cells by the Na/K/2Cl cotransport system

This study is concerned with the relationship between the Na/K/Cl cotransport system and the steady-state volume (MCV) of red blood cells. Cotransport rate was determined in unfractionated and density-separated red cells of different MCV from different donors to see whether cotransport differences contribute to the difference in the distribution of MCVs. Cotransport, studied in cells at their original MCVs, was determined as the bumetanide (10 microM)-sensitive 22Na efflux in the presence of ouabain (50 microM) after adjusting cellular Na (Nai) and Ki to achieve near maximal transport rates. This condition was chosen to rule out MCV-related differences in Nai and Ki that might contribute to differences in the net chemical driving force for cotransport. We found that in both unfractionated and density-separated red cells the cotransport rate was inversely correlated with MCV. MCV was correlated directly with red cell 2,3-diphosphoglycerate (DPG), whereas total red cell Mg was only slightly elevated in cells with high MCV. Thus intracellular free Mg (Mgifree) is evidently lower in red cells with high 2,3-DPG (i.e., high MCV) and vice versa. Results from flux measurements at their original MCVs, after altering Mgifree with the ionophore A23187, indicated a high Mgi sensitivity of cotransport: depletion of Mgifree inhibited and an elevation of Mgifree increased the cotransport rate. The apparent K0.5 for Mgifree was approximately 0.4 mM. Maximizing Mgifree at optimum Nai and Ki minimized the differences in cotransport rates among the different donors. It is concluded that the relative cotransport rate is regulated for cells in the steady state at their original cell volume, not by the number of copies of the cotransporter but by differences in Mgifree. The interindividual differences in Mgifree, determined primarily by differences in the 2,3-DPG content, are responsible for the differences in the relative cotransport activity that results in an inverse relationship with in vivo differences in MCV. Indirect evidence indicates that the relative cotransport rate, as indexed by Mgifree, is determined by the phosphorylated level of the cotransport system.

Characteristics of the volume- and chloride-dependent K transport in human erythrocytes homozygous for hemoglobin C

In human red cells homozygous for hemoglobin C (CC), cell swelling and acid pH increase K efflux and net K loss in the presence of ouabain (0.1 mM) and bumetanide. We report herein, that K influx is also dependent on cell volume in CC cells: cell swelling induces a marked increase in the maximal rate (from 6 to 18 mmol/liter cell X hr) and in the affinity for external K (from 77 +/- 16 mM to 28 +/- 3 mM) of K influx. When the external K concentration is varied from 0 to 140 mM. K efflux from CC and normal control cells is unaffected. Thus, K/K exchange is not a major component of this K movement. K transport through the pathway of CC cells is dependent on the presence of chloride or bromide; substitution with nitrate, acetate or thiocyanate inhibits the volume- and pH-dependent K efflux. When CC cells are separated according to density, a sizable volume-dependent component of K efflux can be identified in all the fractions and is the most active in the least dense fraction. N-ethylmaleimide (NEM) markedly stimulates K efflux from CC cells in chloride but not in nitrate media, and this effect is present in all the fractions of CC cells separated according to density. The persistence of this transport system in denser CC cells suggests that not only cell age, but also the presence of the positively charged C hemoglobin is an important determinant of the activity of this system. These data also indicate that the K transport pathway of CC cells is not an electrodiffusional process and is coupled to chloride.

Regulation of Na+ and K+ contents in rat thymocytes

A modified nystatin technique allowed the investigation of the initial rate of Na+ efflux as a function of internal Na+ content under steady-state conditions in rat thymocytes. This kinetic study showed that 1) ouabain-sensitive Na+ efflux as a function of internal Na+ can be adjusted by a three-sites kinetic model, with a maximal pump rate of 581 +/- 79 mmol.l cells-1.h-1 and an apparent dissociation constant for internal Na+ of 10.0 +/- 1.0 mmol/l cells (mean +/- SE of 3 experiments), 2) bumetanide-sensitive Na+ efflux was extremely low compared with the pump efflux (approximately 1%), and 3) ouabain- and bumetanide-resistant Na+ efflux was almost a linear function of internal Na+ content (as expected for a Na+ leak). This "all-pump" mechanism of thymocyte Na+ regulation was confirmed by non-steady-state experiments showing that 1) ouabain induced a rapid net Na+ gain and K+ depletion in fresh thymocytes and completely blocked the recovery of normal cation contents in Na+-loaded-K+-depleted thymocytes, and 2) bumetanide was unable to modify thymocyte Na+ and K+ contents. Na+ extrusion by Na+-loaded thymocytes was unaffected by prostaglandin E2, isoproterenol, or platelet-aggregating factor (PAF) and was slightly impaired in the adult spontaneously hypertensive rat of the Okamoto strain (10% lower rate constant for net Na+ extrusion, P less than 0.05). Concerning cell Na+ regulation, our results do not support the concept that rat thymocytes are more representative of vascular cells than enucleated erythrocytes.

Clinical profiles and erythrocyte Na+ transport abnormalities of four major types of primary hypertension in Spain

The interaction of three different Na+ transport systems (Na+-K+ pump, Na+-K+ cotransport and Na+-Li+ countertransport), with internal Na+ and the passive Na+ leaks, were measured in erythrocytes from 72 Spanish, essential hypertensive patients and 30 normotensive controls. According to the observed abnormalities in Na+ transport pathways, 93.1% of the patients were classified into the following subsets: 12 (16.7%) exhibited a decreased apparent affinity of Na+-K+ pump for internal Na+ (Pump "-" hypertensives); 20 (27.7%) showed a decreased apparent affinity of Na+-K+ cotransport for internal Na+ (Co "-" hypertensives); 27 (37.5%) showed an accelerated Na+-Li+ countertransport (Counter "+" hypertensives); and 5 (6.9%) exhibited an increased rate constant of passive Na+ leaks (Leak "+" hypertensives). Finally, 5 patients (6.9%) did not show any abnormality in their Na+ transport systems and 3 exhibited more than one. Moreover, distinctive clinical features were recognize in Co "-" and Counter "+" subsets. Blood pressure values were lower in the former and, conversely, Counter "+" hypertensives showed a higher prevalence of moderate or severe hypertension (65.5% vs. 32.6%; P = 0.0059) and higher values of stimulated plasma renin activity (1.63 +/- 0.52 vs. 0.81 +/- 0.15; P = 0.0443). Our results confirm the heterogeneity of Na+ transport abnormalities in essential hypertension and suggest that these subsets of hypertensives could represent clinical entities.

Ca2+-activated K+ efflux limits complement-mediated lysis of human erythrocytes

The lytic effect of complement on human erythrocytes has been reported by others to increase when Na+ is substituted for K+ in the external medium. In this paper we have investigated the hypothesis that net loss of K+ through a K+ transport pathway protects erythrocytes from complement-induced colloidosmotic swelling and lysis. Antibody-sensitized human erythrocytes containing different intracellular cation concentrations (nystatin treatment) were exposed to low concentrations of guinea pig serum in media of different cation composition; complement lysis was assessed by the release of hemoglobin and the volume of the surviving cells estimated by their density distribution profiles. Complement-dependent swelling and lysis of erythrocytes (a) were limited by the presence of an outwardly directed K+ electrochemical gradient and (b) were enhanced by carbocyanine, a specific inhibitor of the Ca2+-activated K+ transport pathway, and by absence of Ca2+ in the external medium. We propose that during complement activation a rising cytosolic calcium triggers the Ca2+-activated K+ permeability pathway, the Gardos effect, produces a net K+, Cl- and water loss, and thus limits the colloidosmotic swelling and lysis of erythrocytes.

The Na-K-Cl cotransport in essential hypertension: cellular functions and genetic environment interactions

The present paper examines factors involved in the expression of the Na-K-Cl cotransport system present in erythrocytes and in vascular cells. This transport system is modulated by vasoactive peptides such as atrial natriuretic factor and bradykinin in vascular smooth muscle and endothelial cells. The Vmax of the Na-K-Cl in human red cells displays large interindividual differences which can be mainly accounted for by genetic factors. Elevation of the Km for Na of the outward Na cotransport is found in red cells of some Caucasian hypertensives and in Black normotensives born of hypertensive parents whose blood pressure increases with salt loading. Reduction of Na intake from 200 to 10 mEq/day does not influence the activity of the cotransport in normotensive individuals but decreases the Km for Na of hypertensive subjects to values similar to those of the normotensives. These findings indicate that the Na-K-Cl cotransport is an important probe of genetic and environmental factors in the hypertensive process.

Interaction of furosemide with lipid membranes

The interaction of furosemide with different phospholipids was investigated. Its influence on the lipid structure was inferred from its effect on the phase transition properties of lipids and on the conductance of planar bilayer membranes. The thermotropic properties of dipalmitoyl phosphatidylcholine, phosphatidylethanolamine (natural), dipalmitoyl phosphatidylethanolamine, brain sphingomyelin, brain cerebrosides and phosphatidylserine in the presence and absence of furosemide were investigated by differential scanning calorimetry. The modifying effect of furosemide seems to be strongest on phosphatidylethanolamine (natural) and sphingomyelin bilayers. The propensity of furosemide to decrease the electrical resistance of planar lipid membranes was also studied and it is shown that the drug facilitates the transport of ions. Partition coefficients of furosemide between lipid bilayers and water were measured.

Co-ordinated variations in chloride-dependent potassium transport and cell water in normal human erythrocytes

1. The capacity of the loop-diuretic-sensitive Na+-K+-Cl- system in normal human erythrocytes shows tenfold interindividual variation between different donors, although the transport rate is constant from month to month for any one donor. 2. The present work shows that this variation in Na+-K+-Cl- transport is inversely correlated with a low-capacity loop-diuretic-insensitive K+ transport, which is chloride dependent and is stimulated by cell swelling in hypotonic media. 3. These variations in K+ transport from donor to donor are related to cell water. Those donors who show high loop-diuretic-sensitive Na+-K+-Cl- co-transport have lower cell water and vice versa.

Asymmetry of Na-K-Cl cotransport in human erythrocytes

The Na-K-Cl cotransport system in human erythrocytes was studied by measuring net influxes and effluxes of Na and K. The influx of K was shown to be stimulated by Na and the influx of Na was stimulated by K, satisfying the fundamental criterion of cotransport. In addition, these mutually stimulating cation influxes had a stoichiometry of 1:1 and were entirely inhibited by furosemide; these results are also consistent with cotransport. Furthermore, the mutually stimulating influxes were entirely dependent on Cl, since they were abolished when nitrate was substituted for Cl. In contrast, cotransport, defined by mutual dependence of fluxes, was not detected in the outward direction over a range of cellular Na and K concentrations from 0 to 50 mmol/l cells. The cotransport pathway did, however, appear to mediate a Na-stimulated K efflux (but no K-stimulated Na efflux), and furosemide-inhibitable effluxes of both Na and K. Nitrate (but not sulfate) appeared to substitute for chloride in promoting Na-stimulated K efflux. Thus the Na-K-Cl cotransport system in human red cells is intrinsically asymmetric, and mediates coupled cation fluxes readily only in the inward direction.

Sodium transport kinetics in erythrocytes from spontaneously hypertensive rats

Rat erythrocytes with five different amounts of Na+ content have been prepared by using a new, nondetrimental Na+-loading method (net NaHPO4-influx through the anion carrier). This method allowed the determination of 1) maximal translocation rates and apparent dissociation constants for internal Na+ of the Na+-K+ pump, outward Na+-K+ cotransport, and Na+-Li+ countertransport and 2) rate constants of Na+ leak in erythrocytes from spontaneously hypertensive rats of the Okamoto strain and Wistar-Kyoto normotensive controls aged 2 to 26 weeks. Two major abnormalities were found in erythrocytes from spontaneously hypertensive rats: 1) a decreased cotransport affinity for internal Na+, which was constantly observed from 2 to 26 weeks of age (mean intracellular Na+ content for half-maximal stimulation of outward Na+-K+ cotransport = 33.1 +/- 7.0 [SD] mmol/L cells in spontaneously hypertensive rats vs 16.7 +/- 4.7 mmol/L cells in Wistar-Kyoto rats; p less than 0.001), and 2) a decreased maximal pump rate in adult (15- to 26-week-old) spontaneously hypertensive as compared with that for age-matched Wistar-Kyoto rats (9-37 vs 34-70 mmol/L cells/hr). Therefore, the low cotransport affinity for internal Na+ appears to be a stable, possibly genetic defect of spontaneously hypertensive rats. Conversely, the decreased maximal pump rate may be a secondary event, possibly reflecting the appearance of endogenous pump inhibitors in the plasma of adult spontaneously hypertensive rats.

The Li+-Na+ exchange and Na+-K+-Cl- cotransport systems in essential hypertension

This review examines the physiological functions of the Li+-Na+ exchanger and Na+-K+-Cl- cotransport system in human red blood cells. Both transporters are family aggregated and determined mainly by genetic factors; they are present in kidney and vascular cells, where they are regulated by vasoactive substances. To assess the physiological function of these two transporters, we investigated their kinetic and equilibrium properties, and their modulation by vasoactive substances. Recent studies in red blood cells indicate that the Li+-Na+ exchanger may be a mode of operation of the Na+-H+ exchanger, which plays an important role in the regulation of cell pH, cell volume, and transtubular sodium transport. In vascular cells, Na+-H+ exchanger is modulated by vasoconstrictors such as growth factors and angiotensin, while Na+-K+-Cl- cotransport is modulated by vasodilators such as atrial natriuretic factor and bradykinin. Kinetic studies in red blood cells of hypertensive patients and their offspring indicate the presence of subsets with elevated Vmax of Li+-Na+ exchange or high Km for cell sodium for outward Na+-K+-Cl- cotransport. The latter alteration is found most frequently in young blacks born of hypertensive parents, and it appears to be dependent on their level of sodium intake. The relationship between the alterations of the red blood cell sodium exchanger and Na+-K+-Cl- cotransport and risk factors for hypertension indicates that they can provide a tool to examine the interaction of genetic, hormonal, and environmental factors in human hypertension.

Na+, K+ and Cl- transport in isolated small intestinal cells from guinea pig. Evidences for the existence of a second Na+ pump

Isolated small intestinal epithelial cells, after incubation at 4 degrees C for 30 min, reach ion concentrations (36 mM K+, 113 mM Na+ and 110 mM Cl-) very similar to those of the incubation medium. Upon rewarming to 37 degrees C, cells are able to extrude Na+, Cl- and water and to gain K+. Na+ extrusion is performed by two active mechanisms. The first mechanism, transporting Na+ by exchanging it for K+, is inhibited by ouabain and is insensitive to ethacrynic acid. It is the classical Na+ pump. The second mechanism transports Na+ with Cl- and water, is insensitive to ouabain but is inhibited by ethacrynic acid. Both mechanisms are inhibited by dinitrophenol and anoxia. The second Na+ extruding mechanism could be the Na+/K+/2Cl- cotransport system. However, this possibility can be ruled out because the force driving cotransport would work inwards, and because Na+ extrusion with water loss continues after substitution of Cl- by NO3-. We propose that enterocytes have a second Na+ pump, similar to that proposed in proximal tubular cells.

Effect of membrane potential on furosemide-inhibitable sodium influxes in human red blood cells

Furosemide-inhibitable Na influx (a measure of Na/K/Cl cotransport) was determined as a function of membrane potential in human red blood cells. The membrane potential was varied from -42 to +118 mV using valinomycin and gradients of K. The furosemide-inhibitable, unidirectional Na influx was independent of membrane potential over the entire range of potentials. The change in flux per mV, 0.443 mumol/(liter cells.hr.mV), was not significantly different from zero. The mean flux was 153 +/- 16 mumol/(liter cells.hr) (+/- SEM, n = 71). The ouabain and furosemide-resistant influxes of Na and K were also measured as functions of membrane potential using either valinomycin and K or a chloride-free, tartrate flux medium to vary membrane potential. The unidirectional Na influx decreased slightly as the membrane potential was increased from negative potentials to about +10 mV. At higher membrane potentials Na influx rose dramatically with potential. This increase was not reversible and was also observed with K influx.

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.

Furosemide-sensitive K+ (Rb+) transport in human erythrocytes: modes of operation, dependence on extracellular and intracellular Na+, kinetics, pH dependency and the effect of cell volume and N-ethylmaleimide

The effect of extracellular and intracellular Na+ (Nao+, Nai+) on ouabain-resistant, furosemide-sensitive (FS) Rb+ transport was studied in human erythrocytes under varying experimental conditions. The results obtained are consistent with the view that a (1 Na+ + 1 K+ + 2 Cl-) cotransport system operates in two different modes: mode i) promoting bidirectional 1:1 (Na+-K+) cotransport, and mode ii) a Nao+-independent 1:1 ki+ exchange requiring Nai+ which, however, is not extruded. The activities of the two modes of operation vary strictly in parallel to each other among erythrocytes of different donors and in cell fractions of individual donors separated according to density. Rb+ uptake through Rbo+/Ki+ exchange contributes about 25% to total Rb+ uptake in 145 mM NaCl media containing 5 mM RbCl at normal Nai+ (pH 7.4). Na+-K+ cotransport into the cells occurs largely additive to K+/K+ exchange. Inward Na+-Rb+ cotransport exhibits a substrate inhibition at high Rbo+. With increasing pH, the maximum rate of cotransport is accelerated at the expense of K+/K+ exchange (apparent pK close to pH 7.4). The apparent KmRbo+ of Na+-K+ cotransport is low (2 mM) and almost independent of pH, and high for K+/K+ exchange (10 to 15 mM), the affinity increasing with pH. The two modes are discussed in terms of a partial reaction scheme of (1 Na+ + 1 K+ + 2 Cl-) cotransport with ordered binding and debinding, exhibiting a glide symmetry (first on outside = first off inside) as proposed by McManus for duck erythrocytes (McManus, T.J., 1987, Fed. Proc., in press). N-ethylmaleimide (NEM) chemically induces a Cl--dependent K+ transport pathway that is independent of both Nao+ and Nai+. This pathway differs in many properties from the basal, Nao+-independent K+/K+ exchange active in untreated human erythrocytes at normal cell volume. Cell swelling accelerates a Nao+-independent FS K+ transport pathway which most probably is not identical to basal K+/K+ exchange. Ko+ less than Nao+ less than Lio+ less than Mgo2+ reduce furosemide-resistant Rb+ inward leakage relative to cholineo+.

Bradykinin and vasopressin stimulate Na+-K+-Cl- cotransport in cultured endothelial cells

We have characterized a Na+-K+-Cl- cotransporter in vascular endothelial cells (EC) cultured from different blood vessels and species that is inhibited by the diuretics furosemide and bumetanide (50% inhibitory concentration for 86Rb influx approximately 20 microM and 0.5 microM, respectively). Inward 86Rb influx mediated via this pathway is greater than 86Rb influx transported by the Na+-K+ pump in cultured EC from bovine and pig aorta, bovine vena cava, and baboon cephalic vein but not in human umbilical or saphenous vein EC. External Na+ or Cl- -stimulated, ouabain-insensitive 86Rb influx is equal to furosemide or bumetanide-sensitive 86Rb influx. Ouabain-insensitive 22Na influx is also partially inhibited by these drugs and stimulated by increasing external K+ or Cl-. Net Na+ extrusion occurs via the Na+-K+-Cl- cotransporter in the absence of external K+, whereas net Na+ influx occurs at higher external K+ (greater than 1 mM). Maximal concentrations (100 nM) of bradykinin and vasopressin increase the initial rate of bumetanide-sensitive 86Rb influx by approximately 60 and 70% (50% effective concentration approximately 1 and 0.6 nM, respectively). Addition of either ethyleneglycol-bis(beta-aminotethylether)-N,N'-tetraacetic acid or LaCl3 (to block calcium influx) prevents bradykinin-stimulated 86Rb influx. When intracellular calcium is elevated using ionomycin (100 nM), a Ca2+ ionophore, bumetanide-sensitive 86Rb influx increases approximately twofold. In contrast, isoproterenol (100 microM) and forskolin (50 microM), adenylate cyclase stimulators, decrease furosemide-sensitive 86Rb influx.(ABSTRACT TRUNCATED AT 250 WORDS)

Modes of operation and variable stoichiometry of the furosemide- sensitive Na and K fluxes in human red cells

We report in this paper different modes of Na and K transport in human red cells, which can be inhibited by furosemide in the presence of ouabain. Experimental evidence is provided for inward and outward coupled transport of Na and K, Ki/Ko and Nai/Nao exchange, and uncoupled Na or K efflux. The outward cotransport of Na and K was defined as the furosemide-sensitive (FS) component of Na and K effluxes into choline medium and as the Cl-dependent or cis-stimulated component of the ouabain-resistant (OR) Na and K effluxes. Inward cotransport of Na and K was defined by the stimulation by external Na (Nao) of the K influx and the stimulation by external K (Ko) of the Na influx in the presence of ouabain. Both effects were FS and Cl dependent. Experimental evidence for an FS Ki/Ko exchange pathway of the Na/K cotransport was provided by (a) the stimulation by external K of FS K influx and efflux, and (b) the stimulation by internal Na or K of FS K influx in the absence of external Na. Evidence for an FS Nai/Nao exchange pathway was provided by the stimulation of FS Na influx by internal Na from a K-free medium (130 mM NaCl). This pathway was four to six times smaller than the Ki/Ko exchange. In cells containing only Na or K, incubated in media containing only Na or K, respectively, there was FS efflux of the cation without simultaneous inward transport (FS uncoupled Na and K efflux). The stoichiometric ratio of FS outward cotransport of Na and K into choline medium varied with the ratio of Nai-to-Ki concentrations, and when Nai/Ki was close to 1, the ratio of FS outward Na to K flux was also 1. In choline media, FS Na efflux was inhibited by external K (noncompetitively), whereas FS k efflux was stimulated. The stimulation of FS K efflux was due to the stimulation by Ko of the Ki/Ko exchange pathway. Thus, the stoichiometry of FS Na and K effluxes also varied in the presence of external K. A minimal model for a reaction scheme of FS Na and K transport accounts for cis stimulation, trans inhibition, and trans stimulation, and for variable stoichiometry of the FS cation fluxes.

Effect of metabolic depletion on the furosemide-sensitive Na and K fluxes in human red cells

We report in this paper the effect of metabolic depletion on several modes of furosemide-sensitive (FS) Na and K transport in human red blood cells. The reduction of ATP content below 100 mumol/liter cells produced a marked decrease in the maximal activation (Vmax) of the outward, FS transport of Na and K into choline medium in the presence of ouabain (0.1 mM) and 1 mM MgCl2. The K0.5 for internal Na to activate the FS Na efflux was not altered by metabolic depletion. However, metabolic depletion markedly decreased the Ki for external K (Ko) to inhibit the FS Na efflux into choline medium (from 25 to 11 mM). Repletion of ATP content by incubation of cells in a substrate-rich medium recovered control levels of Vmax of the FS Na and K fluxes and of Ki for external K to inhibit FS Na efflux. The Vmax of FS Na and K influxes was also markedly decreased when the ATP content dropped below 100 mumol/liter cells. This was mainly due to a decrease in the inward-coupled transport of K and Na (NaO-stimulated K influx and the Ko-stimulated Na influx). The FS Ki/Ko exchange pathway of the Na-K cotransport, estimated from the FS K influx from choline-20 mM Ko medium into cells containing 22 mmol Na/liter cells, was also reduced by starvation. Starvation did not inhibit the FS Nai/Nao exchange pathway, estimated as FS Na influx from a medium containing 130 mM NaCl into cells containing 22 mmol Na/liter cells. The unidirectional FS 22Na efflux and influx were also measured in control and starved cells containing 22 mmol Na/liter cells, incubated in a Na medium (130 mM) at varying external K (0 to 20 mM). In substrate-fed cells, incubated in the absence of external K, FS Na efflux was larger than Na influx. This FS net Na extrusion (400 to 500 mumol/liter cells X hr) decreased when external K was increased, approaching zero around 15 mM Ko. In starved cells the net Na extrusion was markedly decreased and it approached zero at lower Ko than in substrate-fed cells. Our results indicate that the FS Na and K fluxes, and their major component, the gradient driven Na-K-Cl cotransport system, are dependent on the metabolic integrity of the cells.