Role of the furosemide-sensitive Na+/K+ transport system in determining the steady-state Na+ and K+ content and volume of human erythrocytes in vitro and in vivo

The influence of erythrocyte maturity on ion transport and membrane lipid composition in the rat

Significant relationships between ion transport and membrane lipid composition (cholesterol, total phospholipids and sphingomyelins) were found in erythrocytes of salt hypertensive Dahl rats. In these animals mean cellular hemoglobin content correlated negatively with Na(+)-K(+) pump activity and Na(+) leak but positively with Na(+)-K(+) cotransport activity. Immature erythrocytes exhibit lower mean cellular hemoglobin content (MCHC) than mature ones. The aim of the present study was to find a relationship between erythrocyte maturity, membrane lipid composition and ion transport activity in Wistar rats aged three months which were subjected to repeated hemorrhage (blood loss 2 ml/day for 6 days) to enrich circulating erythrocytes with immature forms. Immature and mature erythrocyte fractions in control and hemorrhaged rats were separated by repeated centrifugation. Hemorrhaged rats had increased number of reticulocytes but reduced hematocrit and MCHC compared to control rats. Immature erythrocytes of hemorrhaged rats differed from mature ones of control animals by elevated Na(+)-K(+) pump activity, reduced Na(+)-K(+) cotransport activity and increased Rb(+) leak. These ion transport changes in immature erythrocytes were accompanied by higher concentration of total phospholipids in their cell membranes. Membrane phospholipid content correlated positively with Na(+)-K(+) pump activity and cation leaks but negatively with Na(+)-K(+) cotransport activity. Moreover, they were also negatively related with MCHC which correlated negatively with Na(+)-K(+) pump activity and Rb(+) leak but positively with Na(+)-K(+) cotransport activity. Thus certain abnormalities of erythrocyte ion transport and membrane lipid composition detected in hypertensive animals might be caused by higher incidence of immature cells.

Two different oxygen sensors regulate oxygen-sensitive K+ transport in crucian carp red blood cells

The O2 dependence of ouabain-independent K+ transport mechanisms has been studied by unidirectional Rb+ flux analysis in crucian carp red blood cells (RBCs). The following observations suggest that O2 activates K+-Cl- cotransport (KCC) and deactivates Na+-K+-2Cl- cotransport (NKCC) in these cells via separate O2 sensors that differ in their O2 affinity. When O2 tension (PO2) at physiological pH 7.9 was increased from 0 to 1, 4, 21 or 100 kPa, K+ (Rb+) influx was increasingly inhibited, and at 100 kPa amounted to about 30% of the value at 0 kPa. This influx was almost completely Cl- dependent at high and low PO2, as shown by substituting Cl- with nitrate or methanesulphonate. K+ (Rb+) efflux showed a similar PO2 dependence as K+ (Rb+) influx, but was about 4-5 times higher over the whole PO2 range. The combined net free energy of transmembrane ion gradients favoured net efflux of ions for both KCC and NKCC mechanisms. The KCC inhibitor dihydroindenyloxyalkanoic acid (DIOA, 0.1 mM) abolished Cl- -dependent K+ (Rb+) influx at a PO2 of 100 kPa, but was only partially effective at low PO2 (0-1 kPa). At PO2 values between 0 and 4 kPa, K+ (Rb+) influx was further unaffected by variations in pH between 8.4 and 6.9, whereas the flux at 21 and 100 kPa was strongly reduced by pH values below 8.4. At pH 8.4, where K+ (Rb+) influx was maximal at high and low PO2, titration of K+ (Rb+) influx with the NKCC inhibitor bumetanide (1, 10 and 100 microM) revealed a highly bumetanide-sensitive K+ (Rb+) flux pathway at low PO2, and a relative bumetanide-insensitive pathway at high PO2. The bumetanide-sensitive K+ (Rb+) influx pathway was activated by decreasing PO2, with a PO2 for half-maximal activation (P50) not significantly different from the P50 for haemoglobin O2 binding. The bumetanide-insensitive K+ (Rb+) influx pathway was activated by increasing PO2 with a P50 significantly higher than for haemoglobin O2 binding. These results are relevant for the pathologically altered O2 sensitivity of RBC ion transport in certain human haemoglobinopathies.

Na-K-2Cl cotransporter inhibition impairs human lung cellular proliferation

The widespread presence of the Na-K-2Cl (NKCC) cotransporter protein suggests that chronic administration of inhibitors may result in adverse effects. Inhibition of the NKCC cotransporter by loop diuretics is felt to underlie the diuretic and the pulmonary smooth muscle relaxant effects of this drug class. However, the fundamental regulation of salt and water movement by this cotransporter suggests that it may also mediate cell volume changes occurring during cell cycle progression. Thus we hypothesized that NKCC cotransporter inhibition by loop diuretics would decrease cellular proliferation. Normal human bronchial smooth muscle cells (BSMC) showed a significant concentration-dependent decrease in cell counts after 7 days of exposure to both bumetanide ( n = 5–10) and furosemide ( n = 6–16) compared with controls. Proliferation was similarly inhibited in normal human lung fibroblasts ( n = 5–9). To determine whether this was due to loss of cells, we performed apoptosis assays on BSMC. Both annexin V-propidium iodide staining ( n = 5–10) and single cell gel electrophoresis assays ( n = 4) were negative for necrosis and apoptosis in BSMC exposed to 10 μM bumetanide. Subsequent analysis of the cell cycle by flow cytometry showed that bumetanide-exposed BSMC were delayed in G1 phase compared with controls ( n = 4–8). This is the first evidence for loop diuretic inhibition of airway smooth muscle cell proliferation. NKCC cotransporter inhibition impeded G1-S phase transition without facilitating cell death. Thus although inhibition by loop diuretics relaxes airway smooth muscle, the NKCC cotransporter may have a more important role in cell proliferation regulation.

K(+) transport in red blood cells from human umbilical cord

The current study was designed to characterise K(+) transport in human fetal red blood cells, containing mainly haemoglobin F (HbF, and termed HbF cells), isolated from umbilical cords following normal parturition. Na(+)/K(+) pump activity was comparable to that in normal adult human red cells (which contain HbA, and are termed HbA cells). Passive (ouabain-resistant) K(+) transport was dominated by a bumetanide (10 microM)-resistant component, inhibited by [(dihydroxyindenyl)oxy]alkanoic acid (100 microM), calyculin A (100 nM) and Cl(-) removal, and stimulated by N-ethylmaleimide (1 mM) and staurosporine (2 microM) - all consistent with mediation via the K(+)-Cl(-) cotransporter (KCC). KCC activity in HbF cells was also O(2)-dependent and stimulated by swelling and urea, and showed a biphasic response to changes in external pH. Peak activity of KCC in HbF cells was about 3-fold that in HbA cells. These characteristics are qualitatively similar to those observed in HbA cells, notwithstanding the different conditions experienced by HbF cells in vivo, and the presence of HbF rather than HbA. KCC in HbF cells has a higher total capacity, but when measured at the ambient PO(2) of fetal blood it would be similar in magnitude to that in fully oxygenated HbA cells, and about that required to balance K(+) accumulation via the Na(+)/K(+) pump. These findings are relevant to the mechanism by which O(2) regulates membrane transporters in red blood cells, and to the strategy of promoting HbF synthesis as a therapy for patients with sickle cell disease.

Cation transport and cell volume changes in maturing rat reticulocytes

During maturation, reticulocytes lose membrane material, including transporters, and this is accompanied by a loss of cell water and volume. Here we determined a possible role of ion transport in adjusting cell volume during maturation. Reticulocytes and red blood cells of different ages were prepared from erythropoietin-treated rats by density gradient fractionation. Cell volume and ion transport were measured in freshly prepared cells and in reticulocytes during in vitro maturation. Reticulocytes had an increased K content and cell volume, whereas intracellular Na was decreased. All parameters approached whole blood values after 2 days in culture. Na-K pump was elevated in reticulocytes and decreased during maturation. Na-K-2Cl cotransport (NKCC) activity was lower in reticulocytes and was activated 8- and 20-fold by shrinkage and okadaic acid, respectively, whereas stimulation was barely detectable in high-buoyant density red blood cells. The ouabain- and bumetanide-insensitive Na flux in reticulocytes decreased on maturation. Most of it was inhibited by amiloride, indicating the presence of Na/proton exchange. Our results show that, although the Na-K-pump activity in reticulocytes is very much increased, the enhanced capacity of NKCC is essentially cryptic until stimulated. Both types of capacities (activities) decrease during maturation, indicating a possible loss of transport protein. The decrease was constrained to the period of reticulocyte maturation. Loss of transport capacity appears to exceed the loss of membrane surface area. Reticulocyte age-related changes in the net electrochemical driving force indicate that the increasing NKCC activity might contribute to the reduction in cell water.

Oxygen-sensitive membrane transporters in vertebrate red cells

Oxygen is essential for all higher forms of animal life. It is required for oxidative phosphorylation, which forms the bulk of the energy supply of most animals. In many vertebrates, transport of O(2) from respiratory to other tissues, and of CO(2) in the opposite direction, involves red cells. These are highly specialised, adapted for their respiratory function. Intracellular haemoglobin, carbonic anhydrase and the membrane anion exchanger (AE1) increase the effective O(2)- and CO(2)-carrying capacity of red cells by approximately 100-fold. O(2) also has a pathological role. It is a very reactive species chemically, and oxidation, free radical generation and peroxide formation can be major hazards. Cells that come into contact with potentially damaging levels of O(2) have a variety of systems to protect them against oxidative damage. Those in red cells include catalase, superoxide dismutase and glutathione. In this review, we focus on a third role of O(2), as a regulator of membrane transport systems, a role with important consequences for the homeostasis of the red cell and also the organism as a whole. We show that regulation of red cell transporters by O(2) is widespread throughout the vertebrate kingdom. The effect of O(2) is selective but involves a wide range of transporters, including inorganic and organic systems, and both electroneutral and conductive pathways. Finally, we discuss what is known about the mechanism of the O(2) effect and comment on its physiological and pathological roles.

Swelling-induced activation of Na+,K+,2Cl- cotransport in C6 glioma cells: kinetic properties and intracellular signalling mechanisms

Swelling of C6 glioma cells in hypotonic medium (180 mOsm) results in two- to three-fold activation of K+ (86Rb+) influx suppressed by 10 microM bumetanide. Bumetanide-sensitive transport of 86Rb+ is dependent on extracellular K+, Na+ and Cl- both in iso-osmotic conditions and under hypo-osmotic shock, supporting the notion that it is mediated by Na+,K+,2Cl- cotransport. Inhibitors of protein kinase C (10 microM polymyxin B and l microM staurosporine) had no significant effect on basal cotransport but reduced its hypotonic stimulation by 70-80%. Similar results were obtained with calmodulin antagonist R24571 (10 microM), indicating Ca2+/calmodulin-dependence of the process. Influence of polymyxin B and R24571 was not additive. Swelling-activated Na+,K+,2Cl- cotransport was also suppressed by protein kinase C activator PMA (l microM). By contrast, preincubation of cells with inhibitors of protein phosphatases (100 microM vanadate, 5 mM fluoride and 0.5 microM okadaic acid) activated greatly the bumetanide-sensitive 86Rb+ uptake in isotonic conditions, while a subsequent hypotonic swelling led to smaller or no increment. These results indicate the involvement of Ca2+/calmodulin-dependent staurosporine/polymyxin B-sensitive protein kinase other than protein kinase C in swelling-induced activation of Na+,K+,2Cl- cotransport in glial cells.

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

Urea inhibits NaK2Cl cotransport in human erythrocytes

We examined the effect of urea on NaK2Cl cotransport in human erythrocytes. In erythrocytes from nine normal subjects, the addition of 45 mM urea, a concentration commonly encountered in uremic subjects, inhibited NaK2Cl cotransport by 33 +/- 7%. Urea inhibited NaK2Cl cotransport reversibly, and in a concentration-dependent fashion with half-maximal inhibition at 63 +/- 10 mM. Acute cell shrinkage increased, and acute cell swelling decreased NaK2Cl cotransport in human erythrocytes. Okadaic acid (OA), a specific inhibitor of protein phosphatase 1 and 2A, increased NaK2Cl cotransport by nearly 80%, suggesting an important role for these phosphatases in the regulation of NaK2Cl cotransport. Urea inhibited bumetanide-sensitive K influx even when protein phosphatases were inhibited with OA, suggesting that urea acted by inhibiting a kinase. In cells subjected to shrinking and OA pretreatment, maneuvers expected to increase the net phosphorylation, urea inhibited cotransport only minimally, suggesting that urea acted by causing a net dephosphorylation of the cotransport protein, or some key regulatory protein. The finding that concentrations of urea found in uremic subjects inhibited NaK2Cl cotransport, a widespread transport pathway with important physiological functions, suggests that urea is not only a marker for accumulation of other uremic toxins, but may be a significant uremic toxin itself.

Molecular characterization of the epithelial Na-K-Cl cotransporter isoforms

Recent advances in the molecular characterization of specific isoforms of the Na-K-Cl cotransporter have allowed rapid progress in the study of the structure, function, and regulation of these members of a family of Cl-dependent cation cotransporters. Two distinct isoforms have been identified, one from Cl(-)-secretory epithelia and another found specifically in the diluting segment of the vertebrate kidney, a Cl(-)-absorptive epithelium. The discovery of three alternatively spliced variants of the absorptive isoform, which differ only by 31 amino acids and which appear to be differentially distributed within the mammalian thick ascending limb of the loop of Henle, highlight this spliced region as an important functional component of the protein.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. Na-K-Cl cotransporters are present in a wide variety of cells and tissues, including reabsorptive and secretory epithelia, nerve and muscle cells, endothelial cells, fibroblasts, and blood cells. Na-K-Cl cotransport plays a vital role in renal salt reabsorption and in salt secretion by intestinal, airway, salivary gland, and other secretory epithelia. Cotransport function also appears to be important in the maintenance and regulation of cell volume and of ion gradients by both epithelial and nonepithelial cells. Na-K-Cl cotransport activity is inhibited by "loop" diuretics, including the clinically efficacious agents bumetanide and furosemide. The regulation of Na-K-Cl cotransport is mediated, at least in some cases, through direct phosphorylation of the cotransport protein. Cotransporter regulation is highly tissue specific, perhaps in part related to the presence of different Na-K-Cl cotransporter isoforms. In epithelia, both absorptive (kidney-specific) and secretory isoforms have been identified by cDNA cloning and sequencing and Northern blot analysis; alternatively spliced variants of the kidney-specific isoform have also been identified. The absorptive and secretory isoforms exhibit approximately 60% identity at the amino acid sequence level; these sequences in turn show approximately 45% overall homology with those of thiazide-sensitive, bumetanide-insensitive, Na-Cl cotransport proteins of winter flounder urinary bladder and mammalian kidney. This review focuses on recent developments in the identification of Na-K-Cl cotransport proteins in epithelial and on the regulation of epithelial Na-K-Cl cotransporter function at cellular and molecular levels.

Genetic control of erythrocyte volume regulation: effect of a single gene (rol) on cation metabolism

In laboratory mice we previously defined a gene, rol (resistance to osmotic lysis), based on its effect on erythrocyte osmotic fragility. Here we report a physiological characterization of rol gene action utilizing congenic strains developed for the purpose; these two strains have a common genetic background and differ only by the two alleles of rol, susceptible (rols) or resistant (rolr). In comparison to rols/s erythrocytes, rolr/r cells have a reduced mean cell volume, a higher mean corpuscular hemoglobin concentration and hemolytic volume, and respond differently to swelling induced by ion influx. Rolr/r erythrocytes also have reduced cell water and K, which are associated with a threefold higher activity of the Na-K-Cl cotransporter (measured as ouabain-resistant, bumetanide-sensitive 86Rb influx) and 30% higher Na pump activity. Apart from differences in ion transport and water content, the content of 2,3-diphosphoglycerate (2,3-DPG) in rolr/r cells is 15% lower than in rols/s ones. Analyses of membrane structural components revealed no rol-associated differences in their phospholipid or fatty acid content, nor were strain differences evident among the membrane and cytoskeletal proteins and their posttranslational modifications (phosphorylation and fatty acylation). Rol is not the structural gene for either the alpha- or the beta-chain of hemoglobin and has no effect on erythrocyte production or destruction. The concerted effect of rol variation on erythrocyte volume, water and cation content, cation cotransport, and 2,3-DPG levels is similar in many ways to the variation observed among individual humans for the same characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

The membrane defect in hereditary stomatocytosis

Hereditary stomatocytosis and allied conditions represent a series of diseases in which abnormal movements of univalent cations across the plasma membrane play an important part in cellular disease. The primary problem lies not in the active transporters but in the basal permeability of the membrane, which is always increased, and the extent of the increase correlates with the cellular dysfunction. A number of structural abnormalities have been described in these membranes, but the most consistent and convincing is the deficiency of a hitherto uncharacterized integral membrane protein of molecular weight 31 kDa in the severe, 'overhydrated' form of the disease. The true function of this protein remains enigmatic, but its deficiency in this condition indicates that it may have a role in the regulation of cation transport.

Na+/K+/Cl- cotransport in resealed ghosts from erythrocytes of the Milan hypertensive rats

The erythrocytes (RBC) of the Milan hypertensive rats (MHS) have a smaller volume and faster Na+/K+/Cl- cotransport than RBC from normotensive controls (MNS). The difference in Na+/K+/Cl- cotransport is no longer present in inside-out Vesicles (IOV) of RBC membrane. To differentiate between cytoplasmic or membrane skeleton abnormalities as possible causes of these differences. Resealed ghosts (RG) were used to measure ion transport systems. The following results have been obtained: (1) RG from MHS have a smaller volume than MNS (mean +/- S.E. 20.7 +/- 0.45 vs. 22.09 +/- 0.42 fl, P < 0.05). (2) RG showed a bumetanide-sensitive Na efflux that retains the characteristics of the Na+/K+/Cl- cotransport of the original RBC: it is K(+)- and Cl(-)-sensitive and dependent on the intracellular Na+ concentration. (3) The Na+/K+/Cl- cotransport was faster in RG from MHS than in those from MNS (mean +/- S.E. 0.095 +/- 0.01 vs. 0.066 +/- 0.01 rate constant h-1, P < 0.01). These results, together with those of IOV, support the hypothesis that an abnormality in the membrane skeletal proteins may play a role in the different Na+/K+/Cl- cotransport modulation between MHS and MNS erythrocytes.

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.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

The effects of metabolism on Na(+)-K(+)-Cl- co-transport in ferret red cells

1. The effects of altering metabolism on Na(+)-K(+)-Cl- co-transport were studied in ferret red cells. Na(+)-K(+)-Cl- co-transport was measured as the bumetanide-sensitive uptake of 86Rb. 2. Glucose, but not inosine or adenosine, sustained metabolism and maintained cell ATP content ([ATP]i) at the physiological level. [ATP]i could be reduced by prolonged incubation of cells in a substrate-free medium or more quickly by incubating cells with 2-deoxyglucose or with a mixture of iodoacetamide and glucose. 3. Na(+)-K(+)-Cl- co-transport activity was inhibited when [ATP]i was reduced to below 100 mumol (1 cell)-1 by starvation or by treatment with 2-deoxyglucose. However, a unique relationship between [ATP]i and activity could not be found. [ATP]i and the method and time course of ATP depletion all influenced activity. The inhibition of Na(+)-K(+)-Cl- co-transport, caused by reducing [ATP]i could be partially reversed by restoring [ATP]i to normal. 4. Increasing the concentration of intracellular ionized magnesium [( Mg2+]i) did not stimulate co-transport activity in ATP-depleted cells. This contrasts with the substantial stimulation seen in cells with normal [ATP]i. 5. Vanadate stimulated Na(+)-K(+)-Cl- co-transport activity in ATP-depleted cells but not in cells with normal [ATP]i. Fluoride did not affect activity at any [ATP]i. 6. The effects of some sulphydryl reagents on Na(+)-K(+)-Cl- co-transport were also examined. n-Ethylmaleimide (1 mM) inhibited Na(+)-K(+)-Cl- co-transport while it stimulated bumetanide-resistant potassium transport. Dithiothreitol (1 mM) did not affect activity. Iodoacetamide (6 mM) appeared to reduce the inhibition of cotransport activity seen at low [ATP]i but also greatly increased cell fragility. 7. The data suggest that activity of the Na(+)-K(+)-Cl- co-transport system is controlled by a cycle of phosphorylation and dephosphorylation with the phosphorylated form being active. Phosphorylation and transport appear to be almost maximal in ferret red cells with normal [ATP]i. Reduction of [ATP]i may allow changes in phosphatase activity to manifest as changes in transport rate. Differences in the balance between phosphorylation and dephosphorylation may explain tissue-dependent variations in the response of the system to various stimuli.

Kinetic mechanism of ATP action in Na(+)-K(+)-Cl- cotransport of HeLa cells determined by Rb+ influx studies

The kinetics of Na(+)-K(+)-Cl- cotransport were studied by measuring ouabain-insensitive furosemide-sensitive Rb+ influx (JRb) into HeLa cells while varying the cellular ATP and the extracellular Rb+ and Na+ concentrations. Results reveal that ATP stimulates JRb by increasing the affinity of the cotransporter for Rb+ (K+), and the apparent Michaelis constant (Km) for ATP was 0.95 +/- 0.03 mmol/l cell water. Two ATP molecules may relate to the uptake of one Rb+ by the cotransport pathway, as examined by the nonlinear least-squares method for goodness-of-fit and a Hill plot, JRb was strengthened by an increase in the inward chemical gradient associated with cell swelling on preincubation in a low-Na+ high-K+ medium, accompanying an increase in the affinity of the transporter for ATP. JRb was apparently activated by extracellular Na+, and the activation was enhanced by an increase in the cellular ATP concentration. Lactate production stimulated by 2 microM carbonylcyanide m-chlorophenyl hydrazone (CCCP) was reduced by 10 microM ouabain but not altered by further addition of 0.1 mM furosemide. Increases in cellular adenosine 3',5'-cyclic monophosphate (cAMP) caused by treatment with 0.1 mM isoproterenol plus 0.5 mM 3-isobutyl-1-methylxanthine or with 0.1 mM dibutyryl cAMP did not influence JRb. From this and previous studies, we propose a general and a specific model of Na(+)-K(+)-Cl- cotransport, which elucidate the order of binding of extracellular ions and reaction of cellular ATP.

Thyrotoxic periodic paralysis and the sodium/potassium pump

The hypothesis of altered Na+/K+ transport in thyrotoxic periodic paralysis (TPP) was tested in an investigation of the K+ influx into erythrocytes from two patients with episodes of thyrotoxic muscle weakness. A patient with primary hypokalemic periodic paralysis (HPP) and three healthy volunteers served as controls. The TPP patients were of Oriental and Caucasian origin and differed in their clinical symptoms. For the Caucasian patient, the Na+ content of the erythrocytes was twice the control, for the Oriental patient it was normal. The K+ dependence of the ouabain-inhibitable K+ influx (the pump action) was also abnormal in the Caucasian patient, the flux being 70% of control at 2 mM [K+]e and normal at 4 mM [K+]e. The K+ influx was normal in the Oriental patient. By contrast, the K+ leak of the cells was normal in the Caucasian and was increased in the Oriental patient. The pump/leak ratio was thus reduced in both TPP patients. All parameters investigated were normal in the patient with primary HPP. It is concluded that the ion transport systems of muscle may be altered in TPP, but that the patho-mechanism might be different in the rare Caucasian cases and the rather more common Oriental cases.

Cl-dependent K transport in a pure population of volume-regulating human erythrocytes

Swelling of human red cells activates a putative K-Cl cotransport that is not present at normal cell volume and that disappears after several hours. To determine whether regulatory volume decrease (RVD) is occurring in human erythrocytes and is responsible for the inactivation of K-Cl cotransport, the relationship between cell volume and the inactivation and reactivation of volume-sensitive (VS) K-Cl cotransport was studied. VS K influx into high K cells was transient, whereas influx into low K cells (prepared with nystatin), which are unable to shrink via K efflux, remained fully activated. Likewise, VS K efflux into hypotonic medium disappeared after 100 min in a low K medium but remained activated in a high K medium that prevented cell shrinkage. Cells that had been preincubated in hypotonic medium to inactivate VS K-Cl cotransport showed no significant recovery of VS cotransport after a 6-h incubation in isotonic medium but showed full restoration of VS cotransport after treatment with nystatin in isotonic medium to reequilibrate cell water. A pure fraction of volume-regulating (VR) cells was subsequently isolated by preincubating red cells in hypotonic medium and then subjecting them to further hypotonicity to lyse all non-VR cells. The 2.5% of cells that remained consisted of 16% reticulocytes and exhibited a Cl-dependent RVD in hypotonic medium. VS K-Cl cotransport was enriched 10-fold and Na-K-Cl cotransport was enriched 12-fold in these cells, whereas the enrichment of N-ethylmaleimide (NEM)-activated K-Cl cotransport was only threefold.(ABSTRACT TRUNCATED AT 250 WORDS)

Volume-dependent regulation of ion transport and membrane phosphorylation in human and rat erythrocytes

Osmotic swelling of human and rat erythrocytes does not induce regulatory volume decrease. Regulatory volume increase was observed in shrunken erythrocytes of rats only. This reaction was blocked by the inhibitors of Na+/H+ exchange. Cytoplasmic acidification in erythrocytes of both species increases the amiloride-inhibited component of 22Na influx by five- to eight-fold. Both the osmotic and isosmotic shrinkage of rat erythrocytes results in the 10- to 30-fold increase of amiloride-inhibited 22Na influx and a two-fold increase of furosemide-inhibited 86Rb influx. We failed to indicate any significant changes of these ion transport systems in shrunken human erythrocytes. The shrinking of quin 2-loaded human and rat erythrocytes results in the two- to threefold increase of the rate of 45Ca influx, which is completely blocked by amiloride. The dependence of volume-induced 22Na influx in rat erythrocytes and 45Ca influx in human erythrocytes on amiloride concentration does not differ. The rate of 45Ca influx in resealed ghosts was reduced by one order of magnitude when intravesicular potassium and sodium were replaced by choline. It is assumed that the erythrocyte shrinkage increases the rate of a nonselective Cao2+/(Nai+, Ki+) exchange. Erythrocyte shrinking does not induce significant phosphorylation of membrane protein but increases the 32P incorporation in diphosphoinositides. The effect of shrinkage on the 32P labeling of phosphoinositides is diminished after addition of amiloride. It is assumed that volume-induced phosphoinositide response plays an essential role in the mechanism of the activation of transmembrane ion movements.

Erythrocyte sodium transport in dialyzed uremic patients

To investigate the status of the Na+ concentration and ionic fluxes in red cells of human subjects with dialyzed chronic uremia, the authors measured the Na(+)-K+ pump activity as well as Na(+)-K+ cotransport (CoT), Na(+)-Li+ countertransport (CTT) and Na+ passive permeability in erythrocytes from 37 normal subjects and 23 chronic uremic patients receiving maintenance hemodialysis. The mean intracellular Na+ concentration [Na+]i value in the pre-dialytic group was significantly lower than that in control subjects (p less than .0001), but tended to recover to the normal value of [Na+]i in the post-dialytic group. The mean intracellular K+ concentration value in the post-dialytic group was significantly higher than that of the control group (p less than .001), but not significantly different from that of the pre-dialytic group. It was found that the Na(+)-K+ pump activity of erythrocytes in the pre- and post-dialytic groups markedly decreased over that of the normal control group with statistical significance (p less than .0001, respectively). The Na(+)-K+ pump activity in the post-dialytic group, however, tended to recover, but not significantly. The rate constant for ouabain-sensitive Na+ efflux in the post-dialytic group was significantly decreased over that of the normal controls (p less than .05). The authors observed a significant decrease of the Na+ CoT value (p less than .001 respectively) and rate constant for Na+ CoT (p less than .05, respectively) in the patients with pre- and post-dialytic uremia vs. that of normal subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Abnormal cation transport in uremia. Mechanisms in adipocytes and skeletal muscle from uremic rats

The cause of the abnormal active cation transport in erythrocytes of some uremic patients is unknown. In isolated adipocytes and skeletal muscle from chronically uremic chronic renal failure rats, basal sodium pump activity was decreased by 36 and 30%, and intracellular sodium was increased by 90 and 50%, respectively, compared with pair-fed control rats; insulin-stimulated sodium pump activity was preserved in both tissues. Lower basal NaK-ATPase activity in adipocytes was due to a proportionate decline in [3H]ouabain binding, while in muscle, [3H]ouabain binding was not changed, indicating that the NaK-ATPase turnover rate was decreased. Normal muscle, but not normal adipocytes, acquired defective Na pump activity when incubated in uremic sera. Thus, the mechanism for defective active cation transport in CRF is multifactorial and tissue specific. Sodium-dependent amino acid transport in adipocytes closely paralleled diminished Na pump activity (r = 0.91), indicating the importance of this defect to abnormal cellular metabolism in uremia.

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.

Natural variation in passive sodium permeability in human erythrocytes

The rate of influx of 22Na+ into human erythrocytes (RBC) varies greatly depending upon the donor. A high rate of influx may be related to a congenital predisposition to essential hypertension. In Northern Europeans, we find a threefold difference in the rate of 22Na+ influx between those with the least (LP) and most highly permeable (HP) RBC (from less than 0.15 to greater than 0.60 mmol Na+/liter RBC/hr). In order to further define determinants of these apparently hereditary differences in passive membrane Na+ transport, we identified two groups of normal laboratory and hospital personnel differing markedly (greater than twofold) in RBC 22Na+ influx rate. We find that the loop diuretics furosemide and bumetanide decrease by about 50% the influx of 22Na+ into HP RBC, but have a lesser influence on LP RBC. Impermeant polyanions such as citrate and pyrophosphate also specifically diminish 22Na+ influx into HP, but not LP, RBC. Therefore, the exaggerated 22Na+ influx into HP RBC probably occurs through a discrete pathway (perhaps via "Na/K/Cl cotransport"), which appears to be almost absent in LP RBC. The differences between HP and LP RBC most likely do not involve polymorphisms of RBC anion transport per se. The rate of RBC anion (35SO4(2-)) transport is the same in HP and LP RBC and is equally inhibited by furosemide and (to a lesser extent) bumetanide. Furthermore, the potent inhibitor of RBC anion transport, DIDS (diisothiocyanostilbene disulfonate) does not affect RBC Na+ permeability in either group. Nonetheless, the preferential reduction of Na+ permeation of HP RBC by loop diuretics may be of help in experimentally distinguishing HP from LP phenotypes. This information may be crucial in unraveling the structural basis of intrinsic differences in cell membrane Na+ permeability and their possible relationship to essential hypertension.

A kinetic study of cation transport in erythrocytes from uremic patients

We previously described in red blood cells (RBCs) from uremic patients on dialysis a reduction in sodium (Na) efflux through the Na, potassium (K) cotransport system (Na,K CoT) while Na efflux through the Na,K pump was normal. We then examined Na efflux in fresh cells and in cells loaded to obtain one level of intracellular sodium (Nai) concentration at about 25 mmol/liter cell. In the present study we used similar cation flux methodology to examine the kinetics of cation efflux through the Na,K pump and Na,K CoT in uremic patients on dialysis. RBCs were Na-loaded to attain five different levels of Nai concentration over a range of 5 to 50 mmol/liter cells using the ionophore nystatin. At each level of Na-loading, the Nai achieved was similar in RBCs from controls and patients. Ouabain-sensitive Na efflux through the Na,K pump showed no difference in rate between normals and dialysis patients. When the kinetic parameters of this transport pathway were considered, the apparent affinity (K0.5) for sodium was not significantly different between controls and patients (18.4 +/- 2.3 vs. 20.0 +/- 2.6 mmol/liter cell) and the maximal velocity of efflux (Vmax) was also not different between controls and patients (9.6 +/- 0.7 vs. 8.5 +/- 1.2 mmol/liter cell/hr). Comparison of Nai-activated Na versus K efflux rates through the Na,K CoT in normal subjects demonstrated similar saturation kinetics, (K0.5 15.8 +/- 3.3 vs. 12.2 +/- 2.8 mmol/liter cell, Vmax 0.81 +/- 0.1 vs. 0.78 +/- 0.1 mmol/liter cell/hr) consistent with the known stoichiometric ratio of 1 Na:1 K:2 Cl described for this mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Photolabeling of a 150-kDa (Na + K + Cl) cotransport protein from dog kidney with a bumetanide analogue

(Na + K + Cl) cotransport is the major mechanism of salt transport across the apical membrane of the epithelial cells of the thick ascending limb of Henle's loop of mammalian kidney and the site of action of "loop" diuretics such as furosemide and bumetanide. We have identified a 150-kDa protein in membranes from dog kidney cortex that is photolabeled by a radiolabeled, benzophenone analogue of bumetanide, [3H]4-benzoyl-5-sulfamoyl-3-(3-thenyloxy)benzoic acid ([3H]BSTBA). Several pieces of evidence strongly suggest that this 150-kDa protein is at least part of the (Na + K + Cl) cotransport system. 1) Photoincorporation of [3H]BSTBA into this protein is completely blocked by inclusion of 10 microM unlabeled bumetanide in the photolysis medium. 2) Photoincorporation of [3H]BSTBA into this protein shows a saturable dependence on [3H]BSTBA concentration, with a K 1/2 (approximately 0.1 microM) very similar to that for reversible [3H]BSTBA binding to kidney membranes. 3) Photolabeling of this protein by [3H]BSTBA requires the simultaneous presence of Na, K, and Cl in the photolysis medium. 4) When crude membranes from dog kidney cortex are centrifuged on sucrose density gradients, saturable [3H]bumetanide binding and photoincorporation of [3H]BSTBA in the 150-kDa region show a very similar distribution among the 15 gradient fractions collected. [3H]BSTBA is also photoincorporated into at least two lower molecular mass proteins, the largest of which is approximately 50 kDa.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular calcium content of human erythrocytes: relation to sodium transport systems

To study the possible role of intracellular Ca (Cai) in controlling the activities of the Na+-K+ pump, the Na+-K+ cotransport and the Na+/Li+ exchange system of human erythrocytes, a method was developed to measure the amount of Ca embodied within the red cell. For complete removal of Ca associated with the outer aspect of the membrane, it proved to be essential to wash the cells in buffers containing less than 20 nM Ca. Ca was extracted by HClO4 in Teflon vessels boiled in acid to avoid Ca contaminations and quantitated by flameless atomic absorption. Cai of fresh human erythrocytes of apparently healthy donors ranged between 0.9 and 2.8 mumol/liter cells. The mean value found in females was significantly higher than in males. The interindividual different Ca contents remained constant over periods of more than one year. Sixty to 90% of Cai could be removed by incubation of the cells with A23187 and EGTA. The activities of the Na+-K+ pump, of Na+-K+ cotransport and Na+/Li+ exchange and the mean cellular hemoglobin content fell with rising Cai; the red cell Na+ and K+ contents rose with Cai. Ca depletion by A23187 plus EGTA as well as chelation of intracellular Ca2+ by quin-2 did not significantly enhance the transport rates. It is concluded that the large scatter of the values of Cai of normal human erythrocytes reported in the literature mainly results from a widely differing removal of Ca associated with the outer aspect of the membrane.

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+.

Furosemide-sensitive Na+ and K+ transport and human erythrocyte volume

The relationship between cation transport and cell volume in human erythrocytes was investigated by measuring ouabain-sensitive K+ influx, ouabain-resistant, furosemide-sensitive K+ influx, and ouabain + furosemide-resistant K+ influx, and maximal ouabain binding in microcytic, normocytic and macrocytic red cells. A significant correlation was found between the mean corpuscular volume and furosemide-sensitive K+ influx normalized either to cell number (r = 0.636, P less than 0.001) or to cell volume (r = 0.488, P less than 0.001). No relationship was seen between mean corpuscular volume and ouabain-sensitive K+ influx, and the number of ouabain-binding sites per cell was only weakly correlated with mean corpuscular volume (r = 0.337, P less than 0.05). A slight, negative relationship existed between mean corpuscular volume and ouabain + furosemide-resistant K+ influx expressed per volume of cells (r = -0.359, P less than 0.01), and an apparent relationship between furosemide-sensitive K+ influx and mean corpuscular hemoglobin concentration (r = 0.446, P less than 0.01) disappeared when microcytic samples were excluded from analysis. Furosemide-sensitive transport, including Na+ influx and K+ and Na+ efflux, was completely absent in microcytic cells from one patient with alpha-thalassemia minor. In addition, these cells exhibited a furosemide-resistant, Cl(-)-dependent K+ influx. Exposure of normal erythrocytes to hypotonic conditions (196 mosM) increased furosemide-sensitive K+ influx by a mean of 45% (P less than 0.05), while exposure to hypertonic conditions (386 mosM) had no significant effect. The results indicate that furosemide-sensitive transport and cell volume are interrelated in human erythrocytes. However, the inability to fully recreate this relationship with in vitro manipulation of cell volume suggest that this relationship is established prior to red cell maturation.

Decreased water and potassium content in erythrocytes in essential hypertension

The water content in erythrocytes of subjects with borderline or established essential hypertension was measured by using gas-liquid chromatography and was found to be lower than that in normotensive controls (p less than 0.01). The water content in erythrocytes of normal controls (n = 14), borderline hypertensive subjects (n = 18), and established essential hypertensive subjects (n = 23) was (mean +/- SE) 71.0 +/- 0.2%, 69.9 +/- 0.2%, and 69.3 +/- 0.1% (vol/vol), respectively. A definite negative correlation was found between water content of erythrocytes and mean arterial pressure in normotensive and hypertensive subjects (n = 60, r = -0.59, p less than 0.001). Although there was no statistically significant between-group difference in the sodium content, the potassium content of erythrocytes from subjects with essential hypertension was significantly lower than that of normotensive controls (0.205 +/- 0.003 vs 0.222 +/- 0.004 mumol/mg dry red blood cells; p less than 0.01). There was no between-group correlation of sodium and water content in erythrocytes, but the potassium content correlated with the water content (n = 46, r = 0.49, p less than 0.001).

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

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

Sodium transport in red blood cells from dialyzed uremic patients

Studies on red blood cell (RBC) sodium (Na) transport in chronic renal failure have described abnormalities in the ouabain-sensitive Na, K pump. We now report Na transport in RBC using cation flux methodology, measuring both the ouabain-sensitive Na, K pump and the ouabain-insensitive Na, K cotransport (CoT) and Na, lithium (Li) countertransport (CTT) in 28 subjects on hemodialysis, eight subjects on chronic ambulatory peritoneal dialysis (CAPD) and 29 control subjects. Intracellular cation content and passive permeability of Na were also examined. Mean Na efflux through the ouabain-sensitive Na, K pump was not reduced in dialysis patients when compared to normal subjects, whether measured in fresh cells (1.41 +/- 0.05 vs. 1.30 +/- 0.03 mmole/liter RBC/hr; P less than 0.05) or in Na-loaded cells (7.10 +/- 0.24 vs. 6.90 +/- 0.22; NS). There was, however, a marked and uniform suppression of the CoT pathway in Na-loaded cells from dialysis patients versus controls (0.14 +/- 0.02 vs. 0.41 +/- 0.05 mmole/liter RBC/hr; P less than 0.001). Mean CTT activity, as measured by Li efflux, was not different between dialysis and normal subjects. Uremic and normal RBC had similar intracellular Na or K content as well as passive permeability for either ion. This indicates that intracellular cationic homeostasis is maintained, perhaps secondary to balanced changes in cationic flux activity through these transport pathways.

An unusual pattern of Na+ and K+ movements across the horse erythrocyte membrane

Marked differences in the activities of three monovalent cation transport systems in horse versus human erythrocytes are reported. Whereas horse erythrocytes exhibit a 6-fold higher sodium-lithium countertransport, the unidirectional flux of potassium through the sodium pump is 3-4 times slower and the sodium-potassium cotransport system cannot be detected. In spite of this, horse and human cells are able to maintain similar Na+ and K+ gradients.

Inhibitin: a specific inhibitor of sodium/sodium exchange in erythrocytes

An inhibitor of ouabain-insensitive sodium/sodium exchange in erythrocytes has been isolated from leukemic promyelocytes. To explore the specific effects of this inhibitor, named inhibitin, sodium transport experiments were carried out in human erythrocytes. Inhibitin reduced ouabain-insensitive bidirectional sodium transport. It did not change net sodium fluxes, had no significant effect on rubidium influx, and did not inhibit sodium-potassium-ATPase activity. The inhibitory effect of inhibitin was studied on sodium/sodium exchange and on sodium/lithium countertransport in 140 mM sodium and in sodium-free media. In the presence of sodium, inhibitin reduced sodium and lithium efflux to that observed in sodium-free medium. Inhibitin showed no reduction in sodium or lithium efflux when sodium was replaced by choline chloride or Mg2+. When inhibitin was combined with one or more of the other transport inhibitors (i.e., ouabain, furosemide, or bumetanide and amiloride), its inhibitable component remained distinct and it did not overlap with that of the other inhibitors. These studies show that inhibitin is a specific inhibitor of carrier-mediated sodium/sodium exchange and sodium/lithium countertransport processes in human erythrocytes.

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

This paper reports experiments designed to find the concentrations of internal and external Na and K at which inward and outward furosemide-sensitive (FS) Na and K fluxes are equal, so that there is no net FS movement of Na and K. The red cell cation content was modified by using the ionophore nystatin, varying cell Na (Nai) from 0 to 34 mM (K substitution, high-K cells) and cell K (Ki) from 0 to 30 mM (Na substitution, high-Na cells). All incubation media contained NaCl (Nao = 130 or 120 nM), and KCl (Ko = 0-30 mM). In high-K cells, incubated in the absence of Ko, there was net extrusion of Na through the FS pathway. The net FS Na extrusion increased when Nai was increased. Low concentrations of Ko (0-6 mM) slightly stimulated, whereas higher concentrations of Ko inhibited, FS Na efflux. Increasing Ko stimulated the FS Na influx (K0.5 = 4 mM). Under conditions similar to those that occur in vivo (Nai = 10, Ki = 130, Nao = 130, Ko = 4 mM, Cli/Clo = 0.7), net extrusion of Na occurs through the FS pathway (180-250 mumol/liter cell X h). The concentration of Ko at which the FS Na influx and efflux and the FS K influx and efflux become equal increased when Nai increased in high-K cells and when Ki was increased in high-Na cells. The net FS Na and K fluxes both approached zero at similar internal and external Na and K concentrations. In high-K cells, under conditions when net Na and K fluxes were near zero, the ratio of FS Na to FS K unidirectional flux was found to be 2:3. In high-K cells, the empirical expression (Nai/Nao)2(Ki/Ko)3 remained at constant value (apparent equilibrium constant, Kappeq +/- SEM = 22 +/- 2) for each set of internal and external cation concentrations at which there was no net Na flux. These results indicate that in the physiological region of concentrations of internal and external Na, K, and Cl, the stoichiometry of the FS Na and K fluxes is 2 Na:3 K. In high-Na cells under conditions when net FS Na and K fluxes were near zero, the ratio of FS Na to FS K unidirectional fluxes was 3:2 (1).(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of loop diuretics and anions on passive potassium influx into human red cells

Passive K influx into human red cells was measured with and without Cl ions, Na ions and loop diuretics. Ouabain and loop diuretics appear to inhibit specifically and respectively the Na pump and (Na+K) 'co-transport'. Inhibitors of other pathways, e.g. 4,4'-diisothiocyantostilbene-2,2'-disulphonic acid or amiloride did not inhibit passive K influx. Loop diuretics inhibited with high apparent affinity in Na-containing media and with low apparent affinity in Na-free media where there was a substantial Cl-dependent component. The Cl concentration dependence was measured using six anion substitutions for Cl. With NO3, acetate and gluconate, the curves were sigmoidal and not fully saturable at 150 mM-Cl; with iodide and thiocyanate, the curves were convex; with sulphate, there was saturation at 120 mM-Cl. The half-maximal K influx as a function of [Na]0 was 40 mM for the Cl-dependent flux component and 12 mM for the diuretic-sensitive flux.

An intracellular analysis of gamma-aminobutyric-acid-associated ion movements in rat sympathetic neurones

Double-barrelled ion-sensitive micro-electrodes were used to measure the changes of the intracellular activities of Cl-, K+, and Na+ (aiCl, aiK, aiNa) in neurones of isolated rat sympathetic ganglia during the action of gamma-aminobutyric acid (GABA). The membrane potential of some of the neurones was manually 'voltage clamped' by passing current through the reference barrel of the ion-sensitive micro-electrode. This enabled us to convert the normal depolarizing action of GABA into a hyperpolarization. A GABA-induced membrane depolarization was accompanied by a decrease of aiCl, aiK and no change in aiNa, whereas a GABA-induced membrane hyperpolarization resulted in an increase of aiCl, aiK and also no change in aiNa. GABA did not change the free intracellular Ca2+ concentration, as measured with a Ca2+-sensitive micro-electrode, whereas such an effect was seen during the action of carbachol. pH-sensitive electrodes, on the other hand, revealed a small GABA-induced extracellular acidification. The inward pumping of Cl- following the normal, depolarizing action of GABA required the presence of extracellular K+ as well as Na+, whereas CO2/HCO3--free solutions did not influence the uptake process. Furosemide, but not DIDS, blocked the inward pumping of Cl-. In conclusion, our data show that only changes in intracellular activities of K+ and Cl- are associated with the action of GABA. Furthermore, they indicate that a K+/Cl- co-transport, and not a Cl-/HCO3- counter-transport, may be involved in the homoeostatic mechanism which operates to restore the normal transmembrane Cl- distribution after the action of GABA.