Deoxygenation of sickle red blood cells stimulates KCl cotransport without affecting Na+/H+ exchange

Fluorescence assay of the interaction between hemoglobin and the cytoplasmic domain of erythrocyte membrane band 3

Oxygen tension has emerged as a potent regulator of multiple erythrocyte properties, including glucose metabolism, cell volume, ATP release, and cytoskeletal organization. Because hemoglobin (Hb)(1) binds to the cytoplasmic domain of band 3 (cdb3) in an oxygen dependent manner, with deoxyHb exhibiting significantly greater affinity for cdb3 than oxyHb, the deoxyHb-cdb3 interaction has been hypothesized to constitute the molecular switch for all O2-controlled erythrocyte processes. In this study, we describe a rapid and accurate method for quantitating the interaction of deoxyHb binding to cdb3. For this purpose, enhanced green fluorescent protein (eGFP) is fused to the COOH-terminus of cdb3, and the binding of Hb to the NH2-terminus of cdb3-eGFP is quantitated by Hb-mediated quenching of cdb3-eGFP fluorescence. As expected, the intensity of cdb3-eGFP fluorescence decreases only slightly following addition of oxyHb. However, upon deoxygenation of the same Hb-cdb3 solution, the fluorescence decreases dramatically (i.e. confirming that deoxyHb exhibits much greater affinity for cdb3 than oxyHb). Using this fluorescence quenching method, we not only confirm previously established characteristics of the Hb-cdb3 interaction, but also establish an assay that can be exploited to screen for inhibitors of the sickle Hb-cdb3 interaction that accelerates sickle Hb polymerization.

Current knowledge about the functional roles of phosphorylative changes of membrane proteins in normal and diseased red cells

With the advent of proteomic techniques the number of known post-translational modifications (PTMs) affecting red cell membrane proteins is rapidly growing but the understanding of their role under physiological and pathological conditions is incompletely established. The wide range of hereditary diseases affecting different red cell membrane functions and the membrane modifications induced by malaria parasite intracellular growth represent a unique opportunity to study PTMs in response to variable cellular stresses. In the present review, some of the major areas of interest in red cell membrane research have been considered as modifications of erythrocyte deformability and maintenance of the surface area, membrane transport alterations, and removal of diseased and senescent red cells. In all mentioned research areas the functional roles of PTMs are prevalently restricted to the phosphorylative changes of the more abundant membrane proteins. The insufficient information about the PTMs occurring in a large majority of the red membrane proteins and the general lack of mass spectrometry data evidence the need of new comprehensive, proteomic approaches to improve the understanding of the red cell membrane physiology.

Deoxygenation-induced non-electrolyte pathway in red cells from sickle cell patients

Red cells from patients with sickle cell disease contain HbS rather than the normal HbA (here termed HbS cells). On deoxygenation, HbS cells exhibit a distinctive solute permeability pathway, P(sickle), activated stochastically, and partially inhibited by DIDS and dipyridamole. It is often referred to as a cation channel although its permeability characteristics remain vague and its molecular identity is unknown. We show that, in contrast to normal red cells, a proportion of HbS cells underwent haemolysis when deoxygenated in isosmotic non-electrolyte solutions. Haemolysis was stochastic: cells unlysed after an initial deoxygenation pulse showed lysis when harvested, reoxygenated and subsequently exposed to a second period of deoxygenation. O(2) dependence of haemolysis was similar to that of P(sickle) activation. Haemolysis was accompanied by high rates of sucrose influx, and both haemolysis and sucrose influx were inhibited by DIDS and dipyridamole. Sucrose influx was only detected as ionic strength was reduced below 80 mM. These findings are consistent with the postulate that deoxygenation of HbS cells, under certain conditions, activates a novel non-electrolyte pathway. Their significance lies in understanding the nature of the deoxygenation-induced permeability in HbS cells, together with its relationship with novel pathways induced by a variety of manipulations in normal red cells.

Effect of intracellular magnesium and oxygen tension on K+-Cl- cotransport in normal and sickle human red cells

In red cells from normal individuals (HbA cells), the K+-Cl- cotransporter (KCC) is inactivated by low O2 tension whilst in those from sickle cell patients (HbS cells), it remains fully active. Changes in free intracellular [Mg2+] have been proposed as a mechanism. In HbA cells, KCC activity was stimulated by Mg2+ depletion and inhibited by Mg2+ loading but the effect of O2 was independent of Mg2+. At all [Mg2+]is, the transporter was stimulated in oxygenated cells, minimally active in deoxygenated ones. By contrast, the stimulatory effects of O2 was abolished by inhibitors of protein (de)phosphorylation. HbS cells had elevated KCC activity, which was of similar magnitude in oxygenated and deoxygenated cells, regardless of Mg2+ clamping. In deoxygenated cells, the antisickling agent dimethyl adipimidate inhibited sickling, Psickle and KCC. Results indicate a role for protein phosphorylation in O2 dependence of KCC, with different activities of the relevant enzymes in HbA and HbS cells, probably dependent on Hb.

Protein phosphatase 1alpha is tyrosine-phosphorylated and inactivated by peroxynitrite in erythrocytes through the src family kinase fgr

Protein serine/threonine phosphorylation is a significant component of the intracellular signal that together with tyrosine phosphorylation regulates several processes, including cell-cycle progression, muscle contraction, transcription, and neuronal signaling. Cross-talk between phosphoserine/threonine- and phosphotyrosine-mediated pathways is not yet well understood. In this study we found that peroxynitrite, a physiological oxidant formed by the fast radical-radical reaction between the nitric oxide and the superoxide anion, induced tyrosine phosphorylation of the serine/threonine protein phosphatase 1alpha (PP1alpha) in human erythrocytes through activation of src family kinases. We have previously shown in mouse red cells that upregulation of the src kinase fgr phosphorylates PP1alpha, acting as an upstream negative regulator of PP1alpha, and downregulates K-Cl cotransport. Here we found that PP1alpha is a selective substrate of peroxynitrite-activated fgr and that tyrosine phosphorylation of PP1alpha corresponds to an inhibition of its enzymatic activity. Despite fgr activation and PP1alpha downregulation, peroxynitrite stimulated in a dose-dependent fashion the function of the K-Cl cotransporter. In an attempt to understand the mechanism of K-Cl cotransport activation, we found that the effect of peroxynitrite is completely reversed by dithriothreitol, suggesting that peroxynitrite acts as an oxidizing agent by an SH-dependent and PP1alpha-independent mechanism. These findings highlight a novel function of peroxynitrite in regulating the intracellular signal transduction pathways involving serine/threonine phosphorylation and the functional role of proteins that are targets of these phosphatases.

Ion transport pathology in the mechanism of sickle cell dehydration

Polymers of deoxyhemoglobin S deform sickle cell anemia red blood cells into sickle shapes, leading to the formation of dense, dehydrated red blood cells with a markedly shortened life-span. Nearly four decades of intense research in many laboratories has led to a mechanistic understanding of the complex events leading from sickling-induced permeabilization of the red cell membrane to small cations, to the generation of the heterogeneity of age and hydration condition of circulating sickle cells. This review follows chronologically the major experimental findings and the evolution of guiding ideas for research in this field. Predictions derived from mathematical models of red cell and reticulocyte homeostasis led to the formulation of an alternative to prevailing gradualist views: a multitrack dehydration model based on interactive influences between the red cell anion exchanger and two K(+) transporters, the Gardos channel (hSK4, hIK1) and the K-Cl cotransporter (KCC), with differential effects dependent on red cell age and variability of KCC expression among reticulocytes. The experimental tests of the model predictions and the amply supportive results are discussed. The review concludes with a brief survey of the therapeutic strategies aimed at preventing sickle cell dehydration and with an analysis of the main open questions in the field.

Activation of ferret erythrocyte Na+-K+-2Cl- cotransport by deoxygenation

Deoxygenation of ferret erythrocytes stimulates Na+-K+-2Cl- cotransport by 111% (s.d., 46) compared to controls in air. Half-maximal activation occurs at a PO2 of 24 mmHg (s.d., 2) indicating that physiological changes in oxygen tension can influence cotransport function. Approximately 25-35% of this stimulation can be attributed to the rise of intracellular free magnesium concentration that occurs on deoxygenation (from 0.82 (S.D., 0.07) to 1.40 mm (S.D., 0.17)). Most of the stimulation is probably caused by activation of a kinase which can be prevented or reversed by treating cells with the kinase inhibitors PP1 or staurosporine, or by reducing cell magnesium content to submicromolar levels. Stimulation by deoxygenation is comparable with that caused by calyculin A or sodium arsenite, compounds that cause a 2- to 3-fold increase in threonine phosphorylation of the cotransporter which can be detected with phospho-specific antibodies. However, the same approach failed to detect significant changes in threonine phosphorylation following deoxygenation. The results suggest that deoxygenation causes activation of a kinase that either phosphorylates the transporter, but probably not on threonine, or phosphorylates another protein that in turn influences cotransporter behaviour. They also indicate that more than one kinase and phosphatase are involved in cotransporter phosphorylation.

KCl cotransport mediates abnormal sulfhydryl-dependent volume regulation in sickle reticulocytes

KCl cotransport (KCC) activation by cell swelling and pH was compared in sickle (SS) and normal (AA) red blood cells (RBCs). KCC fluxes had the same relationship to mean corpuscular hemoglobin concentration (MCHC) in SS and AA RBCs when normalized to the maximal volume-stimulated (VSmax) flux (MCHC < 270 g/L [27 g/dL]). Acid-stimulated (pH 6.9) KCC flux in SS RBCs was 60% to 70% of VSmax KCC versus 20% in AA RBCs. Density gradients were used to track changes in reticulocyte MCHC during KCC-mediated regulatory volume decrease (RVD). Swelling to MCHC of 260 g/L (26 g/dL) produced Cl-dependent RVD that resulted in higher MCHC in SS than AA reticulocytes. In acid pH, RVD was also greater in SS than AA reticulocytes. Sulfhydryl reduction by dithiothreitol (DTT) lowered VSmax KCC flux in AA and SS RBCs by one third but did not alter swelling-induced RVD. DTT lowered acid-activated KCC in SS RBCs by 50% and diminished acid-induced RVD in SS reticulocytes. Thus, swelling activation of KCC is normal in SS RBCs but KCC-mediated RVD produces higher MCHC in SS than AA reticulocytes. Acid activation of KCC is exaggerated in SS RBCs and causes dehydration in SS reticulocytes. KCC response to acid stimulation was mitigated by DTT, suggesting that it arises from sulfhydryl oxidation.

Regulation of K-Cl cotransport by Syk and Src protein tyrosine kinases in deoxygenated sickle cells

Protein tyrosine kinases (PTK) of the Src family are thought to suppress K-Cl cotransport (KCC) activity via negative regulation of protein phosphatases. However, some PTK inhibitors reduce KCC activity, suggesting opposite regulation by different PTK families. We have reported previously that deoxygenation of sickle cells stimulates KCC and activates Syk (a Syk family PTK), but not Lyn (an Src family PTK). In this study the same results were obtained when PTK activities were measured under the conditions used to measure KCC activity and which prevent any change in intracellular [Mg(2+)]. Methyl-2,5-dihydroxycinnamate (DHC), a PTK inhibitor, was more selective for Syk than Lyn, while staurosporine (ST), a broad-specificity protein kinase inhibitor, inhibited Lyn more than Syk. Deoxygenation or 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo[3,4- d] pyrimidine (pp2, a specific Src inhibitor) stimulated KCC independently. These effects were not additive and were inhibited by DHC. In contrast, ST-induced KCC activation was resistant to DHC, suggesting a different pathway of activation. Overall, these data indicate that Syk activity is required for KCC activation, either induced by deoxygenation of sickle cells, or mediated by Src inhibition in oxygenated cells, and that Syk and Src PTKs exert opposing and interconnected regulatory effects on the activity of the transporter.

Effect of protein kinase C activation on Na+-H+ exchange in erythrocytes of frog Rana temporaria

The treatment of frog erythrocytes incubated in standard nitrate medium with 100 nM phorbol ester (PMA) induced a sharp increase in the 22Na uptake by the cells and intracellular Na(+) concentration. The PMA-induced enhancement in 22Na uptake was stimulated by the addition of 0.1 mM ouabain to the incubation medium and completely blocked by 1 mM amiloride. The time course of 22Na uptake by frog red cells in the presence of PMA showed a lag phase ( approximately 5 min), after which was linear within 5-15 min. The calculated Na(+) influx in erythrocytes treated with PMA was 49.4+/-3.7 mmol l(-1) cells h(-1) as compared with 1.2+/-0.25 mmol l(-1) h(-1) for control cells. 5-(N-ethyl-N-isopropyl)-amiloride, selective blocker of NHE1, caused a dose-dependent inhibition of the PMA-induced Na(+) influx with IC(50) of 0.27 microM. The PMA-induced Na(+) influx was almost completely inhibited by 0.1 microM staurosporine, protein kinase C blocker. Pretreatment of frog red blood cells for 5, 10 or 15 min with 10 mM NaF, non-selective inhibitor of protein phosphatase, led to a progressive stimulation of the PMA effect on Na(+) influx. Both amiloride and NaF did not affect the basal Na(+) influx in frog erythrocytes. The data indicate that the Na(+)-H(+) exchanger in the frog erythrocytes is quiescent under basal conditions and can be markedly stimulated by PMA.

Dehydration response of sickle cells to sickling-induced Ca(++) permeabilization

Interaction of hemoglobin S polymers with the red blood cell (RBC) membrane induces a reversible increase in permeability ("P(sickle)") to (at least) Na(+), K(+), Ca(2+), and Mg(2+). Resulting changes in [Ca(2+)] and [H(+)] in susceptible cells activate 2 transporters involved in sickle cell dehydration, the Ca(2+)-sensitive K(+) ("Gardos") channel (K(Ca)) and the acid- and volume-sensitive K:Cl cotransport. We investigated the distribution of P(sickle) expression among deoxygenated sickle cell anemia (SS) RBCs using new experimental designs in which the RBC Ca(2+) pumps were partially inhibited by vanadate, and the cells' dehydration rates were detected as progressive changes in the profiles of osmotic fragility curves and correlated with flow cytometric measurements. The results exposed marked variations in (sickling plus Ca(2+))-induced dehydration rates within populations of deoxygenated SS cells, with complex distributions, reflecting a broad heterogeneity of their P(sickle) values. P(sickle)-mediated dehydration was inhibited by clotrimazole, verifying the role of K(Ca), and also by elevated [Ca(2+)](o), above 2 mM. Very high P(sickle) values occurred with some SS discocytes, which had a wide initial density (osmotic resistance) distribution. Together with its previously shown stochastic nature, the irregular distribution of P(sickle) documented here in discocytes is consistent with a mechanism involving low-probability, reversible interactions between sickle polymers and membrane or cytoskeletal components, affecting only a fraction of the RBCs during each deoxygenation event and a small number of activated pathways per RBC. A higher participation of SS reticulocytes in P(sickle)-triggered dehydration suggests that they form these pathways more efficiently than discocytes despite their lower cell hemoglobin concentrations.

Arginine supplementation of sickle transgenic mice reduces red cell density and Gardos channel activity

Nitric oxide (NO), essential for maintaining vascular tone, is produced from arginine by nitric oxide synthase. Plasma arginine levels are low in sickle cell anemia, and it is reported here that low plasma arginine is also found in our sickle transgenic mouse model that expresses human alpha, human beta(S), and human beta(S-Antilles) and is homozygous for the mouse beta(major) deletion (S+S-Antilles). S+S-Antilles mice were supplemented with a 4-fold increase in arginine that was maintained for several months. Mean corpuscular hemoglobin concentration (MCHC) decreased and the percent high-density red cells was reduced. Deoxy K(+) efflux is characteristic of red cells in sickle cell disease and contributes to the disease process by increasing the MCHC and rendering the cells more susceptible to polymer formation. This flux versus the room air flux was reduced in S+S-Antilles red cells from an average value of 1.6 +/- 0.3 mmol per liter of red cells x minute (FU) in nonsupplemented mice to 0.9 +/- 0.3 FU (n = 4, P < .02, paired t test) in supplemented mice. In room air, V(max) of the Ca(++)-activated K(+) channel (Gardos) was reduced from 4.1 +/- 0.6 FU (off diet) to 2.6 +/- 0.4 FU (n = 7 and 8, P < .04, t test) in arginine-supplemented mice versus clotrimazole. In conclusion, the major mechanism by which arginine supplementation reduces red cell density (MCHC) in S+S-Antilles mice is by inhibiting the Ca(++)-activated K(+) channel.

Oxygen-sensitive cation transport in sickle cells

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

K-Cl cotransport modulation by intracellular Mg in erythrocytes from mice bred for low and high Mg levels

Mg is an important determinant of erythrocyte cation transport system(s) activity. We investigated cation transport in erythrocytes from mice bred for high (MGH) and low (MGL) Mg levels in erythrocytes and plasma. We found that K-Cl cotransport activity was higher in MGL than in MGH erythrocytes, and this could explain their higher mean corpuscular hemoglobin concentration, median density, and reduced cell K content. Although mouse KCC1 protein abundance was comparable in MGL and MGH erythrocytes, activities of Src family tyrosine kinases were higher in MGH than in MGL erythrocytes. In contrast, protein phosphatase (PP) isoform 1 alpha (PP1 alpha) enzymatic activity, which has been suggested to play a positive regulatory role in K-Cl cotransport, was lower in MGH than in MGL erythrocytes. Additionally, we found that the Src family kinase c-Fgr tyrosine phosphorylates PP1 alpha in vitro. These findings suggest that in vivo downregulation of K-Cl cotransport activity by Mg is mediated by enhanced Src family kinase activity, leading to inhibition of the K-Cl cotransport stimulator PP1.

Direct estimate of 1:1 stoichiometry of K(+)-Cl(-) cotransport in rabbit erythrocytes

This work was undertaken to obtain a direct measure of the stoichiometry of Na(+)-independent K(+)-Cl(-) cotransport (KCC), with rabbit red blood cells as a model system. To determine whether (86)Rb(+) can be used quantitatively as a tracer for KCC, (86)Rb(+) and K(+) effluxes were measured in parallel after activation of KCC with N-ethylmaleimide (NEM). The rate constant for NEM-stimulated K(+) efflux into isosmotic NaCl was smaller than that for (86)Rb(+) by a factor of 0.68 +/- 0.11 (SD, n = 5). This correction factor was used in all other experiments to calculate the K(+) efflux from the measured (86)Rb(+) efflux. To minimize interference from the anion exchanger, extracellular Cl(-) was replaced with SO, and 4,4'-diisothiocyanothiocyanatodihydrostilbene-2,2'-disulfonic acid was present in the flux media. The membrane potential was clamped near 0 mV with the protonophore 2,4-dinitrophenol. The Cl(-) efflux at 25 degrees C under these conditions is approximately 100,000-fold smaller than the uninhibited Cl(-)/Cl(-) exchange flux and is stimulated approximately 2-fold by NEM. The NEM-stimulated (36)Cl(-) flux is inhibited by okadaic acid and calyculin A, as expected for KCC. The ratio of the NEM-stimulated K(+) to Cl(-) efflux is 1.12 +/- 0.26 (SD, n = 5). We conclude that K(+)-Cl(-) cotransport in rabbit red blood cells has a stoichiometry of 1:1.

Characterization of the phosphatidylserine-exposing subpopulation of sickle cells

Phosphatidylserine (PS), exclusively present in the inner monolayer of the normal red blood cell (RBC) membrane, is exposed in subpopulations of sickle cells. PS-exposing RBCs were found predominantly among the densest and the very light sickle cells. Within the light RBC fraction, PS exposure was found on reticulocytes, transferrin receptor-expressing reticulocytes, and mature RBCs. The last subset contained low-density valinomycin-resistant RBCs, previously shown to have high Na(+) and low K(+) content. This subpopulation contained the highest percentage of PS-exposing cells. The PS-exposing sickle cells did not show the sustained high cytosolic Ca(++) levels that have been shown to activate scramblase activity. Data from this study indicate that PS exposure can occur at different stages in the life of the sickle RBC and that it correlates with the loss of aminophospholipid translocase activity, the only common denominator of the PS-exposing cells. The additional requirement of scramblase activation may occur during transient increases in cytosolic Ca(++). (Blood. 2001;98:860-867)

Dipyridamole inhibits sickling-induced cation fluxes in sickle red blood cells

Sickling-induced cation fluxes contribute to cellular dehydration of sickle red blood cells (SS RBCs), which in turn potentiates sickling. This study examined the inhibition by dipyridamole of the sickling-induced fluxes of Na(+), K(+), and Ca(++) in vitro. At 2% hematocrit, 10 microM dipyridamole inhibited 65% of the increase in net fluxes of Na(+) and K(+) produced by deoxygenation of SS RBCs. Sickle-induced Ca(++) influx, assayed as (45)Ca(++) uptake in quin-2-loaded SS RBCs, was also partially blocked by dipyridamole, with a dose response similar to that of Na(+) and K(+) fluxes. In addition, dipyridamole inhibited the Ca(++)-activated K(+) flux (via the Gardos pathway) in SS RBCs, measured as net K(+) efflux in oxygenated cells exposed to ionophore A23187 in the presence of external Ca(++), but this effect resulted from reduced anion conductance, rather than from a direct effect on the K(+) channel. The degree of inhibition of sickling-induced fluxes was dependent on hematocrit, and up to 30% of dipyridamole was bound to RBC membranes at 2% hematocrit. RBC membrane content of dipyridamole was measured fluorometrically and correlated with sickling-induced flux inhibition at various concentrations of drug. Membrane drug content in patients taking dipyridamole for other clinical indications was similar to that producing inhibition of sickling-induced fluxes in vitro. These data suggest that dipyridamole might inhibit sickling-induced fluxes of Na(+), K(+), and Ca(++) in vivo and therefore have potential as a pharmacological agent to reduce SS RBC dehydration. (Blood. 2001;97:3976-3983)

O2 dependence of K+ transport in sickle cells: the effect of different cell populations and the substituted benzaldehyde 12C79

The molecular basis of sickle cell disease (SCD) is well known but the pathophysiology is poorly understood. It remains intractable to therapy. Hyperactivity of several membrane transport systems, including the K+-Cl- cotransporter (termed KCC), cause HbS-containing red cells (termed HbS cells) to dehydrate and sickle, leading to the development of sickle cell crises (SCCs). Contrary to normal red cells (HbA cells), KCC in HbS cells is active at low O2 tensions (PO2s), remaining responsive to low pH or urea. Since these stimuli are usually encountered in hypoxic regions, the abnormal O2 dependence increases the contribution of KCC to dehydration, and hence development of SCCs. These differences with HbA cells may be due to the younger population of cells or to polymerization of HbS. We used 86Rb+ as a K+ congener to investigate the activity of KCC at different PO2s, and density gradient separation to investigate different red cell fractions. We found no correlation of O2 dependence with cell fractions. We also used the substituted benzaldehyde 12C79 to increase the O2 affinity of HbS and found that its effect on HbS O2 saturation and cell sickling correlated with that on both Cl--independent and Cl--dependent K+ transport, implying that, at low PO2s, KCC activity correlated with HbS polymerization. The importance of these results to understanding the pathophysiology of SCD, and for the design of chemotherapeutic agents to ameliorate or prevent SCC, is discussed.

Elevated temperatures enhance KCCl activity in sickle cells

The maintenance of red blood cell volume is important in the pathophysiology of sickle cell disease. The KCl cotransporter (KCCl) is capable of mediating sickle cell dehydration. In this study, we have determined the effect of increased temperature (over the range 37-41 degrees C) on basal K+ transport and K+ transport following activation of KCCl by urea or N-ethylmaleimide (NEM). An increased temperature was found to have only a small effect (approximately a 20% increase) on basal K+ transport. In contrast, the increase was much greater (about 60%) after activation of KCCl by urea. Following activation of KCCl by NEM, the increase in K+ transport with increasing temperature was small (about 10%). This suggests that it is the signalling system rather than the transporter itself that is sensitive to temperature.

Effect of dimethyl adipimidate on K+ transport and shape change in red blood cells from sickle cell patients

Dimethyl adipimidate (DMA) reduces K+ loss from, and dehydration of, red cells containing haemoglobin S (HbS cells). Three membrane transporters may contribute to these processes: the deoxygenation-induced cation-selective channel (Psickle), the Ca2+-activated K+ channel (or Gardos channel) and the K+-CI- cotransporter (KCC). We show that DMA inhibited all three pathways in deoxygenated HbS cells. The Gardos channel could be activated following Ca2+ loading. Considerable KCC activity was present in oxygenated HbS cells, showing a selective action of DMA on the transporter in deoxygenated cells. Inhibition of sickling correlated strongly with that of Psickle and moderately with that of KCC activity. We conclude that DMA does not inhibit the K+ pathways directly, but acts mainly by preventing HbS polymerisation and sickling. These findings are relevant to the development of novel chemotherapeutic agents for amelioration of sickle cell disease.

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.

Dehydration of mature and immature sickle red blood cells during fast oxygenation/deoxygenation cycles: role of KCl cotransport and extracellular calcium

Sickle red blood cells (RBC) become dehydrated as a consequence of potassium loss. This process depends at least partly on deoxygenation and may be influenced by the presence of oxygenation/deoxygenation cycles and the frequency of cycling. In this study, sickle RBC were subjected to approximately 180 oxygenation/deoxygenation cycles during 4 hours to evaluate RBC dehydration with cycle periods more similar to in vivo cycles than those in previous studies. A continuous-flow, steady-state apparatus circulated a dilute RBC suspension through gas-permeable silicone tubing with segments that were exposed to either nitrogen or ambient oxygen. The percentage of sickling and partial pressure of oxygen were measured by means of sampling ports in the deoxygenation and oxygenation regions. The density increase (dehydration) of young (transferrin receptor-positive) and mature (transferrin receptor-negative) RBC and the requirements for calcium and chloride were evaluated. Density increase correlated with the percentage of sickled cells at the deoxygenation sampling port and was observed only in the presence of calcium, thereby implicating the calcium-dependent potassium channel (Gardos pathway). Density increase was not dependent on the presence of chloride, making it unlikely that KCl cotransport was an important pathway under these conditions.

Oxygen-dependent K+ influxes in Mg2+-clamped equine red blood cells

1. Cl--dependent K+ (86Rb+) influxes were measured in oxygenated and deoxygenated equine red blood cells, whose free [Mg2+]i had been clamped, to examine the effect on O2 dependency of the K+-Cl- cotransporter. 2. Total [Mg2+]i was 2.55 +/- 0.07 mM (mean +/- s.e.m. , n = 6). Free [Mg2+]i was estimated at 0.45 +/- 0.04 and 0.68 +/- 0. 03 mM (mean +/- s.e.m., n = 4) in oxygenated and deoxygenated red cells, respectively. 3. K+-Cl- cotransport was minimal in deoxygenated cells but substantial in oxygenated ones. Cl--dependent K+ influx, inhibited by calyculin A, consistent with mediation via the K+-Cl- cotransporter, was revealed by depleting deoxygenated cells of Mg2+. 4. Decreasing [Mg2+]i stimulated K+ influx, and increasing [Mg2+]i inhibited it, in both oxygenated and deoxygenated red cells. When free [Mg2+]i was clamped, Cl--dependent K+ influxes were always greater in oxygenated cells than in deoxygenated ones, and changes in free [Mg2+]i of the magnitude occurring during oxygenation-deoxygenation cycles had a minimal effect. Physiological fluctuations in free [Mg2+]i are unlikely to provide the primary link coupling activity of the K+-Cl- cotransporter with O2 tension. 5. Volume and H+ ion sensitivity of K+ influx in Mg2+-clamped red cells were increased in O2 compared with those in deoxygenated cells at the same free [Mg2+]i, by about 6- and 2-fold, respectively, but again these features were not responsible for the higher fluxes in oxygenated cells. 6. Regulation of the K+-Cl- cotransporter by O2 is very similar in equine, sheep and in normal human (HbA) red cells, but altered in human sickle cells. Present results imply that, as in sheep red cells, O2 dependence of K+-Cl- cotransport in equine red cells is not mediated via changes in free [Mg2+]i and that cotransport in Mg2+-clamped red cells is still stimulated by O2. This behaviour is contrary to that reported for human sickle (HbS) cells.