The Formation of Transferrin Receptor-Positive Sickle Reticulocytes With Intermediate Density Is Not Determined by Fetal Hemoglobin Content

Prothrombin gene G20210A mutation in acute deep venous thrombosis patients with poor response to warfarin therapy

AIM

The pathogenesis of deep venous thrombosis (DVT) involves an interaction between hereditary and acquired factors. Prothrombin gene mutation is one of the hereditary risk factors. We evaluated the frequency of the prothrombin gene mutation in patients with DVT and its relation to oral warfarin anticoagulant therapy response.

METHODS

Prothrombin gene mutation was looked for in 40 DVT patients with poor response to warfarin. The results were compared with 40 DVT patients with a normal response to warfarin and 30 healthy blood donors. Blood samples were also assessed for protein C, protein S, anti-thrombin III and anticardiolipin antibodies (ACA) levels.

RESULTS

Prothrombin gene mutation was found in normal and poor DVT responders (6/40 and 13/40, respectively; p = NS) as well as in healthy controls (1/30). Patients with recurrent DVT or a family history of DVT were significantly (p<0.0001) more likely to have the prothrombin mutation than other DVT patients. Non prothrombin abnormalities (protein C, anti-thrombin III and ACA) were more common in poor responders than controls (p<0.0037) as were ACA (p<0.034).

CONCLUSIONS

Prothrombin gene mutation is present in several DVT patients, especially those with recurrent DVT or a family history of DVT. This mutation may contribute to a poor response to warfarin.

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.

Regulation of K-Cl cotransport by protein phosphatase 1alpha in mouse erythrocytes

The K-Cl cotransport (KCC) is an electroneutral-gradient-driven-membrane transport system, which is involved in regulation of red cell volume. Although the regulatory cascade of KCC is largely unknown, a signaling pathway involving phosphatases and kinases has been proposed. Here, we investigated the expression and the activity of protein phosphatase 1(PP-1) isoforms in mouse red cells, focusing on two models of abnormally activated KCC: mice genetically lacking the two Src-family tyrosine kinases, Hck and Fgr, (hck-/-fgr-/-) and the SAD transgenic sickle-cell-mice. The PP-1alpha, PP-1gamma, PP-1delta isoforms were expressed at similar levels in wild-type, hck-/-fgr-/- and SAD mouse erythrocytes and in each case were predominantly localized to cytoplasm. The PP-1alpha activity was significantly higher in both membrane and cytosol fractions of hck-/-fgr-/- and of SAD erythrocytes than in those of wild-type red cells, suggesting PP-1alpha as a target of the Hck and Fgr kinases. The PP2, a specific inhibitor of Src-family kinase, significantly increased KCC activity in wild-type mouse red cells, but failed to modify the already increased KCC activity in SAD erythrocytes. The lag-time for activation of KCC was considerably reduced in both hck-/-fgr-/- and SAD erythrocytes, suggesting that the rate limiting activation steps in both strains are freed from their tonic inhibition. Sulfhydryl reduction by dithiothreitol (DTT) lowered KCC activity only in SAD red cells, but did not affect the PP2-treated erythrocytes. These data suggest up-regulation of KCC in SAD red cells is mainly secondary to oxidative damage, which most likely reduces or removes the tonic KCC inhibition resulting from PP-1alpha activity controlled in turn by Src-family kinases.

Gene-gene and gene-environment interactions in mild hyperhomocysteinemia

Mild/moderate hyperhomocysteinemia (HHcy), a highly prevalent condition, is independently associated with an increased risk of arterial and venous thromboembolic diseases. Early reports of the association of mild/moderate HHcy with juvenile venous thromboembolism have shown familiarity for HHcy in relatives of index cases with thrombosis. Similar to inherited thrombophilia defects, inheritance of the HHcy phenotype was accordingly retained important for the definition of HHcy as an independent risk factor for thrombosis. A number of common polymorphisms in genes coding for methylenetetrahydrofolate reductase(MTHFR), methionine-synthase, methionine-synthase reductase and cysthationine beta-synthase (CBS) have been explored for their association with homocysteine levels, fasting and post-methionine load, and with thrombotic diseases. MTHFR thermolability accounts for a 10-fold increase in the risk of mild/moderate HHcy. With the possible exception of the CBS844ins68 insertion, there is no evidence for an increased risk of HHcy for any of these polymorphisms, isolated or in association with MTHFR thermolability. Environmental factors and MTHFR thermolability are main determinants of the HHcy phenotype.If mild/moderate HHcy is a pathogenetic risk factor for thrombosis, intervention aimed to improve the vitamin status appears of major importance, irrespective of common gene polymorphisms of the homocysteine metabolism.

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.

Sickle cell disease: from membrane pathophysiology to novel therapies for prevention of erythrocyte dehydration

Sickle cell anemia is characterized by the presence of dense dehydrated erythrocytes that have lost most of their K content. Due to the unique dependence of Hb S polymerization on intracellular Hb S concentration, preventing this dehydration should markedly reduce polymerization. The erythrocyte intermediate conductance Ca-activated K channel (hSK4 or KCNN4), first described by Gardos, has been shown to be a major pathway for sickle cell dehydration. Studies with the imidazole antimycotic clotrimazole have shown reduction of sickle cell dehydration in vivo in a small number of patients with sickle cell disease; dose-limiting gastrointestinal and liver toxicities were observed. Based on the chemical structure of clotrimazole metabolites, a novel Gardos channel inhibitor, ICA-17043, has been developed. It has shown substantial activity both in vitro and in vivo in transgenic sickle mice. ICA-17043 is currently in phase 2 human trials. Another potential therapeutic target is the K-Cl cotransport. When sickle erythrocytes are exposed to relatively acidic conditions, they undergo cell shrinkage via activation of this pathway. K-Cl cotransport can be blocked by increasing the abnormally low erythrocyte Mg content of sickle erythrocytes. Oral Mg supplementation has been shown to reduce sickle cell dehydration in vivo in transgenic sickle mice and in patients in two separate clinical trials. Oral Mg pidolate is being tested in clinical trials in homozygous sickle cell disease and in Hb S/HbC (SC) disease, either as a single agent or in combination with hydroxyurea. The ongoing trials will determine the clinical effectiveness of therapies aimed at preventing sickle erythrocyte dehydration.

Genetic risk factors in young adults with 'cryptogenic' ischemic cerebrovascular disease

Mutations such as factor V Leiden G1691A (FVL), prothrombin G20210A (FIIM), methylenetetrahydrofolate reductase (MTHFR) C677T, cystathionine beta-synthase (CBS) 844ins68 and endothelial cell protein C receptor (EPCR) 4031ins23 are risk factors for thromboembolism. To assess the role of these mutations in young adults with cerebral ischemia of otherwise undetermined etiology, 93 patients younger than 50 years old with thromboembolic strokes or transient ischemic attacks were studied. One hundred and eighty-six healthy age-matched and sex-matched blood donors served as controls. The FVL mutation was detected in 15/93 patients and 13/186 controls. After adjustment for smoking, arterial hypertension, and hyperlipidemia, the association of the FVL mutation with cerebral ischemia [odds ratio (OR), 3.19; 95% confidence interval (CI), 1.38-7.39] remained significant. One of 93 patients and 6/186 controls were carriers of FIIM (OR, 0.33; 95% CI, 0.04-2.75). We detected the MTHFR TT677 genotype in 9/93 patients and 26/186 controls (OR, 0.66; 95% CI, 0.30-1.47), a CBS 844ins68 mutation in 12/93 patients and 19/186 controls (OR, 1.30; 95% CI, 0.60-2.81), and an EPCR 4031ins23 mutation in 1/93 patients and in no control individual (P = 0.33). In conclusion, in younger adults the FVL mutation is a risk factor for cerebrovascular disease. FIIM, the MTHFR TT677 genotype and the CBS 844ins68 mutation did not contribute to the risk in this group of patients. The EPCR 4031ins23 mutation is very rare, its possible role needs further investigation.

Therapeutic Strategies for Prevention of Sickle Cell Dehydration

The intracellular concentration of Hb S is an important determinant of the kinetic of polymer formation and cell sickling. A variable fraction of dense, dehydrated erythrocytes with high Hb S concentration is seen in the blood of patients with sickle cell disease; these dense cells play an important role in the pathophysiology of the vasoocclusive events of sickle cell disease, due to their higher tendency to polymerize and sickle. Sickle cell dehydration is due to loss of K+, Cl-, and water: the two major determinant pathways of dehydration of sickle erythrocytes are the Ca2+-activated K+ channel (IK1 or Gardos channel) and the K-Cl cotransport (KCC). Specific inhibitors of these pathways being tested in patients with sickle cell disease are Mg2+ pidolate, which inhibits KCC by increasing the sickle cell content of Mg2+, and clotrimazole and derivatives of clotrimazole metabolites, which specifically block the Gardos channel. An inhibitor of Cl- conductance has been shown to reduce dehydration in a transgenic mouse model of sickle cell disease but has not been tested in humans. If clinical efficacy and benefit are demonstrated, an inhibitor of cell dehydration could be used in patients as a single agent or in combination with existing therapies, such as hydroxyurea.

The survival characteristics of dense sickle cells

Sickle red blood cells (RBCs) become depleted of potassium, leading to dehydration and abnormally elevated cellular density. The increased sickling that results is important for both hemolysis and vasocclusion. In this study, sickle cells were subjected to high-speed centrifugation, and the bottom 15% were isolated. This procedure removed light cells and to a variable degree enriched cells that were denser than normal to produce a high-density–enriched (HDE) population of sickle cells. Autologous HDE cells from 3 subjects were labeled with biotin and re-infused. The following determinations were performed: (1) the survival and density changes of HDE cells; (2) the amount of fetal hemoglobin (HbF) in labeled cells after magnetic isolation; (3) the percentage of labeled F cells; (4) the percentage of labeled cells displaying external phosphatidylserine (PS). For patients with 3.5%, 4.5%, and 24% HbF in the HDE RBCs, the circulation half-time was 40, 80, and 180 hours, respectively. The percentage of HbF (measured in all 3 subjects) and of F cells (measured in 2 subjects) in labeled RBCs increased with time after re-infusion, indicating that HDE F cells have longer in vivo survival than HDE non-F cells. The percentage of PS+, biotin-labeled HDE cells showed no consistent increase or decrease with time after re-infusion. These data provide evidence that HDE sickle cells, especially those that do not contain HbF, have a very short in vivo survival, and that the percentage of PS+ cells in a re-infused HDE population does not change in a consistent manner as these cells age in the circulation.

The survival characteristics of dense sickle cells

Sickle red blood cells (RBCs) become depleted of potassium, leading to dehydration and abnormally elevated cellular density. The increased sickling that results is important for both hemolysis and vasocclusion. In this study, sickle cells were subjected to high-speed centrifugation, and the bottom 15% were isolated. This procedure removed light cells and to a variable degree enriched cells that were denser than normal to produce a high-density–enriched (HDE) population of sickle cells. Autologous HDE cells from 3 subjects were labeled with biotin and re-infused. The following determinations were performed: (1) the survival and density changes of HDE cells; (2) the amount of fetal hemoglobin (HbF) in labeled cells after magnetic isolation; (3) the percentage of labeled F cells; (4) the percentage of labeled cells displaying external phosphatidylserine (PS). For patients with 3.5%, 4.5%, and 24% HbF in the HDE RBCs, the circulation half-time was 40, 80, and 180 hours, respectively. The percentage of HbF (measured in all 3 subjects) and of F cells (measured in 2 subjects) in labeled RBCs increased with time after re-infusion, indicating that HDE F cells have longer in vivo survival than HDE non-F cells. The percentage of PS+, biotin-labeled HDE cells showed no consistent increase or decrease with time after re-infusion. These data provide evidence that HDE sickle cells, especially those that do not contain HbF, have a very short in vivo survival, and that the percentage of PS+ cells in a re-infused HDE population does not change in a consistent manner as these cells age in the circulation.

Reticulocyte cellular indices: a new approach in the diagnosis of anemias and monitoring of erythropoietic function

Reticulocyte analysis has been extended from the simple enumeration of reticulocytes to precise measurements of mRNA content and of cellular indices such as volume, hemoglobin (Hb) concentration, and content. Assessment of reticulocyte maturity is based on the fluorescence intensity of reticulocytes, which depends on RNA content. The appearance of high fluorescence reticulocytes has been shown to be associated with engraftment in the setting of bone marrow or peripheral stem cells transplantation, although it is still not clear how this parameter can improve quality or cost of care compared with the traditional use of absolute neutrophil counts. Reticulocyte indices have been studied especially in the setting of iron deficiency and functional iron deficiency during recombinant human erythropoietin (r-HuEPO) therapy. Reticulocyte hemoglobin content (CHr) may allow prompt identification of an imbalance between r-HuEPO therapy and iron availability by detecting the presence in reticulocytes of iron-restricted erythropoiesis. Diagnosis of simple iron deficiency can also be achieved in a more cost-effective fashion by using CHr in conjunction with the regular complete blood count (CBC), rather than relying on the traditional biochemical parameters of iron metabolism. Response to therapy of megaloblastic anemia can also be monitored with CHr. These new reticulocyte parameters provide a real-time assessment of the functional state of erythropoiesis.

Formation of Dense Erythrocytes in SAD Mice Exposed to Chronic Hypoxia: Evaluation of Different Therapeutic Regimens and of a Combination of Oral Clotrimazole and Magnesium Therapies

We have examined the effect of hydroxyurea (HU), clotrimazole (CLT), magnesium oxide (Mg), and combined CLT+Mg therapies on the erythrocyte characteristics and their response to chronic hypoxia in a transgenic sickle mouse (SAD) model. SAD mice were treated for 21 days with 1 of the following regimens (administered by gavage): control (n = 6), HU (200 mg/d; n = 6), CLT (80 mg/kg/d, n = 5), Mg (1,000 mg/kg/d, n = 5), and CLT+Mg (80 and 1,000 mg/kg/d, respectively, n = 6). Nine normal mice were also treated as controls (n = 3), HU (n = 3), and CLT+Mg (n = 3). Treatment with HU induced a significant increase in mean corpuscular volume and cell K content and a decrease in density in SAD mice. Treatment with the CLT and Mg, either alone or in combination, also increased cell K and reduced density in SAD mice. After 21 days of treatment, the animals were exposed to hypoxia (48 hours at 8% O2) maintaining the same treatment. In the SAD mice, hypoxia induced significant cell dehydration. These hypoxia-induced changes were blunted in either HU- or Mg-treated SAD mice and were completely abolished by either CLT or CLT+Mg treatment, suggesting a major role for the Gardos channel in hypoxia-induced dehydration in vivo.

Formation of Dense Erythrocytes in SAD Mice Exposed to Chronic Hypoxia: Evaluation of Different Therapeutic Regimens and of a Combination of Oral Clotrimazole and Magnesium Therapies

We have examined the effect of hydroxyurea (HU), clotrimazole (CLT), magnesium oxide (Mg), and combined CLT+Mg therapies on the erythrocyte characteristics and their response to chronic hypoxia in a transgenic sickle mouse (SAD) model. SAD mice were treated for 21 days with 1 of the following regimens (administered by gavage): control (n = 6), HU (200 mg/d; n = 6), CLT (80 mg/kg/d, n = 5), Mg (1,000 mg/kg/d, n = 5), and CLT+Mg (80 and 1,000 mg/kg/d, respectively, n = 6). Nine normal mice were also treated as controls (n = 3), HU (n = 3), and CLT+Mg (n = 3). Treatment with HU induced a significant increase in mean corpuscular volume and cell K content and a decrease in density in SAD mice. Treatment with the CLT and Mg, either alone or in combination, also increased cell K and reduced density in SAD mice. After 21 days of treatment, the animals were exposed to hypoxia (48 hours at 8% O2) maintaining the same treatment. In the SAD mice, hypoxia induced significant cell dehydration. These hypoxia-induced changes were blunted in either HU- or Mg-treated SAD mice and were completely abolished by either CLT or CLT+Mg treatment, suggesting a major role for the Gardos channel in hypoxia-induced dehydration in vivo.

Two Distinct Pathways Mediate the Formation of Intermediate Density Cells and Hyperdense Cells From Normal Density Sickle Red Blood Cells

In sickle cell anemia (SS), some red blood cells dehydrate, forming a hyperdense (HD) cell fraction (&gt;1.114 g/mL; mean corpuscular hemoglobin concentration [MCHC], &gt;46 g/dL) that contains many irreversibly sickled cells (ISCs), whereas other SS red blood cells dehydrate to an intermediate density (ID; 1.090 to 1.114 g/mL; MCHC, 36 to 46 g/dL). This study asks if the potassium-chloride cotransporter (K:Cl) and the calcium-dependent potassium channel [K(Ca2+)] are participants in the formation of one or both types of dense SS red blood cells. We induced sickling by exposing normal density (ND; 1.080 to 1.090 g/mL; MCHC, 32 to 36 g/dL) SS discocytes to repetitive oxygenation-deoxygenation (O-D) cycles in vitro. At physiologic Na+, K+, and Cl−, and 0.5 to 2 mmol/L Ca2+, the appearance of dense cells was time- and pH-dependent. O-D cycling at pH 7.4 in 5% CO2-equilibrated buffer generated only ID cells, whereas O-D cycling at pH 6.8 in 5% CO2-equilibrated buffer generated both ID and HD cells, the latter taking more than 8 hours to form. At 22 hours, 35% ± 17% of the parent ND cells were recovered in the ID fraction and 18% ± 11% in the HD fraction. Continuous deoxygenation (N2/5% CO2) at pH 6.8 generated both ID and HD cells, but many of these cells had multiple projections, clearly different from the morphology of endogenous dense cells and ISCs. Continuous oxygenation (air/5% CO2) at pH 6.8 resulted in less than 10% dense cell (ID + HD) formation. ATP depletion substantially increased HD cell formation and moderately decreased ID cell formation. HD cells formed after 22 hours of O-D cycling at pH 6.8 contained fewer F cells than did ID cells, suggesting that HD cell formation is particularly dependent on HbS polymerization. EGTA chelation of buffer Ca2+ inhibited HD but not ID cell formation, and increasing buffer Ca2+ from 0.5 to 2 mmol/L promoted HD but not ID cell formation in some SS patients. Substitution of nitrate for Cl− inhibited ID cell formation, as did inhibitors of the K:Cl cotransporter, okadaic acid, and [(dihydroindenyl) oxy]alkanoic acid (DIOA). Conversely, inhibitors of K(Ca2+), charybdotoxin and clotrimazole, inhibited HD cell formation. The combined use of K(Ca2+) and K:Cl inhibitors nearly eliminated dense cell (ID + HD cell) formation. In summary, dense cells formed by O-D cycling for 22 hours at pH 7.4 cycling are predominately the ID type, whereas dense cells formed by O-D cycling for 22 hours at pH 6.8 are both the ID and HD type, with the latter low in HbF, suggesting that HD cell formation has a greater dependency on HbS polymerization. A combination of K:Cl cotransport and the K(Ca2+) activities account for the majority of dense cells formed, and these pathways can be driven independently. We propose a model in which reversible sickling-induced K+ loss by K:Cl primarily generates ID cells and K+ loss by the K(Ca2+) channel primarily generates HD cells. These results imply that both pathways must be inhibited to completely prevent dense SS cell formation and have potential therapeutic implications.

Two Distinct Pathways Mediate the Formation of Intermediate Density Cells and Hyperdense Cells From Normal Density Sickle Red Blood Cells

In sickle cell anemia (SS), some red blood cells dehydrate, forming a hyperdense (HD) cell fraction (>1.114 g/mL; mean corpuscular hemoglobin concentration [MCHC], >46 g/dL) that contains many irreversibly sickled cells (ISCs), whereas other SS red blood cells dehydrate to an intermediate density (ID; 1.090 to 1.114 g/mL; MCHC, 36 to 46 g/dL). This study asks if the potassium-chloride cotransporter (K:Cl) and the calcium-dependent potassium channel [K(Ca2+)] are participants in the formation of one or both types of dense SS red blood cells. We induced sickling by exposing normal density (ND; 1.080 to 1.090 g/mL; MCHC, 32 to 36 g/dL) SS discocytes to repetitive oxygenation-deoxygenation (O-D) cycles in vitro. At physiologic Na+, K+, and Cl−, and 0.5 to 2 mmol/L Ca2+, the appearance of dense cells was time- and pH-dependent. O-D cycling at pH 7.4 in 5% CO2-equilibrated buffer generated only ID cells, whereas O-D cycling at pH 6.8 in 5% CO2-equilibrated buffer generated both ID and HD cells, the latter taking more than 8 hours to form. At 22 hours, 35% ± 17% of the parent ND cells were recovered in the ID fraction and 18% ± 11% in the HD fraction. Continuous deoxygenation (N2/5% CO2) at pH 6.8 generated both ID and HD cells, but many of these cells had multiple projections, clearly different from the morphology of endogenous dense cells and ISCs. Continuous oxygenation (air/5% CO2) at pH 6.8 resulted in less than 10% dense cell (ID + HD) formation. ATP depletion substantially increased HD cell formation and moderately decreased ID cell formation. HD cells formed after 22 hours of O-D cycling at pH 6.8 contained fewer F cells than did ID cells, suggesting that HD cell formation is particularly dependent on HbS polymerization. EGTA chelation of buffer Ca2+ inhibited HD but not ID cell formation, and increasing buffer Ca2+ from 0.5 to 2 mmol/L promoted HD but not ID cell formation in some SS patients. Substitution of nitrate for Cl− inhibited ID cell formation, as did inhibitors of the K:Cl cotransporter, okadaic acid, and [(dihydroindenyl) oxy]alkanoic acid (DIOA). Conversely, inhibitors of K(Ca2+), charybdotoxin and clotrimazole, inhibited HD cell formation. The combined use of K(Ca2+) and K:Cl inhibitors nearly eliminated dense cell (ID + HD cell) formation. In summary, dense cells formed by O-D cycling for 22 hours at pH 7.4 cycling are predominately the ID type, whereas dense cells formed by O-D cycling for 22 hours at pH 6.8 are both the ID and HD type, with the latter low in HbF, suggesting that HD cell formation has a greater dependency on HbS polymerization. A combination of K:Cl cotransport and the K(Ca2+) activities account for the majority of dense cells formed, and these pathways can be driven independently. We propose a model in which reversible sickling-induced K+ loss by K:Cl primarily generates ID cells and K+ loss by the K(Ca2+) channel primarily generates HD cells. These results imply that both pathways must be inhibited to completely prevent dense SS cell formation and have potential therapeutic implications.

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

KCl cotransport activated by swelling of sickle red blood cells (SS RBC)is inhibited by deoxygenation. Yet recent studies found a Cl--dependent increase in sickle reticulocyte density with cyclic deoxygenation. This study sought to demonstrate cotransporter stimulation by deoxygenation of SS RBC in isotonic media with normal pH. Low-density SS RBC exhibited a Cl--dependent component of the deoxygenation-induced net K+ efflux, which was blocked by two inhibitors of KCl cotransport, [(dihydroindenyl)oxy]alkanoic acid and okadaic acid. Cl--dependent K+ efflux stimulated by deoxygenation was enhanced 2.5-fold by clamping of cellular Mg2+ at the level in oxygenated cells using ionophore A-23187. Incubating cells in high external K+ or Rb+ minimized inhibition of KCl cotransport by internal Mg2+, and under these conditions deoxygenation markedly stimulated KCl cotransport in the absence of ionophore. Activation of KCl cotransport by deoxygenation of SS RBC in isotonic media at normal pH is consistent with the generalized dephosphorylation of membrane proteins induced by deoxygenation and activation of the cotransporter by a dephosphorylation mechanism. Na+/H+ exchange activity, known to be modulated by cytosolic Ca2+ elevation and cell shrinkage, remained silent under deoxygenation conditions.