Oxygen-sensitive membrane transporters in vertebrate red cells

miRNA-7145-cuedc2 axis controls hematopoiesis through JAK1/STAT3 signaling pathway

Hematopoiesis ensures tissue oxygenation, and remodeling as well as immune protection in vertebrates. During embryogenesis, hemangioblasts are the source of all blood cells. Gata1a and pu.1 are co-expressed in hemangioblasts before hemangioblasts are differentiated into blood cells. However, the genes that determine the differentiation of hemangioblasts into myeloid or erythroid cell lineages have not been fully uncovered. Here we showed that miRNA-7145, a miRNA with previously unknown function, was enriched in erythrocytes at the definitive wave, but not expressed in myeloid cells. Overexpression and loss-of-function analysis of miRNA-7145 revealed that miRNA-7145 functions as a strong inhibitor for myeloid progenitor cell differentiation while driving erythropoiesis during the primitive wave. Furthermore, we confirmed that cuedc2 is one of miRNA-7145 targeted-genes. Overexpression or knock-down of cuedc2 partially rescues the phenotype caused by miRNA-7145 overexpression or loss-of-function. As well, overexpression and loss-of-function analysis of cuedc2 showed that cuedc2 is required for myelopoiesis at the expense of erythropoiesis. Finally, we found that overexpression of zebrafish cuedc2 in 293 T cell inhibits the JAK1/STAT3 signaling pathway. Collectively, our results uncover a previously unknown miRNA-7145-cuedc2 axis, which regulate hematopoiesis through inhibiting the JAK1/STAT3 signaling pathway.

Signaling mechanisms in red blood cells: A view through the protein phosphorylation and deformability

Intracellular signaling mechanisms in red blood cells (RBCs) involve various protein kinases and phosphatases and enable rapid adaptive responses to hypoxia, metabolic requirements, oxidative stress, or shear stress by regulating the physiological properties of the cell. Protein phosphorylation is a ubiquitous mechanism for intracellular signal transduction, volume regulation, and cytoskeletal organization in RBCs. Spectrin-based cytoskeleton connects integral membrane proteins, band 3 and glycophorin C to junctional proteins, ankyrin and Protein 4.1. Phosphorylation leads to a conformational change in the protein structure, weakening the interactions between proteins in the cytoskeletal network that confers a more flexible nature for the RBC membrane. The structural organization of the membrane and the cytoskeleton determines RBC deformability that allows cells to change their ability to deform under shear stress to pass through narrow capillaries. The shear stress sensing mechanisms and oxygenation-deoxygenation transitions regulate cell volume and mechanical properties of the membrane through the activation of ion transporters and specific phosphorylation events mediated by signal transduction. In this review, we summarize the roles of Protein kinase C, cAMP-Protein kinase A, cGMP-nitric oxide, RhoGTPase, and MAP/ERK pathways in the modulation of RBC deformability in both healthy and disease states. We emphasize that targeting signaling elements may be a therapeutic strategy for the treatment of hemoglobinopathies or channelopathies. We expect the present review will provide additional insights into RBC responses to shear stress and hypoxia via signaling mechanisms and shed light on the current and novel treatment options for pathophysiological conditions.

Influence of ZnO Morphology on the Functionalization Efficiency of Nanostructured Arrays with Hemoglobin for CO2 Capture

In this study, nanostructured ZnO arrays were synthesized by an accessible thermal oxidation (TO) methodology. The Zn films were chemically etched with nitric acid (HNO3) and then oxidized in a furnace at 500 °C for 5 h. Two different morphologies were achieved by modifying the HNO3 concentration in the etching process: (a) ZnO grass-like nanostructures and (b) rod-like nanostructures, with an etching process in HNO3 solution at 2 and 8 M concentration, respectively. The physical and chemical properties of the samples were analyzed by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. Both morphologies were functionalized with hemoglobin, and a difference was found in the efficiency of functionalization, which was monitored by UV–Vis spectroscopy. The sample with the highest efficiency was the ZnO grass-like nanostructures. Afterward, the capture of carbon dioxide was evaluated by monitoring a sodium carbonate solution interacting with the as-functionalized samples. The evaluation was analyzed by UV–Vis spectroscopy and the results showed a CO2 capture of 98.3% and 54% in 180 min for the ZnO grass-like and rod-like nanostructures, respectively.

Does Plasma Inhibit the Activity of KCl Cotransport in Red Cells From LK Sheep?

Red cells from LK sheep represent an important paradigm for control of KCl cotransport activity, as well as being important to sheep erythroid function. A previous report (Godart et al., 1997) suggested that autologous plasma markedly inhibits red cell KCC activity and identified the presence of the bicarbonate/CO₂ buffer system as the probable cause. Findings were restricted, however, to red cells from patients with sickle cell disease (SCD) swollen anisotonically and carried out at a very high O₂ tension (c.700 mmHg). It was therefore important to investigate the generality of the effect described and whether it was also relevant to the two main stimuli for KCC activity encountered most often by circulating red cells in vivo - low pH in active muscle beds during exercise and high urea concentrations in the renal medulla during antidiuresis. Results confirm that inhibition was significant in response to anisotonic swelling with KCC activity in MOPS-buffered saline (MBS) vs. bicarbonate-buffered saline (BBS) and in MBS vs. plasma both reduced (by about 25 and 50%, respectively). By contrast, however, inhibition was absent at low pH and in high concentrations of urea. These findings suggest therefore that red cell KCC activity represents an important membrane permeability in vivo in red cells suspended in plasma. They are relevant, in particular, to sheep red cells, and may also be important by extension to those of other species and to the abnormal red cells found in human patients with SCD.

Adrenergically induced translocation of red blood cell β-adrenergic sodium-proton exchangers has ecological relevance for hypoxic and hypercapnic white seabass

White seabass (Atractoscion nobilis) increasingly experience periods of low oxygen (O₂; hypoxia) and high carbon dioxide (CO₂, hypercapnia) due to climate change and eutrophication of the coastal waters of California. Hemoglobin (Hb) is the principal O₂ carrier in the blood and in many teleost fishes Hb-O₂ binding is compromised at low pH; however, the red blood cells (RBC) of some species regulate intracellular pH with adrenergically stimulated sodium-proton-exchangers (β-NHEs). We hypothesized that RBC β-NHEs in white seabass are an important mechanism that can protect the blood O₂-carrying capacity during hypoxia and hypercapnia. We determined the O₂-binding characteristics of white seabass blood, the cellular and subcellular response of RBCs to adrenergic stimulation, and quantified the protective effect of β-NHE activity on Hb-O₂ saturation. White seabass had typical teleost Hb characteristics, with a moderate O₂ affinity (Po₂ at half-saturation; P₅₀ 2.9 kPa) that was highly pH-sensitive (Bohr coefficient -0.92; Root effect 52%). Novel findings from super-resolution microscopy revealed β-NHE protein in vesicle-like structures and its translocation into the membrane after adrenergic stimulation. Microscopy data were corroborated by molecular and phylogenetic results and a functional characterization of β-NHE activity. The activation of RBC β-NHEs increased Hb-O₂ saturation by ∼8% in normoxic hypercapnia and by up to ∼20% in hypoxic normocapnia. Our results provide novel insight into the cellular mechanism of adrenergic RBC stimulation within an ecologically relevant context. β-NHE activity in white seabass has great potential to protect arterial O₂ transport during hypoxia and hypercapnia but is less effective during combinations of these stressors.

Hemoglobin deoxygenation and methemoglobinemia prevent regulatory volume decrease in crucian carp (Carassius carassius) red blood cells

Fish red blood cells (RBCs) exhibit an oxygen-dependent regulatory volume decrease (RVD) in hypoosmotic environment. In higher vertebrates, membrane-associated hemoglobin is involved in the regulation of osmotic ion movements across the cellular membrane. However, whether the hemoglobin conformational state plays a role in the regulation of osmotic responses in fish red blood cells is still not fully understood. We found that changes in hemoglobin conformation influence the pattern of the regulatory volume decrease in Carassius carassius red blood cells. In oxygenated cells (96.4 ± 3.7% oxygenated hemoglobin), the volume recovery was completed within 125 min. Deoxygenation of hemoglobin (96.5 ± 2.7% of deoxygenated hemoglobin) inhibited the volume decrease in hyposmotically swollen red blood cells. Reoxygenation restored regulatory volume decrease in cells within 5 min. Induced methemoglobinemia (48.4 ± 1.8% of methemoglobin and 41.3 ± 2.3% of deoxygenated hemoglobin) blocked the process of volume recovery and significantly decreased osmotic stability of red blood cells.

The role of WNK in modulation of KCl cotransport activity in red cells from normal individuals and patients with sickle cell anaemia

Abnormal activity of red cell KCl cotransport (KCC) is involved in pathogenesis of sickle cell anaemia (SCA). KCC-mediated solute loss causes shrinkage, concentrates HbS, and promotes HbS polymerisation. Red cell KCC also responds to various stimuli including pH, volume, urea, and oxygen tension, and regulation involves protein phosphorylation. The main aim of this study was to investigate the role of the WNK/SPAK/OSR1 pathway in sickle cells. The pan WNK inhibitor WNK463 stimulated KCC with an EC₅₀ of 10.9 ± 1.1 nM and 7.9 ± 1.2 nM in sickle and normal red cells, respectively. SPAK/OSR1 inhibitors had little effect. The action of WNK463 was not additive with other kinase inhibitors (staurosporine and N-ethylmaleimide). Its effects were largely abrogated by pre-treatment with the phosphatase inhibitor calyculin A. WNK463 also reduced the effects of physiological KCC stimuli (pH, volume, urea) and abolished any response of KCC to changes in oxygen tension. Finally, although protein kinases have been implicated in regulation of phosphatidylserine exposure, WNK463 had no effect. Findings indicate a predominant role for WNKs in control of KCC in sickle cells but an apparent absence of downstream involvement of SPAK/OSR1. A more complete understanding of the mechanisms will inform pathogenesis whilst manipulation of WNK activity represents a potential therapeutic approach.

Regulation of erythrocyte function: Multiple evolutionary solutions for respiratory gas transport and its regulation in fish

Gas transport concepts in vertebrates have naturally been formulated based on human blood. However, the first vertebrates were aquatic, and fish and tetrapods diverged hundreds of millions years ago. Water-breathing vertebrates live in an environment with low and variable O₂ levels, making environmental O₂ an important evolutionary selection pressure in fishes, and various features of their gas transport differ from humans. Erythrocyte function in fish is of current interest, because current environmental changes affect gas transport, and because especially zebrafish is used as a model in biomedical studies, making it important to understand the differences in gas transport between fish and mammals to be able to carry out meaningful studies. Of the close to thirty thousand fish species, teleosts are the most species-numerous group. However, two additional radiations are discussed: agnathans and elasmobranchs. The gas transport by elasmobranchs may be closest to the ancestors of tetrapods. The major difference in their haemoglobin (Hb) function to humans is their high urea tolerance. Agnathans differ from other vertebrates by having Hbs, where cooperativity is achieved by monomer-oligomer equilibria. Their erythrocytes also lack the anion exchange pathway with profound effects on CO₂ transport. Teleosts are characterized by highly pH sensitive Hbs, which can fail to become fully O₂ -saturated at low pH. An adrenergically stimulated Na⁺ /H⁺ exchanger has evolved in their erythrocyte membrane, and plasma-accessible carbonic anhydrase can be differentially distributed among their tissues. Together, and differing from other vertebrates, these features can maximize O₂ unloading in muscle while ensuring O₂ loading in gills.

Regulation of erythrocyte Na+/K+/2Cl- cotransport by an oxygen-switched kinase cascade

Many erythrocyte processes and pathways, including glycolysis, the pentose phosphate pathway (PPP), KCl cotransport, ATP release, Na⁺/K⁺-ATPase activity, ankyrin-band 3 interactions, and nitric oxide (NO) release, are regulated by changes in O₂ pressure that occur as a red blood cell (RBC) transits between the lungs and tissues. The O₂ dependence of glycolysis, PPP, and ankyrin-band 3 interactions (affecting RBC rheology) are controlled by O₂-dependent competition between deoxyhemoglobin (deoxyHb), but not oxyhemoglobin (oxyHb), and other proteins for band 3. We undertook the present study to determine whether the O₂ dependence of Na⁺/K⁺/2Cl^- cotransport (catalyzed by Na⁺/K⁺/2Cl^- cotransporter 1 [NKCC1]) might similarly originate from competition between deoxyHb and a protein involved in NKCC1 regulation for a common binding site on band 3. Using three transgenic mouse strains having mutated deoxyhemoglobin-binding sites on band 3, we found that docking of deoxyhemoglobin at the N terminus of band 3 displaces the protein with no lysine kinase 1 (WNK1) from its overlapping binding site on band 3. This displacement enabled WNK1 to phosphorylate oxidative stress-responsive kinase 1 (OSR1), which, in turn, phosphorylated and activated NKCC1. Under normal solution conditions, the NKCC1 activation increased RBC volume and thereby induced changes in RBC rheology. Because the deoxyhemoglobin-mediated WNK1 displacement from band 3 in this O₂ regulation pathway may also occur in the regulation of other O₂-regulated ion transporters, we hypothesize that the NKCC1-mediated regulatory mechanism may represent a general pattern of O₂ modulation of ion transporters in erythrocytes.

Squeezing for Life - Properties of Red Blood Cell Deformability

Deformability is an essential feature of blood cells (RBCs) that enables them to travel through even the smallest capillaries of the human body. Deformability is a function of (i) structural elements of cytoskeletal proteins, (ii) processes controlling intracellular ion and water handling and (iii) membrane surface-to-volume ratio. All these factors may be altered in various forms of hereditary hemolytic anemia, such as sickle cell disease, thalassemia, hereditary spherocytosis and hereditary xerocytosis. Although mutations are known as the primary causes of these congenital anemias, little is known about the resulting secondary processes that affect RBC deformability (such as secondary changes in RBC hydration, membrane protein phosphorylation, and RBC vesiculation). These secondary processes could, however, play an important role in the premature removal of the aberrant RBCs by the spleen. Altered RBC deformability could contribute to disease pathophysiology in various disorders of the RBC. Here we review the current knowledge on RBC deformability in different forms of hereditary hemolytic anemia and describe secondary mechanisms involved in RBC deformability.

Hypoxia inhibits the regulatory volume decrease in red blood cells of common frog (Rana temporaria)

Red blood cells of vertebrates can restore their cellular volume after hyposmotic swelling. The process strictly depends on oxygen availability in the environment. However, the role of hemoglobin in regulation of cell volume recovery is not clear yet. Little is known about the osmotic reactions and regulatory volume decrease of amphibian red blood cells. We investigated volume recovery process in oxygenated (oxyhemoglobin concentration 97 ± 3% of total hemoglobin) deoxygenated (96 ± 2% of deoxyhemolobin) and oxidized (47 ± 2% of methemoglobin, 41 ± 3% of deoxyhemoglobin) red blood cells of common frog (Rana temporaria) after hyposmotic swelling. Using the low-angle light scattering method we demonstrated the regulatory volume decrease in oxygenated cells and showed that the process was eliminated in hypoxic conditions. Reoxygenation of hypoxic cells restored the regulatory volume decrease. Oxidation of cellular hemoglobin to methemoglobin inhibited the volume recovery response in hyposmotically swollen oxygenated and reoxygenated hypoxic cells.

Electron Pathways through Erythrocyte Plasma Membrane in Human Physiology and Pathology: Potential Redox Biomarker?

Erythrocytes are involved in the transport of oxygen and carbon dioxide in the body. Since pH is the influential factor in the Bohr-Haldane effect, pHi is actively maintained via secondary active transports Na+/H+ exchange and HC3–/Cl– anion exchanger. Because of the redox properties of the iron, hemoglobin generates reactive oxygen species and thus, the human erythrocyte is constantly exposed to oxidative damage. Although the adult erythrocyte lacks protein synthesis and cannot restore damaged proteins, it is equipped with high activity of protective enzymes. Redox changes in the cell initiate various signalling pathways. Plasma membrane oxido-reductases (PMORs) are trans-membrane electron transport systems that have been found in the membranes of all cells and have been extensively characterized in the human erythrocyte. Erythrocyte PMORs transfer reducing equivalents from intracellular reductants to extracellular oxidants, thus their most important role seems to be to enable the cell respond to changes in intra- and extra-cellular redox environments.

So far the activity of erythrocyte PMORs in disease states has not been systematically investigated. This review summarizes present knowledge on erythrocyte electron transfer activity in humans (health, type 1 diabetes, diabetic nephropathy, and chronic uremia) and hypothesizes an integrated model of the functional organization of erythrocyte plasma membrane where electron pathways work in parallel with transport metabolons to maintain redox homeostasis.

Hypoxia Sensing in Plants: On a Quest for Ion Channels as Putative Oxygen Sensors

Over 17 million km2 of land is affected by soil flooding every year, resulting in substantial yield losses and jeopardizing food security across the globe. A key step in resolving this problem and creating stress-tolerant cultivars is an understanding of the mechanisms by which plants sense low-oxygen stress. In this work, we review the current knowledge about the oxygen-sensing and signaling pathway in mammalian and plant systems and postulate the potential role of ion channels as putative oxygen sensors in plant roots. We first discuss the definition and requirements for the oxygen sensor and the difference between sensing and signaling. We then summarize the literature and identify several known candidates for oxygen sensing in the mammalian literature. This includes transient receptor potential (TRP) channels; K+-permeable channels (Kv, BK and TASK); Ca2+ channels (RyR and TPC); and various chemo- and reactive oxygen species (ROS)-dependent oxygen sensors. Identified key oxygen-sensing domains (PAS, GCS, GAF and PHD) in mammalian systems are used to predict the potential plant counterparts in Arabidopsis. Finally, the sequences of known mammalian ion channels with reported roles in oxygen sensing were employed to BLAST the Arabidopsis genome for the candidate genes. Several plasma membrane and tonoplast ion channels (such as TPC, AKT and KCO) and oxygen domain-containing proteins with predicted oxygen-sensing ability were identified and discussed. We propose a testable model for potential roles of ion channels in plant hypoxia sensing.

Reversible binding of hemoglobin to band 3 constitutes the molecular switch that mediates O2 regulation of erythrocyte properties

Functional studies have shown that the oxygenation state of the erythrocyte regulates many important pathways, including glucose metabolism, membrane mechanical stability, and cellular adenosine triphosphate (ATP) release. Deoxyhemoglobin (deoxyHb), but not oxyhemoglobin, binds avidly and reversibly to band 3, the major erythrocyte membrane protein. Because band 3 associates with multiple metabolic, solute transport, signal transduction, and structural proteins, the hypothesis naturally arises that the O₂-dependent regulation of erythrocyte properties might be mediated by the reversible association of deoxyHb with band 3. To explore whether the band 3-deoxyHb interaction constitutes a "molecular switch" for regulating erythrocyte biology, we have generated transgenic mice with mutations in the deoxyHb-binding domain of band 3. One strain of mouse contains a "humanized" band 3 in which the N-terminal 45 residues of mouse band 3 are replaced by the homologous sequence from human band 3, including the normal human band 3 deoxyHb-binding site. The second mouse contains the same substitution as the first, except the deoxyHb site on band 3 (residues 12-23) has been deleted. Comparison of these animals with wild-type mice demonstrates that the following erythrocyte properties are controlled by the O₂-dependent association of hemoglobin with band 3: (1) assembly of a glycolytic enzyme complex on the erythrocyte membrane which is associated with a shift in glucose metabolism between the pentose phosphate pathway and glycolysis, (2) interaction of ankyrin with band 3 and the concomitant regulation of erythrocyte membrane stability, and (3) release of ATP from the red cell which has been linked to vasodilation.

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.

A unique mode of tissue oxygenation and the adaptive radiation of teleost fishes

Teleost fishes constitute 95% of extant aquatic vertebrates, and we suggest that this is related in part to their unique mode of tissue oxygenation. We propose the following sequence of events in the evolution of their oxygen delivery system. First, loss of plasma-accessible carbonic anhydrase (CA) in the gill and venous circulations slowed the Jacobs-Stewart cycle and the transfer of acid between the plasma and the red blood cells (RBCs). This ameliorated the effects of a generalised acidosis (associated with an increased capacity for burst swimming) on haemoglobin (Hb)-O2 binding. Because RBC pH was uncoupled from plasma pH, the importance of Hb as a buffer was reduced. The decrease in buffering was mediated by a reduction in the number of histidine residues on the Hb molecule and resulted in enhanced coupling of O2 and CO2 transfer through the RBCs. In the absence of plasma CA, nearly all plasma bicarbonate ultimately dehydrated to CO2 occurred via the RBCs, and chloride/bicarbonate exchange was the rate-limiting step in CO2 excretion. This pattern of CO2 excretion across the gills resulted in disequilibrium states for CO2 hydration/dehydration reactions and thus elevated arterial and venous plasma bicarbonate levels. Plasma-accessible CA embedded in arterial endothelia was retained, which eliminated the localized bicarbonate disequilibrium forming CO2 that then moved into the RBCs. Consequently, RBC pH decreased which, in conjunction with pH-sensitive Bohr/Root Hbs, elevated arterial oxygen tensions and thus enhanced tissue oxygenation. Counter-current arrangement of capillaries (retia) at the eye and later the swim bladder evolved along with the gas gland at the swim bladder. Both arrangements enhanced and magnified CO2 and acid production and, therefore, oxygen secretion to those specialised tissues. The evolution of β-adrenergically stimulated RBC Na(+)/H(+) exchange protected gill O2 uptake during stress and further augmented plasma disequilibrium states for CO2 hydration/dehydration. Finally, RBC organophosphates (e.g. NTP) could be reduced during hypoxia to further increase Hb-O2 affinity without compromising tissue O2 delivery because high-affinity Hbs could still adequately deliver O2 to the tissues via Bohr/Root shifts. We suggest that the evolution of this unique mode of tissue O2 transfer evolved in the Triassic/Jurassic Period, when O2 levels were low, ultimately giving rise to the most extensive adaptive radiation of extant vertebrates, the teleost fishes.

Tissue oxygen demand in regulation of the behavior of the cells in the vasculature

The control of arteriolar diameters in microvasculature has been in the focus of studies on mechanisms matching oxygen demand and supply at the tissue level. Functionally, important vascular elements include EC, VSMC, and RBC. Integration of these different cell types into functional units aimed at matching tissue oxygen supply with tissue oxygen demand is only achieved when all these cells can respond to the signals of tissue oxygen demand. Many vasoactive agents that serve as signals of tissue oxygen demand have their receptors on all these types of cells (VSMC, EC, and RBC) implying that there can be a coordinated regulation of their behavior by the tissue oxygen demand. Such functions of RBC as oxygen carrying by Hb, rheology, and release of vasoactive agents are considered. Several common extra- and intracellular signaling pathways that link tissue oxygen demand with control of VSMC contractility, EC permeability, and RBC functioning are discussed.

Recent advances in ophthalmic molecular imaging

The aim of molecular imaging techniques is the visualization of molecular processes and functional changes in living animals and human patients before morphological changes occur at the cellular and tissue level. Ophthalmic molecular imaging is still in its infancy and has mainly been used in small animals for pre-clinical research. The goal of most of these pre-clinical studies is their translation into ophthalmic molecular imaging techniques in clinical care. We discuss various molecular imaging techniques and their applications in ophthalmology.

Molecular expression and functional activity of efflux and influx transporters in hypoxia induced retinal pigment epithelial cells

A decrease in tissue oxygen levels (aka hypoxia) mediates a number of vascular retinal diseases. Despite introduction of novel therapeutics, treatment of retinal disorders remains challenging, possibly due to complex nature of hypoxia signaling. To date, the differential effect of hypoxia on expression of efflux and influx transporters in retinal cells has not been studied. Therefore, the objective of this study was to delineate molecular and functional expression of membrane transporters in human retinal pigment epithelial (RPE) cells cultured under normoxic and hypoxic conditions. Quantitative real time polymerase chain reaction (qPCR), ELISA and immunoblot analysis were performed to examine the RNA and protein expression levels of transporters. Further, functional activity was evaluated by performing the uptake of various substrates in both normoxic and hypoxic conditions. qPCR analysis showed elevated expression of efflux transporters (P-glycoprotein, multidrug resistant protein 2, breast cancer resistant protein) and influx transporters (folate receptor-α, cationic and neutral amino acid transporter, sodium dependent multivitamin transporter) in a time dependent manner. Immunoblot analysis further confirmed elevated expression of breast cancer resistant protein and sodium dependent multivitamin transporter. A decrease in the uptake of efflux transporter substrates (digoxin, lopinavir and abacavir) and enhanced uptake of influx transporter substrates (arginine, folic acid and biotin) in hypoxia relative to normoxia further confirmed elevated expression of transporters, respectively. This study demonstrates for the first time that hypoxic conditions may alter expression of efflux and influx transporters in RPE cells. These findings suggest that hypoxia may further alter disposition of ophthalmic drugs.

Oxygen regulates the band 3-ankyrin bridge in the human erythrocyte membrane

The oxygenation state of erythrocytes is known to impact several cellular processes. As the only known O2-binding protein in red blood cells, haemoglobin has been implicated in the oxygenation-mediated control of cell pathways and properties. Band 3, an integral membrane protein linked to the spectrin/actin cytoskeleton, preferentially binds deoxygenated haemoglobin at its N-terminus, and has been postulated to participate in the mechanism by which oxygenation controls cellular processes. Because the ankyrin-binding site on band 3 is located near the deoxyHb (deoxygenated haemoglobin)-binding site, we hypothesized that deoxyHb might impact the association between band 3 and the underlying erythrocyte cytoskeleton, a link that is primarily established through band 3-ankyrin bridging. In the present paper we show that deoxygenation of human erythrocytes results in displacement of ankyrin from band 3, leading to release of the spectrin/actin cytoskeleton from the membrane. This weakening of membrane-cytoskeletal interactions during brief periods of deoxygenation could prove beneficial to blood flow, but during episodes of prolonged deoxygenation, such as during sickle cell occlusive crises, could promote unwanted membrane vesiculation.

Sickle hemoglobin disturbs normal coupling among erythrocyte O2 content, glycolysis, and antioxidant capacity

Energy metabolism in RBCs is characterized by O2-responsive variations in flux through the Embden Meyerhof pathway (EMP) or the hexose monophosphate pathway (HMP). Therefore, the generation of ATP, NADH, and 2,3-DPG (EMP) or NADPH (HMP) shift with RBC O2 content because of competition between deoxyhemoglobin and key EMP enzymes for binding to the cytoplasmic domain of the Band 3 membrane protein (cdB3). Enzyme inactivation by cdB3 sequestration in oxygenated RBCs favors HMP flux and NADPH generation (maximizing glutathione-based antioxidant systems). We tested the hypothesis that sickle hemoglobin disrupts cdB3-based regulatory protein complex assembly, creating vulnerability to oxidative stress. In RBCs from patients with sickle cell anemia, we demonstrate in the present study constrained HMP flux, NADPH, and glutathione recycling and reduced resilience to oxidative stress manifested by membrane protein oxidation and membrane fragility. Using a novel, inverted membrane-on-bead model, we illustrate abnormal (O2-dependent) association of sickle hemoglobin to RBC membrane that interferes with sequestration/inactivation of the EMP enzyme GAPDH. This finding was confirmed by immunofluorescent imaging during RBC O2 loading/unloading. Moreover, selective inhibition of inappropriately dispersed GAPDH rescues antioxidant capacity. Such disturbance of cdB3-based linkage between O2 gradients and RBC metabolism suggests a novel mechanism by which hypoxia may influence the sickle cell anemia phenotype.

Interaction of deoxyhemoglobin with the cytoplasmic domain of murine erythrocyte band 3

The partial pressure of oxygen constitutes an important factor in the regulation of human erythrocyte physiology, including control of cell volume, membrane structure, and glucose metabolism. Because band 3 is thought to be involved in all three processes and because binding of hemoglobin (Hb) to the cytoplasmic domain of band 3 (cdb3) is strongly oxygen-dependent, the possibility that the reversible association of deoxyhemoglobin (deoxyHb) with cdb3 might constitute an O(2)-dependent sensor that mediates O(2)-regulated changes in erythrocyte properties arises. While several lines of evidence support this hypothesis, a major opposing argument lies in the fact that the deoxyHb binding sequence on human cdb3 is not conserved. Moreover, no effect of O(2) pressure on Hb-band 3 interactions has ever been demonstrated in another species. To explore whether band 3-Hb interactions might be widely involved in O(2)-dependent regulation of erythrocyte physiology, we undertook characterization of the effect of O(2) on band 3-Hb interactions in the mouse. We report here that murine band 3 binds deoxyHb with significantly greater affinity than oxyHb, despite the lack of significant homology within the deoxyHb binding sequence. We further map the deoxyHb binding site on murine band 3 and show that deletion of the site eliminates deoxyHb binding. Finally, we identify mutations in murine cdb3 that either enhance or eliminate its affinity for murine deoxyHb. These data demonstrate that despite a lack of homology in the sequences of both murine band 3 and murine Hb, a strong oxygen-dependent association of the two proteins has been conserved.

Acute effect of TSH on oxygenation state and volume of erythrocytes from subjects thyroidectomized for differentiated thyroid carcinoma

We previously reported the presence in the membrane erythrocyte of a TSH receptor (TSHR), a G-protein coupled receptor, which responds to TSH with increased cAMP level. Since there is evidence for a role of G protein receptors as oxygen sensor(s) implicated in cell volume regulation, we hypothesized that erythrocyte TSHR, by TSH stimulation, could modify the erythrocyte volume and the oxygenation state of erythrocytes. We determined the effect of TSH on the gas analysis in 35 thyroidectomized patients for stage I differentiated thyroid cancer enrolled for recombinant human thyroid-stimulating hormone (rhTSH) test during chronic treatment with synthetic l-thyroxine. Moreover, we explored the influence of TSH on the shape of erythrocytes. Venous blood-gas analysis before and after TSH were determined with a pH/blood gas electrolyte and 682 CO-Oxymeter. In a subgroup of subjects (n=10), the isolated red blood cells (RBC) were analyzed by flow cytometry for morphological changes. After TSH stimulation, we found a significant decrease in PCO(2) (P<0.001), an increase in pH (P<0.01) and an increase of % O(2)-Hb (P<0.05) and pO(2) (P<0.05). By flow cytometry, the erythrocytes after TSH showed a significant enrichment on the mean number in the selected region R1 corresponding to bigger volumes (P<0.05, n=10). Finally, by contrast phase microscopy, when the cell area was measured, a mean increased volume was observed in erythrocytes after TSH compared to the basal before TSH (P<0.05). In conclusion, our results indicate that acute stimulation of TSH by rhTSH modifies the oxygenation state and volume of erythrocyte.

Effects of chronic acetazolamide administration on gas exchange and acid-base control in pulmonary circulation in exercising horses

REASONS FOR PERFORMING STUDY

Carbonic anhydrase (CA) catalyses the hydration/dehydration reaction of CO(2) and increases the rate of Cl(-) and HCO(3)(-) exchange between the erythrocytes and plasma. Therefore, chronic inhibition of CA has a potential to attenuate CO(2) output and induce greater metabolic and respiratory acidosis in exercising horses.

OBJECTIVES

To determine the effects of Carbonic anhydrase inhibition on CO(2) output and ionic exchange between erythrocytes and plasma and their influence on acid-base balance in the pulmonary circulation (across the lung) in exercising horses with and without CA inhibition.

METHODS

Six horses were exercised to exhaustion on a treadmill without (Con) and with CA inhibition (AczTr). CA inhibition was achieved with administration of acetazolamide (10 mg/kg bwt t.i.d. for 3 days and 30 mg/kg bwt before exercise). Arterial, mixed venous blood and CO(2) output were sampled at rest and during exercise. An integrated physicochemical systems approach was used to describe acid base changes.

RESULTS

AczTr decreased the duration of exercise by 45% (P < 0.0001). During the transition from rest to exercise CO(2) output was lower in AczTr (P < 0.0001). Arterial PCO(2) (P < 0.0001; mean ± s.e. 71 ± 2 mmHg AczTr, 46 ± 2 mmHg Con) was higher, whereas hydrogen ion (P = 0.01; 12.8 ± 0.6 nEq/l AczTr, 15.5 ± 0.6 nEq/l Con) and bicarbonate (P = 0.007; 5.5 ± 0.7 mEq/l AczTr, 10.1 ± 1.3 mEq/l Con) differences across the lung were lower in AczTr compared to Con. No difference was observed in weak electrolytes across the lung. Strong ion difference across the lung was lower in AczTr (P = 0.0003; 4.9 ± 0.8 mEq AczTr, 7.5 ± 1.2 mEq Con), which was affected by strong ion changes across the lung with exception of lactate.

CONCLUSIONS

CO(2) and chloride changes in erythrocytes across the lung seem to be the major contributors to acid-base and ions balance in pulmonary circulation in exercising horses.

The Properties of Red Blood Cells from Patients Heterozygous for HbS and HbC (HbSC Genotype)

Sickle cell disease (SCD) is one of the commonest severe inherited disorders, but specific treatments are lacking and the pathophysiology remains unclear. Affected individuals account for well over 250,000 births yearly, mostly in the Tropics, the USA, and the Caribbean, also in Northern Europe as well. Incidence in the UK amounts to around 12-15,000 individuals and is increasing, with approximately 300 SCD babies born each year as well as with arrival of new immigrants. About two thirds of SCD patients are homozygous HbSS individuals. Patients heterozygous for HbS and HbC (HbSC) constitute about a third of SCD cases, making this the second most common form of SCD, with approximately 80,000 births per year worldwide. Disease in these patients shows differences from that in homozygous HbSS individuals. Their red blood cells (RBCs), containing approximately equal amounts of HbS and HbC, are also likely to show differences in properties which may contribute to disease outcome. Nevertheless, little is known about the behaviour of RBCs from HbSC heterozygotes. This paper reviews what is known about SCD in HbSC individuals and will compare the properties of their RBCs with those from homozygous HbSS patients. Important areas of similarity and potential differences will be emphasised.

Effects of chronic metabolic acidosis on splanchnic protein turnover and oxygen consumption in human beings

BACKGROUND & AIMS

Although metabolic acidosis stimulates protein catabolism, its effects on splanchnic protein turnover and energy expenditure have not been measured in human beings. We investigated the effects of chronic metabolic acidosis (CMA) on splanchnic protein dynamics and oxygen consumption in human beings by using a leucine tracer and mass-balance techniques.

METHODS

Five subjects were studied after 6 days of HCl-, CaCl(2)-, and NH(4)Cl-induced acidosis; 8 subjects served as controls. Blood samples were collected from the radial artery and the hepatic veins. Measurements were performed on plasma and whole-blood samples.

RESULTS

Based on plasma measurements, subjects who had undergone CMA had lower rates of splanchnic proteolysis (-35%) and protein synthesis (-50%; P < .05) than controls, as well as a negative leucine kinetic balance (-6.81 +/- 2.48 micromol/kg/min/1.73 m(2) body surface [BS](-1)), compared with the neutral balance in control plasma samples (0.76 +/- 2.11 micromol/kg/min/1.73; P < .05 between groups). Based on measurements from whole blood, splanchnic proteolysis and protein synthesis did not differ significantly between CMA and control samples, and the net leucine kinetic balance was neutral in both groups (CMA, -0.69 +/- 1.57; controls, -0.74 +/- 3.45 micromol/kg/min/1.73). In CMA whole-blood measurements, splanchnic oxygen consumption (44.8 +/- 4.3 mL/min/1.73 m(2) BS) was slightly lower than in controls (57.5 +/- 8.4 mL/min/1.73 m(2) BS; P = NS). Splanchnic protein synthesis correlated with oxygen consumption (r = 0.82; P < .001).

CONCLUSIONS

CMA reduces splanchnic protein turnover and results in a negative leucine balance--an effect that apparently is offset by the contribution of blood cells to organ leucine (and protein) dynamics. Protein synthesis is a major contributor (about 67%) to energy expenditure in splanchnic organs.

Hypoxia limits antioxidant capacity in red blood cells by altering glycolytic pathway dominance

The erythrocyte membrane is a newly appreciated platform for thiol-based circulatory signaling, and it requires robust free thiol maintenance. We sought to define physiological constraints on erythrocyte antioxidant defense. Hemoglobin (Hb) conformation gates glycolytic flux through the hexose monophosphate pathway (HMP), the sole source of nicotinamide adenine dinucleotide phosphate (NADPH) in erythrocytes. We hypothesized elevated intraerythrocytic deoxyHb would limit resilience to oxidative stress. Human erythrocytes were subjected to controlled oxidant (superoxide) loading following independent manipulation of oxygen tension, Hb conformation, and glycolytic pathway dominance. Sufficiency of antioxidant defense was determined by serial quantification of GSH, NADPH, NADH redox couples. Hypoxic erythrocytes demonstrated greater loss of reduction potential [Delta GSH E(hc) (mV): 123.4+/-9.7 vs. 57.2+/-11.1] and reduced membrane thiol (47.7+/-5.7 vs. 20.1+/-4.3%) (hypoxia vs. normoxia, respectively; P<0.01), a finding mimicked in normoxic erythrocytes after HMP blockade. Rebalancing HMP flux during hypoxia restored resilience to oxidative stress at all stages of the system. Cell-free studies assured oxidative loading was not altered by oxygen tension, heme ligation, or the inhibitors employed. These data indicate that Hb conformation controls coupled glucose and thiol metabolism in erythrocytes, and implicate hypoxemia in the pathobiology of erythrocyte-based vascular signaling.

Regulation of nitrite transport in red blood cells by hemoglobin oxygen fractional saturation

Allosteric regulation of nitrite reduction by deoxyhemoglobin has been proposed to mediate nitric oxide (NO) formation during hypoxia. Nitrite is predominantly an anion at physiological pH, raising questions about the mechanism by which it enters the red blood cell (RBC) and whether this is regulated and coupled to deoxyhemoglobin-mediated reduction. We tested the hypothesis that nitrite transport by RBCs is regulated by fractional saturation. Using human RBCs, nitrite consumption was faster at lower fractional saturations, consistent with faster reactions with deoxyheme. A membrane-based regulation was suggested by slower nitrite consumption with intact versus lysed RBCs. Interestingly, upon nitrite addition, intracellular nitrite concentrations attained a steady state that, despite increased rates of consumption, did not change with decreasing oxygen tensions, suggesting a deoxygenation-sensitive step that either increases nitrite import or decreases the rate of nitrite export. A role for anion exchanger (AE)-1 in the control of nitrite export was suggested by increased intracellular nitrite concentrations in RBCs treated with DIDS. Moreover, deoxygenation decreased steady-state levels of intracellular nitrite in AE-1-inhibited RBCs. Based on these data, we propose a model in which deoxyhemoglobin binding to AE-1 inhibits nitrite export under low oxygen tensions allowing for the coupling between deoxygenation and nitrite reduction to NO along the arterial-to-venous gradient.

Oxygen-dependent ion transport in erythrocytes

The present contribution reviews current knowledge of apparently oxygen-dependent ion transport in erythrocytes and presents modern hypotheses on their regulatory mechanisms and physiological roles. In addition to molecular oxygen as such, reactive oxygen species, nitric oxide, carbon monoxide, regional variations of cellular ATP and hydrogen sulphide may play a role in the regulation of transport, provided that they are affected by oxygen tension. It appears that the transporter molecules themselves do not have direct oxygen sensors. Thus, the oxygen level must be sensed elsewhere, and the effect transduced to the transporter. The possible pathways involved in the regulation of transport, including haemoglobin as a sensor, and phosphorylation/dephosphorylation reactions both in the transporter and its upstream effectors, are discussed.

Quantitative evaluation of respiration induced metabolic oscillations in erythrocytes

The changes in the partial pressures of oxygen and carbon dioxide (P(O(2)) and P(CO(2))) during blood circulation alter erythrocyte metabolism, hereby causing flux changes between oxygenated and deoxygenated blood. In the study we have modeled this effect by extending the comprehensive kinetic model by Mulquiney and Kuchel [P.J. Mulquiney, and P.W. Kuchel. Model of 2,3-bisphosphoglycerate metabolism in the human erythrocyte based on detailed enzyme kinetic equations: equations and parameter refinement, Biochem. J. 1999, 342, 581-596.] with a kinetic model of hemoglobin oxy-/deoxygenation transition based on an oxygen dissociation model developed by Dash and Bassingthwaighte [R. Dash, and J. Bassingthwaighte. Blood HbO(2) and HbCO(2) dissociation curves at varied O(2), CO(2), pH, 2,3-DPG and temperature levels, Ann. Biomed. Eng., 2004, 32(12), 1676-1693.]. The system has been studied during transitions from the arterial to the venous phases by simply forcing P(O(2)) and P(CO(2)) to follow the physiological values of venous and arterial blood. The investigations show that the system passively follows a limit cycle driven by the forced oscillations of P(O(2)) and is thus inadequately described solely by steady state consideration. The metabolic system exhibits a broad distribution of time scales. Relaxations of modes with hemoglobin and Mg(2+) binding reactions are very fast, while modes involving glycolytic, membrane transport and 2,3-BPG shunt reactions are much slower. Incomplete slow mode relaxations during the 60 s period of the forced transitions cause significant overshoots of important fluxes and metabolite concentrations - notably ATP, 2,3-BPG, and Mg(2+). The overshoot phenomenon arises in consequence of a periodical forcing and is likely to be widespread in nature - warranting a special consideration for relevant systems.

Environmental stress, erythrocyte dysfunctions, inflammation, and the metabolic syndrome: adaptations to CO2 increases?

In the Western world, the prevalence of the metabolic syndrome is increasing exponentially. Chronic subacute inflammation characterizes the syndrome, suggesting that inflammation might be a common denominator that links obesity to its pathologic sequelae. Potential mechanisms for the activation of inflammation include current air pollution inhalation and/or excess food intake. Both of these environmental factors have, in fact, been shown to promote oxidation followed by the release of proinflammatory cytokines. Potential sources of systemic inflammation include oxidized erythrocytes. Increased exogenous or endogenous CO2 deoxygenates hemoglobin, thereby increasing the fraction of hemoglobin reacting with nitrite to form methemoglobin together with release of superoxide and nitric oxide. These may form peroxynitrite, which may oxidize erythrocytes. Macrophages may then recognize and engulf these cells, thereby releasing proinflammatory cytokines. Therefore, studies should focus on the red blood cell and its proteins to finely target and appropriately treat a world pandemic ominously related to CO2 increases.

Further characterization of cation channels present in the chicken red blood cell membrane

In this paper, we provide an update on cation channels in nucleated chicken erythrocytes. Patch-clamp techniques were used to further characterize the two different types of cation channels present in the membrane of chicken red blood. In the whole-cell mode, with Ringer in the bath and internal K+ saline in the pipette solution, the membrane conductance was generated by cationic currents, since the reversal potential was shifted toward cations equilibrium when the impermeant cation NMDG was substituted to small cations. The membrane conductance could be increased by application of mechanical deformation or by the addition of agonists of the cAMP-dependent pathway. At the unitary level, two different types of cationic channels were revealed and could account for the cationic conductance observed in whole-cell configuration. One of them belongs to the family of stretch-activated cationic channel showing changes in activity under conditions of membrane deformation, whereas the second one belongs to the family of the cAMP activated cationic channels. These two channels could be distinguished according to their unitary conductances and drug sensitivities. The stretch-activated channel was sensitive to Gd(3+) and the cAMP-dependent channel was sensitive to flufenamic acid. Possible role of these channels in cell volume regulation process is discussed.

Lacking deoxygenation-linked interaction between cytoplasmic domain of band 3 and HbF from fetal red blood cells

AIM

Several of the red blood cell's metabolic and membrane functions display dependence on haemoglobin oxygenation. In adult human red cells, the increased glycolytic rate at low O2 tension results from binding of deoxygenated HbA at negatively charged, N-terminal, cytoplasmic domain of the membrane protein band 3, which liberates glycolytic enzymes from this site. This study aims to investigate the role of fetal HbF (that has lower anion-binding capacity than HbA) in fetal red cells (that are subjected to low O2 tensions), and to elucidate possible linkage (e.g. via the major red cell membrane organising centre, band 3) between the individual oxygenation-linked reactions encountered in red cells.

METHODS

The interaction between band 3 and Hb is analysed in terms of the effects, measured under different conditions, of a 10-mer peptide that corresponds to the N-terminus of human band 3 protein, on the oxygenation reaction of HbF and HbA, isolated from umbilical chord red cells.

RESULTS

Contrasting with the unequivocal interaction of the peptide with HbA that with fetal HbF is weak, and annihilated in the presence of autochthonous red cell O2 affinity modulators (chloride and organic phosphates).

CONCLUSION

The data indicate that HbF does not function as a transducer mediating O2 dependence of glycolysis in fetal red cells, in accordance with the different O2 and metabolic profiles compared to those in HbA-bearing adult red cells. In conjunction with the previously discovered O2 dependence of K+ transport in HbF-rich fetal cells, they moreover argue against linkage between different, physiologically relevant, O2-dependent red cell functions.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

Characterization of the deoxyhemoglobin binding site on human erythrocyte band 3: implications for O2 regulation of erythrocyte properties

Band 3, the major protein of the human erythrocyte membrane, associates with multiple metabolic, ion transport, and structural proteins. Functional studies demonstrate that the oxygenation state of the erythrocyte regulates cellular properties performed by these and/or related proteins. Because deoxyhemoglobin, but not oxyhemoglobin, binds band 3 reversibly with high affinity, these observations raise the hypothesis that hemoglobin might regulate erythrocyte properties through its reversible, oxygenation-dependent association with band 3. To explore this hypothesis, we have characterized the binding site of deoxyHb on human erythrocyte band 3. We report that (1) deoxyHb binds to residues 12-23 of band 3; (2) mutation of residues on either side of this sequence greatly enhances affinity of deoxyHb for band 3, suggesting that evolution of a higher affinity interaction would have been possible had it been beneficial for survival; (3) Hb does not bind to 2 other sequences in band 3 despite their high sequence homology to residues 12-23, and (4) the Hb binding site on band 3 lies proximal to binding sites for glycolytic enzymes, band 4.1 and ankyrin, suggesting possible mechanisms through which multifarious erythrocyte properties might be regulated by the oxygenation state of the cell.

Oxygen and reactive oxygen species in articular cartilage: modulators of ionic homeostasis

Articular cartilage is an avascular tissue dependent on diffusion mainly from synovial fluid to service its metabolic requirements. Levels of oxygen (O(2)) in the tissue are low, with estimates of between 1 and 6%. Metabolism is largely, if not entirely, glycolytic, with little capacity for oxidative phosphorylation. Notwithstanding, the tissue requires O(2) and consumes it, albeit at low rates. Changes in O(2) tension also have profound effects on chondrocytes affecting phenotype, gene expression, and morphology, as well as response to, and production of, cytokines. Although chondrocytes can survive prolonged anoxia, low O(2) levels have significant metabolic effects, inhibiting glycolysis (the negative Pasteur effect), and also notably matrix production. Why this tissue should respond so markedly to reduction in O(2) tension remains a paradox. Ion homeostasis in articular chondrocytes is also markedly affected by the extracellular matrix in which the cells reside. Recent work has shown that ion homeostasis also responds to changes in O(2) tension, in such a way as to produce significant effects on cell function. For this purpose, O(2) probably acts via alteration in levels of reactive oxygen species. We discuss the possibility that O(2) consumption by this tissue is required to maintain levels of ROS, which are then used physiologically as an intracellular signalling device. This postulate may go some way towards explaining why the tissue is dependent on O(2) and why its removal has such marked effects. Understanding the role of oxygen has implications for disease states in which O(2) or ROS levels may be perturbed.

Oxygen-sensitive regulatory volume increase and Na transport in red blood cells from the cane toad,Bufo marinus

The red blood cells (RBCs) of cane toad, Bufo marinus, are only partially saturated with oxygen in most of the circulation due to cardiac shunts that cause desaturation of arterial blood. The present study examines the oxygen dependency of RBC ouabain-insensitive unidirectional Na transport,using 22Na, in control cells and in cells exposed to hyperosmotic shrinkage or the β-adrenergic agonist isoproterenol. Deoxygenation per se induced a slow, but significant Na influx, which was paralleled by a slow increase in RBC volume. Hyperosmotic shrinkage by a calculated 25% activated a robust Na influx that in the first 30 min had a strong PO2 dependency with maximal activation at low PO2 values and a P50of ∼5.5 kPa. This activation was completely abolished by the Na/H exchanger (NHE) inhibitor EIPA (10–4 mol l-1). Hyperosmotic shrinkage is particularly interesting in B. marinus as it withstands considerable elevation in extracellular osmolarity following dehydration. Parallel studies showed that deoxygenated B. marinusRBCs had a much faster regulatory volume increase (RVI) response than air-equilibrated RBCs, reflecting the difference in magnitude of Na influxes at the two PO2 values. The extent of RVI(∼60%) after 90 min, however, was similar under the two conditions,reflecting a more prolonged elevation of the shrinkage-induced Na influx in air-equilibrated RBCs. There were no significant differences in the ability to perform RVI between whole blood cells at a PCO2of 1 and 3 kPa or washed RBCs, and 10–4 mol l-1amiloride reduced the RVI under all conditions, whereas 10–5mol l-1 bumetanide had no effect. Isoproterenol(10–5 mol l-1) induced a significant and prolonged increase in an EIPA-sensitive and bumetanide-insensitive Na influx at low PO2 under iso-osmotic conditions, whilst there was no stimulation by isoproterenol for up to 45 min in air-equilibrated RBCs. The prolonged β-adrenergic activation of the Na influx at low PO2 is distinctly different from the rapid and transient stimulation in teleost RBCs, suggesting significant differences in the signal transduction pathways leading to transporter activation between vertebrate groups.

Effects of chronic acetazolamide administration on fluid flux from the pulmonary vasculature at rest and during exercise in horses

REASONS FOR PERFORMING STUDY

Horses develop high pulmonary pressures during exercise, which force fluid out of pulmonary capillaries. Specific airway diseases in horses, especially those associated with hypoxaemia, hypercapnoea and acidosis may influence pulmonary haemodynamics and pulmonary interstitial fluid equilibrium.

OBJECTIVES

This study was designed to determine fluid flux (J(V-A) l/min) across the lung in exercising horses treated chronically with acetazolamide.

METHODS

Six horses were exercised on a treadmill until fatigue without (Con) and with chronic carbonic anhydrase (CA) inhibition (AczTr) and associated hypercapnoea and acidosis. Carbonic anhydrase inhibition was achieved with administration of acetazolamide (Acz). Arterial and mixed venous blood were sampled, and VCO2 and VO2 measured. Blood volume changes across the lung (deltaBV%) were calculated from changes in plasma protein, haemoglobin and packed cell volume (PCV). Cardiac output (Q) was calculated using Fick principle. J(V-A) across the alveolar-capillary barrier was then quantified based on Q and deltaBV. Variables were analysed using 2-way repeated-measures ANOVA (P<0.05). A significant F ratio was further analysed using Tukey post hoc analysis.

RESULTS

Treatment had a significant effect on J(V-A) (P = 0.002). At rest there was no J(V-A) in Con (0.63 +/- 0.6 l/min) and AczTr (0.84 +/- 0.3 l/min). During exercise Con fluid moved from the pulmonary circulation into the pulmonary interstitium (mean +/- s.e. J(V-A) 9.4 +/- 2.4 l/min). This was different from AczTr (mean +/- s.e. J(V-A) 1.8 +/- 1.9 l/min), where no transvascular fluxes from pulmonary circulation were present during exercise (P = 0.008).

CONCLUSIONS

Chronic Acz treatment with associated hypercapnoea and acidosis affects J(V-A) in lungs of exercising horses. Lung fluid dynamics adapted to hypercapnoea and acidosis with reduction of fluid flow from the pulmonary circulation.

POTENTIAL RELEVANCE

The current data provide comprehensive evidence of in vivo fluid homeostasis in lungs of exercising horses without and with CA inhibition.

In vitro effects of oxygen on physico-chemical properties of horse erythrocyte membrane

Whether direct exposure to different concentrations (0%, 13%, 100%) of oxygen may affect horse erythrocyte membrane fluidity (EMF) and fatty acid (FA) composition was studied during 1 (T60) and 2h (T120) exposure. EMF was investigated at the head group level and hydrophobic core thanks to phosphorus nucleus 31 ((31)P) nuclear magnetic resonance ((31)P NMR) and electronic paramagnetic resonance (EPR) using two spin probes: 5-nitroxydestearic acid and 16-doxylstearic acid. Lipid structure of the membranes was studied by gas liquid chromatography. 4-Hydroxy-2E-nonenal was also analyzed as a marker of lipid peroxidation. It increased at T120 13% and 100% oxygen whereas there were no significant changes in membrane dynamic or structure. Correlation was demonstrated between EMF and partial pressure of oxygen in the blood ( [Formula: see text] ). In vitro high rate of oxygenation was efficient to induce lipid peroxidation but did not change membrane dynamics. This may be due to a low free radical production in vitro or to the high red blood cells antioxidant properties.

The effect of O2 tension on pH homeostasis in equine articular chondrocytes

OBJECTIVE

To determine the effects of varying O(2) on pH homeostasis, based on the hypothesis that the function of articular chondrocytes is best understood at realistic O(2) tensions.

METHODS

Cartilage from equine metacarpophalangeal/tarsophalangeal joints was digested with collagenase to isolate chondrocytes, and then loaded with the pH-sensitive fluorophore 2',7'-bis-2-(carboxyethyl)-5(6)-carboxylfluorescein. The radioisotope(22)Na(+) was used to determine the kinetics of Na(+)/H(+) exchange (NHE) and the activity of the Na(+)/K(+) pump, and ATP levels were assessed with luciferin assays. Levels of reactive oxygen species (ROS) were determined using 2',7'-dichlorofluorescein diacetate.

RESULTS

The pH homeostasis was unaffected when comparing tissue maintained at 20% O(2) (the level in water-saturated air at 37 degrees C) with that at 5% O(2) (which approximates the normal level in healthy cartilage); however, an O(2) tension of <5% caused a fall in intracellular pH (pH(i)) and slowed pH(i) recovery following acidification, an effect mediated via inhibition of NHE activity (likely through acid extrusion by NHE isoform 1). The Na(+)/K(+) pump activity and intracellular ATP concentration were unaffected by hypoxia, but the levels of ROS were reduced. Hypoxic inhibition of NHE activity and the reduction in ROS levels were reversed by treatment with H(2)O(2), Co(2+), or antimycin A. Treatment with calyculin A also prevented hypoxic inhibition of NHE activity.

CONCLUSION

The ability of articular chondrocytes to carry out pH homeostasis is compromised when O(2) tensions fall below those normally experienced, via inhibition of NHE. The putative signal is a reduction in levels of ROS derived from mitochondria, acting via altered protein phosphorylation. This effect is relevant to both physiologic and pathologic states of lowered O(2), such as in chronic inflammation.

Oxygen-dependent diseases in the retina: Role of hypoxia-inducible factors

The function of the retina is sensitive to oxygen tension. Any change in the perfusion pressure of the eye affects the retina although the eye is able to autoregulate its hemodynamics. Systemic hypoxemia (lung or heart disease) or a vascular disease in the retina can cause retinal hypoxia. All the hypoxia-dependent events in cells appear to share a common denominator: hypoxia-inducible factor (HIF), which is a heterodimeric transcription factor, a protein. HIF comprises a labile alpha subunit (1-3), which is regulated, and a stable beta subunit, which is constitutively expressed. Both are helix-loop-helix factors and belong to the PAS-domain family of transcription factors. Oxygen plays the key role in stabilizing HIF-1alpha and its function. When the oxygen tension is normal, HIF-1alpha is rapidly oxidized by hydroxylase enzymes, but when cells become hypoxic, HIF-1alpha escapes the degradation and starts to accumulate, triggering the activation of a large number of genes, like vascular endothelial growth factor (VEGF) and erythropoietin. HIF-1alpha has been shown to have, either clinically or experimentally, a mediating or contributing role in several oxygen-dependent retinal diseases such as von Hippel-Lindau, proliferative diabetic retinopathy, retinopathy of prematurity and glaucoma. In retinitis pigmentosa and high-altitude retinopathy, however, the evidence is still indirect. There are three different strategies available for treating retinal diseases, which have all shown promising results: retinal cell transplantation or replacement, gene replacement, and pharmacological intervention. Specifically, recent results show that the HIF pathway can be used as a therapeutic target, although there is still a long way to go from bench to clinic. HIF can be stabilized by inhibiting prolyl hydroxylase or by blocking the VHL:HIF-alpha complex if angiogenesis is the goal, as in retinitis pigmentosa. On the other hand, the downregulation of HIF has a pivotal role if we are to inhibit neovascularization, as in proliferative diabetic retinopathy. To date, several small-molecule inhibitors of HIF have been developed and are entering clinical trials. HIF is a remarkable example of a single transcription factor that can be regarded as a "master switch" regulating all the oxygen-dependent retinal diseases.

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.