The effect of buffer composition and deoxygenation on the concentration of ionized magnesium inside human red blood cells

Metabolite and protein shifts in mature erythrocyte under hypoxia

As the only cell type responsible for oxygen delivery, erythrocytes play a crucial role in supplying oxygen to hypoxic tissues, ensuring their normal functions. Hypoxia commonly occurs under physiological or pathological conditions, and understanding how erythrocytes adapt to hypoxia is fundamental for exploring the mechanisms of hypoxic diseases. Additionally, investigating acute and chronic mountain sickness caused by plateaus, which are naturally hypoxic environments, will aid in the study of hypoxic diseases. In recent years, increasingly developed proteomics and metabolomics technologies have become powerful tools for studying mature enucleated erythrocytes, which has significantly contributed to clarifying how hypoxia affects erythrocytes. The aim of this article is to summarize the composition of the cytoskeleton and cytoplasmic proteins of hypoxia-altered erythrocytes and explore the impact of hypoxia on their essential functions. Furthermore, we discuss the role of microRNAs in the adaptation of erythrocytes to hypoxia, providing new perspectives on hypoxia-related diseases.

Emerging drug targets for sickle cell disease: shedding light on new knowledge and advances at the molecular level

INTRODUCTION

In sickle cell disease (SCD), a single amino acid substitution at β6 of the hemoglobin (Hb) chain replaces glutamate with valine, forming HbS instead of the normal adult HbA. Loss of a negative charge, and the conformational change in deoxygenated HbS molecules, enables formation of HbS polymers. These not only distort red cell morphology but also have other profound effects so that this simple etiology belies a complex pathogenesis with multiple complications. Although SCD represents a common severe inherited disorder with life-long consequences, approved treatments remain inadequate. Hydroxyurea is currently the most effective, with a handful of newer treatments, but there remains a real need for novel, efficacious therapies.

AREAS COVERED

This review summarizes important early events in pathogenesis to highlight key targets for novel treatments.

EXPERT OPINION

A thorough understanding of early events in pathogenesis closely associated with the presence of HbS is the logical starting point for identification of new targets rather than concentrating on more downstream effects. We discuss ways of reducing HbS levels, reducing the impact of HbS polymers, and of membrane events perturbing cell function, and suggest using the unique permeability of sickle cells to target drugs specifically into those more severely compromised.

From Experiments to Simulation: Shear-Induced Responses of Red Blood Cells to Different Oxygen Saturation Levels

Red blood cells (RBC) carry and deliver oxygen (O₂) to peripheral tissues through different microcirculatory regions where they are exposed to various levels of shear stress (SS). O₂ affinity of hemoglobin (Hb) decreases as the blood enters the microcirculation. This phenomenon determines Hb interactions with RBC membrane proteins that can further regulate the structure of cytoskeleton and affect the mechanical properties of cells. The goal of this study is to evaluate shear-induced RBC deformability and simulate RBC dynamics in blood flow under oxygenated and deoxygenated conditions. Venous blood samples from healthy donors were oxygenated with ambient air or deoxygenated with 100% nitrogen gas for 10 min and immediately applied into an ektacytometer (LORRCA). RBC deformability was measured before and after the application of continuous 5 Pa SS for 300 s by LORRCA and recorded as elongation index (EI) values. A computational model was generated for the simulation of blood flow in a real carotid artery section. EI distribution throughout the artery and its relationships with velocity, pressure, wall SS and viscosity were determined by computational tools. RBC deformability significantly increased in deoxygenation compared to oxygenated state both before and after 5 Pa SS implementation (p < 0.0001). However, EI values after continuous SS were not significant at higher SS levels (>5.15 Pa) in deoxygenated condition. Simulation results revealed that the velocity gradient dominates the generation of SS and the shear thinning effect of blood has a minor effect on it. Distribution of EI was calculated during oxygenation/deoxygenation which is 5-10 times higher around the vessel wall compared to the center of the lumen for sections of the pulsatile flow profile. The extent of RBC deformability increases as RBCs approach to the vessel wall in a real 3D artery model and this increment is higher for deoxygenated condition compared to the oxygenated state. Hypoxia significantly increases shear-induced RBC deformability. RBCs could regulate their own mechanical properties in blood flow by increasing their deformability in hypoxic conditions. Computational tools can be applied for defining hypoxia-mediated RBC deformability changes to monitor blood flow in hypoxic tissues.

Oxidative stress and phosphatidylserine exposure in red cells from patients with sickle cell anaemia

Phosphatidylserine (PS) exposure increases as red cells age, and is an important signal for the removal of senescent cells from the circulation. PS exposure is elevated in red cells from sickle cell anaemia (SCA) patients and is thought to enhance haemolysis and vaso-occlusion. Although precise conditions leading to its externalisation are unclear, high intracellular Ca2+ has been implicated. Red cells from SCA patients are also exposed to an increased oxidative challenge, and we postulated that this stimulates PS exposure, through increased Ca2+ levels. We tested four different ways of generating oxidative stress: hypoxanthine and xanthine oxidase, phenazine methosulphate, nitrite and tert-butyl hydroperoxide, together with thiol modification with N-ethylmaleimide (NEM), dithiothreitol and hypochlorous acid (HOCl), in red cells permeabilised to Ca2+ using bromo-A23187. Unexpectedly, our findings showed that the four oxidants significantly reduced Ca2+ -induced PS exposure (by 40-60%) with no appreciable effect on Ca2+ affinity. By contrast, NEM markedly increased PS exposure (by about 400%) and slightly but significantly increased the affinity for Ca2+ . Dithiothreitol modestly reduced PS exposure (by 25%) and HOCl had no effect. These findings emphasise the importance of thiol modification for PS exposure in sickle cells but suggest that increased oxidant stress alone is not important.

Inhibitors of second messenger pathways and Ca(2+)-induced exposure of phosphatidylserine in red blood cells of patients with sickle cell disease

The present work investigates the contribution of various second messenger systems to Ca²⁺-induced phosphatidylserine (PS) exposure in red blood cells (RBCs) from sickle cell disease (SCD) patients. The Ca²⁺ dependence of PS exposure was confirmed using the Ca²⁺ ionophore bromo-A23187 to clamp intracellular Ca²⁺ over 4 orders of magnitude in high or low potassium-containing (HK or LK) saline. The percentage of RBCs showing PS exposure was significantly increased in LK over HK saline. This effect was reduced by the Gardos channel inhibitors, clotrimazole and charybdotoxin. Nevertheless, although Ca²⁺ loading in the presence of an outwardly directed electrochemical gradient for K⁺ stimulated PS exposure, substantial exposure still occurred in HK saline. Under the conditions used inhibitors of other second messenger systems (ABT491, quinacrine, acetylsalicylic acid, 3,4-dichloroisocoumarin, GW4869 and zVAD-fmk) did not inhibit the relationship between [Ca²⁺] and PS exposure. Inhibitors of phospholipase A2, cyclooxygenase, platelet-activating factor, sphingomyelinase and caspases, therefore, were without effect on Ca²⁺-induced PS exposure in RBCs, incubated in either HK or LK saline.

Deoxygenation-induced and Ca(2+) dependent phosphatidylserine externalisation in red blood cells from normal individuals and sickle cell patients

Phosphatidylserine (PS) is usually confined to the inner leaflet of the red blood cell (RBC) membrane. It may become externalised in various conditions, however, notably in RBCs from patients with sickle cell disease (SCD) where exposed PS may contribute to anaemic and ischaemic complications. PS externalisation requires both inhibition of the aminophospholipid translocase (or flippase) and activation of the scramblase. Both may follow from elevation of intracellular Ca(2+). Flippase inhibition occurs at low [Ca(2+)](i), about 1μM, but [Ca(2+)](i) required for scrambling is reported to be much higher (around 100μM). In this work, FITC-labelled lactadherin and FACS were used to measure externalised PS, with [Ca(2+)](i) altered using bromo-A23187 and EGTA/Ca(2+) mixtures. Two components of Ca(2+)-induced scrambling were apparent, of high (EC(50) 1.8±0.3μM) and low (306±123μM) affinity, in RBCs from normal individuals and the commonest SCD genotypes, HbSS and HbSC. The high affinity component was lost in the presence of unphysiologically high [Mg(2+)] but was unaffected by high K(+) (90mM) or vanadate (1mM). The high affinity component accounted for PS scrambling in ≥2/3rd RBCs. It is likely to be most significant in vivo and may be involved in the pathophysiology of SCD or other conditions involving eryptosis.

Role of calcium in phosphatidylserine externalisation in red blood cells from sickle cell patients

Phosphatidylserine exposure occurs in red blood cells (RBCs) from sickle cell disease (SCD) patients and is increased by deoxygenation. The mechanisms responsible remain unclear. RBCs from SCD patients also have elevated cation permeability, and, in particular, a deoxygenation-induced cation conductance which mediates Ca²⁺ entry, providing an obvious link with phosphatidylserine exposure. The role of Ca²⁺ was investigated using FITC-labelled annexin. Results confirmed high phosphatidylserine exposure in RBCs from SCD patients increasing upon deoxygenation. When deoxygenated, phosphatidylserine exposure was further elevated as extracellular [Ca²⁺] was increased. This effect was inhibited by dipyridamole, intracellular Ca²⁺ chelation, and Gardos channel inhibition. Phosphatidylserine exposure was reduced in high K⁺ saline. Ca²⁺ levels required to elicit phosphatidylserine exposure were in the low micromolar range. Findings are consistent with Ca²⁺ entry through the deoxygenation-induced pathway (P_sickle), activating the Gardos channel. [Ca²⁺] required for phosphatidylserine scrambling are in the range achievable in vivo.

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.

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

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

Chemical and pathological oxidative influences on band 3 protein anion-exchanger

BACKGROUND/AIMS

The erythrocyte is a cell exposed to a high level of oxygen pressure and to oxidative chemical agents. This stress involves SH-groups oxidation, cell shrinkage by activation of K-Cl co-transport (KCC) and elevation of the band 3 tyrosine phosphorylation level. The aim of our study was to test whether oxidative stress could influence band 3-mediated anion transport in human red blood cells.

METHODS

To evaluate this hypothesis, normal and pathological (glucose 6 phosphate dehydrogenase (G6PDH) defficient) erythrocytes were treated with known sulphydryl-blocking or thiol-oxidizing agents, such as N-ethylmaleimide (NEM), azodicarboxylic acid bis[dimethylamide] (diamide), orthovanadate, Mg2+ and tested for sulphate (SO4-) uptake, K+ efflux, G6PDH activity and glutathione (GSH) concentration.

RESULTS

In normal red blood cells, the rate constants of SO4- uptake decreased by about 28 % when cells were incubated with NEM, diamide and orthovanadate. In G6PDH-deficient red blood cells, in which oxidative stress occurs naturally, the rate constant of sulphate uptake was decreased by about 40% that of normal red cells. Addition of oxidizing and phosphatase inhibitor agents to pathological erythrocytes further decreased anion transport. In contrast, G6PDH activity was increased under oxidative stress in normal as well as in pathological cells and was lower in the presence of exogenous Mg2+ in parallel to a significant increase in sulphate transport. In both cells, the oxidizing agents increased K+ efflux with depletion of GSH.

CONCLUSION

The data are discussed in light of the possible opposite effects exerted by oxidative agents and Mg2+ on KCC and on the protein tyrosine kinase (PTK)-protein tyrosine phosphatase (PTP) equilibrium. The decreased sulphate uptake observed in the experimental and pathological conditions could be due to band 3 SH-groups oxidation or to oxidative stress-induced K-Cl symport-mediated cell shrinkage with concomitant band 3 tyrosine phosphorylation.

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

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

Stimulation of Na+/Mg2+ antiport in rat erythrocytes by intracellular Cl-

Mg(2+) efflux from rat erythrocytes was measured in NaCl, NaNO(3), NaSCN and Na gluconate medium. Substitution of extracellular and intracellular Cl(-) with the permeant anions NO(3)(-) and SCN(-) reduced Mg(2+) efflux via Na(+)/Mg(2+) antiport. After substitution of extracellular Cl(-) with the non-permeant anion gluconate, Mg(2+) efflux was not significantly reduced. In Na gluconate medium, an influence of the changed membrane potential and intracellular pH on Mg(2+) efflux could be excluded. The results indicate the existence of Cl(-)-independent Na(+)/Mg(2+) antiport and of Na(+)/Mg(2+) antiport stimulated by intracellular Cl(-). Intracellular Cl(-), as determined by means of (36)Cl(-), was found to stimulate Na(+)/Mg(2+) antiport through a cooperative effect according to a sigmoidal kinetics. The Hill coefficient for intracellular Cl(-) amounted to 1.4-1.8, indicating that two intracellular Cl(-) may be simultaneously active. With respect to specificity, Cl(-) was most effective, followed by Br(-), J(-), and F(-). Stimulation of Na(+)/Mg(2+) antiport by intracellular Cl(-) together with intracellular Mg(2+) may play a role during deoxygenation of erythrocytes and in essential hypertension.

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

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

Simultaneous determination of low free Mg2+ and pH in human sickle cells using 31P NMR spectroscopy

The concentrations of free magnesium, [Mg(2+)](free), [H(+)], and [ATP] are important in the dehydration of red blood cells from patients with sickle cell anemia, but they are not easily measured. Consequently, we have developed a rapid, noninvasive NMR spectroscopic method using the phosphorus chemical shifts of ATP and 2,3-diphosphoglycerate (DPG) to determine [Mg(2+)](free) and pH(i) simultaneously in fully oxygenated whole blood. The method employs theoretical equations expressing the observed chemical shift as a function of pH, K(+), and [Mg(2+)](free), over a pH range of 5.75-8.5 and [Mg(2+)](free) range 0-5 mm. The equations were adjusted to allow for the binding of hemoglobin to ATP and DPG, which required knowledge of the intracellular concentrations of ATP, DPG, K(+), and hemoglobin. Normal oxygenated whole blood (n = 33) had a pH(i) of 7.20 +/- 0.02, a [Mg(2+)](free) of 0.41 +/- 0.03 mm, and [DPG] of 7.69 +/- 0.47 mm. Under the same conditions, whole sickle blood (n = 9) had normal [ATP] but significantly lower pH(i) (7.10 +/- 0.03) and [Mg(2+)](free) (0.32 +/- 0.05 mm) than normal red cells, whereas [DPG] (10.8 +/- 1.2 mm) was significantly higher. Because total magnesium was normal in sickle cells, the lower [Mg(2+)](free) could be attributed to increased [DPG] and therefore greater magnesium binding capacity of sickle cells.

Comparison of the potentiometric, (31)P NMR, and zero-point titration methods of determining ionized magnesium in erythrocytes

A simple and rapid method of determining ionized magnesium in erythrocytes using a potentiometric clinical analyzer, Microlyte 6 (Kone, Finland), was investigated. The erythrocyte cell membranes were destroyed using ultrasound. The results were compared with those obtained with the (31)P nuclear magnetic resonance spectroscopy method and the zero-point titration method using atomic absorption spectrometry. The results obtained from potentiometry and from the other methods did not differ significantly (Student t test, alpha = 0.01). Total magnesium concentration was determined using atomic absorption spectrometry.

Altered cellular magnesium responsiveness to hyperglycemia in hypertensive subjects

Previous studies by our group have identified ionic aspects of insulin resistance in hypertension, in which cellular responses to insulin were influenced by the basal intracellular ionic environment-the lower the cytosolic free magnesium (Mg(i)), the less Mg(i) increased following insulin stimulation. To investigate whether this ionic insulin resistance represents a more general abnormality of cellular responsiveness in hypertension, we studied Mg(i) responses to nonhormonal signals such as hyperglycemia (15 mmol/L) and used (31)P-nuclear magnetic resonance (NMR) spectroscopy to measure Mg(i) in erythrocytes from normal (NL, n=14) and hypertensive (HTN, n=12) subjects before and 30, 60, 120, and 180 minutes after in vitro glucose incubations. Basal Mg(i) levels were significantly lower in HTN subjects than in NL subjects (169+/-10 versus 205+/-8 micromol.L(-1), P<0.01). In NL cells, hyperglycemia significantly lowered Mg(i), from 205+/-8 micromol.L(-1) (basal, T=0) to 181+/-8, 162+/-6, 152+/-7, and 175+/-9 micromol.L(-1) (T=30, 60, 120, and 180, respectively; P<0.005 versus T=0 at all times). In HTN cells, maximal Mg(i) responses to hyperglycemia were blunted, from 169+/-10 micromol.L(-1) (basal, T=0) to 170+/-11, 179+/-12, 181+/-14, and 173+/-15 micromol.L(-1) (T=30, 60, 120, and 180, respectively; P=NS versus T=0 at all times). For all subjects, Mg(i) responses to hyperglycemia were closely related to basal Mg(i) levels: the higher the Mg(i), the greater the response (n=26, r=0.620, P<0.001). Thus, (1) erythrocytes from hypertensive vis-à-vis normotensive subjects are resistant to the ionic effects of extracellular hyperglycemia on Mg(i) levels, and (2) cellular ionic responses to glucose depend on the basal Mg(i) environment. Altogether, these data support a role for altered extracellular glucose levels in regulating cellular magnesium metabolism and also suggest the importance of ionic factors in determining cellular responsiveness to nonhormonal as well as hormonal signals.

Model of 2,3-bisphosphoglycerate metabolism in the human erythrocyte based on detailed enzyme kinetic equations: equations and parameter refinement

Over the last 25 years, several mathematical models of erythrocyte metabolism have been developed. Although these models have identified the key features in the regulation and control of erythrocyte metabolism, many important aspects remain unexplained. In particular, none of these models have satisfactorily accounted for 2,3-bisphosphoglycerate (2,3-BPG) metabolism. 2,3-BPG is an important modulator of haemoglobin oxygen affinity, and hence an understanding of the regulation of 2,3-BPG concentration is important for understanding blood oxygen transport. A detailed, comprehensive, and hence realistic mathematical model of erythrocyte metabolism is presented that can explain the regulation and control of 2,3-BPG concentration and turnover. The model is restricted to the core metabolic pathways, namely glycolysis, the 2,3-BPG shunt and the pentose phosphate pathway (PPP), and includes membrane transport of metabolites, the binding of metabolites to haemoglobin and Mg(2+), as well as pH effects on key enzymic reactions and binding processes. The model is necessarily complex, since it is intended to describe the regulation and control of 2,3-BPG metabolism under a wide variety of physiological and experimental conditions. In addition, since H(+) and blood oxygen tension are important external effectors of 2,3-BPG concentration, it was important that the model take into account the large array of kinetic and binding phenomena that result from changes in these effectors. Through an iterative loop of experimental and simulation analysis many values of enzyme-kinetic parameters of the model were refined to yield close conformity between model simulations and 'real' experimental data. This iterative process enabled a single set of parameters to be found which described well the metabolic behaviour of the erythrocyte under a wide variety of conditions.

Refinement and evaluation of a model of Mg2+ buffering in human red cells

The total Mg2+ content of human red cells ([Mg]T,i) is partitioned between free and bound forms. The main cytoplasmic Mg2+ buffers are ATP and 2,3 bisphosphoglycerate. Haemoglobin binds free ATP and bisphosphoglycerate, preferentially in the deoxygenated state. Thus, the free ionized Mg2+ concentration ([Mg2+]i) oscillates with the oxy-deoxy condition of the cells. The binding reactions are also modulated by the pH changes that accompany the oxygenation-deoxygenation transitions. The complex interactions between Mg2+, its ligands and Hb can be encoded in a set of equilibrium equations representing all the known binding reactions of the system. To develop a comprehensive understanding of the Mg2+ homeostasis of intact red cells it is necessary to correct and refine the equations and parameters of the model by systematic comparisons between model predictions and measured cytoplasmic Mg2+ buffering curves under a variety of experimental conditions. Earlier models largely underestimated total Mg2+ binding in intact cells. We carried out experiments in which [Mg]T,i and [Mg2+]i were controlled over a wide range ([Mg]T,i between 0.1 and 23 mM) by the use of the ionophore A23187, under diverse metabolic conditions, and the results were used to interpret the adjustments required for good model fits. By the inclusion of low-affinity Mg2+ binding to ATP and bisphosphoglycerate, and also binding of Mg2+ to haemoglobin (four ions per tetramer) with an apparent dissociation constant of 45 mM we were able to realistically model, for the first time, all the experimentally observed changes in [Mg2+]i in human red cells under diverse metabolic conditions.

Deoxygenation and elevation of intracellular magnesium induce tyrosine phosphorylation of band 3 in human erythrocytes

Deoxygenation increases the level of tyrosine phosphorylation of band 3 by approximately 25% in human red blood cells (RBCs), as determined by Western blotting. The effect is much more pronounced in osmotically shrunken RBCs or in the presence of vanadate. When the rise in intracellular free Mg2+ concentration in deoxygenated RBCs is simulated via clamping of the intracellular magnesium in oxygenated RBCs by ionomycin, band 3 phosphorylation is elevated by up to 10-fold. Phosphorylated band 3 is preferentially retained by RBC skeletons, after mild extraction with Triton X-100. Elevation of intracellular free Mg2+ leads to band 3 phosphorylation and is accompanied by rigidification of the membrane skeleton as determined by analysis of RBC membrane mechanical fluctuations. These findings suggest that the visco-elastic properties of human erythrocytes may be regulated by band 3 tyrosine phosphorylation.

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

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

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.

Oxygen-dependent K+ fluxes in sheep red cells

1. This study was designed to investigate the O2 dependence of K+ influx in sheep red cells. Influx was determined using 86Rb+ as a tracer for K+; glass tonometers coupled to a gas mixing pump were used to equilibrate cell samples to the requisite oxygen tension (PO2). 2. Both volume- and H(+)-stimulated K+ influxes in low potassium-containing (LK) sheep red cells were approximately doubled on equilibration with O2 relative to influxes measured in N2.O2-dependent influxes were abolished when Cl- was replaced with NO3-, consistent with mediation by the KCl cotransporter. At pH 7, PO2 required for half-maximal stimulation was 56 +/- 1 mmHg (mean +/- S.E.M., 3 sheep) for the O2-dependent component of K+ influx: thus PO2 values over the physiological range affected K+ influx. 3. K+ influx in fully deoxygenated sheep red cells showed substantial volume and H+ sensitivity. These residual components in N2 were also Cl- dependent, indicating that the KCl cotransporter of LK sheep red cells was active in the absence of O2. 4. Volume-sensitive K+ influxes in high potassium-containing (HK) sheep red cells responded in a similar way to those in cells from LK sheep, although much smaller in magnitude, showing that intracellular [K+] had no significant effect on the O2 dependence of the cotransporter. 5. Intracellular [Mg2+] ([Mg2+]i) was altered by incubating sheep red cells with A23187 (20 microM) and different values of extracellular [Mg2+] ([Mg2+]o). Total [Mg2+]i was determined by atomic absorption spectroscopy and free [Mg2+]i from [Mg2+]o and the Donnan ratio. Total [Mg2+]i was 1.29 +/- 0.08 mM (mean +/- S.E.M., n = 5), similar to that reported in the literature. Estimates of free [Mg2+]i showed an increase from 0.39 +/- 0.05 in oxygenated cells to 0.52 +/- 0.04 mM (mean +/- S.E.M., n = 5; P < 0.05) in deoxygenated ones. 6. Finally, although K+ influxes were altered by pharmacological loading or depletion of cells with Mg2+, the free [Mg2+]i required to affect influxes significantly was outside the physiological range. Results are difficult to reconcile with PO2 modulating KCl cotransport activity directly via changes in free [Mg2+]i or [Mg(2+)-ATP]i.

Free magnesium-ion concentration in erythrocytes by 31P NMR: the effect of metabolite-haemoglobin interactions

The effects that haemoglobin-metabolite interactions have on estimates of free magnesium-ion concentration in human erythrocytes, determined by 31P NMR [Gupta, R. K. et al., J. Biol. Chem. 253, 6172-6176 (1978)], were investigated. If the metabolite-haemoglobin association constants of Berger et al. [Eur. J. Biochem. 38, 553-562 (1973)] are used in the analysis then the estimates of intracellular free magnesium-ion concentration made by Gupta et al. (0.25 and 0.67 mM) become 0.43 and 0.60 mM, for oxygenated and deoxygenated cells, respectively. In oxygenated cells, this difference is primarily due to the lower value of KHbMgATP, given by Berger et al. These newly calculated concentrations are in closer agreement with those of Flatman (0.40 mM for oxygenated cells; 0.62 mM for deoxygenated cells) [Flatman, P. W., J. Physiol. 300, 19-30 (1980)] obtained with the 'zero-point titration' method. In addition, the assumptions that the chemical shift separations between the alpha- and beta-phosphorus resonances of ATP and MgATP are unchanged on association with Hb were shown to be false. Under normal intracellular conditions this may lead to errors of 5-10%. Much larger errors would be possible in cases where significant amounts of ATP or MgATP are bound to Hb. These outcomes place doubt on measurements of intracellular free Mg2+ concentration made using 31P NMR if there is no consideration given to the total concentration of 2,3-bisphosphoglycerate (BPG), ATP and Hb in the sample; the same principle would apply to other cell-types.

Model of the pH-dependence of the concentrations of complexes involving metabolites, haemoglobin and magnesium ions in the human erythrocyte

The rate of glucose consumption and the concentrations of glycolytic intermediates in human erythrocytes have long been known to be pH sensitive. Despite the extensive literature on modelling erythrocyte metabolism, no model developed so far can adequately describe all of these pH-dependent changes. None of these models have included all the significant association reactions between metabolites, Hb and Mg2+ that will influence metabolism. As part of a larger enterprise to develop a detailed model of erythrocyte glycolysis, we present a sub-model which predicts, as a function of pH and oxygenation state, the concentrations of free and Mg(2+)-bound metabolites that are substrates, co-factors and effectors of glycolysis. This model shows that pH changes around physiological values can cause large changes in the distribution of metabolites between free, bound and Mg(2+)-complexed forms, based on binding interactions alone; in oxygenated cells, at pH 7.2-7.6, many glycolytic intermediates undergo changes in concentration of 50-100%. The model also predicts intracellular concentrations of free Mg2+ in erythrocytes to be 0.4 mM and 0.64 mM in oxygenated and deoxygenated cells, respectively, assuming a total magnesium concentration of 3 mM (approximately 88% of the total magnesium usually found in erythrocytes). This is in close agreement with the values found by Flatman [Flatman, P. W. (1980) J. Physiol. 300, 19-30] and the finding by Flatman and Lew [Flatman, P. & Lew, V. L. (1977) Nature 267, 360-362] that the main Mg2+ buffer systems bind approximately 90% of Mg2+ in the cell. Hexokinase has a high 'flux control coefficient' in human erythrocyte glycolysis, so the dependence of its rate on the pH and oxygenation state of haemoglobin is important. With a low oxygen tension and an intracellular pH of 7.34, the major inhibitor of its activity (2,3-bisphosphoglycerate) is 85% bound to either haemoglobin or Mg2+, and the maximum possible flux of substrate via it would be 2.05 mmol L erythrocytes-1 h-1. However, if the haemoglobin were saturated with oxygen, and the pH were 7.2, it was calculated that the maximum rate would be 1.48 mmol L erythrocytes-1 h-1; this is primarily due to a doubling of the free 2,3-bisphosphoglycerate concentration. However, the full extent of the inhibition is counteracted because the concentration of the Mg(2+)-2,3-bisphosphoglycerate would be approximately doubled. Many other similar comparisons are possible with this new model, which highlights the complex network of interactions between haemoglobin, Mg2+, H+ and the metabolites as substrates and effectors of the glycolytic reactions.

Pathophysiology of sickle cell anemia

The anemia results from the markedly shortened circulatory survival of SS cells, together with a limited erythropoietic response. Both independent properties of Hb S-polymerization of the deoxy-Hb and instability of the oxy-Hb-contribute to early red cell destruction by effects on the Hb and on the red cell membranes. The erythroid response is limited mainly by the low oxygen affinity of SS cells, caused by the polymer and the increased 2,3-DPG. But the worst culprits in these processes are the dense, dehydrated SS cells (including the ISCs), most of which are formed rapidly from non-Hb F-reticulocytes by cation transport mechanisms triggered by polymerization. Since the clinical consequences of microvascular occlusion far exceed those of anemia per se, measures to lessen the anemia must also inhibit polymerization and sickling.

The effects of oxygenation upon the Cl-dependent K flux pathway in equine red cells

The effects of oxygen tension (PO2) upon the K influx pathways of equine red cells have been studied using 86Rb+ as congener for K. Equilibration of cells in 100% nitrogen led to a low and Cl-independent K flux. Change to an atmosphere of 100% air led to a rapid sixfold increase in K flux. The oxygen-activated flux was entirely Cl dependent and was maintained for up to 3 h. Oxygenation-evoked activation was dependent upon PO2 over the physiological range with little effect up to 70% saturation of haemoglobin with oxygen but significant effects between 70 and 100%. K flux at low PO2 was unaffected by acidification to pH 7 or by hypotonic cell swelling. By contrast, at high PO2 both manipulations caused a substantial increase in Cl-dependent K flux. N-Ethylmaleimide (NEM; 1 mM) caused a progressive activation of KCl cotransport in cells held under nitrogen. The protein phosphatase inhibitor, calyculin A (100 nM), applied during NEM-evoked activation caused a "clamping" of K influx at that level. This "clamped" activity was unaffected by subsequent oxygenation. We conclude that oxygenation exerts a primary control over cotransport activity and that acidification and cell swelling are secondary modulators. It appears that oxygenation-evoked activation of the Cl-dependent K flux involves a serine/threonine phosphorylation event. Regulating the PO2 of the solution before and during experiments is important in controlling the activity of the KCl cotransporter and cell volume.

Effect of intracellular magnesium on calcium extrusion by the plasma membrane calcium pump of intact human red cells

1. The effect of varying the concentration of intracellular magnesium on the Ca(2+)-saturated Ca(2+)-extrusion rate through the Ca2+ pump (phi max) was investigated in human red blood cells with the aid of the divalent cation ionophore A23187. The aim was to characterize the [Mg2+]i dependence of the Ca2+ pump in the intact cell. 2. The initial experimental protocol consisted of applying a high ionophore concentration to obtain rapid sequential Mg2+ and [45Ca]CaCl2 equilibration, prior to measuring phi max at constant internal [MgT]i by either the Co2+ block method or by ionophore removal. With this protocol, competition between Ca2+ and Mg2+ through the ionophore prevented Ca2+ equilibration at high [Mg2+]o. To provide rapid and comparable Ca2+ loads and maintain intracellular ATP within normal levels it was necessary to separate the Mg2+ and the Ca2+ loading-extrusion stages by an intermediate ionophore and external Mg2+ removal step, and to use different metabolic substrates during Mg2+ loading (glucose) and Ca2+ loading-extrusion (inosine) periods. 3. Intracellular Co2+ was found to sustain Ca2+ extrusion by the pump at subphysiological [Mg2+]i. Ionophore removal was therefore used to estimate the [Mg2+]i dependence of the pump at levels below [MgT]i (approximately 2 mmol (340 g Hb)-1), whereas both ionophore removal and Co2+ block were used for higher [MgT]i levels. 4. [Mg2+]i was computed from measured [MgT]i using known cytoplasmic Mg(2+)-buffering data. The phi max of the Ca2+ pump increased hyperbolically with [Mg2+]i. The Michaelis parameter (K 1/2) of activation was 0.12 +/- 0.04 mmol (1 cell water)-1 (mean +/- S.E.M.). Increasing [MgT]i and [Mg2+]i to 9 mmol (340 g Hb)-1 and 2.6 mmol (1 cell water)-1, respectively, failed to cause significant inhibition of the phi max of the Ca2+ pump. 5. The results suggest that within the physiological and pathophysiological range of [Mg2+]i, from 0.3 mmol (1 cell water)-1 in the oxygenated state to 1.2 mmol (1 cell water)-1 in the deoxygenated state, the Ca(2+)-saturated Ca2+ pump remains unaffected by [Mg2+]i at normal ATP levels.

Effects of deoxygenation on active and passive Ca2+ transport and on the cytoplasmic Ca2+ levels of sickle cell anemia red cells

Elevated [Ca2+]i in deoxygenated sickle cell anemia (SS) red cells (RBCs) could trigger a major dehydration pathway via the Ca(2+)-sensitive K+ channel. But apart from an increase in calcium permeability, the effects of deoxygenation on the Ca2+ metabolism of sickle cells have not been previously documented. With the application of 45Ca(2+)-tracer flux methods and the combined use of the ionophore A23187, Co2+ ions, and intracellular incorporation of the Ca2+ chelator benz-2, in density-fractionated SS RBCs, we show here for the first time that upon deoxygenation, the mean [Ca2+]i level of SS discocytes was significantly increased, two- to threefold, from a normal range of 9.4 to 11.4 nM in the oxygenated cells, to a range of 21.8 to 31.7 nM in the deoxygenated cells, closer to K+ channel activatory levels. Unlike normal RBCs, deoxygenated SS RBCs showed a two- to fourfold increase in pump-leak Ca2+ turnover. Deoxygenation of the SS RBCs reduced their Ca2+ pump Vmax, more so in reticulocyte- and discocyte-rich than in dense cell fractions, and decreased their cytoplasmic Ca2+ buffering. Analysis of these results suggests that both increased Ca2+ influx and reduced Ca2+ pump extrusion contribute to the [Ca2+]i elevation.

Effects of deoxygenation on active and passive Ca2+ transport and cytoplasmic Ca2+ buffering in normal human red cells

1. The effects of deoxygenation on cytoplasmic Ca2+ buffering, saturated Ca2+ extrusion rate through the Ca2+ pump (Vmax), passive Ca2+ influx and physiological [Ca2+]i level were investigated in human red cells to assess whether or not their Ca2+ metabolism might be altered by deoxygenation in capillaries and venous circulation. 2. The study was performed in fresh human red cells maintained in a tonometer either fully oxygenated or deoxygenated. Cytoplasmic Ca2+ buffering was estimated from the equilibrium distribution of 45Ca2+ induced by the divalent cation ionophore A23187 and the Vmax of the Ca2+ pump was measured either by the Co(2+)-exposure method or following ionophore wash-out. The passive Ca2+ influx and physiological [Ca2+]i were determined in cells preloaded with the Ca2+ chelator benz-2 and resuspended in autologous plasma. 3. Deoxygenation increased the fraction of ionized Ca2+ in cell water by 34-74% and reduced the Vmax of the Ca2+ pump by 18-32%. 4. To elucidate whether or not these effects were secondary to deoxygenation-induced pH shifts, the effects of deoxygenation on cell and medium pH, and of pH on cytoplasmic Ca2+ binding and Ca2+ pump Vmax in oxygenated cells were examined in detail. 5. Deoxygenation generated large alkaline pH shifts that could be explained if the apparent isoelectric point (pI) of haemoglobin increased by 0.2-0.4 pH units in intact cells, consistently higher than the value of 0.15 reported for pure haemoglobin solutions. 6. In oxygenated cells, the fraction of ionized cell calcium, alpha, was little affected by pH within the 7.0-7.7 range. Ca2+ pump Vmax was maximal at a medium pH of about 7.55. Comparison between pH effects elicited by HCl-NaOH additions and by replacing Cl- with gluconate suggested that Vmax was inhibited by both internal acidification and external alkalinization. Since deoxygenation alkalinized cells and medium within a range stimulatory for Vmax, the inhibition observed was not due to pH. 7. There was no significant effect of deoxygenation on passive Ca2+ uptake, or steady-state physiological [Ca2+]i level. 8. The deoxygenation-induced reduction in Ca2+ binding capacity may result from the increased protonation of haemoglobin on deoxygenation and from binding of 2,3-diphosphoglyceric acid (2,3-DPG) and ATP to deoxyhaemoglobin; inhibition of the Ca2+ pump may result from shifts in the [Mg2+]i/[ATP]i ratio away from a near optimal stimulatory value in the oxygenated state.

Oxygenation-deoxygenation cycle of erythrocytes modulates submicron cell membrane fluctuations

Low frequency submicron fluctuations of the cell membrane were recently shown to be characteristic for different cell types, nevertheless their physiological role is yet unknown. Point dark-field microscopy based recordings of these local displacements of cell membrane in human erythrocytes, subjected to cyclic oxygenation and deoxygenation, reveals a reversible decrease of displacement amplitudes from 290 +/- 49 to 160 +/- 32 nm, respectively. A higher rate of RBC adhesion to a glass substratum is observed upon deoxygenation, probably due to a low level of fluctuation amplitudes. The variation in the amplitude of these displacements were reconstituted in open RBC ghosts by perfusing them with composite solutions of 2,3 diphosphoglycerate, Mg+2, and MgATP, which mimic the intracellular metabolite concentrations in oxygenated and deoxygenated erythrocytes. The mere change in intracellular Mg+2 during oxygenation-deoxygenation cycle is sufficient to explain these findings. The results imply that the magnitude of fluctuations amplitude is directly connected with cell deformability. This study suggests that the physiological cycle of oxygenation-deoxygenation provides a dynamic control of the bending deformability and adhesiveness characteristics of the RBC via a Mg+2-dependent reversible assembly of membrane-skeleton proteins. The existing coupling between oxygenation-deoxygenation of the RBC and its mechanical properties is expected to play a key role in blood microcirculation and may constitute an example of a general situation for other circulating blood cells, where the metabolic control of cytoskeleton dynamics may modulate their dynamic mechanical properties.

25Mg NMR studies of magnesium binding to erythrocyte constituents

The binding of Mg2+ ion to ATP, ADP, AMP, 2,3-bisphosphyoglycerate (DPG), and hemoglobin has been studied by 25Mg NMR spectroscopy at 9.4 T. Addition of any of these ligands to a solution of 2 mM 25MgCl2 at pH 7.2 caused a progressive increase in linewidth, with no discernible chemical shift. ATP and ADP, which form tight 1:1 complexes with Mg2+, did not cause maximal broadening until present in several-fold excess, implying that bis(nucleotide) complexes also form. The studies showed progressively weaker Mg2+ binding to ATP, ADP, DPG, and AMP, consistent with published binding constants. Hemoglobin cause fairly little broadening, consistent with its known weak affinity for Mg2+. Competition studies determined ATP affinities for Ca2+ and H+ that were also in good agreement with published values. 25Mg NMR spectra of 2 mM bound 25Mg2+ were obtained with good signal to noise in less than 1 hr. The technique may now be a practical means for studying the binding of Mg2+ within erythrocytes and other cells.

Deoxygenation permeabilizes sickle cell anaemia red cells to magnesium and reverses its gradient in the dense cells

1. Our findings of a low total magnesium content in the dense fraction (over 1.118 g ml-1) of sickle cell anaemia (SS) red cells seemed inconsistent with the low Mg2+ permeability and outward Mg2+ gradient seen in normal red cells, and prompted studies of the Mg2+ permeability and equilibria in the SS cells. 2. Deoxygenation and sickling induced Mg2+ permeabilization in SS cells, supporting non-specificity of the sickling-induced cation permeabilization, previously described for Na+, K+ and Ca2+. The extent of Mg2+ permeabilization was comparable in SS cells with normal or high density. 3. Compared with normal-density SS cells and normal red cells, the dense SS cells showed a much larger increase in the fraction of ionized magnesium ([Mg2+]i) on deoxygenation, resulting in [Mg2+]i levels sufficient to reverse the normal inward direction of the transmembrane Mg2+ gradient. 4. The molar ratio of 2,3-diphosphoglycerate (2,3-DPG) to haemoglobin was markedly reduced in the dense SS cells. Since 2,3-DPG and ATP are the main cytoplasmic Mg2+ buffers, their further reduction upon binding to deoxyhaemoglobin accounts for the high [Mg2+]i in the deoxygenated dense SS cells; the resulting outward electrochemical Mg2+ gradient, together with sickling-induced Mg2+ permeabilization, could explain the decreased total magnesium content of these cells. 5. The above findings suggested that the documented low sodium pump fluxes in dense SS cells may result from an increased Mg2+:ATP ratio, which is known to inhibit Na(+)-K+ exchange fluxes through the sodium pump. If so, deoxygenation, by increasing the Mg2+:ATP ratio, should inhibit the pump further, whereas increasing ATP should relieve the inhibition. Experiments designed to test this possibility showed that in these dense SS cells, the ouabain-sensitive K(86Rb) influx was low in oxygenated cells, was reduced further by deoxygenation, but was substantially increased after treatment with inosine, pyruvate and phosphate to increase their organic phosphate pool. These results were thus consistent with such a mechanism for Na+ pump inhibition in the dense SS cells.

Further studies on alterations in magnesium binding during cold storage of erythrocytes

Free intracellular Mg2+ concentration ([Mg2+]i) was measured in cold-stored human erythrocytes by the method of null-point titration with ionophore A23187. [Mg2+]i was 311 +/- 41 microM (mean +/- S.D.) for cells stored 0-10 days, increasing to 458 +/- 64 microM for cells stored 22-48 days. The values for stored cells were higher than those previously determined by a 31P-NMR method (Bock et al. (1985) Blood 65, 1526-1530); however, the null-point method requires extensive washing of the cells, which we have found to increase NMR-measured [Mg2+]i. The null-point values still represent a small fraction of total cell Mg2+, and confirm that binding of Mg2+ to ligands other than ATP and 2,3-bisphosphoglycerate must increase during storage. As an initial test of whether this may imply suboptimal availability of Mg2+ for cell preservation, we used A23187 to prepare erythrocytes with altered Mg2+ content, then removed ionophore and stored the cells in plasma-free medium for up to 2 weeks. Higher Mg2+ content had a very significant positive correlation (P less than 0.0001) with higher cell ATP concentrations. Storage did not significantly affect basal or Na+-stimulated efflux of Mg2+ from Mg2+-loaded red cells.

Manganous chloride stimulation of adenylate cyclase responsiveness in ocular ciliary process membranes

Manganous chloride was compared with magnesium chloride in supporting maximal stimulations of the adenylate cyclase system in ocular ciliary process membranes by isoproterenol, vasoactive intestinal peptide, sodium fluoride, guanosine 5'-(beta, gamma-imino) triphosphate (GppNHp), or forskolin, and in supporting synergism between isoproterenol and forskolin. The primary effect of Mn2+ (2 mM) was due to an interaction at the catalytic unit. Mn2+ had no significant effect on the function of the GTP-binding stimulatory G-protein (Gs) which couples beta-adrenergic receptors to the catalytic unit of adenylate cyclase. However, Gs-protein function was impaired by Mn2+ relative to Mg2+ when GppNHp was used instead of GTP as the ligand for the Gs-protein. Compared with Mg2+, Mn2+ caused a 4-5.5-fold increase in adenylate cyclase responsiveness to all the activators tested (except GppNHp, where the increase was 2.5-3.5-fold). Thus Mn2+ ions appeared to be intrinsically more effective at divalent cation binding sites on the catalytic unit that control its enzymatic activity. Ciliary process membranes differ from erythrocyte and S49 lymphoma cell membranes where 2 mM Mn2+ strongly inhibits hormone-Gs-protein-mediated stimulations of adenylate cyclase. Divalent cations bound to the catalytic unit also affected the degree of synergism between hormone-activated Gs and forskolin to stimulate adenylate cyclase activity. In the presence of MgCl2 all effective doses of isoproterenol and forskolin in combination showed marked synergism. In contrast, with MnCl2 there was no synergism with high-dose isoproterenol-forskolin combinations, which gave only additive responses.

The effects of calcium on potassium transport in ferret red cells

1. The effects of changes in the concentration of intracellular calcium on potassium transport were investigated in ferret red cells. Bumetanide was used to divide potassium transport into three components: total, bumetanide sensitive and bumetanide resistant. The bumetanide-sensitive component is equivalent to sodium-potassium-chloride co-transport. 2. Internal calcium concentration was controlled with the ionophore A23187 which was present throughout the experiments. 3. Changes in internal ionized calcium over the range 5 X 10(-10) M to 7 X 10(-7) M did not affect any component of potassium uptake. 4. Increasing the internal ionized calcium concentration above 10(-6) M stimulated bumetanide-resistant potassium transport. Half-maximal stimulation of this system was achieved with 3 X 10(-6) M-internal calcium. The system spontaneously inactivated after the initial activation by calcium and ionophore. Transport was inhibited by 1 mM-quinine. 5. Increasing the internal ionized calcium concentration to 10(-5) M had no effect on bumetanide-sensitive transport. 6. Concentrations of intracellular ionized calcium above 10(-5) M inhibited all three components of transport. Inhibition of the bumetanide-sensitive component was only slightly reversed when internal calcium concentration was reduced to normal. 7. Physiological changes in internal ionized calcium concentration do not affect sodium-potassium-chloride co-transport in ferret red cells. Very high concentrations of calcium inhibit transport, probably by an indirect mechanism. 8. In the course of the experiments the concentration of ionized intracellular magnesium in oxygenated ferret red cells was found to be about 0.65 mM.

Free oxygen radicals decrease electrical resistance of microvascular endothelium in brain

The effect of free oxygen radicals on the electrical resistance of brain venular endothelium was studied in anesthetized frogs. The technique allowed continuous recording of the electrical resistance of the vascular wall reflecting its ionic permeability. The oxygen radicals were generated by an enzymatic reaction between xanthine oxidase and hypoxanthine supplied to the surface of the exposed brain. Electrical resistance of the venular endothelium decreased within 1-2 s after the reaction was initiated. Hypoxanthine (1 mM) and xanthine oxidase at a concentration of 10, 25, 50, 100, and 250 mU ml-1 lowered resistance to 1.0, 0.9, 0.8, 0.5 and 0.2 X control value, respectively, within a 3 min period of administration. The effect induced by 25 and 50 mU ml-1 of xanthine oxidase was readily reversible, whereas that induced by the two highest concentrations was irreversible within the observation time. The response was totally blocked by allopurinol as well as by superoxide dismutase plus catalase. Pretreatment with methylprednisolone or BW755C (an inhibitor of cyclo- and lipoxygenase) did not inhibit the response, nor did removal of calcium or magnesium from the extracellular medium. Free oxygen radicals are powerful agents that rapidly induce dynamic changes in the electrical resistance of brain vessels, supporting the notion that they may be important mediators of vascular endothelial damage in the brain.

Blood oxygen equilibria and theoretical models. II. A critical estimation

1. Theoretical models can fit the oxygen equilibria of trout and human red cell suspensions, and describe the apparent oxygenation process in the red cell. 2. The assumption that full oxygenation is attained at atmospheric O2 pressures can result in biphasic Hill plots and high Hill coefficients for high O2 saturations. This phenomenon must not be confused with aggregation of hemoglobin. 3. Problems specific to measurements of red cell suspensions, regarding the ionic cellular composition and its stability with time, are approached. Changes of buffer osmolarity, and--for trout--addition of adrenaline, within physiological proportions, have no impact on the results. 4. This tends to validate the general significance of equilibrium data obtained on this material, regarding the effects of protons and organic phosphates, although complex ionic movements across the red cell membrane are known to occur in the animal under certain circumstances.

Effect of common agonists on cytoplasmic ionized calcium concentration in platelets. Measurement with 2-methyl-6-methoxy 8-nitroquinoline (quin2) and aequorin

Because of controversy regarding the relationship of cytoplasmic ionized calcium concentration ([Cai2+]) to platelet activation, we studied the correlation of platelet aggregation and ATP secretion with [Cai2+] as determined by 2-methyl-6-methoxy 8-nitroquinoline (quin2) and aequorin in response to ADP, epinephrine, collagen, the Ca2+ ionophore A23187, and thrombin. Both indicators showed a concentration-dependent increase in [Cai2+] in response to all agonists except epinephrine when gel-filtered platelets were suspended in media containing 1 mM Ca2+. With epinephrine, a rise in [Cai2+] was indicated by aequorin, but not by quin2; [Cai2+] signals, aggregation, and secretion were suppressed by EGTA. ADP [0.5 microM] produced a rise in [Cai2+] that was registered by both aequorin and quin2 in platelets in Ca2+-containing media; addition of EGTA to the medium raised the threshold concentration of ADP to 5.0 microM for both indicators. Collagen produced progressive concentration-related increases in [Cai2+] and aggregation in aspirin-treated aequorin-loaded platelets. Quin2 failed to indicate a rise in [Cai2+]at lower collagen concentrations with EGTA or aspirin. [Cai2+] response to A23187 and thrombin was reduced by addition of EGTA to platelets loaded with either aequorin or quin2. With all five agonists in all conditions tested, aequorin [Cai2+] signals occurred at the same agonist concentration as that or lower than that which produced platelet shape change, aggregation, or secretion. Platelet activation was better correlated with changes in [Cai2+] indicated by aequorin than with the response of quin2, possibly because aequorin is more sensitive to local zones of [Cai2+] elevation.

Dependence on Ca2+ of the activities of phosphatidylinositol 4,5-bisphosphate phosphodiesterase and inositol 1,4,5-trisphosphate phosphatase in smooth muscles of the porcine coronary artery

The activities of phosphatidylinositol 4,5-bisphosphate (PIP2) phosphodiesterase (PDE) and inositol 1,4,5,-trisphosphate (IP3) phosphatase in the particulate and cytosol fractions prepared from porcine coronary artery smooth muscles were examined using 32P-labelled PIP2 and IP3 as substrates, respectively. The activity of PIP2 PDE, as assessed from the production of IP3, in the cytosol fraction was about 10-fold higher than that in the particulate fraction. In the absence of MgCl2, the activity of PIP2 PDE in both fractions showed no causal relation to the free Ca2+ concentration in the physiological range of 10(-7)-10(-5) M, but was enhanced remarkably by 10(-4) M free Ca2+. The addition of 1 mM-MgCl2 to the assay medium markedly inhibited the activity of PIP2 PDE in both fractions in the presence of free Ca2+ (10(-8)-10(-5) M). In the absence of MgCl2, 10(-5)M-acetylcholine (ACh) produced IP3, and this action was blocked by 3 X 10(-6) M-atropine. The ACh-induced activation of PIP2 PDE ceased in the presence of 1 mM-MgCl2; however, the reactivation occurring on the addition of 10 microM-guanosine 5'-[gamma-thio]triphosphate did not depend on the free Ca2+ concentrations (10(-7)-10(-5)M). The activities of IP3 phosphatase, determined from decrease in the amount of IP3 in the particulate and cytosol fractions, had much the same potency in both fractions. The activity of IP3 phosphatase in the cytosol fraction was enhanced by MgCl2 in a concentration-dependent manner, the maximal value occurring at 1 mM-MgCl2, and was also enhanced in the presence of physiological concentrations of free Ca2+ (10(-7)-10(-6) M). These findings suggest that the activation of PIP2 PDE which occurs with application of ACh in the presence of guanine nucleotides and 1 mM-MgCl2 is independent of the free Ca2+ concentration, and that the hydrolysis of IP3 by phosphatase increases, depending on the concentration of free Ca2+.

Intracellular free magnesium in neurones of Helix aspersa measured with ion-selective micro-electrodes

Cytoplasmic free Mg2+ concentration, [Mg2+]i, was measured in identified neuronal cell bodies of the suboesophageal ganglia of Helix aspersa, using Mg2+-selective micro-electrodes. In calibration solutions, the electrodes showed significant interference from K+, but not from Na+, or Ca2+, at concentrations found intracellularly. Therefore, in order to calibrate the electrodes properly, it was necessary first to obtain an accurate value for intracellular free K+ concentration [( K+]i). The mean value for [K+]i was 91 mM (S.E. of the mean +/- 2.2 mM, n = 8), measured with K+-sensitive 'liquid ion exchanger micro-electrodes'. In seven experiments, which met stringent criteria for satisfactory impalement and electrode calibration, the mean [Mg2+]i was 0.66 mM (S.E. of the mean +/- 0.05 mM). The mean [Mg2+]i in cells that had spontaneous spike activity was not significantly different from that in quiescent cells. If Mg2+ was in electrochemical equilibrium, the ratio [Mg2+]i/[Mg2+]o would be about 55. Mg2+ is therefore not passively distributed across the neuronal membrane and an outwardly directed extrusion mechanism must exist to keep [Mg2+]i low and constant, even in cells undergoing spike activity.

Intracellular Mg2+ homeostasis of Ehrlich ascites tumor cells

Treatment of Ehrlich ascites tumor cells with the cation ionophore A23187 results in cellular accumulation of calcium from the extracellular medium only when cellular metabolism is inhibited by uncouplers and iodoacetate. In the metabolically active cells A23187 induces a substantial loss of intracellular magnesium at extracellular Mg2+ concentrations below 1 mM. By contrast, in the inhibited cells A23187 induces a conspicuous loss of magnesium at concentrations of magnesium in the medium ranging from 0.2 to 5 mM. This cellular magnesium loss is accompanied by an increase in intracellular calcium. At 10 mM external Mg2+, A23187 provokes an intracellular magnesium increase in both metabolically active and inhibited cells with a corresponding H+ ejection. Intracellular free Mg2+ in both control and metabolically inhibited cells was estimated through a null-point titration by monitoring changes in extracellular Mg2+ concentration due to the addition of A23187 to cell suspensions with the metallochromic indicator Antipyrylazo III. Free intracellular Mg2+ was 0.4 and 1.2 mM in metabolically active cells and in cells treated with metabolic inhibitors, respectively. Under these conditions the cellular ATP was 30.9 and 2.3 nmol/mg dry wt, respectively. The results lead to the conclusion that in intact Ehrlich ascites tumor cells the level of ATP mainly regulates the intracellular free Mg2+ concentration and, in turn, intracellular free Mg2+ determines the entry of calcium into the cell after A23187 addition.

Inhibition of human red cell sodium and potassium transport by divalent cations

The influx and efflux of Na and K across the human red cell membrane by the bumetanide-sensitive (Na-K co-transport) and residual (ouabain- and bumetanide-insensitive) routes were inhibited by increasing concentrations of external Mg. Ca, Sr, Ba, Mn and Co also inhibited bumetanide-sensitive and residual K transport. External Mg inhibited choline uptake and the Na-dependent fractions of L-alanine and L-serine uptakes. External Mg reduced the maximal rate (app. Vmax) but not the affinity (app. Km) of the bumetanide-sensitive K and Na influxes when they were measured as functions of external K and Na respectively. The inhibitory effect of Mg was not due to a small reduction in zeta potential since much larger reductions in zeta potential produced by neuraminidase did not affect transport. Internal Mg stimulated the ouabain-sensitive K influx but inhibited the co-transport and residual components of K influx. Bumetanide was a poor co-transport inhibitor in red cells pre-treated with A23187 and EDTA. It was concluded that the inhibitory effects of external Mg were probably not due to changes in the ionic composition of the diffuse double layer adjacent to the cell membrane. Mg and other divalent cations should not be used as 'inert' ionic substitutes in human red cell Na and K transport studies.

Hormone-sensitive magnesium transport in murine S49 lymphoma cells: characterization and specificity for magnesium

1. The hormone-sensitive transport of Mg(2+) into murine S49 lymphoma cells and its relationship to other divalent cation transport systems have been investigated.2. Mg(2+) influx, measured with (28)Mg(2+), is saturable with an apparent extracellular ion concentration at half-maximal influx (K(in)) for Mg(2+) of 330 muM and a maximal influx rate of 360 p-mole/min.10(7) cells (2.9 n-mole/min.mg cell protein or a flux rate of about 0.12 p-mole/sec.cm(2)). Efflux of Mg(2+) is biphasic with half-times of 55 and 240 min at 37 degrees C and is temperature-sensitive.3. beta-Adrenergic agonists inhibit influx but not efflux of Mg(2+) in S49 cells. Efflux of Mg(2+) is also unaffected by extracellular [Mg(2+)] or [Ca(2+)]. These results imply that the mechanism of the transport system does not involve Mg-Mg exchange.4. Mn(2+) is a non-competitive inhibitor of Mg(2+) influx with an inhibition constant, K(i), of about 200 muM. The weak inhibition exhibited by Ca(2+) (K(i) > 5 mM) is also non-competitive. La(3+) inhibits Mg(2+) transport half-maximally at about 100 muM; Ni(2+), Zn(2+), Co(2+) and Sc(3+) are all less effective than La(3+). The Ca(2+)-channel blockers cis-diltiazem, verapamil, and nifedipine and the monovalent cations Na(+) and K(+) also have no effect on Mg(2+) influx. However, increasing the extracellular pH stimulates Mg(2+) influx.5. Total cellular Mg(2+) is about 85 n-mole/10(7) cells; however, at apparent isotopic equilibrium with (28)Mg(2+) less than 3% of total cellular Mg(2+) has been exchanged. This indicates that cellular Mg(2+) is highly compartmented and that recently transported Mg(2+) exchanges very slowly with bulk intracellular Mg(2+).6. Ca(2+) influx has a K(in) of 80 muM and is much slower than Mg(2+) influx. V(max) varied in different experiments from 3 to 15 p-mole/min.10(7) cells (25-125 p-mole/min.mg cell protein). Efflux of Ca(2+) is biphasic with half-times of 22 and 200 min and is temperature-sensitive. Hormonal stimulation has no effect on either influx or efflux of Ca(2+). Mg(2+) is a competitive inhibitor of Ca(2+) influx (K(i) = 3 mM).7. Two kinetic components of Mn(2+) influx are present with apparent K(in)s of 4 muM and 100 muM. Maximal influx rates are 5 and 60 p-mole/min.10(7) cells (40 and 480 p-mole/min.mg cell protein), respectively. Influx of Mn(2+) is not altered by beta-adrenergic agonist.8. Uptake of Na(+) or K(+) is unaltered by beta-adrenergic stimulation. These data in the S49 lymphoma cell indicate that (a) Mg(2+) is translocated by a transport system independent of those that transport other divalent cations, (b) hormonal inhibition of divalent ion transport is specific for Mg(2+) and (c) cellular Mg(2+) is highly compartmented.

Uniform ionophore A23187 distribution and cytoplasmic calcium buffering in intact human red cells

The divalent cation-selective ionophore A23187 has been used to characterize cytoplasmic Ca and Mg buffering, Ca2+-pump parameters and the properties of a Ca2+-activated K+-channel in intact red cells. A critical assumption in these studies has been that the ionophore causes a uniform increase in divalent cation-permeability in all the cells. This has now been tested directly in ATP-depleted human red cells by analysing the kinetics of ionophore-induced 45Ca-tracer and net Ca2+ fluxes. The experimental curves were all adequately fitted by single-exponentials at all ionophore concentrations tested. Moreover, statistical analysis of 61 individual tracer influx curves and of pooled data showed no trend towards fast second exponential components. These results demonstrate uniformity of ionophore distribution, ionophore-induced Ca2+-permeability, and cytoplasmic Ca-buffering among all the cells. Experiments involving mixing of cell suspensions with high and low original ionophore content, and involving ionophore extraction by albumin, demonstrate a rapid redistribution of ionophore among the cells, indicating that homogeneity of ionophoric effects is achieved through dynamic ionophore redistribution.

Binding of magnesium and chloride ions to human hemoglobin A. Mg2+ concentrations in solutions simulating red cell conditions

Ultrafiltration and direct determination of [Mg2+] by ion exchange were used to study the binding of Mg2+ and Cl- to isoionic human hemoglobin. A value of 58.8 +/- 1.7 l/mol was determined for the volume of hydrated hemoglobin from ultrafiltration of hemoglobin solutions containing 0.15-0.8 M glucose. In solutions with 5.7 mmol hemoglobin, 150 mmol KCl and 0.5-3.5 mmol MgCl2/l total water, 0.6 mol and 2 mol Cl- were bound/mol oxygenated and deoxygenated hemoglobin respectively. A value of about 11 l/mol was determined for the association constant of Mg2+ to hemoglobin monomer. Free Mg2+ concentrations were measured in hemoglobin solutions containing KCl, MgCl2, ATP and D-glycerate-2,3-bisphosphate at concentrations close to those of red cells. The experiments yielded 0.65 mmol/l free Mg2+ after oxygenation and 0.82 mmol/l after deoxygenation. The data indicate that only small changes of free Mg2+ levels in red cells are caused by physiological changes of pO2.

Lack of effect of the Ca2+ ionophore A23187 on tumour cells

The Ca2+ ionophore A23187 increases intracellular calcium content in normal thymic cells, while it is without effect on the corresponding neoplastic cell (Ascites thymoma) and on Ehrlich ascites tumour cells. The A23187-induced total cell calcium increase in normal thymocytes takes place both in control and energy-depleted cells, while it is lacking in neoplastic cells. In addition the ionophore stimulates aerobic glycolysis of normal thymocytes, whereas it is ineffective on neoplastic cells. The study of intracellular calcium exchange properties reveals that in normal cells the ionophore A23187 provokes a 60% increase of the exchangeable pool together with a more significant, 4-fold enlargement of the unexchangeable pool. These effects are lacking in cancer cells. The data give rise to interesting considerations concerning the regulation and compartmentalization of calcium in neoplastic cells. The results will be also discussed in relation to the models that predict altered cell calcium metabolism as a cause of cancer cell high aerobic glycolysis and uncontrolled growth.

The magnesium dependence of sodium-pump-mediated sodium-potassium and sodium-sodium exchange in intact human red cells

1. The magnesium content of human red blood cells was controlled by varying the magnesium concentration in the medium in the presence of the ionophore A23187. The new magnesium levels attained were very stable, which allowed the magnesium dependence of the sodium pump to be investigated.2. The effects of magnesium were shown to occur at the inner surface of the red cell membrane for the range of magnesium concentrations tested (10(-7) to 6 x 10(-3)m).3. At intracellular ionized magnesium concentrations below 0.8 mm the activation of ouabain-sensitive sodium-potassium exchange by internal ionized magnesium could be resolved into two or three components: (a) a small component, about 5% of the maximum flux, which is apparently independent of the ionized magnesium concentration below 2 mum, (b) a saturating component with a K((1/2)) of between 30 and 45 mum, and possibly (c) a component which increases linearly with ionized magnesium concentration and which only becomes apparent at concentrations above 0.1 mm.4. At intracellular ionized magnesium concentrations below 0.8 mm, activation of ouabain-sensitive sodium-sodium exchange by internal ionized magnesium could be resolved into two components: (a) a small component, about 6% of the maximal flux, which is apparently independent of the ionized magnesium concentration below 2 mum, and (b) a saturating component with a K((1/2)) of about 9 mum. At ionized magnesium concentrations between about 0.2 and 0.8 mm the rate of sodium-sodium exchange remained constant at the maximal level.5. The intracellular concentration of ATP decreased and the ADP concentration increased as the magnesium content of the cells was reduced from the normal level. A small increase in ATP and a small decrease in ADP was seen when the magnesium content was increased above the normal level. The variation in the ATP: ADP ratio from 2.5 at very low magnesium levels to about 6 at normal magnesium levels can account, at least in part, for the different K((1/2)) values of sodium-potassium and sodium-sodium exchange.6. When the concentration of ionized magnesium was increased above about 0.8 mm both sodium-potassium and sodium-sodium exchange were inhibited. Sodium-sodium exchange was more strongly inhibited than sodium-potassium exchange.7. The possible sites of action of magnesium in the sodium pump cycle are discussed.

Magnesium buffering in intact human red blood cells measured using the ionophore A23187

1. A method was developed for measuring the cytoplasmic magnesium buffering of intact red cells using the divalent cation selective ionophore A23187. Addition of A23187 to a suspension of red cells induces rapid equilibration of ionized magnesium across the cell membrane. 2. Entry of magnesium into red cells is associated with cell swelling and depolarization of the membrane potential. 3. At an external ionized magnesium concentration of about 0.15 mM corresponding to an internal ionized concentration of 0.4 mM the addition of A23187 did not produce a change in the magnesium content of the cells. This indicates that the normal ionized magnesium concentration inside the oxygenated red cell is about 0.4 mM. 4. The magnesium buffering curve for oxygenated, inosine-fed human red blood cells is adequately described by the existence of three buffer systems of increasing capacity and decreasing affinity. These are 0.15 mM with a Km < 10(-7) M, probably structural magnesium bound within the cell proteins; 1.6 mM with a Km approximately equal to 0.08 mM, mainly ATP and other nucleotides; and about 21-25 mM with a Km approximately equal to 3.6 mM, a major portion of this being organic phosphates. It is suggested that the contribution of 2,3-DPG to the low affinity site involves each phosphate group acting as an independent binding site for magnesium.