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

Evaluation of lipid peroxidation and the level of some elements in rat erythrocytes during separate and combined vanadium and magnesium administration

The impact of vanadium (V) and magnesium (Mg) as sodium metavanadate (SMV, 0.125 mg V/ml) and magnesium sulfate (MS, 0.06 mg Mg/ml) on lipid peroxidation (LPO) and selected elements in the rat erythrocytes (RBCs) was investigated. Relationships between some indices determined in RBC were also studied. SMV alone (Group II) elevated the malondialdehyde level (MDA_RBC) (by 95% and 60%), compared with the control (Group I) and MS-supplemented rats (Group III), respectively, reduced the concentration of Cu_RBC (by 23.5%), in comparison with Group I, but did not change the levels of Na_RBC, K_RBC, and Ca_RBC, whereas MS alone (Group III) only reduced the Cu_RBC concentration (by 22%), compared with Group I. The SMV + MS combination (Group IV) reduced and elevated the Cu_RBC (by 24%) and Ca_RBC (by 111%) concentrations, respectively, in comparison with Groups I and III, and these changes were induced by the V-Mg antagonistic and synergistic interaction, respectively. The combined SMV + MS effect also enhanced the MDA_RBC level, compared with Groups I (by 79%) and III (by 47%) and slightly limited its concentration, compared with Group II, which, in turn, resulted from the distinct trend toward the V-Mg antagonistic interaction. We can conclude that V (as SMV) is able to stimulate LPO in rat RBCs and that V-Mg interactive effects are involved in changes in Cu_RBC, Ca_RBC, and MDA_RBC. Further studies are needed to elucidate the exact mechanisms of the V-Mg antagonistic/synergistic interactions and to provide insight into the biochemical mechanisms of changes in rats suffering from anemia [1], characterized by a disrupted antioxidant barrier in RBCs [2] and an intensified free radical process in these cells.

Ion transport pathology in the mechanism of sickle cell dehydration

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

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.

Distribution of plasma membrane Ca2+ pump activity in normal human red blood cells

The plasma membrane calcium pump (PMCA) is the only active Ca2+ transporter in human red blood cells (RBCs). Previous measurements of maximal Ca2+ extrusion rates (Vmax) reported only mean values in the RBC population. Despite early evidence for differences in Ca2+ extrusion capacity among RBCs, the precise Vmax distribution remained unknown. It was important to characterize this distribution to assess the range and modality (uni- or multimodal) of PMCA Vmax variation and the likelihood of RBCs with elevated [Ca2+]i in the circulation participating in physiologic and pathologic processes. We report here the application of a new method to investigate the detailed distribution of PMCA Vmax activity in RBCs. The migrating profile of osmotic lysis curves was used to identify and quantify the fraction of cells that extrude a uniform Ca2+ load at different rates. The results revealed that RBCs from single donors have large variations in PMCA activity that follow a unimodal, broad distribution pattern consistently skewed toward higher Vmax values, suggesting an excess of cells with Vmax higher than the mean value. The method applied may provide a way of evaluating whether the observed variation in PMCA Vmax is related to cell age.

Up-regulation of Na+-dependent Mg2+ transport by nitric oxide and cyclic GMP pathway in renal epithelial cells

A putative, Na(+)-dependent Mg(2+) transport pathway controls the intracellular free Mg(2+) concentration ([Mg(2+)](i)) in various mammalian cells. The characteristics of this Mg(2+) transport pathway have not been clarified. Herein, we examined the regulatory mechanism of Na(+)-dependent Mg(2+) efflux in renal epithelial NRK-52E cells. Mg(2+) removal from the extracellular bathing solution induced an Na(+)-dependent [Mg(2+)](i) decrease in Mg(2+) (5 mM)-loaded cells but not in control cells. Amiloride inhibited the [Mg(2+)](i) decrease in a dose-dependent manner (IC(50) = 3 microM). Similarly, atomic absorption spectrophotometry showed that Mg(2+) removal decreased intracellular Mg(2+) content, while it increased Na(+) content. Calphostin C (1 microM), a protein kinase C inhibitor, and genistein, a tyrosine kinase inhibitor (10 microM), blocked the [Mg(2+)](i) decrease. The [Mg(2+)](i) decrease was accompanied by an increase in intracellular nitric oxide (NO) and cyclic GMP contents. (E)-4-methyl-2-[(E)-hydoxyimino]-5-nitro-6-methoxy-3-hexenamide (0.1 mM), an NO donor, and 8-bromo-cyclic GMP (0.1 mM), a membrane-permeable cyclic GMP analogue, accelerated the [Mg(2+)](i) decrease. In contrast, N(G)-monomethyl-L-arginine (L-NMMA, 0.1 mM), an NO competitive inhibitor, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 microM), an NO-sensitive guanylate cyclase inhibitor, significantly blocked the [Mg(2+)](i) decrease. These results indicate that a decrease in extracellular Mg(2+) concentration induces the production of NO and cyclic GMP, which leads to the up-regulation of Na(+)-dependent Mg(2+) efflux.

Normal Ca2+ extrusion by the Ca2+ pump of intact red blood cells exposed to high glucose concentrations

The ATPase activity of the plasma membrane Ca2+ pump (PMCA) has been reported to be inhibited by exposure of red blood cell (RBC) PMCA preparations to high glucose concentrations. It has been claimed that this effect could have potential pathophysiological relevance in diabetes. To ascertain whether high glucose levels also affect PMCA transport function in intact RBCs, Ca2+ extrusion by the Ca2+-saturated pump [PMCA maximal velocity (V(max))] was measured in human and rat RBCs exposed to high glucose in vivo or in vitro. Preincubation of normal human RBCs in 30-100 mM glucose for up to 6 h had no effect on PMCA V(max). The mean V(max) of RBCs from 15 diabetic subjects of 12.9 +/- 0.7 mmol. 340 g Hb(-1). h(-1) was not significantly different from that of controls (14.3 +/- 0.5 mmol. 340 g Hb(-1). h(-1)). Similarly, the PMCA V(max) of RBCs from 11 streptozotocin-diabetic rats was not affected by plasma glucose levels more than three times normal for 6-8 wk. Thus exposure to high glucose concentrations does not affect the ability of intact RBCs to extrude Ca2+.

A fast and simple screening test to search for specific inhibitors of the plasma membrane calcium pump

No specific inhibitors of the plasma membrane Ca(2+) pump have been found to date, limiting research on the particular contribution of this pump to the Ca(2+) homeostasis of animal cells. The search for Ca(2+) pump inhibitors may have been hampered by the lack of an efficient screening method to measure pump activity that would provide an alternative to the lengthy and costly adenosine triphosphatase or Ca(2+)-flux measurements. We propose here a novel screening method in which Ca(2+) pump inhibition is translated into easily measurable cell dehydration. Intact human red cells, suspended in Ca(2+)-containing, low-K(+) buffers were exposed to sequential additions of (1) ionophore A23187 (t = 0) to load the cells with Ca(2+); (2) CoCl(2) (t = 1 minute) to block ionophore-mediated Ca(2+) transport and to allow complete extrusion of the Ca(2+) load by the pump in less than 5 minutes; and (3) NaSCN (t = 6 minutes) to accelerate cell dehydration via Ca(2+)-sensitive K(+) channels when the Ca(2+) load is retained as a result of Ca(2+) pump inhibition. Samples were taken at 10 to 25 minutes after ionophore addition and delivered into hypotonic media containing about 45 mmol/L NaCl. Non-dehydrated cells-with normal, uninhibited pumps-instantly underwent lysis, whereas dehydrated cells-with inhibited pumps-resisted lysis, resulting in translucent or opaque samples, respectively, which were quantifiable by light-absorption measurements. Vanadate was used as a test substance to assess the effect of putative pump inhibitors. This method offers a cost-efficient and easily automated alternative for testing large numbers of natural or synthetic agents.

Functional state of the plasma membrane Ca2+ pump in Plasmodium falciparum-infected human red blood cells

The active Ca2+ transport properties of malaria-infected, intact red blood cells are unknown. We report here the first direct measurements of Ca2+ pump activity in human red cells infected with Plasmodium falciparum, at the mature, late trophozoite stage. Ca2+ pump activity was measured by the Co2+-exposure method adapted for use in low-K+ media, optimal for parasitised cells. This required a preliminary study in normal, uninfected red cells of the effects of cell volume, membrane potential and external Na+/K+ concentrations on Ca2+ pump performance. Pump-mediated Ca2+ extrusion in normal red cells was only slightly lower in low-K+ media relative to high-K+ media despite the large differences in membrane potential predicted by the Lew-Bookchin red cell model. The effect was prevented by clotrimazole, an inhibitor of the Ca2+-sensitive K+ (KCa) channel, suggesting that it was due to minor cell dehydration. The Ca2+-saturated Ca2+ extrusion rate through the Ca2+ pump (Vmax) of parasitised red cells was marginally inhibited (2-27 %) relative to that of both uninfected red cells from the malaria-infected culture (cohorts), and uninfected red cells from the same donor kept under identical conditions (co-culture). Thus, Ca2+ pump function is largely conserved in parasitised cells up to the mature, late trophozoite stage. A high proportion of the ionophore-induced Ca2+ load in parasitised red cells is taken up by cytoplasmic Ca2+ buffers within the parasite. Following pump-mediated Ca2+ removal from the host, there remained a large residual Ca2+ pool within the parasite which slowly leaked to the host cell, from which it was pumped out.

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.

Antiarrhythmic effects of magnesium on rat papillary muscle and guinea pig ventricular myocytes

1. Despite widespread use of magnesium ion (Mg2+) for antiarrhythmic purposes, little direct information is available regarding its antiarrhythmic mechanisms. To elucidate the possible cellular mechanism, the effects of Mg2+ on early afterdepolarization (EAD), delayed afterdepolarization (DAD), triggered activity (TA), transient inward current (TI) and aftercontraction (AC) were examined in various cardiac preparations. The effects of Mg2+ on myoplasmic Ca2+ concentration were also studied. 2. The effects of Mg2+ on AC, induced by overdrive stimulation, were studied in isolated rat ventricular papillary muscle superfused with low K+ solution. In enzymatically isolated guinea pig myocytes, EAD, DAD and/or TA were induced after overdrive stimulation under conditions of superfusion with low K+ solution, using the whole-cell current-clamp method, and TI was also induced by the whole-cell voltage-clamp method. 3. Immediately after changing the solutions, containing varying concentrations of Mg2+, the effects of Mg2+ were examined. In addition, effects of Mg2+ on Ca transient were studied, using fura-2. 4. We found that: (1) in the rat papillary muscle, 10 mM Mg2+ effectively inhibited AC, which was produced after stimulation at both 3.3 Hz and 5 Hz, although 5 mM Mg2+ was without effect in the case of AC induced after 5-Hz stimulation; (2) in the myocytes, 5 mM Mg2+ did not inhibit DADs, EADs and TA, but 10 mM Mg2+ inhibited them completely; (3) the amplitude and frequency of TI decreased significantly in the presence of 10 mM Mg2+; and finally (4) 10 mM Mg2+ inhibited the Ca transient underlying DAD and/or TA. 5. The findings suggest, but do not prove unequivocally, that Mg's actions are probably due to a combination of a shift of the threshold of various ion channels to less negative potentials, a decrease in Ca2+ influx via Ca channels, a block of several K channels, and/or a block of Na-Ca exchanger. In conclusion, the present study indicates that extracellular Mg2+, via whatever mechanism, exerts antiarrhythmic activities.

Activation of cardiac chloride conductance by the tyrosine kinase inhibitor, genistein

1. Genistein (GST), an inhibitor of protein tyrosine kinase (PTK), Na3VO4 (VO4), an inhibitor of phosphotyrosine phosphatase (PTPase), and forskolin (FSK), an activator of the cyclic AMP-dependent, cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel, were applied to guinea-pig ventricular myocytes to probe for a possible role of tyrosine phosphorylation in the regulation of cardiac Cl- channels. 2. Myocytes in the standard whole-cell configuration were pulsed to various potentials and Cl- current (ICl) measured as the difference from control background current. GST (1-500 microM) activated a current that had similar biophysical properties (time- and voltage-independent; Cl(-)-dependent reversal potential and outward rectification) as ICl activated by 5 microM FSK. The EC50 for activation of Cl- conductance (gCl) by GST was approximately 100 microM, and gCl activated by GST (500 microM) was as large as gCl activated by maximally-effective FSK (5 microM). Daidzein, a GST analogue with little effect on PTK, was at least one order less effective than GST. 3. GST responses were rapidly and reversibly inhibited by 0.1-1 mM VO4 treatments that had little effect on FSK-activated ICl. 4. Niflumic acid (100-200 microM) reversibly depressed GST (100 microM)-activated gCl by 55%. 5. GST (50 microM) strongly incremented current in myocytes with cyclic AMP-dependent CFTR ICl already activated by maximally-effective FSK 5 microM. 6. Based on these results, and on evidence of a synergistic interaction between GST and FSK, we conclude that inhibition of tyrosine phosphorylation by GST causes an activation of cardiac CFTR that is not mediated by an elevation of cyclic AMP.