Passive permeability of human red blood cells to calcium

Passive transport pathways for Ca(2+) and Co(2+) in human red blood cells. (57)Co(2+) as a tracer for Ca(2+) influx

The passive transport of calcium and cobalt and their interference were studied in human red cells using (45)Ca and (57)Co as tracers. In ATP-depleted cells, with the ATP concentration reduced to about 1μM, the progress curve for (45)Ca uptake at 1mM rapidly levels off with time, consistent with a residual Ca-pump activity building up at increasing [Ca(T)](c) to reach at [Ca(T)](c) about 5μmol(lcells)(-1) a maximal pump rate that nearly countermands the passive Ca influx, resulting in a linear net uptake at a low level. In ATP-depleted cells treated with vanadate, supposed to cause Ca-pump arrest, a residual pump activity is still present at high [Ca(T)](c). Moreover, vanadate markedly increases the passive Ca(2+) influx. The residual Ca-pump activity in ATP-depleted cells is fuelled by breakdown of the large 2,3-DPG pool, rate-limited by the sustainable ATP-turnover at about 40-50μmol(lcells)(-1)h(-1). The apparent Ca(2+) affinity of the Ca-pump appears to be markedly reduced compared to fed cells. The 2,3-DPG breakdown can be prevented by inhibition of the 2,3-DPG phosphatase by tetrathionate, and under these conditions the (45)Ca uptake is markedly increased and linear with time, with the unidirectional Ca influx at 1mM Ca(2+) estimated at 50-60μmol(lcells)(-1)h(-1). The Ca influx increases with the extracellular Ca(2+) concentration with a saturating component, with K(½(Ca)) about 0.3mM, plus a non-saturating component. From (45)Ca-loaded, ATP-depleted cells the residual Ca-pump can also be detected as a vanadate- and tetrathionate-sensitive efflux. The (45)Ca efflux is markedly accelerated by external Ca(2+), both in control cells and in the presence of vanadate or tetrathionate, suggesting efflux by carrier-mediated Ca/Ca exchange. The (57)Co uptake is similar in fed cells and in ATP-depleted cells (exposed to iodoacetamide), consistent with the notion that Co(2+) is not transported by the Ca-pump. The transporter is thus neither SH-group nor ATP or phosphorylation dependent. The (57)Co uptake shows several similarities with the (45)Ca uptake in ATP-depleted cells supplemented with tetrathionate. The uptake is linear with time, and increases with the cobalt concentration with a saturating component, with J(max) about 16μmol(lcells)(-1)h(-1) and K(½(Co)) about 0.1mM, plus a non-saturating component. The (57)Co and (45)Ca uptake shows mutual inhibition, and at least the stochastic Ca(2+) influx is inhibited by Co(2+). The (57)Co and (45)Ca uptake are both insensitive to the 1,4-dihydropyridine Ca-channel blocker nifedipine, even at 100μM. The (57)Co uptake is increased at high negative membrane potentials, indicating that the uptake is at least partially electrogenic. The (57)Co influx amounts to about half the (45)Ca influx in ATP-depleted cells. It is speculated that the basal Ca(2+) and Co(2+) uptake could be mediated by a common transporter, probably with a channel-like and a carrier-mediated component, and that (57)Co could be useful as a tracer for at least the channel-like Ca(2+) entry pathway in red cells, since it is not itself transported by the Ca-pump and, moreover, is effectively buffered in the cytosol by binding to hemoglobin, without interfering with Ca(2+) buffering. The molecular identity of the putative common transporter(s) remains to be defined.

Decreased 45calcium uptake in red cells of patients with essential hypertension

Decreased 45calcium uptake was observed in red cells of 20 patients with essential hypertension. Equilibration of extracellular 45calcium with intracellular calcium was not achieved within 60 min in red cells of either hypertensive patients or control subjects. By introducing the ionophore A23187, equilibrium conditions were attained for red cells of both categories of individuals. Still the discrepancy in 45calcium uptake was preserved between them. These results support the view that red cells of hypertensive patients have an altered membrane permeability to calcium, possibly reflecting also a greater exchangeable pool of cytosolic free calcium.

Effect of the ionic strength and prostaglandin E2 on the free Ca2+ concentration and the Ca2+ influx in human red blood cells

Human red blood cells (RBCs) were loaded with the Ca(2+)-sensitive fluorescent dye fura-2 to investigate the effects of media ionic strength and prostaglandin E2 (PGE2) on the intracellular free Ca2+ concentration ([Ca2+]i). [Ca2+]i of intact RBCs in a Ca(2+)-containing physiological (high) ionic strength (HIS) solution was 75.1 +/- 8.3 nM after 5 min incubation, increasing to 114.9 +/- 9.6 nM after 1 h. In Ca(2+)-containing low ionic strength (LIS) solutions, [Ca2+]i was significantly lower than in the Ca(2+)-containing HIS solution (p = 0.041 or 0.0385 for LIS solutions containing 200 or 250 mM sucrose, respectively), but, as in HIS solution, an increase of [Ca2+]i was seen after 1 h. In Ca(2+)-free (0 Ca2+ plus 15 microM EGTA) media, [Ca2+]i decreased (ranging from 15 to 21 nM), but were not significantly different in HIS or LIS, and did not change following 1 h incubation. The effect of the ionic strength and PGE2 on passive Ca2+ influx was investigated on ATP-depleted RBCs. Ca2+ influx was faster during the initial 10 min in comparison with the subsequent time period (10-45 min), both in HIS and LIS media, decreasing from 20.3 +/- 1.9 to 12.9 +/- 1.3 micromol/(lcells x h) in HIS, and from 36.7 +/- 5.3 to 8.6 +/- 1.2 micromol/(lcells x h) in LIS. Prostaglandin E2 (PGE2; 10(-7)-10(-11) M), dissolved in deionised water or in ethanol, did not affect [Ca2+]i in either normal or in ATP-depleted RBCs suspended in Ca(2+)-containing HIS medium. Finally, the addition of carbachol (100 microM) did not affect [Ca2+]i. The present findings suggest that stimulation of the Ca(2+)-activated K+ channel by PGE2, reported in [J. Biol. Chem. 271 (1996) 18651], cannot be mediated via increased [Ca2+]i.

Reconstitution of the basal calcium transport in resealed human red blood cell ghosts

The (45)Ca(2+) influx into right-side-out resealed ghosts (RG) prepared from human red blood cells (RBC) was measured. The (45)Ca(2+) equilibration occurred with t(1/2)=2.5 min and the steady-state was reached after 17 min with the level of 22+/-2 micromol/L(packed cells) at 37 degrees C. The rate of the influx was 97+/-17 micromol/L(packed cells)h. The (45)Ca(2+) influx was saturated with [Ca(2+)](0) at 4 mmol/L and was optimal at pH 6.5 and 30 degrees C. Divalent cations (10(-4)-10(-6)mol/L), nifedipine (10(-5)-10(-4)mol/L), DIDS (up to 10(-4)mol/L), and quinidine (10(-4)-10(-3)mol/L), inhibited the (45)Ca(2+) influx while uncoupler (10(-6)-10(-5)mol/L) stimulated it. In contrast to intact RBC, vanadate inhibited the (45)Ca(2+) influx when added to the external medium, however, the stimulation was observed when vanadate was present in media during both lysis and resealing. PMA had no effect under conditions found to stimulate the Ca(2+) influx in intact RBC. The results show that the Ca(2+) influx into RG is a carrier-mediated process but without control by protein kinase C and that the influx and efflux of Ca(2+) are coupled via the H(+) homeostasis similarly as in intact RBC but with modified mechanism.

Properties of the basal calcium influx in human red blood cells

The basal (45)Ca(2+) influx in human red blood cells (RBC) into intact RBC was measured. (45)Ca(2+) was equilibrated with cells with t(1/2)=15-20 s and the influx reached the steady state value in about 90-100 s and the steady state level was 1.5+/-0.2 micromol/l(packed cells) (n=6) at 37 degrees C. The average value of the Ca(2+) influx rate was 43.2+/-8.9 micromol/l(packed cells) hour. The rate of the basal influx was pH-dependent with a pH optimum at pH 7.0 and on the temperature with the temperature optimum at 25 degrees C. The basal Ca(2+) influx was saturable with Ca(2+) up to 5 mmol/l but at higher extracellular Ca(2+) concentrations caused further increase of basal Ca(2+) influx. The (45)Ca(2+) influx was stimulated by addition of submicromolar concentrations of phorbol esters (phorbol 12-myristate-13-acetate (PMA) and phorbol-12,13-dibutyrate (PDBu)) and forskolin. Uncoupler (3,3',4',5-tetrachloro-salicylanilide (TCS) 10(-6)-10(-5) mol/l) inhibited in part the Ca(2+) influx. The results show that the basal Ca(2+) influx is mediated by a carrier and is under control of intracellular regulatory circuits. The effect of uncoupler shows that the Ca(2+) influx is in part driven by the proton-motive force and indicates that the influx and efflux of Ca(2+) are coupled via the RBC H(+) homeostasis.

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

Characterization of voltage-dependent calcium influx in human erythrocytes by fura-2

Thus far, the methods used to determine erythrocyte Ca2+ influx have not allowed the assessment of the kinetics of ion uptake. To overcome this drawback, we studied a new method, using the fluorescent Ca2+-chelator fura-2, which directly quantifies intracellular Ca2+ changes in human erythrocytes. This method has the advantage over previous techniques that it monitors continuously cellular Ca2+ levels. The Ca2+ influx is modulated by cellular membrane potential in the presence of a transmembrane Ca2+ concentration gradient and exhibits a first slow increase of the intracellular Ca2+ concentration, followed, after the reachment of a threshold value of 125 +/- 13 nM Ca2+, by a faster increase until a plateau is reached. The influx rate is inhibited by dihydropyridines in the micromolar range. These findings support the hypothesis that erythrocyte Ca2+ influx is mediated by a carrier similar to the slow Ca2+ channels and is dependent on membrane depolarization.

Changes to erythrocyte membrane cation permeability induced by a bacterial toxin

Vibrio parahaemolyticus secretes an exotoxin prepared as Kanagawa haemolysin (KH) which causes marked alterations to the function of epithelial, cardiac and other cell types, but whose cellular mode of action is poorly understood. Using human red cells as a model system with radiotracer flux techniques, we have shown that KH (1) markedly elevated the basal leak to K+, (2) raised Ca2+ influx, and as a consequence of this, (3) stimulated the Ca2+-activated K+ channel. These results suggest that an important deleterious effect of this toxin is to elevate cation permeability, which will have both direct and indirect actions on the behaviour of a variety of cell types in vivo.

Leukocyte modulation inhibits endotoxin-induced disruption of intracellular calcium homeostasis

OBJECTIVE

Sepsis is associated with disruption of intracellular calcium homeostasis. The specific mechanisms responsible for these changes remain unclear. This study attempts to modify endotoxin-induced alterations in erythrocyte intracellular calcium dynamics through modulation of the activated leukocyte and its products.

METHODS

Paired anticoagulated whole-blood specimens were obtained from healthy adult volunteers (n = 30). Specimens were incubated with 2 micrograms/mL endotoxin [lipopolysaccharide (LPS)] or saline control in the presence and absence of the white blood cell. Studies were repeated in specimens pretreated with allopurinol, superoxide dismutase, and pentoxifylline (PTX). After incubation, erythrocytes were separated, washed, and loaded with the fluorescent calcium chelator, FURA-2. Free cytosolic calcium concentration was determined on 10(6) cells using fluorescent spectroscopy. Values were computer-calculated every 1.8 seconds for 1 minute, and mean results were used for analysis. Differences were evaluated by analysis of variance.

RESULTS

The LPS resulted in a significant increase in intracellular calcium concentration (LPS 70.95 nM vs. control 44.04 nM). This increase was dependent on the presence of the white blood cell and could not be induced in its absence (control 30.15 --> LPS 32.78). Pretreatment inhibited these endotoxin-induced alterations: allopurinol, 50.49 nM; superoxide dismutase, 49.12 nM; and PTX, 40.23 nM (p < 0.01).

CONCLUSIONS

Endotoxin induces a significant increase in intracellular calcium concentration. This alteration seems to be mediated by activated neutrophils and can be ameliorated by both leukocyte modulation (PTX) and free radical scavengers.

Alzheimer amyloid beta-peptides exhibit ionophore-like properties in human erythrocytes

There is growing evidence that the amyloid beta-peptide (beta 1-40) is involved in the aetiology of Alzheimer's disease also implicating an altered calcium homeostasis of affected cells. Beta 1-40 has been proposed to form calcium channels in synthetic bilayer membranes [1]. We wanted to investigate in the present study whether beta 1-40 (or fragments thereof) could act as ionophores in a biological membrane like the one in human erythrocytes. Incubation of the cells for 2 h and 4 h at 37 degrees C together with 6 mumol L-1 of beta 1-40 or of fragments beta 1-28 and beta 25-35, resulted in a significantly decreased energy charge qualitatively similar to the one obtained by a known calcium ionophore (A 23187, 0.05 mumol L-1). Moreover, beta 1-40 and its two fragments induced a significant alteration of 45Ca permeability in human red blood cells of the same type as the one achieved by the calcium ionophore. The ionophoric action of beta 1-40 and its two fragments may lead to an increase of the intracellular calcium ion concentration, in turn resulting in enhanced Ca(2+)-ATPase activity and a decrease in energy charge. This may be valid also for neuronal plasma membranes and could, therefore, be a possible aetiological mechanism in Alzheimer's disease.

Effect of sepsis on erythrocyte intracellular calcium homeostasis

OBJECTIVES

To examine erythrocyte intracellular calcium dynamics in clinical sepsis and experimental endotoxemia.

DESIGN

Prospective, multiexperimental study utilizing in vitro manipulation and evaluation of human erythrocytes.

SETTING

University research laboratory.

PATIENTS

Healthy, elective surgical patients, "septic" surgical patients, and normal volunteers.

INTERVENTIONS

For all experimental studies, whole blood specimens were incubated with 2 micrograms/mL of Escherichia coli endotoxin (experimental) or an equivalent volume of phosphate buffered saline (control). Incubations were performed in specimens pretreated with 0.4 mM of verapamil and/or 50 mM of dantrolene. Incubations were performed in the presence and absence of extracellular calcium. Incubations were also performed utilizing pre- and posttreatment with 1 mM of adenosine 5'-triphosphate (ATP) and/or 30 mM of adenosine.

MEASUREMENTS AND MAIN RESULTS

Free cytosolic calcium concentration was determined by fluorescent spectroscopy, utilizing the calcium chelator, FURA-2AM. Sepsis was associated with a significant increase in erythrocyte intracellular calcium concentration as compared with nonseptic controls (96.26 vs. 45.38 nM; p < .001). Similar changes could be induced by endotoxin incubation of whole blood (84.52 vs. 40.45 nM; p < .001). This endotoxin-induced increase was independent of extracellular calcium concentration and was only partially ameliorated by calcium-channel blockade. Inhibition of intracellular calcium release was ineffective in altering the endotoxin-induced increase in the erythrocyte intracellular calcium value. In contrast, pretreatment with either adenosine or ATP minimized these increases. Posttreatment with ATP, but not adenosine, allowed partial reversal of this endotoxin-induced increase in intracellular calcium.

CONCLUSIONS

Sepsis induces alterations of erythrocyte intracellular calcium homeostasis. A significant increase in free cytosolic concentrations of intracellular calcium is characteristic of this altered homeostasis. These changes are reproducible by the incubation of whole blood with endotoxin. This increase in cytosolic calcium concentration appears to be independent of extracellular calcium concentration, transmembrane calcium channels, and/or intracellular calcium stores. It can, however, be modulated through provision of high-energy phosphates and/or their precursors to the cell itself.

Erythrocyte Ca2+ handling in the spontaneously hypertensive rat, effect of vanadate ions

Cytosolic Ca2+ concentration ([Ca2+]i) and 45Ca2+ influx were investigated in erythrocytes from conscious spontaneously hypertensive rats (SHR) and their normotensive controls Wistar-Kyoto (WKY). [Ca2+]i was evaluated with fura-2 and intra- and extra-cellular calibration parameters were compared. Irrespective of the calibration parameters used, erythrocyte [Ca2+]i was always significantly higher in SHR than in WKY and Wistar rats (by 25 and 40%, p < 0.01 and 0.001). A rise of the external Ca2+ concentration from 1 to 2 mmol/l increased less [Ca2+]i in SHR than in WKY erythrocytes (17 vs 37%, p < 0.01). SHR erythrocytes incorporated more 45Ca2+ than those from WKY, with an initial rate of 45Ca2+ uptake higher by 57% than that of WKY erythrocytes (p < 0.05). Vanadate ions, after corrections of their quenching effect on red cell and fura-2 fluorescence signals, increased [Ca2+]i by 19% in WKY erythrocytes (p = 0.05), but did not modify the SHR values. They also increased 45Ca2+ accumulation and the initial rate of 45Ca2+ influx in WKY erythrocytes only (p < 0.01). This study indicates that, when compared to WKY rats, erythrocytes from SHR are characterized by higher [Ca2+]i values, higher initial rate of Ca2+ influx and low sensitivity to vanadate ions.

Mechanisms of perturbation of erythrocyte calcium homeostasis in favism

Favism is an acute hemolytic anemia triggered by ingestion of fava beans in genetically susceptible subjects with severe deficiency of glucose-6-phosphate dehydrogenase (G6PD) activity. Erythrocytes from 10 favic patients had constantly and markedly increased calcium levels, as compared with values detected in 4 asymptomatic G6PD-deficient controls. Correspondingly, the calcium permeability of erythrocytes, estimated as the fraction of intracellular calcium exchangeable with externally added 45Ca2+, was invariably enhanced in favism and returned to normal patterns after several months from the acute hemolytic crisis. In favic patients, the levels of erythrocyte calcium ATPase activities showed wide variability, ranging from 2.0-12.9 mumol Pi/ml RBC/h, while control values in asymptomatic G6PD-deficient subjects were 10.62 +/- 2.03 mumol Pi/ml RBC/h. Analysis of the calcium ATPase in situ in erythrocyte membranes from favic patients showed the same molecular mass of 134 kD as observed in the control subjects. Exposure of G6PD-deficient erythrocytes in vitro to autoxidizing divicine, a pyrimidine aglycone strongly implicated in the pathogenesis of favism which leads to late accumulation of intracellular calcium, caused: (i) a marked inactivation of calcium ATPase, without changes in the molecular mass of 134 kD; and (ii) the concomitant loss of spectrin, band 3 and band 4.1, all known substrates of the calcium activated procalpain-calpain proteolytic system. Thus, the increased intraerythrocytic calcium apparently results in the degradation of calcium ATPase observed in some favic patients. It is proposed that both enhanced calcium permeability and a calcium-stimulated degradation of the calcium pump are the mechanisms responsible for the perturbation of erythrocyte calcium homeostasis in favism.

Control of the erythrocyte free Ca2+ concentration in essential hypertension

Since Ca2+ ions seem to directly participate in the control of erythrocyte membrane structure and deformability and because cell Ca2+ metabolism has been repeatedly proposed to be modified in hypertension, the intracellular calcium ion concentration ([Ca2+]i) was investigated in red blood cells from hypertensive and normotensive subjects. [Ca2+]i was measured by using the fluorescent Ca2+ chelator fura-2. Red blood cell [Ca2+]i was increased in hypertensive compared with normotensive subjects in the whole population and further increased when hypertensive were compared with age-matched normotensive subjects. An inverse relation between age and [Ca2+]i was observed when calculated with blood pressure adjusted. In hypertensive patients, high [Ca2+]i values were associated with a reduced erythrocyte deformability. The initial rate of 45Ca2+ uptake did not differ between the two blood pressure groups. Similarly, when the extracellular Ca2+ concentration was elevated from 1 to 2 mmol/l, [Ca2+]i increased by 16 +/- 4% (p less than 0.03) in red blood cells from both groups, thus maintaining a significant difference between hypertensive and normotensive subjects. Under these conditions, the addition of 10(-7) mol/l nicardipine, a dihydropyridine Ca2+ antagonist, decreased [Ca2+]i by 15 +/- 4% (p less than 0.05) and 7 +/- 5% in erythrocytes from hypertensive and normotensive subjects, respectively, thereby reducing the difference in [Ca2+]i observed between these two groups. This nicardipine effect was positively correlated to the initial [Ca2+]i. In the presence of 5 mumol/l W7, a calmodulin antagonist, [Ca2+]i increased significantly only in erythrocytes from hypertensive patients (26 +/- 6%, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium homeostasis of human erythrocytes and its pathophysiological implications

In human red cells, Ca is mainly bound to the inner side of the plasma membrane. A smaller part may be present within intracellular Ca storing vesicles, while only a few percent of total red cell Ca is in ionized form. In some hemolytic anemias (sickle cell anemia, beta-thalassemia), an increased number of endocytotic vesicles storing Ca is probably responsible for the elevation of total red cell Ca content. Red cell Ca inward transport, which is partially susceptible to inhibition by Ca entry blockers, has been reported to be enhanced by physiological shear stress and enrichment in membrane cholesterol, as well as in some hemolytic anemias. Normal intracellular ionized Ca levels have been assessed in several diseases where elevated Ca inward transport rates or decreased Ca efflux through the Ca pump (hemolytic anemias, cystic fibrosis, essential hypertension) had been observed previously. Thus, red cell Ca homeostasis is apparently capable of keeping ionized Ca levels within the physiological range of 20-60 nM under most pathological conditions investigated so far. Conceptually, changes in red cell Ca homeostasis (or also in other red cell membrane parameters) may be of pathophysiological importance in two respects: 1) A disturbance may be directly responsible for some of the symptoms associated with a disease. This is the case in sickle cell anemia, where red cell dehydration is possibly caused by transient elevations of intracellular ionized calcium, which may activate K efflux through the Ca-activated K channel. The presence of dehydrated red cells will, in turn, lead to microvascular occlusion, a pathophysiologically important phenomenon in sickle cell anemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of ABH blood group antigens in the stimulation of a DIDS-sensitive Ca2+ influx pathway in human erythrocytes by Ulex europaeus agglutinin I and a monoclonal anti A1 antibody

Of eleven agglutinating lectins tested, only one, Ulex europaeus agglutinin I (UEA1), stimulated Ca2+ uptake in quin2-loaded erythrocytes by about 2-fold. UEA1 is known to be an alpha-L-fucose and ABH blood group specific lectin. The 45Ca2+ influx induced by UEA1 was absent in the presence of extracellular fucose (5 and 15 mM) and depended on the ABH blood group of the donor, the stimulatory potency of the lectin decreasing in the order H greater than A2 greater than A1. Ca2+ entry blockers, such as cobalt and verapamil, did not affect the 45Ca2+ influx induced by UEA1. 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) inhibited dose-dependently with a Ki of 1-2 microM. 10 microM DIDS, 10 microM 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS) and 20 microM dipyridamole fully blocked the 45Ca2+ influx induced by UEA1. The effect of UEA1 on 45Ca2+ influx was absent in K+ and Mg2+ media and was less pronounced in choline than in Na+ media. The 45Ca2+ influx induced by the lectin was abolished by preincubation with 12-O-tetradecanoylphorbol 13-acetate (TPA, 60 ng/ml). A monoclonal antibody raised against A1 erythrocytes (Bric 54) accelerated 45Ca2+ influx in quin2 loaded A1 erythrocytes by about 2-fold. No effect was seen in A2 and H erythrocytes. The 45Ca2+ influx elicited by Bric 54 exhibited a sensitivity towards inhibition by DIDS and TPA, as well as a dependence on the cation composition of the incubation medium similar to that observed with UEA1. The effects of UEA1 and Bric 54 were not additive. These observations suggest that the Ca2+ influx induced by UEA1 and Bric 54 is mediated by the same transport pathway. Since both the lectin and the antibody exhibit ABH blood group specificity, it appears reasonable to conclude that ABH antigens can serve as recognition sites for activation of a Ca2+ influx pathway in human erythrocytes, which is sensitive to inhibitors of the band 3 anion-exchanger.

Erythrocyte rheology

Two main subjects of erythrocyte rheology, deformation and aggregation, are discussed in detail, on the basis of biochemical structure. The close relationship between the life span (or cell aging) and the rheology of individual erythrocytes is also briefly described. A currently important problem is emphasized, that is, the molecular aspect of the dynamic cytoskeletal structure and the mechanism of its regulation. This concerns not only the rheological function and the survival of circulating erythrocytes, but also the pathophysiology of abnormal erythrocytes.

Alterations of human erythrocyte membrane fluidity by oxygen-derived free radicals and calcium

Two possible reasons for the structural alterations of cell membranes caused by free radicals are lipid peroxidation and an increase in the intracellular calcium ion concentration. To characterize the alterations in membrane molecular dynamics caused by oxygen-derived free radicals and calcium, human erythrocytes were spin-labeled with 5-doxyl stearic acid, and alterations in membrane fluidity were quantified by electron spin resonance spectrometry. The in vitro generation of oxygen free radicals, using hypoxanthine (0.43 mM) plus xanthine oxidase (0.07 U/mL) decreased membrane fluidity, and the addition of superoxide dismutase and catalase inhibited the effect on membrane fluidity of the hypoxanthine-xanthine oxidase system. Hydrogen peroxide (0.1 and 1 mM) also decreased membrane fluidity and caused alterations to erythrocyte morphology. In addition, a decrease in membrane fluidity was observed in erythrocytes incubated with 2.8 mM CaCl2. On the other hand, incubation of erythrocytes with calcium-free solution decreased the changes in membrane fluidity caused by hydrogen peroxide. These results suggest that changes in membrane fluidity are directly due to lipid peroxidation and are indirectly the result of increased intracellular calcium concentration. We support the hypothesis that alterations of the biophysical properties of membranes caused by free radicals play an important role in cell injury, and that the accumulation of calcium amplifies the damage to membranes weakened by free radicals.

Distinction of two components of passive Ca2+ transport into human erythrocytes by Ca2+ entry blockers

The nature of downhill Ca2+ net-transport into human erythrocytes was investigated using the experimental models of Ca2+ pump inhibition by vanadate and of intracellular chelation of Ca2+ by quin2. Ca2+ uptake by erythrocytes loaded with 0.5 mM vanadate and suspended in 145 mM Na+ -5 mM K+ media was reduced by about 60% when medium K+ was raised to 80 mM. Organic and inorganic Ca2+ entry blockers such as nifedipine (10(-5) M), verapamil (10(-4) M), diltiazem (10(-4) M), Co2+ (1.5 mM) and Cu2+ (0.1 mM) as well as the K+ channel blocker quinidine (1mM) inhibited Ca2+ uptake in 145 mM Na+ -5 mM K+ media by 60-75%. Flunarizine was less effective. In vanadate-loaded cells suspended in 70 mM Na+ -80 mM K+ media, in contrast, flunarizine exerted a dose-dependent inhibition of Ca2+ uptake by up to 80% at 10(-5) M, the other blockers being ineffective (except for verapamil at 10(-4) M). A similar pattern of inhibition was seen in quin2-loaded erythrocytes. The different susceptibility towards inhibitors may indicate that passive Ca2+ uptake by vanadate-loaded erythrocytes suspended in 145 mM Na+ -5 mM K+ media, on the one hand, and by vanadate-loaded erythrocytes suspended in 70 mM Na+ -80 mM K+ media as well as by quin2-loaded erythrocytes, on the other hand, is mediated by two different transport components.

Calcium content of the erythrocytes: a sensitive and easy handling method for measuring free calcium ions, and modulation of the Ca2+ ion concentration by the calcium antagonists nifedipine and pentoxifylline

1. A method for determining free Ca2+-ions in the erythrocyte is described, using a commercially available ORION-Ca-electrode and calomel reference electrode assembly, where changes in free Ca2+-ion concentration upon addition of 0.01% digitonin could be measured. 2. The average value found for fresh cells from 20 healthy donors at 37 degrees C (pH = 7.4) was 0.20 +/- 0.04 mumol/L referred to a haematocrit of 10%. 3. Decrease of the simultaneously determined adenosinetriphosphate (ATP) concentration indicates that ATP is presumably needed to activate the Ca-ATPase. 4. In vitro addition of the calcium antagonists pentoxifylline and nifedipine, respectively, induced a normalization of the intraerythrocytic Ca2+-ion concentration after previous increase with the ion carrier ionophore A23187. 5. The advantages and possible clinical applications of this method are discussed.

Effects of long-chain acyl carnitine on membrane fluidity of human erythrocytes

Amphiphilic compounds such as long-chain acyl carnitines accumulate in ischemic myocardium and potentially contribute to the myocardial damage. To characterize alterations in membrane molecular dynamics produced by palmitoylcarnitine, human erythrocytes were spin-labeled with 5-doxylstearic acid, and membrane fluidity was quantified by measuring the changes in the order parameter derived from ESR spectra. Palmitoylcarnitine induced triphasic alterations in membrane fluidity of human erythrocytes. The membrane fluidity increased for 5 min, then decreased in a concentration-dependent manner. At higher concentrations (100 and 150 microM) of palmitoylcarnitine, membrane fluidity increased again after 30 and 20 min of the incubation, respectively. Addition of 2.8 mM CaCl2 resulted in a significant decrease in membrane fluidity and enhanced the alterations in membrane fluidity caused by palmitoylcarnitine. The results suggest that alterations in molecular dynamics are one mechanism through which long-chain acyl carnitine could play an important role in ischemic injury.

Fluorescence measurements of free Ca2+ concentration in human erythrocytes using the Ca2+-indicator fura-2

We report here the use of the fluorescent Ca2+-chelator fura-2 to directly measure free Ca2+ concentration within intact human erythrocytes and the influence of viscosity on the fluorescence of this probe. The bright fluorescence of fura-2 has permitted the use of low concentrations of indicator and cells, thus minimizing the screening effect and the intrinsic fluorescence of haemoglobin. Erythrocytes (10(8) cells/ml) were loaded with 0.5 microM fura-2AM then diluted at 10(7) cells per ml for measurements. The extracellular signal was suppressed by addition of manganese ions just before recording spectra. Under these conditions, a blood sample of 100 microliter was sufficient for analysis. To study the influence of viscosity on fura-2 fluorescence, gelatin and polyvinylpyrrolidone at various concentrations were added to a physiological buffer to perform fura-2-Ca fluorescence standard curves. Fluorescence intensities and the apparent affinity constant for Ca2+ were modified by viscosity. When intra-erythrocytic viscosity was simulated with 21 g/l polyvinylpyrrolidone to obtain a mean viscosity of 14 mPa.s similar to that observed in human erythrocytes, the mean value of free Ca2+ concentration measured in erythrocytes from healthy subjects was 78 +/- 16 nM (mean +/- S.D., n = 29).

The calcium permeability of cold-stored erythrocytes

The calcium permeability of erythrocytes was studied by a new technique in which cells were loaded with an intracellular calcium chelator, quin 2, followed by measurement of 45Ca influx. The erythrocyte calcium permeability did not change during cold storage of blood in citrate-phosphate-dextrose for 6 weeks. Previous reports of an increased Ca2+ permeability in cold-stored erythrocytes have been based on an ATP-depletion technique which yields values for Ca2+ influx which are influenced by erythrocyte 2,3DPG levels.

Intracellular calcium content of human erythrocytes: relation to sodium transport systems

To study the possible role of intracellular Ca (Cai) in controlling the activities of the Na+-K+ pump, the Na+-K+ cotransport and the Na+/Li+ exchange system of human erythrocytes, a method was developed to measure the amount of Ca embodied within the red cell. For complete removal of Ca associated with the outer aspect of the membrane, it proved to be essential to wash the cells in buffers containing less than 20 nM Ca. Ca was extracted by HClO4 in Teflon vessels boiled in acid to avoid Ca contaminations and quantitated by flameless atomic absorption. Cai of fresh human erythrocytes of apparently healthy donors ranged between 0.9 and 2.8 mumol/liter cells. The mean value found in females was significantly higher than in males. The interindividual different Ca contents remained constant over periods of more than one year. Sixty to 90% of Cai could be removed by incubation of the cells with A23187 and EGTA. The activities of the Na+-K+ pump, of Na+-K+ cotransport and Na+/Li+ exchange and the mean cellular hemoglobin content fell with rising Cai; the red cell Na+ and K+ contents rose with Cai. Ca depletion by A23187 plus EGTA as well as chelation of intracellular Ca2+ by quin-2 did not significantly enhance the transport rates. It is concluded that the large scatter of the values of Cai of normal human erythrocytes reported in the literature mainly results from a widely differing removal of Ca associated with the outer aspect of the membrane.