Nuclear magnetic resonance measurement of cytosolic free calcium levels in human red blood cells

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Enhanced Ca2+ influx in mechanically distorted erythrocytes measured with 19F nuclear magnetic resonance spectroscopy

We present the first direct nuclear magnetic resonance (NMR) evidence of enhanced entry of Ca²⁺ ions into human erythrocytes (red blood cells; RBCs), when these cells are mechanically distorted. For this we loaded the RBCs with the fluorinated Ca²⁺ chelator, 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA), and recorded ¹⁹F NMR spectra. The RBCs were suspended in gelatin gel in a special stretching/compression apparatus. The 5FBAPTA was loaded into the cells as the tetraacetoxymethyl ester; and ¹³C NMR spectroscopy with [1,6-¹³C]D-glucose as substrate showed active glycolysis albeit at a reduced rate in cell suspensions and gels. The enhancement of Ca²⁺ influx is concluded to be via the mechanosensitive cation channel Piezo1. The increased rate of influx brought about by the activator of Piezo1, 2-[5-[[(2,6-dichlorophenyl)methyl]thio]-1,3,4-thiadiazol-2-yl]-pyrazine (Yoda1) supported this conclusion; while the specificity of the cation-sensing by 5FBAPTA was confirmed by using the Ca²⁺ ionophore, A23187.

Multiple genetic loci define Ca++ utilization by bloodstream malaria parasites

Background

Bloodstream malaria parasites require Ca⁺⁺ for their development, but the sites and mechanisms of Ca⁺⁺ utilization are not well understood. We hypothesized that there may be differences in Ca⁺⁺ uptake or utilization by genetically distinct lines of P. falciparum. These differences, if identified, may provide insights into molecular mechanisms.

Results

Dose response studies with the Ca⁺⁺ chelator EGTA (ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid) revealed stable differences in Ca⁺⁺ requirement for six geographically divergent parasite lines used in previous genetic crosses, with the largest difference seen between the parents of the HB3 x Dd2 cross. Genetic mapping of Ca⁺⁺ requirement yielded complex inheritance in 34 progeny clones with a single significant locus on chromosome 7 and possible contributions from other loci. Although encoded by a gene in the significant locus and a proposed Ca⁺⁺ target, PfCRT (P. falciparum chloroquine resistance transporter), the primary determinant of clinical resistance to the antimalarial drug chloroquine, does not appear to contribute to this quantitative trait. Stage-specific application of extracellular EGTA also excluded determinants associated with merozoite egress and erythrocyte reinvasion.

Conclusions

We have identified differences in Ca⁺⁺ utilization amongst P. falciparum lines. These differences are under genetic regulation, segregating as a complex trait in genetic cross progeny. Ca⁺⁺ uptake and utilization throughout the bloodstream asexual cycle of malaria parasites represents an unexplored target for therapeutic intervention.

A kinetic fluorescence assay reveals unusual features of Ca⁺⁺ uptake in Plasmodium falciparum-infected erythrocytes

Background

To facilitate development within erythrocytes, malaria parasites increase their host cell uptake of diverse solutes including Ca⁺⁺. The mechanism and molecular basis of increased Ca⁺⁺ permeability remains less well studied than that of other solutes.

Methods

Based on an appropriate Ca⁺⁺ affinity and its greater brightness than related fluorophores, Fluo-8 was selected and used to develop a robust fluorescence-based assay for Ca⁺⁺ uptake by human erythrocytes infected with Plasmodium falciparum.

Results

Both uninfected and infected cells exhibited a large Ca⁺⁺-dependent fluorescence signal after loading with the Fluo-8 dye. Probenecid, an inhibitor of erythrocyte organic anion transporters, abolished the fluorescence signal in uninfected cells; in infected cells, this agent increased fluorescence via mechanisms that depend on parasite genotype. Kinetic fluorescence measurements in 384-well microplates revealed that the infected cell Ca⁺⁺ uptake is not mediated by the plasmodial surface anion channel (PSAC), a parasite nutrient channel at the host membrane; it also appears to be distinct from mammalian Ca⁺⁺ channels. Imaging studies confirmed a low intracellular Ca⁺⁺ in uninfected cells and higher levels in both the host and parasite compartments of infected cells. Parasite growth inhibition studies revealed a conserved requirement for extracellular Ca⁺⁺.

Conclusions

Nondestructive loading of Fluo-8 into human erythrocytes permits measurement of Ca⁺⁺ uptake kinetics. The greater Ca⁺⁺ permeability of cells infected with malaria parasites is apparent when probenecid is used to inhibit Fluo-8 efflux at the host membrane. This permeability is mediated by a distinct pathway and may be essential for intracellular parasite development. The miniaturized assay presented here should help clarify the precise transport mechanism and may identify inhibitors suitable for antimalarial drug development.

39K nuclear magnetic resonance and a mathematical model of K+ transport in human erythrocytes

(39)K nuclear magnetic resonance was used to measure the efflux of K(+) from suspensions of human erythrocytes [red blood cells (RBCs)], that occurred in response to the calcium ionophore, A23187 and calcium ions; the latter activate the Gárdos channel. Signals from the intra- and extracellular populations of (39)K(+) were selected on the basis of their longitudinal relaxation times, T (1), by using an inversion- recovery pulse sequence with the mixing time, tau(1), chosen to null one or other of the signals. Changes in RBC volume consequent upon efflux of the ions also changed the T (1) values so a new theory was implemented to obviate a potential artefact in the data analysis. The velocity of the K(+) efflux mediated by the Gárdos channel was 1.19+/-0.40 mmol (L RBC)(-1) min(-1) at 37 degrees C.

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.

Effects of chronic hypoxia on maternal vasodilation and vascular reactivity in guinea pig and ovine pregnancy

During pregnancy, exposure to chronic hypoxia is thought to be associated with an increased risk of preeclampsia and fetal intrauterine growth restriction (IUGR). While some studies suggest that this process may be mediated through effects of chronic hypoxia on uterine artery vasodilation and growth, these observations are likely to be species specific and may represent genetic variability in maternal adaptation to hypoxia. This review is a comparative analysis of the effects of chronic hypoxia on vascular reactivity in pregnant and nonpregnant guinea pig and sheep. Data suggest that exposure to chronic hypoxia is associated with enhanced uterine artery blood flow in the sheep, whereas, in the guinea pig, blood flow is decreased.

Cellular ionic alterations with age: relation to hypertension and diabetes

BACKGROUND

Cytosolic free calcium (Cai) and magnesium (Mgi) are vital to cellular homeostasis and function.

OBJECTIVE

To evaluate cellular divalent cations in normal subjects at different ages and their relationship to ion levels in essential hypertension and diabetes.

DESIGN

A cross-sectional study.

SETTING

A university hospital in New York.

PARTICIPANTS

A total of 103 subjects (32 older, 71.1 +/- 1.2 y/o, and 71 young/middle aged subjects, 51.1 +/- 2.3 y/o).

INTERVENTION

Oral glucose tolerance test.

MEASUREMENTS

19F and 31P NMR spectroscopy were used to measure Cai and Mgi levels in erythrocytes from normal (>65 y/o, n = 11; <65 y/o, n = 26), hypertensive (EH) (>65 y/o, n = 9; <65 y/o, n = 30), and type 2 diabetic (DM) (>65 y/o, n = 12; <65 y/o, n = 15) subjects; these levels were also compared with glucose and insulin levels before and after oral glucose loading.

RESULTS

Fasting Mgi levels were lower (207 +/- 7.8 vs 236 +/- 7.5 microM; P < .05) and Cai higher (32.2 +/- 3.0 vs 20.3 +/- 1.8 nM; P < .05) in older than in younger normal subjects. For all normal subjects, the greater the age, the higher the Cai (r = 0.622, P = .004) and the lower the Mgi (r = -0.423; P = .011). However, no significant (P = NS) differences in Mgi or Cai levels were observed between older normal and young/middle-aged subjects with EH (Mgi = 189.7 +/- 5.9 vs 182.6 +/- 9.8 microM; Cai = 33.8 +/- 4.9 vs 35.6 +/- 4.0 nM) or DM (Mgi = 182.8 +/- 10.9 vs 180.8 +/- 8.1 microM; Cai = 33.6 +/- 4.3 vs 39.7 +/- 5.9 nM). Significant relationships were also found between cellular ion content, blood pressure, and glycemic indices.

CONCLUSIONS

Aging is associated with the onset of altered Cai and Mgi levels, indistinguishable from those observed in hypertension and diabetes, independent of age. We suggest that these ionic changes may be clinically significant, underlying the predisposition of older subjects to cardiovascular and metabolic diseases.

Hyporeactivity of rat diaphragmatic arterioles after exposure to hypoxia in vivo. Role of the endothelium

The effect of prior in vivo hypoxia on the in vitro responses to changes in transmural pressure, alpha-adrenoceptor activation, and depolarization with KCl were evaluated in first-order diaphragmatic arterioles. Rats (n = 14 per group) were exposed to normoxia (controls) or to hypoxia (inspired O2 concentration = 10%) for 12 or 48 h. The arteriolar pressure-diameter relationships were recorded over a pressure range from 10 to 200 mm Hg. In separate groups of arterioles (n = 12 per group), the diaphragmatic arteriolar responses to phenylephrine (10(-8) to 10(-5 M) or KCl (10 to 100 mM) were determined after exposure to either room air or hypoxia for 48 h. In half of the arterioles studied, the endothelium was removed. After 12 h of hypoxia, the pressure-diameter relationship was normal in endothelialized arterioles but was shifted upward in de-endothelialized vessels (p < 0.05). After 48 h of hypoxia, the constrictor response to increasing transmural pressure was severely suppressed in all arterioles. The intraluminal diameters during activation with phenylephrine and KCl were larger in arterioles from rats exposed to hypoxia (103 +/- 8 and 81 +/- 7 microns, respectively) than in control arterioles (41 +/- 5 and 54 +/- 6 microns, respectively; p < 0.05 for differences). During maximum phenylephrine- and KCl-induced constriction in de-endothelialized arterioles, diameters averaged 125 +/- 8 and 105 +/- 8 microns, respectively, for arterioles from hypoxic rats and 32 +/- 6 and 40 +/- 5 microns, respectively, for arterioles from control vessels. Exposure to hypoxia results in impairment of diaphragmatic arteriolar smooth muscle reactivity and reversal of the normal inhibitory influence of the endothelium on diaphragmatic arteriolar tone.

A peroxidative model of human erythrocyte intracellular Ca2+ changes with in vivo cell aging: measurement by 19F-NMR spectroscopy

Numerous changes occur with human erythrocyte aging in vivo, including an increase in free ionic intracellular calcium concentration ([Ca2+]i) (N.R. Aiken et al. (1992) Biochim. Biophys. Acta 1136, 155-160). An attractive hypothesis of cell aging suggests that oxidative stress is responsible for many age-related changes. To determine whether oxidative stress leads to increased intracellular Ca2+ concentrations, we used the fluorinated calcium probe 5,5'-difluoroBAPTA and fluorine nuclear magnetic resonance spectroscopy (19F-NMR) to measure [Ca2+]i following mild hydrogen peroxide (H2O2) stress to young red cells. Cells were separated using density centrifugation, exposed to 815 microM H2O2, loaded with the calcium probe, and [Ca2+]i measured. Intracellular [Ca2+] increased from 62 nM (+/- 4, S.E.) in untreated young cells to 173 nM (+/- 11) in peroxide treated cohort young cells. This value approached our previously reported [Ca2+]i of 221 nM (+/- 25) in old human erythrocytes. Pretreatment of young cells with (a) cobalt, which blocks Ca2+ influx through calcium channels, or (b) carbon monoxide, which prevents methemoglobin formation, inhibited the peroxide-induced increase in ionic intracellular calcium. These findings are consistent with the hypothesis that oxidative stress of erythrocytes contributes to the increased [Ca2+]i found in senescent cells, and that this is due to increased membrane Ca2+ leak resulting from oxidatively induced methemoglobin-cytoskeletal protein crosslinking.

19F NMR magnetization transfer between 5-FBAPTA and its complexes. An alternative means for measuring free Ca2+ concentration, and detection of complexes with protein in erythrocytes

The 19F NMR Ca(2+)-indicator molecule 5,5'-difluoro-1,2-bis(o- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (5-FBAPTA) was used in a procedure that enhances its range of applications to measuring free Ca2+ concentrations in buffer solutions and human erythrocytes. Even if the signal from the Ca-5-FBAPTA complex was not visible, the concentration of the complex could be calculated from saturation transfer spectra. This was demonstrated with well characterized buffer solutions in vitro and shown to also apply to concentrated haemolysates. The analysis required a precise estimate of the dissociation rate constant of the complex; this was found to be 295/s at 37 degrees C and the corresponding association rate constant was 4.1 x 10(8) L/mol/s. These values differ from those obtained previously in different buffer conditions and by two different NMR methods. A series of spectra were acquired from haemolysates containing 5-FBAPTA, in which saturating irradiation was applied at a frequency that was progressively off-set from the carrier frequency. Saturation transfer to the free 5-FBAPTA was seen from irradiation at frequencies different from that of Ca-5-FBAPTA, thus suggesting the presence of complexes with proteins.

Intracellular ionic consequences of dietary salt loading in essential hypertension. Relation to blood pressure and effects of calcium channel blockade

To study the ionic basis of salt sensitivity in hypertension, 19F-, 13P-, and 23Na-nuclear magnetic resonance techniques were used to measure cytosolic free calcium (Cai), pH (pHi), free magnesium (Mgi), and sodium (Nai) in erythrocytes of essential hypertensive subjects (n = 19). Individuals were studied for 2 mo each on low- (UNaV < 50 meq/d) and high- (UNaV > 200 meq/d) salt diets, with the concomitant administration of nifedipine (10 mg t.i.d.) or placebo tablets for 1 mo of each diet. Salt loading elevated Cai and Nai while suppressing Mgi and pHi; these changes occurred predominantly in salt-sensitive subjects (n = 9). Nifedipine blunted the pressor response to salt loading > 50% (delta diastolic BP [high-low salt vs placebo] = 5 +/- 2 vs 14 +/- 2 mmHg, P < 0.05) and reversed salt-induced ionic changes, lowering Cai and elevating Mgi and pHi. Regardless of the definition of salt sensitivity, continuous relationships were observed between the pressure response to salt loading, the levels of Cai (r = 0.726, P < 0.001), Nai (r = 0.747, P < 0.001), and pHi (r = -0.754, P < 0.001), and the salt-induced change in Mgi (r = -0.757, P < 0.001). Altogether, these results emphasize the reciprocal and coordinate nature of intracellular ionic changes in response to dietary salt loading and calcium channel blockade in essential hypertension. They suggest that salt sensitivity is mediated by cellular calcium accumulation from the extracellular space, in association with magnesium depletion and acidification. Lastly, interpretation of intracellular ion measurements in the future will require concurrent assessment of dietary salt intake.

In vivo 19F NMR studies of hyperthermia: hydrophobic environments probed by halothane

The steady-state distribution of the general anesthetic halothane in different rat tissues, including a renal adenocarcinoma with and without hyperthermia treatment, has been evaluated by in vivo 19F NMR spectroscopy. The 19F spectra of halothane (which is a hydrophobic probe) from within tissue show differences in the partitioning between normal rat tissues and adenocarcinoma. Muscle, as a control tissue, exhibits a single large resonance around 0 ppm. However, the adenocarcinoma exhibits two slow-exchanging resonances separated by 0.3 ppm with the one at the more hydrophobic chemical shift being more sensitive to hyperthermia treatment. The results from this tumor model suggest that 19F NMR spectroscopy may be useful first in detecting a change in hydrophobic environments using a lipophilic probe such as halothane, and secondly in monitoring the effects of hyperthermia, a treatment whose effectiveness may involve changes at the level of the plasma membrane. Under conditions of continuous delivery, a resonance which is not detected in the spectra of halothane in excised tissue appears 5 ppm downfield from the resonance for halothane localized in tissues. A rotating frame experiment is used to show that this resonance is derived from anesthetic absorbed on the tissue surface.

Measurement of intracellular Ca2+ in young and old human erythrocytes using 19F-NMR spectroscopy

Elevated cell calcium has been implicated in functional changes with human erythrocyte aging. However, until recently it has been difficult to measure free ionic intracellular calcium in red cells. We have made use of a fluorinated calcium chelator probe (5,5'-difluoroBAPTA) and fluorine nuclear magnetic resonance (19F-NMR) techniques to measure changes of intracellular Ca2+ concentrations ([Ca2+]i) with cell aging. We have demonstrated in these studies that human erythrocyte [Ca2+]i is significantly elevated as a function of in-vivo aging. Young cells, the least dense fraction of density-separated erythrocytes, contained an average of 62 (+/- 4) nM Ca2+ (+/- S.E.), whereas the oldest, most dense cell fraction contained 221 nM Ca2+ (+/- 25). Mechanisms by which intracellular [Ca2+] increases with in-vivo aging are currently under investigation.

Epidermal growth factor stimulates Ca2+ uptake of human erythrocytes

To examine the functional significance of epidermal growth factor (EGF) binding sites present on the human erythrocyte membrane [Engelmann et al. (1992) Am J Hematol 39:239-241], the effect of EGF on 45Ca2+ uptake and on 22Na+ efflux from these cells has been studied. In all cases media contained 1.25 mM Ca2+, whereas Na+ and K+ were varied. In 140 mM Na+/5 mM K+ medium EGF (250 ng/ml) stimulated 45Ca2+ uptake by 50%-90% in quin-2-loaded cells, and by up to threefold in untreated cells. Increasing extracellular K+ up to 75 mM at the expense of extracellular Na2+ stimulated the EGF-induced 45Ca2+ uptake by about twofold compared to 145 mM Na+ medium both in quin-2-loaded and in untreated cells. In 145 mM K+ medium, however, no EGF-induced 45Ca2+ uptake was detectable in quin-2-loaded cells, while in untreated cells Ca2+ entry was stimulated twofold by EGF. After increasing intracellular Na+ from 6 mmol/l cells to 18 mmol/l cells in untreated cells suspended in 145 mM K+ medium, 45Ca2+ uptake induced by EGF gradually increased. In contrast, in 140 mM Na+/5 mM K+ as well as in 70 mM Na+/75 mM K+ medium, 45Ca2+ uptake accelerated by EGF was largely unaffected by a modified red cell Na+ content. When 22Na-loaded untreated red cells were suspended in 145 mM K+ medium EGF stimulated red cell 22Na+ efflux by more than threefold. In 140 mM Na+/5 mM K+ as well as in 70 mM Na+/75 mM K+ medium, no 22Na+ efflux induced by the growth factor was evident.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin activation of the Ca2+ pump revealed by fluorescent chelator dyes in human red blood cell ghosts

Ca2+ transport in red blood cell ghosts was monitored with fura2 or quin2 incorporated as the free acid during resealing. This is the first report of active transport monitored by the fluorescent intensity of the chelator dyes fura2 (5-50 microM) or quin2 (250 microM) in hemoglobin-depleted ghosts. Since there are no intracellular compartments in ghosts and the intracellular concentrations of all assay chelator substances including calmodulin (CaM), the dyes, and ATP could be set, the intracellular concentrations of free and total Ca [( Cafree]i and [Catotal]i) could be calculated during the transport. Ghosts prepared with or without CaM rapidly extruded Ca2+ to a steady-state concentration of 60-100 nM. A 10(4)-fold gradient for Ca2+ was routinely produced in medium containing 1 mM Ca2+. During active Ca2+ extrusion, d[Cafree]i/dt was a second order function of [Cafree]i and was independent of the dye concentration, whereas d[Catotal]i/dt increased as a first order function of both the [Cafree]i and the concentration of the Ca:dye complex. CaM (5 microM) increased d[Catotal]i/dt by 400% at 1 microM [Cafree]i, while d[Cafree]i/dt increased by only 25%. From a series of experiments we conclude that chelated forms of Ca2+ serve as substrates for the pump under permissive control of the [Cafree]i, and this dual effect may explain cooperativity. Free Ca2+ is extruded, and probably also Ca2+ bound to CaM or other chelators, while CaM and the chelators are retained in the cell.

Relationship between total magnesium concentration and free intracellular magnesium in sheep red blood cells

The cellular free magnesium concentration of ionophore A23187 permeabilized high potassium sheep erythrocytes was measured by 31P nuclear magnetic resonance spectroscopy, and the total cellular magnesium concentration was determined by atomic absorption spectroscopy. The free versus total cellular magnesium concentrations yield a linear relationship on a log-log scale in the concentration range from 0.3 to 1.92 mmol Mg/liter cells. Thus, free intracellular magnesium concentrations can be calculated from atomic absorption data. The method permits the estimation of physiologically or experimentally induced variations of intracellular free magnesium concentrations between 7 and 405 microM magnesium in cell water. This range encompasses the free magnesium concentration of 335 +/- 60 microM in cell water determined for untreated erythrocytes.

19F-NMR study of the effect of lead on intracellular free calcium in human platelets

Lead has been shown to affect calcium homeostasis. However, there is no prior evidence to indicate an effect of low concentrations of lead in the environment (approximately 1 microM) on the intracellular free Ca2+ concentration in any human tissue. We have investigated the effect of lead on the intracellular free Ca2+ concentration of human blood platelets using 19F-NMR and a fluorinated intracellular Ca2+ indicator. We report a basal intracellular free Ca2+ value of 172 +/- 8 nM. Treatment with 1, 5, 10 and 25 microM Pb2+ resulted in average increases in intracellular free Ca2+ of 39%, 91%, 135% and 172%, respectively. The percent increase in intracellular free Ca2+ was linearly and positively correlated with the log of Pb2+ concentration. Using atomic absorption spectroscopy, a significant increase in total calcium of approx. 10 nmol/mg protein was found in 25 microM Pb2+ treated platelets. This indicates that influx of external Ca2+ contributes to the observed increase in free Ca2+. The results provide an explanation for the previously reported effects of lead on platelet function, and suggest a mechanism for low level lead-induced hypertension.

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)

Use of chlortetracycline fluorescence for the detection of Ca storing intracellular vesicles in normal human erythrocytes

The uptake of chlortetracycline (CTC) and the nature of the fluorescence of CTC was studied in intact human erythrocytes from apparently healthy donors. The uptake of CTC at 22 degrees C proceeded with a t1/2 of about 3 min, and after 15 min a stable equilibrium was achieved with an intracellular accumulation by a factor of 5-6 relative to the medium concentration. The accumulation did not change in the range of CTC concentrations tested (20-500 microM). The Ca specificity of the CTC fluorescence spectrum was confirmed by Ca depletion of red cells using A23187 in the presence of EGTA and 0.2 mM Mg. This procedure decreased the total intracellular calcium content by about 70% and reduced the fluorescence intensity to one-fourth. Fluorescence microscopy of red cells incubated with 100 microM CTC at 22 degrees C showed that the fluorescence originated mainly from the red cell membrane. In addition, in about 15% of erythrocytes one or more fluorescent dots (diameter greater than 0.2 less than 1 microns) were detected. The fluorescence of the dots and membranes was related to calcium, as evidenced by the reduction of their intensity in Ca depleted cells. The number of erythrocytes with fluorescent dots and the frequency of the dots per cell was largely unaffected by lowering the incubation temperature to 0 degrees C, indicating that the dots most probably do not represent endocytotic artifacts induced by CTC. The number of dots was increased in erythrocytes preincubated with primaquine, demonstrating that CTC fluorescence can be applied to monitor the appearance of intracellular Ca storing vesicles. It is concluded that in (at least) 15% of erythrocytes obtained from apparently healthy donors intracellular vesicles containing Ca can be detected by CTC fluorescence microscopy.

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.

The effect of in vitro and in vivo cellular aging on the active calcium transport in human inside-out red cell membrane vesicles

Modelling of the in vivo and in vitro aging processes in the human red cell has stressed the following features of the active calcium uptake by inside-out vesicles: 1) it is higher in the outdated, in vitro aged, than in the fresh red cell (p less than 0.0005), and in the densest, in vivo aged fraction than in the lightest, young fraction (p = 0.08); 2) it increases following stimulation by excess calmodulin to values that are not significantly different; 3) it decreases to the same value in the absence of endogenous calmodulin and inhibitor, with and without exogenous calmodulin; 4) it is the target of a non-competitive inhibition, that is stronger in the fresh than in the outdated red cell. We conclude that the aging process does not involve neither membrane Ca-ATPase nor calmodulin, but rather the interaction of the calcium pump with the inhibitor of Ca-ATPase.

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).

Cellular Ca2+ homeostasis and Ca2+-mediated cell processes as critical targets for toxicant action: conceptual and methodological pitfalls

Because of the central role of the calcium messenger system in diverse functions of tissues, organs, and cells, Ca2+ homeostasis and function may prove to be critical cellular and molecular targets for a diverse range of toxicants. Experimental proof of these targets as a specific site of toxicant action is challenging and technically difficult as a result of the complexity of Ca2+ homeostatic and Ca2+-mediated processes. However, as the investigation of normal physiological control of Ca2+ and function will continue to be an active and productive area of basic research for several years to come, it is anticipated that these insights will be increasingly applied to the understanding of the mechanisms of action of toxic agents.

Ion transport by lasalocid A across red-blood-cell membranes. A multinuclear NMR study

Na+ and K+ fluxes mediated by lasalocid A across erythrocyte membranes have been determined from 23Na-NMR peak areas and chemical shifts, respectively. In similar experiments, Cl- transport has been monitored by NMR signal intensities. Taking into account the external pH variations, the results are readily explainable in terms of charge-balance conservation. The effect of disodium 4,4'-diisothiocyanostilbene-2,2'-disulfonate, an anion-exchange inhibitor, has also been studied.

Elevation in cytosolic free calcium concentration early in myocardial ischemia in perfused rat heart

Changes in cytosolic free calcium concentration during myocardial ischemia were measured by 19F NMR in 5FBAPTA-loaded perfused rat hearts. The hearts were perfused with Krebs-Henseleit buffer containing 5 microM of the acetoxymethyl ester of 5FBAPTA, which was hydrolyzed by cytosolic esterases to achieve cytosolic concentrations of 5FBAPTA of 0.12 to 0.65 mM. Cytosolic free calcium concentrations were calculated as the product of the ratio of peak areas for bound and free 5FBAPTA in the NMR spectra and the dissociation constant (708 nM). The basal cytosolic calcium concentration, measured in potassium or magnesium arrested hearts, was 252 nM, and the time-average calcium concentration in beating hearts was 630 nM. Following the onset of total ischemia, there was no immediate substantial change in cytosolic calcium despite a rapid decline in creatine phosphate and ATP and a marked increase in inorganic phosphate as monitored by 31P NMR, but by 10 minutes, there was a substantial increase in free calcium concentration. The ratio of peak areas of bound and free 5FBAPTA returned to the preischemic value during reperfusion, and there was no detectable loss of 5FBAPTA from the heart. Creatine phosphate was also restored to its preischemic level during reperfusion. These results indicate that cytosolic free calcium increases during ischemia and is not immediately associated with lethal injury. This increase in cytosolic calcium may activate degradative enzymes that eventually could compromise myocyte viability.

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.