Determination of membrane potential and cell volume by 19F NMR using trifluoroacetate and trifluoroacetamide probes

Erythrocyte plasma membrane potential: past and current methods for its measurement

The plasma membrane functions both as a natural insulator and a diffusion barrier to the movement of ions. A wide variety of proteins transport and pump ions to generate concentration gradients that result in voltage differences, while ion channels allow ions to move across the membrane down those gradients. Plasma membrane potential is the difference in voltage between the inside and the outside of a biological cell, and it ranges from ~- 3 to ~- 90 mV. Most of the most significant discoveries in this field have been made in excitable cells, such as nerve and muscle cells. Nevertheless, special attention has been paid to some events controlled by changes in membrane potential in non-excitable cells. The origins of several blood disorders, for instance, are related to disturbances at the level of plasma membrane in erythrocytes, the structurally simplest red blood cells. The high simplicity of erythrocytes, in particular, made them perfect candidates for the electrophysiological studies that laid the foundations for understanding the generation, maintenance, and roles of membrane potential. This article summarizes the methodologies that have been used during the past decades to determine Δψ in red blood cells, from seminal microelectrodes, through the use of nuclear magnetic resonance or lipophilic radioactive ions to quantify intra and extracellular ions, to continuously renewed fluorescent potentiometric dyes. We have attempted to highlight the advantages and disadvantages of each methodology, as well as to provide a description of the technical aspects involved.

MCT2 mediates concentration-dependent inhibition of glutamine metabolism by MOG

α-Ketoglutarate (αKG) is a key node in many important metabolic pathways. The αKG analog N-oxalylglycine (NOG) and its cell-permeable prodrug dimethyloxalylglycine (DMOG) are extensively used to inhibit αKG-dependent dioxygenases. However, whether NOG interference with other αKG-dependent processes contributes to its mode of action remains poorly understood. Here we show that, in aqueous solutions, DMOG is rapidly hydrolyzed, yielding methyloxalylglycine (MOG). MOG elicits cytotoxicity in a manner that depends on its transport by monocarboxylate transporter 2 (MCT2) and is associated with decreased glutamine-derived tricarboxylic acid-cycle flux, suppressed mitochondrial respiration and decreased ATP production. MCT2-facilitated entry of MOG into cells leads to sufficiently high concentrations of NOG to inhibit multiple enzymes in glutamine metabolism, including glutamate dehydrogenase. These findings reveal that MCT2 dictates the mode of action of NOG by determining its intracellular concentration and have important implications for the use of (D)MOG in studying αKG-dependent signaling and metabolism.

The NMR 'split peak effect' in cell suspensions: Historical perspective, explanation and applications

The physicochemical environment inside cells is distinctly different from that immediately outside. The selective exchange of ions, water and other molecules across the cell membrane, mediated by integral, membrane-embedded proteins is a hallmark of living systems. There are various methodologies available to measure the selectivity and rates (kinetics) of such exchange processes, including several that take advantage of the non-invasive nature of NMR spectroscopy. A number of solutes, including particular inorganic ions, show distinctive NMR behaviour, in which separate resonances arise from the intra- and extracellular solute populations, without the addition of shift reagents, differences in pH, or selective binding partners. This 'split peak effect/phenomenon', discovered in 1984, has become a valuable tool, used in many NMR studies of cellular behaviour and function. The explanation for the phenomenon, based on the differential hydrogen bonding of the reporter solutes to water, and the various ways in which this phenomenon has been used to investigate aspects of cellular biochemistry and physiology, are the topics of this review.

NMR magnetization-transfer analysis of rapid membrane transport in human erythrocytes

Nuclear magnetic resonance (NMR) magnetization-transfer (MT) experiments provide a convenient tool for studying rapid sub-second membrane-transport processes in situ in metabolically active cells. These experiments are used with membrane-permeable substances when separate (resolved) NMR signals are observed from their populations inside and outside the cells. Here, we provide a description of the theory and practice of the most common NMR MT experiments that have been used to study membrane-transport processes in human erythrocytes (red blood cells; RBCs). The procedures, involved in the analysis of the experimental data for defining mechanisms of transport, and for estimating values of kinetic parameters in the corresponding mathematical models, are given special attention.

Electrophysiological-metabolic modeling of microbes: applications in fuel cells and environment analysis

A formalism for simulating coupled metabolic and electrophysiological processes is presented. The resulting chemical kinetic and electrophysiological equations are solved numerically to create a cell simulator. Metabolic features of this simulator were adapted from Karyote, a multi-compartment biochemical cell modeling simulator. We present the mathematical formalism and its computational implementation as an integrated electrophysiological-metabolic model. Applications to Geobacter sulfurreducens in the environment and in a fuel cell are discussed.

Interaction of 5-fluorouracil loaded nanoparticles with 1,2-dimyristoyl-sn-glycero-3-phosphocholine liposomes used as a cellular membrane model

Nuclear magnetic resonance (NMR) spectroscopy and steady-state fluorescence anisotropy were used to study the behavior and interaction of 5-fluorouracil, both in a free form (5FU) and included in the polymer matrix of poly(butylcyanoacrylate) nanoparticles (5FUPBCN) with a phospholipid bilayer of large unilammellar vesicles composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), as a model system of biomembranes. The results confirm an interaction and penetration of 5FU into the phospholipid bilayer of DMPC liposomes. Different mechanisms of drug transfer from the aqueous environment into the model membrane environment, for the free drug and that incorporated into polymer nanoparticles, are suggested: (i) concentration-dependent reversible diffusion of the free 5FU and (ii) sustained 5FU release from nanoparticles adsorbed on the liposome surface resulting in continuous delivery of the drug into the phospholipid bilayers of the DMPC liposomes.

Enhancement of intracellular Cl- concentrations induced by extracellular ATP in guinea pig ventricular muscle

We investigated effects of extracellular ATP on intracellular chloride activities ([Cl-]i) and possible contribution of the Cl--HCO3- exchange to this increase in [Cl-]i in isolated guinea pig ventricular muscles. The [Cl-]i and intracellular pH (pHi) were recorded in quiescent ventricular muscles using double-barreled ion-selective microelectrode techniques. MgATP at a concentration higher than 0.1 mM, induced an increase in [Cl-]i, and this increase in [Cl-]i was dependent on the concentration of ATP but not on the concentration of magnesium ions present in the perfusion solution. NaADP, but not NaAMP, at a concentration of 0.5 mM induced a similar increase in [Cl-]i as that induced by MgATP. However, the NaADP-induced increase in [Cl-]i was transient and gradually returned to the control level even though NaADP was continuously present. Furthermore, ATP also triggered a transient acidification of pHi, and both increases in [Cl-]i and intracellular H+ induced by ATP were prevented when preparations were pretreated with stilbene derivatives, SITS and DIDS, or perfused with a Cl--free solution. Our findings showed that the increased extracellular ATP concentrations might trigger an increase in [Cl-]i in ventricular muscles. In light of previous studies showing that cardiac ischemia induced increases in extracellular nucleotide concentrations and [Cl-]i in ventricular muscles, we propose that ischemia-induced accumulation of ATP concentration in the extracellular space may be an important factor to trigger increment of [Cl-]i during ischemic conditions.

Development of an 19F NMR method for the analysis of fluorinated acids in environmental water samples

This investigation was carried out to evaluate 19F NMR as an analytical tool for the measurement of trifluoroacetic acid (TFA) and other fluorinated acids in the aquatic environment. A method based upon strong anionic exchange (SAX) chromatography was also optimized for the concentration of the fluoro acids prior to NMR analysis. Extraction of the analyte from the SAX column was carried out directly in the NMR solvent in the presence of the strong organic base, DBU. The method allowed the analysis of the acid without any prior cleanup steps being involved. Optimal NMR sensitivity based upon T1 relaxation times was investigated for seven fluorinated compounds in four different NMR solvents. The use of the relaxation agent chromium acetylacetonate, Cr(acac)3, within these solvent systems was also evaluated. Results show that the optimal NMR solvent differs for each fluorinated analyte. Cr(acac)3 was shown to have pronounced effects on the limits of detection of the analyte. Generally, the optimal sensitivity condition appears to be methanol-d4/2M DBU in the presence of 4 mg/mL of Cr-(acac)3. The method was validated through spike and recovery for five fluoro acids from environmentally relevant waters. Results are presented for the analysis of TFA in Toronto rainwater, which ranged from < 16 to 850 ng/L. The NMR results were confirmed by GC-MS selected-ion monitoring of the fluoroanalide derivative.

Intracellular chloride activity increases in guinea pig ventricular muscle during simulated ischemia

We investigated the effects of simulated ischemia on intracellular Cl- activity ([Cl-]i) in isolated guinea pig ventricular papillary muscles using ion-selective microelectrode techniques. Simulated ischemia in ventricular muscles was produced by stopping the flow of superfusion and immersing preparations in mineral oil as previously described [B. Vanheel, L. Leybaert, A. De Hemptinne, and I. Leusen. Am. J. Physiol. 257 (Cell Physiol. 26): C365-C379, 1989; Z. F. Lai, J. Liu, and K. Nishi. Jpn. J. Pharmacol. 72: 161-174, 1996]. When preparations were exposed to paraffin oil for 15 min, [Cl-]i markedly increased and the peak magnitude of [Cl-]i reached 55.3 +/- 2.5 mM from 18.7 +/- 3.5 mM, whereas membrane potentials (Vm) depolarized from -82.5 +/- 1.1 to -54.7 +/- 2.4 mV (n = 6 muscles from 6 animals). SITS (0.5 mM), a known blocker of the Cl-/HCO-3 exchanger, suppressed the ischemia-induced depolarization of Vm and delayed the onset of the ischemia-induced increase in [Cl-]i but did not suppress the magnitude of the increase of [Cl-]i. Under Cl--free conditions created by replacing Cl- with equimolar gluconate, the increase in [Cl-]i during ischemia was transient and suppressed by >60% compared with that in normal-Cl- conditions (peak value was 20. 3 +/- 1.7 mM, n = 6 muscles from 6 animals). The present results provide direct evidence that [Cl-]i in ventricular muscle increases in ischemic conditions in quiescent guinea pig ventricular muscle, suggesting that activation of the Cl-/HCO-3 exchanger by ischemia would partially contribute to the elevation of [Cl-]i during the initial stage of ischemia.

19F NMR study of the uptake of 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil in erythrocytes: evidence of transport by facilitated and nonfacilitated pathways

The 19F NMR resonances of intra- and extracellular 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU) in suspensions of human erythrocytes are well resolved. This phenomenon allows its transport behavior to be monitored in a 19F NMR time-course experiment. The rate of L-FMAU uptake at 25 degrees in a suspension containing L-FMAU at an initial extracellular concentration of 4 mM was 7.6 +/- 1.0 x 10(-7) pmol cell(-1) sec(-1) (N = 5). Concentration-dependent uptake studies of L-FMAU indicate the existence of both saturable and nonsaturable transport mechanisms, with a Km for the saturable uptake of approximately 1 mM. Although the transport of L-FMAU at 25 degrees was inhibited significantly (54-65%) by nitrobenzylthioinosine (NBTI) and dipyridamole, consistent with the participation of the nucleoside transporter, these inhibitors did not achieve complete blockage of L-FMAU uptake. The participation of the nucleobase transporter in L-FMAU uptake was ruled out by the absence of competition with uracil uptake, and by the lack of inhibition by papaverine. In addition, the NBTI-insensitive uptake of L-FMAU was not affected by pretreatment of the cells with the sulfhydryl reagent, p-chloromercuriphenylsulfonic acid (pCMBS). However, the NBTI- and dipyridamole-insensitive transport of L-FMAU was found to increase upon treatment of the erythrocytes with butanol, an agent that affects membrane fluidity. The partition coefficient of L-FMAU in octanol/phosphate-buffered saline determined by absorption spectrophotometry was 0.31. These data indicate that under the conditions of the studies, L-FMAU uptake by erythrocytes proceeds by both the nucleoside transporter and nonfacilitated membrane diffusion.

Fluorine-19 NMR studies of glucosyl fluoride transport in human erythrocytes

Fluorine-19 magnetization transfer studies have been used to measure the transport rate of glucopyranosyl fluorides under equilibrium exchange conditions. Although rate constants and permeabilities could be determined for beta-D-glucopyranosyl fluoride, the exchange rate for alpha-D-glucopyranosyl fluoride was found to be too slow for determination using this method. The time-dependent decomposition of the beta-glucopyranosyl fluoride also limits the accuracy of the numerical results for this species; however, it is clear that the permeabilities of the alpha and beta forms differ significantly, i.e., P beta > P alpha. This observation is in contrast to recent observations for n-fluoro-n-deoxyglucose, for which P alpha > P beta for n = 2, 3, 4, or 6. The difference can be explained in terms of a simple alternating conformation model in which one of the conformations (with an external sugar-binding site) exhibits a preference for the beta form of the molecule, while the second conformation (with an internal sugar binding site) exhibits a preference for the alpha form. Fluorine/hydroxyl substitutions unmask these preferences by selectively reducing the binding to one of the conformations, depending on the specific site of fluorination.

A 19F NMR study of lomefloxacin in human erythrocytes and its interaction with hemoglobin

19F NMR spectroscopy of a model fluoroquinolone, lomefloxacin, in an erythrocyte suspension showed separate resonances for the intra- and extra-cellular compartments. The intra-cellular peak revealed significant line broadening of the fluorine signals of lomefloxacin. Line broadening also occurred in the presence of oxyhemoglobin (HbO2), hematin, globin and iron. This evidence indicated that lomefloxacin interacted with these compounds; however, ultrafiltration experiments indicated that there was only weak binding (5%) of lomefloxacin to HbO2. 19F and 31P NMR spectroscopy revealed that lomefloxacin may compete with 2,3-diphosphoglycerate for its binding site on HbO2. An apparent partition coefficient of 1.90 +/- 0.15 was observed for lomefloxacin in human erythrocytes, utilizing LC analysis.

Nuclear magnetic resonance analysis of cell metabolism

Nuclear magnetic resonance (NMR) continues to be a useful tool for the study of cellular metabolism. A variety of NMR techniques have been developed or newly applied to the analysis of cell systems. Many of these techniques are particularly useful for the analysis of immobilized cell bioreactors. The use of several NMR techniques has been an integral part of recent comprehensive metabolic studies. Novel computer-based models and methods have been developed which may make NMR study of metabolism more accessible and powerful.

The minimal metabolism of inhaled 1,1,1,2-tetrafluoroethane to trifluoroacetic acid in man as determined by high sensitivity 19F nuclear magnetic resonance spectroscopy of urine samples

In this work, oxidative metabolism of the new propellant, 1,1,1,2-tetrafluoroethane to trifluoroacetic acid in man is shown to be minimal. Alternative propellants and refrigerants are under development to replace the currently used chlorofluorocarbons which lead to stratospheric ozone depletion. One potentially useful replacement is the hydrofluorocarbon, 1,1,1,2-tetrafluoroethane (HFA-134a). Before it can be used, however, particularly as a propellant in an aerosol pharmaceutical formulation whereby the compound is in effect dosed to people, it is important that the safety of this compound is established. As a part of this safety evaluation it is necessary to understand the metabolism of HFA-134a. In this work the production of the potential oxidative metabolite of HFA-134a, trifluoroacetic acid (TFA) has been studied in human urine following inhalation dosing with HFA-134a. The concentrations of TFA in urine have been measured using a highly sensitive 19F nuclear magnetic resonance procedure with a limit of detection of 10 ng ml-1 based on an acquisition time of only 2.25 h per sample. TFA is the only fluorinated species observed in the urine samples and only at very low levels, indicating that the oxidative route of metabolism can occur in vivo in man, but this metabolism is minimal in terms of percentage of administered dose.

Anomeric dependence of fluorodeoxyglucose transport in human erythrocytes

The transport of several n-fluoro-n-deoxy-D-glucose derivatives across the human erythrocyte membrane has been studied under equilibrium exchange conditions using one- and two-dimensional nuclear magnetic resonance (NMR) techniques. This approach is based on the intracellular 19F shift, which was found to depend on the anomeric form and on the F/OH substitution position. Since the transport behavior of both glucose anomers can be followed simultaneously, this approach is particularly sensitive to differences in anomeric permeability. For 2-, 3-, 4-, and 6-fluorodeoxyglucose analogs, the alpha anomers permeate more rapidly, and the P alpha/P beta ratio is dependent on the position of fluorination, with values of 1.1, 1.3, 2.5, and 1.6, respectively, obtained at 37 degrees C. These results have been analyzed in terms of a simple alternating conformation model for the glucose transporter. Although mutarotase activity has been reported for red cells, mutarotation behavior for all anomers was found to be completely negligible on the transport and spin-lattice relaxation time scales. Metabolic transformation of the fluorinated glucose analogs, primarily to fluorinated gluconate and sorbitol analogs, is very slow and does not significantly interfere with the transport measurements. A mean ratio of 2.6 was found for the extracellular/intracellular fluorine spin-lattice relaxation rates.

Measurement of cytosolic free calcium in perfused rat heart using TF-BAPTA

The feasibility and usefulness of loading 1,2-bis(2-amino-5,6-difluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (TF-BAPTA), a new high-dissociation constant (KD) (65 microM) Ca2+ indicator, into perfused rat heart is demonstrated. TF-BAPTA-loaded perfused rat heart showed less than a 10% reduction in left ventricular developed pressure. In addition, loading perfused rat heart with TF-BAPTA had no effect on cell high-energy phosphates measured by 31P-nuclear magnetic resonance (NMR). Cytosolic free Ca2+ (Ca2+i) can be monitored in TF-BAPTA-loaded perfused rat heart using 19F-NMR. TF-BAPTA has a Ca(2+)-insensitive resonance (6F) and a Ca(2+)-sensitive fluorine (5F) that responds to changes in Ca2+ binding with fast exchange kinetics at magnetic fields < or = 8.5 T. Thus the shift difference between the 5F and 6F resonances is a measure of Ca2+i. Given the high KD and the slight differences in intra- vs. extracellular fluorine shifts, TF-BAPTA is not well suited for measuring basal Ca2+i, but it is useful for measuring increases in Ca2+i above this level. For studies in which intracellular pH changes are significant, e.g., during ischemia, pH-dependent corrections must be made to obtain an accurate Ca2+i value. Given its fast exchange kinetics, TF-BAPTA is also useful for measurement of free Ca2+ in different compartments or cells with different Ca2+i. We show that the rise in Ca2+i is not uniform during prolonged global ischemia (60 min); several different Ca2+i values are present. Thus TF-BAPTA is a useful new indicator for measuring elevations in Ca2+i or compartmentation of Ca2+i.(ABSTRACT TRUNCATED AT 250 WORDS)

Continuous measurement of cell volume changes in perfused rat salivary glands by proton NMR

Changes in intracellular and extracellular water content have been measured in perfused rat salivary glands by repetitive application of an inversion recovery (IR) pulse sequence. The relaxation reagent Gd-DTPA (10 mM) was included in the perfusate so that the intracellular and extracellular water proton signals could be distinguished by their different longitudinal relaxation times. Changes in water content in response to altered perfusion pressure and perfusate osmolarity were determined at 30-s intervals and indicated a clear separation of the intracellular and extracellular components. Using a modification of the IR pulse sequence, changes in intracellular water content were also measured at 6-s intervals. With this time resolution, differences in the rates of cell shrinkage in response to hyperosmotic perfusates and the secretomotor agonist acetylcholine were observed. The results suggest that this approach offers a relatively noninvasive method for studying cell volume regulation in intact, perfused tissues and organs.

Characterisation of erythrocyte transmembrane exchange of trifluoroacetate using 19F-NMR: evidence for transport via the monocarboxylate transporter

The transport of trifluoroacetate (TFA) and difluorophosphate (DFP) into and out of human and sheep erythrocytes was measured using 19F-NMR. The pathways for the transport in human erythrocytes were characterised by differentiating between the transport inhibition caused by different reagents. (1) Pre-treatment of human erythrocytes with N-ethylmaleimide (10 mM) caused a decrease of the membrane-permeability coefficients for TFA influx and efflux to 0.74 +/- 0.05 and 0.83 +/- 0.09-times, respectively, of those determined in the absence of inhibition. Concomitantly there was no apparent effect on the band-3-mediated transport of DFP. Thus, the decrease of the permeability of TFA is consistent with the inhibition being that of the monocarboxylate transporter. (2) Inhibition of TFA and DFP exchange was also seen in human erythrocytes treated with p-chloromercuriphenylsulfonate (pCMBS). The extent of inhibition reached a maximum value for the pCMBS concentrations beyond which further inhibition was not achieved and there was substantial residual exchange of the two solutes. (3) Residual flux of TFA was found in the presence of high concentrations of the inhibitors, alpha-cyano-4-hydroxycinnamate (> or = 4 mM) or 4,4'-dinitrostilbene-2,2'-disulfonate (> or = 1 mM) when each compound was used alone. (4) Complete inhibition of TFA uptake was obtained when human erythrocytes were treated with both alpha-cyano-4-hydroxycinnamate (4 mM) and a stilbene disulfonate. It was, therefore, concluded that simple diffusion of TFA via the lipid bilayer was negligible in human erythrocytes and that incomplete inhibition of the monocarboxylate transporter occurred when the compounds were used alone.

Bulk magnetic susceptibility induced broadening in the 19F NMR of suspended leukemic cells

The relevance of bulk magnetic susceptibility (BMS) induced broadening to in vivo NMR studies of intact cells has been examined and the significance of the contribution of BMS difference to the resolution of intra- and extracellular resonances was demonstrated. BMS difference between intra- and extracellular compartments was found to limit the resolution of intra- and extracellular 19F resonances of fluoro compounds in leukemic cells.

Transmembrane 19F NMR chemical shift difference of fluorinated solutes in liposomes, erythrocytes and erythrocyte ghosts

In erythrocytes suspended in isotonic medium, a number of fluorinated anions showed well resolved 19F NMR resonances from the solute populations in the intra- and extracellular compartments; the intracellular resonances were shifted to higher frequency (low field). In addition 19F NMR resonances of extracellular solutes were shifted to higher frequency when bovine serum albumin was incorporated into the extracellular medium. The dependence of 19F NMR chemical shift on protein concentration was also demonstrated using resealed red cell ghosts and liposomes; in the presence of external hemoglobin, lysozyme and bovine serum albumin, the shift of the external resonances was to higher frequency. In addition, significant high frequency shifts of 19F NMR resonances were evident along with an increase of temperature. The results of the present study further support the contention that the principal physical basis for the shifts is the disruption of direct hydrogen bonds between 19F of the solutes and (primarily) solvent H2O by protein hydration. The 'split peak' phenomenon is of general importance in biological systems where a transmembrane protein-concentration difference exists.

Aqueous shift reagents for high-resolution cation NMR. VI. Titration curves for in vivo 23Na and 1H2O MRS obtained from rat blood

Frequency shift/concentration calibration curves applicable to the use of shift reagents (SRs) for in vivo 23Na MRS studies can be obtained from experiments with whole blood. Here, they are reported for titrations of rat blood with the SRs DyTTHA3- and TmDOTP5-. There are a number of considerations that must be made in order to derive accurate calibration curves from the experimental data. These include the effects of bulk magnetic susceptibility (BMS, since the SRs are paramagnetic), the effects of water flux (since addition of the SR stock solution to blood renders the plasma hyperosmotic), and the consequences of restricted distribution of the SR anion in the erythrocyte suspension. We give in some detail the BMS shift theory that obtains in this case and show also how it applies to excised perfused organ as well as in vivo studies. Also, we report significant effects of adjuvant Ca2+ additions in the TmDOTP5- titrations. These are very important to the successful use of this SR in vivo. Finally, our considerations of BMS lead naturally to an understanding of its manifestations in the shifts of the 1H2O resonance frequencies of cell suspensions and tissues induced by SRs. Since these are being increasingly reported, and often misinterpreted, we devote an experiment and some discussion to this subject. We show that this phenomenon cannot be used to quantitatively discriminate intra- and extracellular 1H2O signals.

19F NMR studies of changes in membrane potential and intracellular volume during dexamethasone-induced apoptosis in human leukemic cell lines

The induction of apoptosis in leukemic cells by dexamethasone is well known, but the mechanism of this type of cell death and of dexamethasone resistance by some variants is still poorly understood. Apoptotic cell death is preceded by many changes in cellular properties, such as glucose metabolism, cell size, cell density, and others. In this study, 19F-NMR has been used to characterize changes in cell membrane potential and intracellular accessible volume during dexamethasone induced apoptosis. One dex-sensitive (CEM-C7) and three dex-resistant variants (CEM-C1, CEM-ICR27, and CEM-4R4) were examined. We have observed separate intracellular and extracellular resonances for trifluoroacetate and trifluoroacetamide added to suspended leukemic cells. From the equilibrium distribution of these fluoro-compounds between intra and extracellular spaces, the changes in membrane potential and intracellular accessible volume were calculated. The membrane potential for CEM-C7 cells was found to significantly decrease in the presence of dexamethasone (9-mV decrease within 18 h of dexamethasone treatment), while that of CEM-ICR27 was found in some samples to increase on dexamethasone incubation. The membrane potential for CEM-C1 decreased slightly, while that of CEM-4R4 was not appreciably affected by dexamethasone. The reduction of membrane potential seems to be an early step in the mechanism of dexamethasone induced apoptosis. Although the intracellular volume varied with cell type and dexamethasone incubation (for CEM-C7), the fractional intracellular volume (alpha = Vin/Vcell) was found to be the same (0.82 +/- 0.06) for all the cell lines in the presence and absence of dexamethasone.

Band-3 mediated uptake of beryllofluoride complexes by human erythrocytes

Beryllium forms several multivalent fluoride complexes in aqueous solution; the relative concentration of each is governed by the relative concentrations of the constituent ions and pH. In 9Be NMR spectra the 9Be (spin = 3/2) and 19F (spin = 1/2) spin coupling gave rise to an overlapping resonance triplet, quartet, and quintet of BeF2, BeF3-, and BeF4(2-), respectively. The low frequency shift of the quartet (0.31 ppm) and the quintet (0.62 ppm) from the triplet correlated with an increase in the number of 19F-ions in each complex. 19F NMR spectra of the complexes showed that the spin-coupled quartet of each complex was progressively shifted to higher frequency with an increase in the number of F- ions in the complex. Using 9Be and 19F NMR, the multiple equilibrium mixture of complexes was found to shift substantially to favor the BeF3- and BeF4(2-) with a relative increase of NaF concentration. The association constants for BeF2, BeF3-, and BeF4(2-) at 25 degrees C were determined directly from the peak intensities of the spectra, and by a numerical fitting procedure for multiple spectra, and were 0.51 +/- 0.17 mM-2, 0.26 +/- 0.03 mM-1, and 1.0 x 10(-2) +/- 0.1 x 10(-2) mM-1, respectively. 19F NMR spectra of human erythrocytes to which Be2+ and F- were added showed separate resonances from the intracellular populations of the complexes and these were shifted to higher frequency from their extracellular counterparts.(ABSTRACT TRUNCATED AT 250 WORDS)

Ionophore-induced Cl- transport in human erythrocyte suspensions: a multinuclear magnetic resonance study

To investigate the effect of ionophores on Cl- distribution in human erythrocyte suspensions, we measured the membrane potential by using 19F and 31P NMR methods. Incubation of human erythrocytes with 0.005 mM of the neutral ionophores valinomycin and nonactin resulted in membrane potentials of -21.2 and -17.8 mV in the presence and absence of DIDS. However, 0.020 mM of the carboxylic ionophores lasalocid, monensin, and nigericin yielded membrane potentials similar to those measured in the absence of ionophore (-9.4 mV). In methanol, the 35Cl- NMR linewidth in the presence of valinomycin was twice as broad as those observed in the presence of carboxylic ionophores, suggesting that neutral ionophores induce Cl- efflux in part via ion pairing.

The phenomenon of separate intra- and extracellular resonances of difluorophosphate in 31P and 19F NMR spectra of erythrocytes

Trifluoroacetate and trifluoroacetamide, when added to a suspension of human red blood cells, give rise to separate 19F NMR signals from the intra- and extracellular species. This phenomenon has recently been exploited for measuring the membrane potential of erythrocytes. However, the separation of the peaks was incorrectly ascribed to a difference in magnetic susceptibility between the intra- and extracellular environments. Previously, we have reported well-resolved resonances in 31P NMR spectra for the intra- and extracellular populations of some phosphoryl compounds; in these cases, however, the intracellular peak is shifted to low frequency which is the opposite to the situation with the fluorinated compounds. By using difluorophosphate, which rapidly equilibrates across the membrane of human erythrocytes and which has both the phosphoryl and fluorine functional groups, we observed the separate intra- and extracellular resonances. But, the intracellular resonance was shifted to high frequency of the extracellular resonance in the 19F spectra and to low frequency in the 31P spectra. The basis for the phenomenon in both cases is thought to be the reduced hydrogen bonding inside the cells between the solvent water and the phosphoryl oxygen or fluorine atoms.

A novel editing technique for 19F MRI: molecule-specific imaging

A novel technique is proposed to facilitate the selective imaging of specific molecules from a mixture. The application of the technique presented here demonstrates the ability to selectively produce 19F MR images of either trifluoroacetic acid or the perfluorocarbon emulsion Oxypherol-ET (perfluorotributylamine), when both molecules are present simultaneously. Selective detection is based on the presence of homonuclear J-modulation in one molecule and differential spin-spin relaxation time (T2). Perfluorotributylamine, an A3B2 system, is subject to homonuclear J-modulation, which produces a null signal from the antiphase components of the triplet (A3) when an echo time (TE) = 1/2J is used in a spin-echo image. At this echo time the second molecule, in this example trifluoroacetic acid, a non-coupled spin system, is selectively imaged. At longer echo times, e.g., TE = 1/J there is substantial recovery of the J-modulated signal, which may be solely observed due to T2 decay of the trifluoroacetic acid signal. The method is demonstrated both using phantoms and in vivo.