Comparison of indirect probes of membrane potential utilized in studies of human neutrophils

Simultaneous Measurement of Changes in Neutrophil Granulocyte Membrane Potential, Intracellular pH, and Cell Size by Multiparametric Flow Cytometry

Neutrophils provide rapid and efficient defense mechanisms against invading pathogens. Upon stimulation with proinflammatory mediators, including complement factors and bacterial peptides, neutrophils respond with changes in their membrane potential, intracellular pH, and cellular size. This study provides an approach to quantify these important changes simultaneously using multiparametric flow cytometry, thereby revealing a typical sequence of neutrophil activation consisting of depolarization, alkalization, and increase in cellular size. Additionally, the time resolution of the flow cytometric measurement is improved in order to allow changes that occur within seconds to be monitored, and thus to enhance the kinetic analysis of the neutrophil response. The method is appropriate for the reliable semiquantitative detection of small variations with respect to an increase, no change, and decrease in those parameters as demonstrated by the screening of various proinflammatory mediators. As a translational outlook, the findings are put into context in inflammatory conditions in vitro as well as in a clinically relevant whole blood model of endotoxemia. Taken together, the multiparametric analysis of neutrophil responsiveness regarding depolarization, alkalization, and changes in cellular size may contribute to a better understanding of neutrophils in health and disease, thus potentially yielding innovative mechanistic insights and possible novel diagnostic and/or prognostic approaches.

Granulocyte concentrates: how can we assess their quality?

The use of granulocyte transfusions to treat and prevent life-threatening infection in patients lacking neutrophil numbers or function may become increasingly important in aiding advances in the treatment of haematological malignancies. A critical factor in determining the outcome of granulocyte transfusion is the number of cells transfused, and collection of sufficiently high concentration of cells from donors remains challenging. A number of tests of granulocyte function can be performed in vitro to assess the quality of granulocyte concentrates, which may be useful in helping to optimize granulocyte collection, processing and storage methods. Studies that have examined neutrophil function in granulocyte concentrates to date have tended to focus on the assessment of viability, chemotaxis, phagocytosis and oxidative killing. How useful in vitro tests of neutrophil function are in predicting neutrophil function following granulocyte transfusion remains to be established in conjunction with well-designed clinical trials.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Discrete intracellular Ca(2+) pools coupled to two distinct Ca(2+) influx pathways in human platelets

Ca(2+) influx is an important event associated with platelet activation and regulated by the content of intracellular Ca(2+). Previous studies have suggested two different Ca(2+) pools and two Ca(2+) influx pathways exist in platelets. In the present study, we have investigated the regulation of thrombin- and thapsigargin-induced Ca(2+) entry into human platelets, using fluorescent indicators to monitor Ca(2+) mobilization and membrane potential. It was found that depletion of thapsigargin-sensitive Ca(2+) stores was coupled to Ca(2+) influx through a Ca(2+)-selective pathway. Additional release of Ca(2+) from the thapsigargin-insensitive pool by thrombin caused the opening of a nonselective cation channel.

Ortho substituted polychlorinated biphenyls elevate intracellular [Ca(2+)] in human granulocytes

The perturbation of Ca(2+)-homeostasis in human granulocytes exposed to ortho and non ortho substituted polychlorinated biphenyls (PCB) was investigated. Ortho substituted PCB congeners increased intracellular free calcium, [Ca(2+)]i, in a concentration-dependent manner. The increase in [Ca(2+)]i was inversely proportional to the total surface area of the ortho substituted congeners. The effect of ortho substituted PCB congeners was dependent upon external Ca(2+) and phospholipase C activation, except for a tetra-ortho substituted congener, 2,2',6,6'-TeCB, that was not phospholipase C-dependent. We suppose that PCBs activate phospholipase C which leads to the production of ins(1,4,5)P3. This will release Ca(2+) from intracellular stores and subsequently activation of Ca(2+) release activated Ca(2+) channels (CRAC) in the plasma membrane. It is also possible that PCBs activate CRAC in a more direct manner. Our findings show that ortho substituted PCB congeners stimulate [Ca(2+)]i elevation in human granulocytes, and this could in part account for the effects of PCB on the immune system.

Superoxide release is involved in membrane potential changes in mouse peritoneal macrophages

Participation of reactive oxygen species (ROS) in the changes in macrophage membrane potential resulted from effects of different agonists has been studied. Treatment of macrophages with chemotactic peptide fMLP or platelet-activating factor (PAF) caused a brief depolarization followed by a long-lasting hyperpolarization. Lipopolysaccharide and interferon-gamma only depolarized the plasma membrane. Chemiluminescence measurements indicated that only fMLP and PAF activated macrophages to release ROS. The hyperpolarization response of the cell was significantly decreased in the presence of superoxide dismutase (but not catalase). Moreover, the O2.- -generating system, xanthine plus xanthine oxidase, caused a marked hyperpolarization. In all the cases, the hyperpolarization induced by fMLP, PAF and O2.- -generating system was found to depend on the concentration of intracellular Ca2+ and extracellular K+. Furthermore, in the presence of quinidine, a blocker of Ca2+-dependent K+ conductance fMLP and PAF caused only prolonged depolarization while the effect of O2.- was reduced to a minimum. These data suggest that the macrophage hyperpolarization response to fMLP and PAF involves superoxide-mediated Ca2+-dependent alteration of the relative membrane permeability to K+.

ATP stimulates lysosomal sulphate transport at neutral pH: evidence for phosphorylation of the lysosomal sulphate carrier

ATP markedly stimulated sulphate uptake by rat liver lysosomes that had been treated with N-ethylmaleimide to block the effects of the lysosomal proton-translocating ATPase (H+-ATPase). Maximal stimulation required millimolar concentrations of ATP and neutral buffer pH. ATP-stimulated transport exhibited saturation kinetics with a Km of 175 microM, identical with the Km for lysosomal sulphate uptake at pH 5.0, a process that does not require ATP. The requirement for ATP was specific: other nucleotides such as AMP, ADP, CTP, GTP, ITP and UTP failed to stimulate transport. Adenosine 5'-[beta,gamma-imido]triphosphate, the non-hydrolysable analogue of ATP, also failed to stimulate sulphate uptake, suggesting a requirement for ATP hydrolysis. Lysosomal pH, membrane potential and glucose transport were unchanged by the presence of ATP under the experimental conditions, consistent with a direct effect of ATP on the sulphate transporter. Exposure of lysosomes to protein kinase A and protein kinase C inhibitors did not alter the stimulation of sulphate transport by ATP. The lysosomal sulphate transport protein might be subject to regulation by a phosphorylation pathway that is not dependent on protein kinase A or protein kinase C.

Radioiodinated (aminostyryl)pyridinium (ASP) dyes: new cell membrane probes for labeling mixed leukocytes and lymphocytes for diagnostic imaging

We prepared [125I/131I]iodo-(aminostyryl)pyridinium dyes from tributylstannyl precursors. ASP 7a and 7b labeled leukocytes ex vivo (70-94%) using saline with or without washing plasma from cells. Viability of peripheral blood lymphocytes (PBLs) (dogs, rats) and splenic lymphocytes (rats) labeled with 7a and 7b (71-82%) was unchanged after labeling (> or = 88%). Canine 7b-leukocytes showed higher uptake in inflammatory lesions than did 111In-oxine leukocytes. At 3 h, aspirates contained more radioiodine than 111In (1.65:1 to 22:1) and radioiodine was cell bound. ROI measurements (3 h) gave abscess to contralateral knee ratios of 12.3 and 10.6 for 131I-7b vs. 4.8 and 2.3 for 111In-oxine.

Simultaneous detection of free radical release and membrane current during phagocytosis

Stimulation of macrophages induces the "respiratory burst" response which is associated with the generation of superoxide (O2-), a drop in cytoplasmic pH, and a pronounced depolarization of the membrane potential. The purpose of the present studies was to determine whether an increase in O2- was temporally related to changes in membrane potential and transmembrane current. Release of O2- at the single cell level was photometrically monitored during phagocytosis of immune complexes while simultaneously measuring whole-cell current. Membrane depolarization and the generation of a non-selective current followed an increase in O2- production with a variable lag time which was correlated with the state of cellular maturation in culture. In the absence of phagocytosis, the exposure of macrophages to O2- generated by a xanthine-xanthine oxidase reaction activated a non-selective current similar to that seen after phagocytosis. These results provide the first demonstration of the relationship between free radical release and the ensuing electrophysiological signaling events which are linked to particle engulfment in phagocytic cells.

Flow cytometric determination of absolute membrane potential of cells

Membrane potential measurements using fluorescent membrane potential indicator dyes report on relative changes but usually do not result in an absolute value of the measured parameter. The method developed in this paper is based on the assumption that the negatively charged bis-oxonol distributes across the cytoplasmic membrane according to the Nernst equation. It is further supposed that the fluorescence intensity measured from a given stained cell is a single-value function of the intracellular dye concentration. The protocol suggested incorporates the construction of a calibration curve (fluorescence intensity measured from stained cells vs. extracellular dye concentration). This allows the evaluation of the membrane potential in millivolts using fluorescence readings of the cells both in the depolarized state and in the state of interest. Good agreement was found between absolute membrane potential data of human peripheral blood lymphocytes by our method and results of parallel patch clamp measurements.

Activation of murine macrophages by hydrogen peroxide

Hydrogen peroxide at concentrations from 0.1 to 20 microM enhances phagocytosis and oxidative burst of murine peritoneal macrophages. The activation of these macrophage functions is paralleled by prolonged hyperpolarization and a transient increase in cytoplasmic free calcium concentration. All the effects are dose- and time-dependent. The results obtained for H2O2 are compared with those for a natural activator, peptide N-formyl-methionyl-leucyl-phenylalanine. The data demonstrate the ability of small doses of hydrogen peroxide to stimulate macrophages through the intracellular mechanisms of ion transduction.

Whole cell Cl- currents in human neutrophils induced by cell swelling

The properties of the conductive Cl- transport pathway underlying regulatory volume decrease (RVD) in human neutrophils were investigated using the whole cell patch-clamp technique. Cell swelling was induced during whole cell recordings by making the patch pipette solution hyperosmotic (approximately 20%) relative to the bath by addition of sucrose. Immediately after establishment of the whole cell configuration, no measurable Cl- currents were evident. Over a period of several minutes the outwardly rectifying Cl- current that developed displayed no apparent voltage dependence of activation and did not inactivate with time during voltage steps over the range of -80 to +80 mV. Reduction of Cl- currents by application of suction to the interior of the pipette implied that the swelling-induced Cl- channels are activated by membrane stretch. Based on reversal potential measurements, the volume-induced Cl- conductance was found to discriminate poorly among Cl-, Br-, I-, and NO3-, to possess a finite permeability to glucuronate (Pglucuronate/PCl approximately 0.1) and to be impermeable to cations. Single-channel conductance was estimated to be 1.5 pS from analysis of the variance of membrane current fluctuations. The activated Cl- currents were blocked by 100 microM of the compound MK-447 analogue A (inhibitor constant Ki = 37 microM) and by 200 microM 3,5-diiodosalicylate, 500 microM 4-acetamido-4'-iodothiocyanostilbene-2,2'-disulfonic acid, and 200 microM UK-5099. These results suggest that the initial event triggering RVD in neutrophils may be activation of stretch sensitive Cl- channels in the plasma membrane.

EMD 52692 (bimakalim), a new potassium channel opener, attenuates luminol-enhanced chemiluminescence and superoxide anion radical formation by zymosan-activated polymorphonuclear leukocytes

We investigated the relationship of potassium channel activation on modulation of oxidative respiratory bursts in canine neutrophils. Generation of superoxide anion radicals in opsonized zymosan-activated cells was determined using the technique of ferricytochrome c reduction. Preincubation of cells with the selective potassium channel opener, EMD 52692 (1-100 microM), attenuated superoxide anion radical production. Furthermore, EMD 52692 also produced a concentration-dependent inhibition of luminol-enhanced chemiluminescence by activated neutrophils. Glyburide, a selective antagonist of ATP-sensitive potassium channels, prevented the modulatory effect of EMD 52692 on both superoxide anion generation and luminol-enhanced chemiluminescence. The results suggest that ATP-sensitive potassium channels may play a significant role in regulating oxygen-derived free radical production in neutrophil-induced tissue injury.

Protein kinase C activates an H+ (equivalent) conductance in the plasma membrane of human neutrophils

The rate of metabolic acid generation by neutrophils increases greatly when they are activated. Intracellular acidification is prevented in part by Na+/H+ exchange, but a sizable component of H+ extrusion persists in the nominal absence of Na+ and HCO3-. In this report we determined the contribution to H+ extrusion of a putative H+ conductive pathway and its mode of activation. In unstimulated cells, H+ conductance was found to be low and unaffected by depolarization. An experimental system was designed to minimize the metabolic acid generation and membrane potential changes associated with neutrophil activation. By using this system, beta-phorbol esters were shown to increase the H+ (equivalent) permeability of the plasma membrane. The direction of the phorbol ester-induced fluxes was dictated by the electrochemical H+ gradient. Moreover, the parallel migration of a counterion through a rheogenic pathway was necessary for the displacement of measurable amounts of H+ equivalents across the membrane. These findings suggest that the H+ flux is conductive. The effect of beta-phorbol esters was mimicked by diacylglycerol and mezerein and was blocked by staurosporine, whereas alpha-phorbol esters were ineffective. Together, these findings indicate that stimulation of protein kinase C induces the activation of an H+ conductance in the plasma membrane of human neutrophils. Preliminary evidence for activation of a separate, bafilomycin A1-sensitive H+ extrusion mechanism, likely a vacuolar type H(+)-ATPase, is also presented.

Altered electrophysiologic and pharmacologic response of smooth muscle cells on exposure to electrical fields generated by blood flow

The flow of the blood past the vascular wall gives rise to an electrical potential. This field is calculated to have a periodic waveform with a transluminal peak-to-peak amplitude of approximately 1.35 V/m-1. Digital imaging fluorescent microscopy was used to measure changes in the membrane potentials of smooth muscle cells by following changes in the fluorescence of the potential sensitive dye, 3,3'-dipropyloxacarbocyanine iodide (di-O-C5[3]). The effect of the low level electrical field on the membrane potentials of cultured smooth muscle vascular cells was shown to cause a steady-state depolarization of approximately 10 mV. The degree of steady-state depolarization was shown to directly vary with the frequency of the applied field and the effect was not dependent on the presence of extracellular Ca+2 or Mg+2. These effects are though to be most consistent with an electroconformational coupling mechanism. The presence of this electrokinetic field was also shown to alter the electrophysiological response of smooth muscle cells treated with 5-hydroxytryptamine. Cells exposed concurrently to both 5-HT and the electrical field showed an increased membrane depolarization thus implying that the electrokinetic field may be important in both normal and pathologic cellular responses.

Characteristics of taurine transport in rat liver lysosomes

Taurine (2-aminoethanesulfonic acid) is a unique sulfur amino acid derivative that has putative nutritional, osmoregulatory, and neuroregulatory roles and is highly concentrated within a variety of cells. The permeability of Percoll density gradient purified rat liver lysosomes to taurine was examined. Intralysosomal amino acid analysis showed trace levels of taurine compared to most other amino acids. Taurine uptake was Na(+)-independent, with an overshoot between 5-10 minutes. Trichloroacetic acid extraction studies and detergent lysis confirmed that free taurine accumulated in the lysosomal space. Kinetic studies revealed heterogeneous uptake with values for Km1 = 31 +/- 1.82 and Km2 greater than 198 +/- 10.2 mM. The uptake had a pH optimal of 6.5 and was stimulated by the potassium specific ionophore valinomycin. The exodus rate was fairly rapid, with a t1/2 of 5 minutes at 37 degrees C. Analog inhibition studies indicated substrate specificity similar to the plasma membrane beta-alanine carrier system, with inhibition by beta-alanine, hypotaurine, and taurine. alpha-Alanine, 2-methylaminoisobutyric acid (MeAIB), and threonine were poor inhibitors. No effects were observed with sucrose and the photoaffinity derivative of taurine NAP-taurine [N-(4-azido-2-nitrophenyl)-2-aminoethanesulfonate]. In summary, rat liver lysosomes possess a high Km system for taurine transport that is sensitive to changes in K+ gradient and perhaps valinomycin induced diffusional membrane potential. These features may enable lysosomes to adapt to changing intracellular concentrations of this osmotic regulatory substance.

Immunological aspects of fungal pathogenesis

The incidence of infection with the pathogenic fungi continues to escalate, especially in the era of the acquired immune deficiency syndrome. To the clinician, this heterogeneous group of organisms poses both a diagnostic and a therapeutic challenge. Consequently, growing numbers of investigators are seeking to elucidate the pathogenetic mechanisms involved in disease caused by medically important fungi. In this review, many of the recent scientific advances that have been made in the immunological aspects of the pathogenesis of fungal infections are presented. The topics covered include 1) the receptors for fungi on the surface of professional phagocytes; 2) the mechanisms for killing and growth inhibition of fungi by phagocytes; 3) the means by which fungi evade host defenses; 4) the role of humoral immunity in fungal infection; 5) immunoregulation in fungal infections; and 6) the influence of cytokines on host defenses against pathogenic fungi.

The glycosyl phosphatidylinositol-linked Fc gamma RIIIPMN mediates transmembrane signaling events distinct from Fc gamma RII

To investigate the ability of FcgammaRIII(PMN), the GPI-anchored isoform of FcgammaRIII (CD16) in polymorphonuclear leukocytes (PMN), to mediate transmembrane signaling events, we measured changes in membrane potential with DiOC(5) and in intracellular calcium with indo-1. FcgammaR were ligated by anti-FcgammaRIII mAb 3G8 (IgG and Fab), anti-FcgammaRII mAb IV.3 (IgG and Fab), and human IgG aggregates. Cell bound mAbs were also crosslinked by goat F(ab')(2) anti-mouse IgG. 3G8 IgG elicited a rapid change in [Ca(2+)](i), which was unaffected by EGTA, Vibrio cholerae toxin (CT), or Bordetella pertussis toxin (PT), and was abolished by BAPTA . Univalent receptor binding with 3G8 Fab gave no response but crosslinking with F(aV)2 GAM gave a rapid [Ca2,](i) response. Neither IV.3 Fab, IV.3 IgG, nor crosslinking of IV.3 Fab elicited a calcium signal. PI-PLC-treated PMN with the density of FcgammaRIII(PMN) reduced to that of FcgammaRII showed an unattenuated change in [Ca(2+)](i), with a 3G8 stimulus. The effects of IgG aggregates paralleled those of 3G8 mAb. These data indicate that multivalent ligation of FcgammaRIII(PMN) initiates an increase in [Ca(2+)];, derived from intracellular stores, that is distinct from both the FMLP- and FcgammaRII-induced responses. Ligand-dependent interaction with FcgammaRII is not required. Since FcgammaRIII(PMN) can internalize the FcgammaRIII-specific probe Con A-opsonized E and lyse anti-FcgammaRIII heteroantibody-opsonized chick E, this GPI-anchored molecule mediates both signal transduction and integrated cell responses.

Simultaneous imaging of cell and mitochondrial membrane potentials

The distribution of charged membrane-permeable molecular probes between intracellular organelles, the cytoplasm, and the outside medium is governed by the relative membrane electrical potentials of these regions through coupled equilibria described by the Nernst equation. A series of highly fluorescent cationic dyes of low membrane binding and toxicity (Ehrenberg, B., V. Montana, M.-D. Wei, J. P. Wuskell, and L. M. Loew, 1988. Biophys. J. 53:785-794) allows the monitoring of these equilibria through digital imaging video microscopy. We employ this combination of technologies to assess, simultaneously, the membrane potentials of cells and of their organelles in situ. We describe the methodology and optimal conditions for such measurements, and apply the technique to concomitantly follow, with good time resolution, the mitochondrial and plasma membrane potentials in several cultured cell lines. The time course of variations induced by chemical agents (ionophores, uncouplers, electron transport, and energy transfer inhibitors) in either or both these potentials is easily quantitated, and in accordance with mechanistic expectations. The methodology should therefore be applicable to the study of more subtle and specific, biologically induced potential changes in cells.

In vitro infection of cell lines with HTLV-I and SIVmac results in altered intracellular free calcium concentration and increased membrane polarization

This study was designed to determine whether retrovirus infection alters basal concentrations of intracellular free calcium, the magnitude of intracellular free calcium change upon receptor-mediated cell-stimulation, and cell membrane polarization. Two retrovirus-infected cell lines were utilized: HTLV-I-infected MT-2 cells and SIV-infected H-9 cells. Uninfected H-9 cells were used as controls. The results show that the retrovirally-infected cells had an increased basal concentration of intracellular free calcium when compared with the control cells and also a reduced magnitude of intracellular free calcium response to receptor-mediated cell stimulants. The retrovirally-infected cell membranes were also hyperpolarized when compared with the control cells.

Platelet activating factor and leukotriene B4 induce hyperpolarization of human endothelial cells but depolarization of neutrophils

We studied one expression of cell activation in neutrophils (PMN) and endothelial cells (EC), membrane potential changes [assessed by the fluorescent dye, di-C-O5(3)]. Human neutrophils responded with depolarization after exposure to fMLP, LTB4, A23187, PAF and PMA. In contrast, only PAF and LTB4 induced membrane potential changes in human umbilical vein EC, which responded with increased fluorescence, possibly indicating membrane hyperpolarization. These discordant responses may reflect processes of significance for interactions between EC and PMN.

Time dependence of transmembrane potential changes and intracellular calcium flux in stimulated human monocytes

An important characteristic of the functional differentiation of the blood monocyte is the development of its capacity to recognize and respond to stimuli. This ability is mediated to a large extent by specific receptor glycoproteins located on the cell surface. Stimulation of mononuclear phagocytes via these receptors results in a rapid rise in intracellular Ca++ concentration, accompanied or followed by a change in membrane potential, generation of oxidative products, degranulation, and effector functions such as phagocytosis, aggregation, or locomotion. While the development of these characteristics is difficult to characterize in vivo, several investigators have demonstrated in vitro changes in these cells that correlate with the development of effector function. To examine the mechanisms of specific membrane-stimulus interactions of monocytes as they differentiate into macrophage-like cells, we studied the responses of human monocytes and of monocytes incubated in serum-containing medium for up to 96 hr to the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP). Freshly isolated monocytes exhibited little change in transmembrane potential following stimulation with an optimal concentration of peptide and underwent a significant increase only after 48 hr in culture. While constant resting intracellular Ca++ concentrations were maintained during the culture period, intracellular Ca++ levels following fMLP stimulation increased with with incubation in serum, for up to 96 hr. In contrast, fMLP-induced respiratory burst activity increased from 0 to 24 hr in culture; it remained elevated at 48 hr but declined again by 96 hr. Incubation of the cells for 24 hr increased their random (unstimulated) motility in modified Boyden chambers but did not alter the cells' directed (chemotactic) response to fMLP in comparison to the response of freshly isolated monocytes. Peptide binding to the cells did not increase during the incubation period, indicating that an increase in receptor number or in affinity for fMLP was not responsible for the enhanced responsiveness to fMLP as incubation time increased. These studies indicate that incubation of monocytes in serum-containing medium leads to a complex, altered series of responses to fMLP that correlate with the differentiation of the original monocytes in vitro and may relate to the in vivo differentiation of monocytes to macrophages.

Fluorometer based multi-parameter analysis of phagocytic cell activation

Simultaneous measurements of the calcium rise, membrane potential change, and 90 degrees light scatter (shape change) responses exhibited by neutrophils upon activation, can be obtained with identical result as that obtained when independently performing each measurement. The putative intracellular mediator diacylglycerol depolarizes membrane potential and causes a decrease in light scatter. Leukotriene B4 causes a rise in calcium and a decrease in light scatter. The chemotactic peptide, N-formylmethionyl-leucyl-phenylalanine, causes a depolarization of membrane potential, a calcium rise, and a decrease in light scatter. The fura 2 measurements of intracellular free calcium indicate that the calcium concentration of unstimulated cells is much lower than previously thought based on quin 2 studies.

Cystine transport in purified rat liver lysosomes

Amino acid efflux from highly purified rat liver lysosomes exposed to the methyl ester derivatives of leucine, methionine, tyrosine and cystine was examined. The lysosomal efflux of leucine, methionine and tyrosine was unaffected by the presence of MgATP, whereas cystine efflux was stimulated by MgATP. Exposure of lysosomes to 2 mM-MgATP resulted in lysosomal acidification and a 0.5 pH unit increase in the lysosomal pH gradient through the action of a proton-pumping ATPase. Cystine efflux was also stimulated when the lysosomal proton gradient was increased through changes in buffer pH. Decreasing the lysosomal proton gradient with ionophores resulted in diminished cystine efflux. Bivalent cations had no effect on the lysosomal efflux of leucine, methionine and tyrosine. However, cystine efflux was stimulated by the presence of bivalent cations even when the lysosomal proton gradient was minimized. Cation-stimulated cystine efflux was inhibited by the presence of the calcium ionophore A23187, which altered the lysosomal membrane potential. Cystine efflux from lysosomes appears to be uniquely dependent on pH gradients and cation concentrations.

Characterization of the amiloride-sensitive Na+-H+ antiport of human neutrophils

In response to infection, neutrophils undergo a metabolic burst associated with a marked increase in acid generation. However, cytoplasmic pH (pHi) remains normal or even becomes slightly alkaline. Regulation of pHi is due, at least in part, to a Na+-H+ exchange mechanism. The basic properties of this antiport were investigated in human blood neutrophils using fluorescein derivatives as pHi indicators and by means of electronic cell sizing. Addition of external Na+ (Na+o) to acid-loaded cells resulted in intracellular alkalinization due to transmembrane H+ (equivalent) flux. The alkalinization was associated with Na+ uptake, and both processes were blocked by amiloride, suggesting Na+-H+ countertransport. The rate of amiloride-sensitive H+ efflux could be calculated from the rate of change of pHi, using a buffering power of 28 mmol X l-1 X pH unit-1, determined by titration with NH+4 or propionate-. The rate of Na+o-H+i exchange was a saturable function of Na+o (apparent Km = 73 mM). Forward (Na+o-H+i) exchange was inhibited by elevating external [H+] or internal [Na+] and competitively by amiloride (apparent Ki = 24 microM). The antiport was virtually inactive in unstimulated cells at pHi greater than or equal to 7.2 but was markedly stimulated by cytoplasmic acidification. This behavior is consistent with a role in pHi homeostasis. The possible mechanisms of stimulation of Na+-H+ countertransport in resting and activated neutrophils are discussed.

Voltage-sensitive cyanine dye fluorescence signals in lymphocytes: plasma membrane and mitochondrial components

The origin of the cyanine dye fluorescence signal in murine and human peripheral blood leukocytes was investigated using the oxa- and indo-carbocyanines di-O-C5(3) and di-I-C5(3). Fluorescence signals from individual cells suspended with nanomolar concentrations of the dyes were measured in a flow cytometer modified to permit simultaneous four-parameter analysis (including two-color fluorescence or fluorescence polarization measurements). The contributions of mitochondrial membrane potential (psi m) and plasma membrane potential (psi pm) to the total voltage-sensitive fluorescence signal were found to depend on the equilibrium extracellular dye concentration, manipulated in these experiments by varying the ratio of dye to cell density. Hence, conditions could be chosen that amplified either the psi m or the psi pm component. Selective depolarization of lymphocytes or polymorphonuclear leukocytes (PMN) in mixed cell suspensions demonstrated that defining the partition of dye between cells and medium is requisite to assessing the heterogeneity of cell responses by cyanine dye fluorescence. At extracellular dye concentrations exceeding 5 nM in equilibrated cell suspensions, both mitochondrial and plasma membrane dye toxicity were observed. In murine splenic lymphocytes, plasma membrane toxicity (dye-induced depolarization) was selective for the B lymphocytes. Certain problems in calibration of psi pm with valinomycin at low dye concentrations and perturbations of psi pm by mitochondrial inhibitors are presented. These findings address the current controversy concerning psi m and psi pm measurement in intact cells by cyanine dye fluorescence. The finding of selective toxicity at low cyanine dye concentrations suggest that purported differences in resting psi m among cells or changes in psi pm with cell activation may reflect variable susceptibility to dye toxicity rather than intrinsic cell properties.

Plasma membrane potential of neutrophils generated by the Na+ pump

The plasma membrane potential of human neutrophils was monitored using the anionic dye oxonol-V. The cells maintain a potential of -75 +/- 17 mV when suspended in physiological saline solutions. The cells are scarcely depolarized by extracellular K+ and the depolarization induced by the chemotactic peptide fMet-Leu-Phe is of similar magnitude for cells suspended in 5 or 155 mM K+. Neutrophils are, however, depolarized by suspension in K+-free media or after treatment with ouabain. Neutrophils catalyse Na+-H+ exchange and possess other electroneutral ion transport systems. We propose that the neutrophil membrane potential is generated by an electrogenic Na+ pump, that osmotic stability is achieved by electroneutral ion transport systems and that electrical stability is maintained by anion leakage. Similar mechanisms may also operate in other biological membranes.

Calcium-induced heterogeneous changes in membrane potential detected by flow cytofluorimetry

Ionophore-induced changes in the cell-associated fluorescence of samples of approx. 50000 individual murine L1210 leukemia cells which had been incubated with the voltage-sensitive dye 3,3'-dihexyloctacarbocyanine iodide (DiOC6(3] were monitored by flow cytometry. The K+ ionophore valinomycin (1 microM) produced homogeneous changes in the fluorescence of the entire population, the magnitude of which was dependent upon the concentration of extracellular K+. These changes allowed the estimation of the potassium equilibrium potential of the cells, by the null-point method, to be -11.9 mV. The Ca2+ ionophore A23187 (500 nM) produced heterogeneous changes in fluorescence, with populations of both hyperpolarised and depolarised cells. In addition, the depolarised population underwent an apparent size change, with a reduction in cell volume. This heterogeneity of response resulted in a minimal change in the median fluorescence value for the whole population, which suggests that it would not have been detectable by methods dependent upon net population-averaged changes in fluorescence. Removal of extracellular Na+ or preincubation of cells with amiloride (500 microM) effectively eliminated the depolarised population. Removal of extracellular K+ increased the hyperpolarised population. These findings provide evidence for the presence of Ca2+-induced Na+ exchange and Ca2+-induced K+ efflux mechanisms in these cells which may be expressed simultaneously in the cell population.