Potential difference and the distribution of ions across the human red blood cell membrane; a study of the mechanism by which the fluorescent cation, diS-C3-(5) reports membrane potential

Transmembrane potential, an indicator in situ reporting cellular senescence and stress response in plant tissues

BACKGROUND

Plant cells usually sustain a stable membrane potential due to influx and/or efflux of charged ions across plasma membrane. With the growth and development of plants, different tissues and cells undergo systemic or local programmed decline. Whether the membrane potential of plasma membrane could report senescence signal of plant tissues and cells is unclear.

RESULTS

We applied a maneuverable transmembrane potential (TMP) detection method with patch-clamp setup to examine the senescence signal of leaf tissue cells in situ over the whole life cycle in Arabidopsis thaliana. The data showed that the TMPs of plant tissues and cells were varied at different growth stages, and the change of TMP was higher at the vegetative growth stage than at the reproductive stage of plant growth. The distinct change of TMP was detectable between the normal and the senescent tissues and cells in several plant species. Moreover, diverse abiotic stimuli, such as heat stress, hyperpolarized the TMP in a short time, followed by depolarized membrane potential with the senescence occurring. We further examined the TMP of plant chloroplasts, which also indicates the senescence signal in organelles.

CONCLUSIONS

This convenient TMP detection method can report the senescence signal of plant tissues and cells, and can also indicate the potential of plant tolerance to environmental stress.

Antibacterial isoamphipathic oligomers highlight the importance of multimeric lipid aggregation for antibacterial potency

Cationic charge and hydrophobicity have long been understood to drive the potency and selectivity of antimicrobial peptides (AMPs). However, these properties alone struggle to guide broad success in vivo, where AMPs must differentiate bacterial and mammalian cells, while avoiding complex barriers. New parameters describing the biophysical processes of membrane disruption could provide new opportunities for antimicrobial optimization. In this work, we utilize oligothioetheramides (oligoTEAs) to explore the membrane-targeting mechanism of oligomers, which have the same cationic charge and hydrophobicity, yet show a unique ~ 10-fold difference in antibacterial potency. Solution-phase characterization reveals little difference in structure and dynamics. However, fluorescence microscopy of oligomer-treated Staphylococcus aureus mimetic membranes shows multimeric lipid aggregation that correlates with biological activity and helps establish a framework for the kinetic mechanism of action. Surface plasmon resonance supports the kinetic framework and supports lipid aggregation as a driver of antimicrobial function.

Joseph Brown et al. use oligothioetheramides (oligo TEAs) to show that multimeric lipid aggregation in Staphylococcus aureus mimetic membranes correlates with the biological activity of oligoTEAs. These results may explain why antimicrobial peptides with identical cationic charge and hydrophobicity show different biological activity.

Effect of Composition on Antibacterial Activity of Sequence-Defined Cationic Oligothioetheramides

In response to the urgent need for new antibiotic development strategies, antimicrobial peptides and their synthetic mimetics are being investigated as promising alternatives to traditional antibiotics. To facilitate their development into clinically viable candidates, we need to understand what molecular features and physicochemical properties are needed to induce cell death. Within the context of sequence-defined oligothioetheramides (oligoTEAs), we explore the impact of the cationic pendant group and backbone hydrophobicity on the potency and selectivity of antibacterial oligoTEAs. Through antibacterial, cytotoxicity, membrane destabilization, and membrane depolarization assays, we find a strong dependency on the nature of the cationic group and improved selectivity toward bacteria by tuning backbone hydrophobicity. In particular, compounds with the guanidinium headgroup are more potent than those with amines. Finally, we identify a promising oligoTEA, PDT-4G, with enhanced activity in vitro (minimum inhibitory concentration (MIC) ∼ 0.78 μM) and moderate activity in a mouse thigh infection model of methicillin-resistant Staphylococcus aureus. The studies outlined in this work provide insights into the effect of macromolecular physicochemical properties on antibacterial potency. This knowledge base will be vital for researchers engaged in the ongoing development of clinically viable antibacterial agents.

Methodological aspects of measuring absolute values of membrane potential in human cells by flow cytometry

The bis-barbituric acid oxonol, DiBAC(4)(3) is used as a standard potentiometric probe in human cells. However, its fluorescence depends not only on membrane potential but also varies with nonpotential related changes in the amount of intracellular free and bound dye. This study demonstrates the influence of different experimental conditions on this nonspecific fluorescence proportion. IGR1 melanoma cells as a model were specifically altered in cell volume and protein content by depolarizing treatments or cell cycle synchronization. Flow cytometry was performed over a wide range of extracellular DiBAC(4)(3) concentrations. Fixation and increase in protein content led to a nonspecifically enhanced fluorescence, while changes in the amount of free intracellular dye as a result of altered cell volume proved to be negligible. To establish a calibration curve using totally depolarized cells, the pore-forming action of gramicidin should be preferred to fixation. Below 100 nM DiBAC(4)(3), the logarithmic relation between cell fluorescence and dye concentration turned into a virtually linear function intersecting with zero. Consequently, calibration can then be confined to determination of the fluorescence of depolarized cells stained with the same concentration as used for the actual measurement of membrane potential. Unexpectedly, quenching of fluorescence occurred in totally depolarized cells at concentrations higher than 6,250 nM. Linearity and quenching could be confirmed by additional experiments on Chinese hamster ovary CHO-K1 and B lymphoblastoid LCL-HO cells.

Formation of two different types of ion channels by amphotericin B in human erythrocyte membranes

The polyene antibiotic amphotericin B (AmB) is known to form aqueous pores in lipid membranes and biological membranes. Here, membrane potential and ion permeability measurements were used to demonstrate that AmB can form two types of selective ion channels in human erythrocytes, differing in their interaction with cholesterol. We show that AmB induced a cation efflux (negative membrane polarization) across cholesterol-containing liposomes and erythrocytes at low concentrations (< or =1.0 x 10(-6) M), but a sharp reversal of such polarization was observed at concentrations greater than 1.0 x 10(-6) M AmB, an indication that aqueous pores are formed. Cation-selective AmB channels are also formed across sterol-free liposomes, but aqueous pores are only formed at AmB concentrations 10 times greater. The effect of temperature on the AmB-mediated K+ efflux across erythrocytes revealed that the energies of activation for channel formation are negative and positive at AmB concentrations that lead predominantly to the formation of cation-selective channels and aqueous pores, respectively. These findings support the conclusion that the two types of AmB channels formed in human erythrocytes differ in their interactions with cholesterol and other membrane components. In effect, a membrane lipid reorganization, as induced by incubation of erythrocytes with tetrathionate, a cross-linking agent of the lipid raft-associated protein spectrin, led to differential changes in the activation parameters for the formation of both types of channels, reflecting the different lipid environments in which such structures are formed.

Targeting of cancer energy metabolism

The main purpose of this review is to update and analyze the effect of several antineoplastic drugs (adriamycin, apoptodilin, casiopeinas, cisplatin, clotrimazole, cyclophosphamide, ditercalinium, NSAIDs, tamoxifen, taxol, 6-mercaptopurine, and alpha-tocopheryl succinate) and energy metabolism inhibitors (2-DOG, gossypol, delocalized lipophilic cations, and uncouplers) on tumor development and progression. The possibility that these antineoplastic drugs currently used in in vitro cancer models, in chemo-therapy, or under study in phase I to III clinical trials induce tumor cellular death by altering also metabolite concentration (i.e., ATP), enzyme activities, and/or energy metabolism fluxes is assessed. It is proposed that the use of energy metabolic therapy, as an alternative or complementary strategy, might be a promising novel approach in the treatment of cancer.

The Escherichia coli enterotoxin STb permeabilizes piglet jejunal brush border membrane vesicles

The membrane-permeabilizing ability of the Escherichia coli enterotoxin STb was evaluated using brush border membrane vesicles isolated from piglet jejunum and a membrane-potential-sensitive fluorescent probe, 3,3'-dipropylthiadicarbocyanine iodide. A strong membrane potential was generated by the efflux of K+ ions from the vesicles in the presence of the potassium ionophore valinomycin. Under these conditions, preincubation of the vesicles with STb efficiently depolarized the membrane in a dose-dependent and saturable manner. This activity was independent of pH, however, at least between pH 5.5 and 8.0. On the other hand, in the absence of valinomycin, STb had no significant influence on the measured fluorescence levels, indicating that it was unable to modify the ionic selectivity of the intact membrane. In agreement with the fact that the integrity of the disulfide bridges of STb is known to be essential for its biological activity, a reduced and alkylated form of the toxin was unable to depolarize the membrane in the presence of valinomycin. Furthermore, two previously described poorly active STb mutants, M42S and K22A-K23A, showed no membrane-permeabilizing capacity. These results demonstrate for the first time that STb can permeabilize its target membrane and suggest that it does so by forming nonspecific pores.

Influences of lovastatin on membrane ion flow and intracellular signaling in breast cancer cells

Lovastatin, an inhibitor of cellular cholesterol synthesis, has an apparent anti-cancer property, but the detailed mechanisms of its anti-cancer effects remain poorly understood. We investigated the molecular mechanism of Lovastatin anti-tumor function through the study of its effect on membrane ion flow, gap junctional intercellular communication (GJIC), and the pathways of related signals in MCF-7 mammary cancer cells. After treatment for 24–72 h with 4, 8 or 16 μmol/L Lovastatin, cellular proliferation was examined via the MTT assay, and changes in membrane potential and cellular [Ca²⁺]_i were monitored using confocal laser microscopy. In addition, the expression of plasma membrane calcium ATPase isoform 1 (PMCA1) mRNA was analyzed via RT-PCR, the GJIC function was examined using the scrape-loading dye transfer (SLDT) technique, and MAPK phosphorylation levels were tested with the kinase activity assay. The results showed that Lovastatin treatment significantly inhibited the growth of MCF-7 breast cancer cells. It also increased the negative value of the membrane potential, leading to the hyperpolarization of cells. Moreover, Lovastatin treatment continuously enhanced [Ca²⁺]_i, although the levels of PMCA1 mRNA were unchanged. GJIC was also upregulated in MCF-7 cells, with transfer of LY Fluorescence reaching 4 to 5 rows of cells from the scraped line after treatment with 16 μmol/L Lovastatin for 72 h. Finally, downregulation of ERK1 and p38^MAPK phosphorylation were found in Lovastatin-treated MCF-7 cells. It could be deduced that Lovastatin can induce changes in cellular hyperpolarization and intracellular Ca²⁺ distributions, and increase GJIC function. These effects may result in changes in the downstream signal cascade, inhibiting the growth of MCF-7 cells.

Ion pairing between Cl- or ClO4- and alkali metal complexes of ionophore antibiotics in organic solvents: a multinuclear NMR and FT-IR study

The extent of ion pairing in chloride and perchlorate salts was studied by measurement of the Cl- and ClO4- resonances and the observation of the perchlorate stretching frequency by use of nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FT-IR), respectively, for a variety of ionophores in various solutions and in large unilaminar vesicles (LUVs). The NMR line widths of chloride and perchlorate were larger in solutions containing the neutral ionophores valinomycin (Val) and nonactin (Non) than in solutions containing the negatively charged ionophores nigericin (Nig), lasalocid (Las), and monensin (Mon). The viscosity-corrected perchlorate NMR line widths in solutions containing Val and Las were significantly negatively correlated (r2 > or = 0.99) with the dielectric constant of the solvent. Solvents with low dielectric constants favored ion pair formation. From methanolic solutions containing the Li+, Na+, K+, and Cs+ salts of Cl- and ClO4-, it was determined that the cation with the highest selectivity for the ionophore affords the most ion pairing. A decrease in pH from 7 to 3 had no significant effect on the NMR line widths of chloride and perchlorate in methanolic solutions containing Val, whereas a similar decrease in pH in a methanolic solution containing Mon caused a 2-fold increase in the line widths. The FT-IR difference spectrum of KClO4 in a methanolic solution containing Val showed splitting at the perchlorate stretching frequency. No band splitting was observed in the FT-IR difference spectrum of KClO(4) in methanolic solutions containing Las. The efflux of 35Cl in LUVs containing the neutral ionophore Val followed first-order kinetics with an efflux constant of 1.70 x 10(-3) x min(-1), as determined by 35Cl NMR spectroscopy. The induction of increased membrane permeability in LUVs by the ionophore was determined to be negligible for Val and Nig by fluorescence spectroscopy.

Electrodiffusion, barrier, and gating analysis of DIDS-insensitive chloride conductance in human red blood cells treated with valinomycin or gramicidin

Current-voltage curves for DIDS-insensitive Cl- conductance have been determined in human red blood cells from five donors. Currents were estimated from the rate of cell shrinkage using flow cytometry and differential laser light scattering. Membrane potentials were estimated from the extracellular pH of unbuffered suspensions using the proton ionophore FCCP. The width of the Gaussian distribution of cell volumes remained invariant during cell shrinkage, indicating a homogeneous C1- conductance among the cells. After pretreatment for 30 min with DIDS, net effluxes of K+ and Cl- were induced by valinomycin and were measured in the continued presence of DIDS; inhibition was maximal at approximately 65% above 1 microM DIDS at both 25 degrees C and 37 degrees C. The nonlinear current-voltage curves for DIDS-insensitive net Cl- effluxes, induced by valinomycin or gramicidin at varied [K+] o, were compared with predictions based on (1) the theory of electrodiffusion, (2) a single barrier model, (3) single occupancy, multiple barrier models, and (4) a voltage-gated mechanism. Electrodiffusion precisely describes the relationship between the measured transmembrane voltage and [K+]o. Under our experimental conditions (pH 7.5, 23 degrees C, 1-3 microM valinomycin or 60 ng/ml gramicidin, 1.2% hematocrit), the constant field permeability ratio PK/PCl is 74 +/- 9 with 10 microM DIDS, corresponding to 73% inhibition of PCl. Fitting the constant field current-voltage equation to the measured Cl- currents yields PCl = 0.13 h-1 with DIDS, compared to 0.49 h-1 without DIDS, in good agreement with most previous studies. The inward rectifying DIDS-insensitive Cl- current, however, is inconsistent with electrodiffusion and with certain single-occupancy multiple barrier models. The data are well described either by a single barrier located near the center of the transmembrane electric field, or, alternatively, by a voltage-gated channel mechanism according to which the maximal conductance is 0.055 +/- 0.005 S/g Hb, half the channels are open at -27 +/- 2 mV, and the equivalent gating charge is -1.2 +/- 0.3.

Prostaglandin E2 stimulates a Ca2+-dependent K+ channel in human erythrocytes and alters cell volume and filterability

To understand the mechanism by which human red blood cells (RBCs) contribute to hemostasis and thrombosis, we have examined the effects of metabolites released by activated platelets on intact RBCs. Prostaglandin E2 (PGE2), a signal molecule produced by activated platelets, was observed to lower the filterability of human erythrocytes by approximately 30% at 10(-10) M. PGE2 also caused a reduction in mean cell volume of approximately 10%. The shrinkage of red cells after PGE2 treatment was confirmed by documenting a decrease in osmotic fragility and an increase in cell density following exposure to the hormone. Careful analysis, however, revealed that only approximately 15% of the erythrocytes responded to stimulation with PGE2. Examination of the cause of cell shrinkage showed that induction of a PGE2-stimulated K+ efflux pathway leading to rapid loss of cellular K+ was responsible. The PGE2-stimulated K+ loss was also observed to be Ca2+-dependent, suggesting the possible involvement of the Gardos channel. Gardos channel participation was supported by the observation that two Gardos channel inhibitors, charybdotoxin and clotrimazole, independently blocked the PGE2-stimulated K+ efflux. Further evidence for Gardos channel activation came from experiments aimed at characterizing the efflux pathway followed by the obligatory counterion. Thus, K+ efflux was readily stimulated even when NO3- was substituted for Cl-, suggesting that neither KCl cotransport nor Na/K/2Cl cotransport plays a prominent role in the PGE2-induced cell shrinkage. Further, the anion transporter band 3 was implicated as the counterion efflux route, since DIDS inhibited the PGE2-stimulated cell volume change without blocking the change in membrane potential. Taken together, we propose that release of PGE2 by activated platelets constitutes part of a mechanism by which activated platelets may recruit adjacent erythrocytes to assist in clot formation.

Slow fluorescent indicators of membrane potential: a survey of different approaches to probe response analysis

Basic tenets related to the use of three main classes of potentiometric redistribution fluorescent dyes (carbocyanines, oxonols, and rhodamines) are discussed in detail. They include the structure/function relationship, formation of nonfluorescent (H-type) and fluorescent (J-type) dimers and higher aggregates, probe partitioning between membranes and medium and binding to membranes and intracellular components (with attendant changes in absorption and emission spectra, fluorescence quantum yield and lifetime). The crucial importance of suitable probe-to-cell concentration ratio and selection of optimum monitored fluorescence wavelength is illustrated in schematic diagrams and possible artifacts or puzzling results stemming from faulty experimental protocol are pointed out. Special attention is paid to procedures used for probe-response calibration (potential clamping by potassium in the presence of valinomycin, use of gramicidin D in combination with N-methylglucamine, activation of Ca-dependent K-channels by A23187, the null-point technique). Among other problems treated are dye toxicity, interaction with mitochondria and other organelles, and possible effects of intracellular pH and the quantity of cytosolic proteins and/or RNA on probe response. Individual techniques using redistribution dyes (fluorescence measurements in cuvettes, flow cytometry and microfluorimetry of individual cells including fluorescence confocal microscopy) are discussed in terms of reliability, limitations and drawbacks, and selection of suitable probes. Up-to-date examples of application of slow dyes illustrate the broad range of problems in which these probes can be used.

Use of voltage-sensitive dyes and optical recordings in the central nervous system

Understanding the spatio-temporal features of the information processing occurring in any complex neural structure requires the monitoring and analysis of the activity in populations of neurons. Electrophysiological and other mapping techniques have provided important insights into the function of neural circuits and neural populations in many systems. However, there remain limitations with these approaches. Therefore, complementary techniques which permit the monitoring of the spatio-temporal activity in neuronal populations are of continued interest. One promising approach to monitor the electrical activity in populations of neurons or on multiple sites of a single neuron is with voltage-sensitive dyes coupled with optical recording techniques. This review concentrates on the use of voltage-sensitive dyes and optical imaging as tools to study the activity in neuronal populations in the central nervous system. Focusing on 'fast' voltage-sensitive dyes first, several technical issues and developments in optical imaging will be reviewed. These will include more recent developments in voltage-sensitive dyes as well as newer developments in optical recording technology. Second, studies using voltage-sensitive dyes to investigate information processing questions in the central nervous system and in the invertebrate nervous system will be reviewed. Some emphasis will be placed on the cerebellum, but the major goal is to survey how voltage-sensitive dyes and optical recordings have been utilized in the central nervous system. The review will include optical studies on the visual, auditory, olfactory, somatosensory, auditory, hippocampal and brainstem systems, as well as single cell studies addressing information processing questions. Discussion of the intrinsic optical signals is also included. The review attempts to show how voltage-sensitive dyes and optical recordings can be used to obtain high spatial and temporal resolution monitoring of neuronal activity.

Iron transport mechanisms in reticulocytes and mature erythrocytes

The mechanism of iron transport into erythroid cells was investigated using rabbit reticulocytes and mature erythrocytes incubated with 59Fe-labelled Fe(II) in isotonic sucrose or in solutions in which the sucrose was replaced with varying amounts of isotonic NaCl or KCl. Iron uptake was inhibited at all concentrations of NaCl, in a concentration-dependent manner, but with KCl inhibition occurred only at concentrations up to 10 mM. Higher KCl concentrations stimulated iron uptake to the cytosol of the cells, but inhibited its incorporation into heme. This effect became more marked as the iron concentration was raised. It was found that KCl inhibits iron incorporation into heme and stimulates iron uptake by mature erythrocytes, as well as by reticulocytes. It is concluded that erythroid cells can take up nontransferrin-bound Fe(II) by two mechanisms. One is a high-affinity mechanism that is limited to reticulocytes, saturates at a low iron concentration, and is inhibited by metabolic inhibitors. The other is a low-affinity process that is found in both reticulocytes and erythrocytes, becomes more prominent at higher iron concentrations, and is stimulated by KCl, as well as RbCl, LiCl, CsCl, and choline Cl. The KCl stimulation is inhibited by amiloride, but not by metabolic inhibitors, and its operation is not dependent on changes in cell volume or membrane potential, but it does require the presence of a permeant extracellular anion. Iron uptake by this process appears to occur by facilitated transport and is possibly associated with exchange of Na+. A further aspect of this study was a comparison of iron uptake by reticulocytes from Fe(II)-sucrose and Fe(II)-ascorbate using a variety of incubation conditions. No major differences were observed.

Transmembrane potentials in cells: a diS-C3(3) assay for relative potentials as an indicator of real changes

The mechanism by which the fluorescent cationic dye diS-C3(3) reports on cellular transmembrane potential has been investigated in murine haemopoietic cells. Due to the large molar absorbance of diS-C3(3) and its high quantum yield of fluorescence in cells, this dye can be used at very low labelling concentrations (5 x 10(-8) to 2 x 10(-7) M). In contrast to the quenching of fluorescence observed for the most commonly used voltage-sensitive dyes of the carbocyanine class, the fluorescence intensity of diS-C3(3) increases when the dye accumulates in the cells. The method of synchronous emission spectroscopy was used to resolve the intracellular and extracellular components of the diS-C3(3) fluorescence of suspensions of labelled cells. In comparing changes in these signals consequent on changes in transmembrane potential induced by varying the extracellular concentration of potassium ions in the presence of valinomycin, the logarithm of the ratio of intensities of these two components, as predicted theoretically, was found to be a good linear measure of transmembrane potential under these conditions. The dye was also demonstrated to be suitable for flow-cytofluorimetric analysis, the logarithm of the mean population signal similarly being found to provide a good linear measure of the transmembrane potential. The conditions under which such linearity may be expected with respect to possible effects due to changes in the capacity for binding of the dye to proteins and various cytosolic structures are delineated and their validity with respect to the possibly contentious role of mitochondria in such measurements examined in particular. The use of the method in indicating changes in the transmembrane potential and/or changes in the transport numbers of the major ions determining transmembrane potential between different physiological states, the possible extension to determinations of absolute differences in potential between different cell states without calibration or comparison with potassium-ion potentials, and the conditions for validity and limitations of these partially complementary measurements, are discussed.

Evaluation of rhodamine 123 as a probe for monitoring mitochondrial function in Trypanosoma brucei spp

Rhodamine 123, a membrane potential-specific dye, has been evaluated as a probe to monitor the function of the mitochondrion in long slender bloodstream and procyclic trypomastigotes of several Trypanosoma brucei spp. By epifluorescence microscopy, mitochondrial development has been followed in long slender bloodstream and procyclic organisms stained with rhodamine 123. To photograph stained long slender bloodstream forms, it was necessary to develop a method to completely immobilize viable organisms. In both parasite forms, as the cell cycle progressed, the mitochondrion developed from a thread-like structure to a highly branched organelle. A dramatic reorganization occurred preceding cytokinesis to produce two progeny thread-like structures which were partitioned into newly formed daughter cells. The organelle within the long slender trypomastigote was found to stain optimally at 0.3 microgram/ml of rhodamine 123, while the procyclic form required 3.0 micrograms/ml. The results suggest that the plasma membrane potential is higher in the long slender parasite than in the procyclic form. The effects of inhibitors that disrupt mitochondrial function were examined in long slender and procyclic parasites, and some of these agents were shown to affect rhodamine 123 accumulation and retention. In long slender trypomastigotes the trypanosome alternative oxidase does not appear to be coupled to proton pumping, whereas in procyclic organisms the effects of inhibitors indicate that this oxidase may be coupled to a pathway that is branched preceding an antimycin A1-sensitive site.

Deoxygenation-induced cation fluxes in sickle cells. III. Cation selectivity and response to pH and membrane potential

Deoxygenation of sickle cells increases membrane permeability to Na, K, and Ca and contributes to cellular cation depletion. This study examines the physiological nature of the pathway that mediates deoxygenation-induced movements of monovalent cations. Deoxygenation-induced Rb influx was a linear function of external Rb concentration, with no evidence of saturation. Activation of the deoxygenation-induced pathway was fostered by alkaline pH (7.5), whereas ion movements via the activated pathway were optimal between pH 6.9 and 7.0 in cells incubated in media in which NO3 replaced Cl to eliminate KCl cotransport. The deoxygenation-induced pathway exhibited no selectivity among the alkali metal cations Li, Na, K, Rb, or Cs, but the monovalent organic cations tetramethylammonium, tetraethylammonium, and N-methylglucamine were excluded. Sickle cells incubated in low-Cl media (external Cl, 40 mM) to depolarize the membrane exhibited increased deoxygenation-induced K efflux and reduced Na influx. Cells treated with valinomycin to hyperpolarize the membrane showed increased deoxygenation-induced Na influx. These characteristics of the deoxygenation-induced transport pathway, linear concentration dependence, lack of cation selectivity, and response to membrane potential, argue against a carrier-mediated mechanism in favor of a diffusional process. The exclusion of small organic cations, however, suggests that factors other than ion size influence deoxygenation-induced permeability.

Cation transport and volume regulation in sickle red blood cells

Cellular dehydration is one of several pathological features of the sickle cell. Cation depletion is quite severe in certain populations of sickle cells and contributes to the rheological dysfunction that is the root cause of vascular occlusion in this disease. The mechanism of dehydration of sickle cells in vivo has not been ascertained, but three transport pathways may play important roles in this process. These include the deoxygenation-induced pathway that permits passive K+ loss and entry of Na+ and Ca2+; the K(+)-Cl- cotransport pathway, activated by acidification or cell swelling; and the Ca(2+)-activated K+ channel, or Gardos pathway, presumably activated by deoxygenation-induced Ca2+ influx. Recent evidence suggests that these pathways may interact in vivo. Heterogeneity exists among sickle cells as to the rate at which they become dense, suggesting that other factors may affect the activity or interactions of these pathways. Understanding the mechanism of dehydration of sickle cells may provide opportunities for pharmacological manipulation of cell volume to mitigate some of the symptoms of sickle cell disease.

Ion concentrations, fluxes and electrical properties of the embryonic chicken lens

The membrane properties of embryonic chicken lenses were characterized using isotopic and electrical techniques. The lenses had a relatively high water content (80%) and large extracellular space (12.5%). Isotopic uptake measurements indicated that the lens cytoplasm contained 118 mM K+ and 26 mM Cl-. A value for intracellular Na+ of 14 mM was obtained using Na(+)-sensitive microelectrodes. A double-exponential model was used to fit the efflux of 86Rb+, 22Na+, 36Cl- and [3H]mannitol (an extracellular space marker) from the lens. When perfused with artificial aqueous humor (AAH) solution, embryonic lenses exhibited membrane potentials of between -20 and -40 mV. The more negative values were generally observed in lenses from older embryos. A ouabain-sensitive component, contributing -7 mV to the membrane potential, was also identified. The relatively depolarized membrane potentials suggested that the lens membranes were only weakly selective for K+ over Na+. To test this further, lenses were perfused with AAH containing varying concentrations of K+. The resulting changes in potential were interpreted in terms of the Goldman model. The best fit of the Goldman potential equation indicated that, in the presence of ouabain, the chicken lens membranes had a relative permeability to K+, Na+ and Cl- of 1.0, 0.36, 0.51 respectively. Replacing most or all of the Na+ in the AAH caused only a small change in the membrane potential rather than the large hyperpolarization towards the K+ equilibrium potential predicted by the Goldman model. Including the K+ ionophore valinomycin in the low Na(+)-AAH solutions caused a large increase in 86Rb+ efflux but did not result in additional hyperpolarization. This suggested that the insensitivity of the membrane potential to reduced extracellular Na+ was not due to voltage or pH inactivation of lens K+ channels.

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.

Isolation of human salivary polymorphonuclear leukocytes and their stimulation-coupled responses

A simple method was developed to isolate viable human salivary polymorphonuclear leukocytes (SPMN) from the oral cavity, and stimulation-coupled responses of these cells were examined. From morphological characteristics and the presence of neutrophil-specific annexin protein (39-kDa protein), we found that these cells seemed to be very similar to human peripheral polymorphonuclear leukocytes (PPMN), although they were in rather young stages. Stimulation-coupled responses of these cells were observed in terms of superoxide (O2.-) genration, luminol chemiluminescence response (LCL), membrane depolarization, and changes in intracellular calcium ion concentration ([Ca2+]i). The rates of superoxide generation by various stimuli, such as formylmethionylleucylphenylalanine (FMLP), phorbol 12-myristate 13-acetate (PMA) and opsonized zymosan (OZ) were different. Superoxide generation and strong chemiluminescence response were observed without addition of any stimuli. This endogenous LCL was inhibited by azide and superoxide dismutase (SOD), but not by uric acid (UA). The intensity of the endogenous LCL decreased with time after isolation from the oral cavity. This decrease was accompanied by the appearance of a FMLP-coupled response. Furthermore, the endogenous activity which produced active oxygen species was maintained in the medium at 4 degrees C for a long period after isolation. From these results, it is suggested that SPMN have the ability to show characteristic responses to various stimuli, and that SPMN play important roles in the defense mechanisms in the oral cavity.

Stimulation of non-selective cation channels providing Ca2+ influx into platelets by platelet-activating factor and other aggregation inducers

To elucidate the mechanism of the receptor-stimulated Ca2+ entry into human platelets, the influence of Ca(2+)-mobilizing agonists on plasma membrane potential (Em) has been studied. Em changes were registered using potentiometric probe 3,3'-dipropyl-2,2'-thiadicarbocyanine iodide. The agonist effect on Em varied from hyperpolarization to slight and slow rise. On the contrary, after loading of platelets with intracellular Ca2+ indicator quin2, platelet-activating factor (PAF), thrombin, vasopressin, ADP and thromboxane-A2-mimetic U46619 cause substantial transient membrane depolarization. Similar effects were observed after platelet loading with other Ca2+ chelators fura-2 and indo-1. Agonist-induced depolarization considerably reduced if quin2-loaded platelets were suspended in isoosmotic choline-containing medium. Using Ba2+ as a substitute of Ca2+, we have demonstrated that in choline-containing medium PAF-induced Ba2+ entry into platelets results in membrane depolarization. Dependence on Ba2+ concentration and depolarization kinetics correlates with the dose dependence and kinetics of Ba2+ entry detected by quin2 fluorescence. The agonists also stimulate considerable Na+, Li+ and Cs+ inward currents into platelets. Na(+)-dependent depolarization is 2-5-fold suppressed by extracellular Ca2+ [median inhibitory concentration (IC50) approximately 0.3 mM]. Ni2+ and Cd2+ at similar concentrations block Ca2+ entry and agonist-induced Na2+ current (IC50 for both cations approximately 50 microM). Agonist-induced depolarization is blocked by the adenylate cyclase stimulator prostaglandin E1 and the protein kinase C stimulator phorbol ester. It is concluded that agonists stimulate Ca2+ entry into human platelets via receptor-operated channels which are not strictly selective toward divalent cations and are permeable to Na+, Li+ and Cs+.

Assessment of membrane potential changes using the carbocyanine dye, diS-C3-(5): synchronous excitation spectroscopy studies

The fluorescence of the voltage sensitive dye, diS-C3-(5), has been analyzed by means of synchronous excitation spectroscopy. Using this rather rare fluorescence technique we have been able to distinguish between the slightly shifted spectra of diS-C3-(5) fluorescence from cells and from the supernatant. It has been found that diS-C3-(5) fluorescence in the supernatant can be selectively monitored at lambda exc = 630 nm and lambda em = 650 nm, while the cell associated fluorescence can be observed at lambda exc = 690 nm and lambda em = 710 nm. A modified theory for the diS-C3-(5) fluorescence response to the membrane potential is presented, according to which a linear relationship exists between the logarithmic increment of the dye fluorescence intensity in the supernatant, 1n I/I degrees, and the underlying change in the plasma membrane potential, delta psi p = psi p - psi p degrees. The theory has been tested on human myeloid leukemia cells (line ML-1) in which membrane potential changes were induced by valinomycin clamping in various K+ gradients. It has been demonstrated that the membrane potential change, delta psi p, can be measured on an absolute scale.

Determination of the membrane potential of cultured mammalian Schwann cells and its sensitivity to potassium using a thiocarbocyanine fluorescent dye

The membrane potential of cultured rat sciatic nerve Schwann cells was determined with conventional microelectrode and voltage-sensitive fluorescent dye, Di-S-C3(5), optical techniques. The value for membrane potential obtained with microelectrodes was -42.1 +/- 4.7 mV (n = 8). Using optically determined fluorescent intensity changes caused by changes in external potassium ion concentration, in the presence or absence of valinomycin (null point method), the membrane potential was estimated at -45.7 +/- 6.2 mV (n = 7); with a gramicidin and valinomycin double ionophore method it was -52.2 +/- 9.1 (n = 4). The membrane potential of Schwann cells was found to be potassium sensitive at and above the physiological range of [K+] at 27.5 mV/10x delta[K+], which is approximately half the Nernstian value. This result suggests that other ion permeabilities strongly influence the resting membrane potential of cultured Schwann cells. Since Na+ had little effect on the membrane potential, it is concluded that Cl- is a likely candidate for the other permeant ionic species. The optical method has been shown to be a useful tool for the systematic study of the membrane potential of Schwann cells in culture and for the characterization of its ionic basis and regulation.

Regulation of stimulus-induced calcium transport pathways in human T (Jurkat) lymphoblasts

In human T (Jurkat) lymphoblasts we have studied the calcium signals induced by monoclonal antibodies reacting with the T-cell antigen receptor complex (TCR and CD3). Jurkat cells were preloaded with the fluorescent calcium indicator Indo-1 and the stimulus-induced rise in cytoplasmic free calcium concn was followed in the absence or in the presence of external calcium. The technique allowed the separate investigation of the intracellular calcium release and the external calcium influx processes. The changes in the membrane potential of Jurkat cells were followed simultaneously by using fluorescent indicators. We found that the activation of protein kinase C by phorbol ester (PMA) or by the permeable diacyl glycerol, DiC8, rapidly eliminated the calcium signal, independently of the presence or absence of external calcium, while these treatments did not appreciably change the membrane potential. In contrast, cell membrane depolarization achieved by various treatments selectively blocked the stimulus-induced calcium influx, while did not affect stimulus-induced calcium release from internal stores. The magnitude of the stimulus-induced calcium influx was found to be largely independent of the external calcium concns between about 2-2500 microM. It is demonstrated that the inhibitory effect of membrane depolarization on calcium influx is not simply due to the reduction of the inward calcium gradient under these conditions. These observations indicate a significant down-regulation of the stimulus-induced calcium signal by protein kinase C activation and a selective inhibition of the receptor-operated calcium channels by membrane depolarization.

Membrane depolarization selectively inhibits receptor-operated calcium channels in human T (Jurkat) lymphoblasts

Jurkat lymphoblasts were stimulated by a monoclonal antibody against the CD3 membrane antigen and the evoked calcium signal was followed by the intracellular fluorescent calcium indicator indo-1. The technique applied allowed us to separately investigate the stimulus-induced intracellular calcium release and the calcium-influx pathways, respectively. In the same cells membrane potential was estimated by the fluorescent dye diS-C3-(5). The resting membrane potential of Jurkat lymphoblasts under normal conditions was between -55 and -60 mV. Membrane depolarization, obtained by increasing external K+ concentration, removing external Cl-, or by increasing the Na+/K+ leak permeability with gramicidin or PCMBS, did not induce calcium influx in the resting cells and did not influence the CD3 receptor-mediated internal calcium release, while strongly inhibited the receptor-mediated calcium influx pathway. Half-maximum inhibition of this calcium influx was observed at membrane potential values of about -35 to -40 mV and this inhibition did not depend on the external calcium concentration varied between 5 and 2500 microM. Membrane hyperpolarization by valinomycin did not affect either component of the calcium signal. The observed selective inhibition of the receptor-operated calcium influx pathway by membrane depolarization is probably an important modulator of calcium-dependent cell stimulation.

Membrane depolarization inhibits thrombin-induced calcium influx and aggregation in human platelets

The relationship between thrombin-evoked changes in intracellular calcium concentration [( Ca2+]i) and aggregation was examined in Indo-1-loaded human platelets. The stimulus-induced intracellular calcium release and external calcium influx, as well as platelet aggregation, were studied in the same cell preparation. A close correlation between the sustained high [Ca2+]i level, depending on calcium entry, and the aggregation response was found. Gramicidin, at a concentration high enough to induce membrane depolarization, strongly inhibited the calcium influx and aggregation, but did not influence the thrombin-induced intracellular calcium release. We conclude that calcium influx through depolarization-inhibited calcium channels is a prerequisite of thrombin-induced platelet aggregation.

Plasma separator for encapsulating fluorescent probe in erythrocytes

Using a plasma separator, it is possible to wash large volumes of whole blood free of plasma proteins in short periods. The novelty, however, is that the same apparatus can be used to reversibly hemolyze and reseal the red blood cells (RBCs) using hypotonic and hypertonic dialysates, respectively. This technique was assessed using the fluorescent exogenous agent uranin (fluorescein sodium). Results indicate that the encapsulation of the RBCs can be completed in under 2 h. The encapsulation efficiency of the RBCs was approximately 80% with a RBC recovery rate of 75%.

Two mechanisms by which fluorescent oxonols indicate membrane potential in human red blood cells

Optical potentiometric indicators have been used to monitor the transmembrane electrical potential (Em) of many cells and organelles. A better understanding of the mechanisms of dye response is needed for the design of dyes with improved responses and for unambiguous interpretation of experimental results. This paper describes the responses to delta Em of 20 impermeant oxonols in human red blood cells. Most of the oxonols interacted with valinomycin, but not with gramicidin. The fluorescence of 15 oxonols decreased with hyperpolarization, consistent with an "on-off" mechanism, whereas five oxonols unexpectedly showed potential-dependent increases in fluorescence at less than 2 microM [dye]. Binding curves were determined for two dyes (WW781, negative response and RGA451, positive response) at 1 mM [K]o (membrane hyperpolarized with gramicidin) and at 90 mM [K]o (delta Em = 0 with gramicidin). Both dyes showed potential-dependent decreases in binding. Changes in the fluorescence of cell suspensions correlated with changes in [dye]bound for WW781, in accordance with the "on-off" mechanism, but not for RGA451. Large positive fluorescence changes (greater than 30%) dependent on Em were observed between 0.1 and 1.0 microM RGA451. A model is suggested in which RGA451 moves between two states of different quantum efficiencies within the membrane.

Transmembrane potential measurement with carbocyanine dye diS-C3-(5): fast fluorescence decay studies

The mechanism of carbocyanine dye diS-C3-(5) fluorescence intensity variations with transmembrane potential changes has been studied using time-resolved fluorescence spectroscopy. Clear evidence is given of the transmembrane-potential-dependent partition of the dye among various sites with different fluorescence lifetimes. It was found that fluorescence decay profiles reflect the transmembrane potential changes.

Continuous monitoring of transport by fluorescence on cells and vesicles

Fluorescence techniques are gaining wider applicability in the field of membrane transport due to their high temporal resolution, modest demand for biological material and the kinetic information which is made available by fluorescence tracings. The development of novel fluorescent substrates for particular transport systems and of novel fluorescent indicators for permeant ions, have opened the way for studying transport kinetics and regulation of transport in a variety of cellular and vesicular systems. The various methods of continuous monitoring of transport by fluorescence (CMTF) which are presently in use, are reviewed with emphasis on both analytical and applicative properties.

K+ transport and membrane potentials in isolated rat parotid acini

42K+ transport properties of isolated rat parotid acini were characterized concomitant with measurements of membrane potentials (Em) by means of the fluorescent dye diSC3-(5). In unstimulated acini suspended in a 5 mM K+ buffer, Em was governed by the K+ and Cl- gradients and amounted to about -59 mV, a value that remained unaffected on cholinergic stimulation. In unstimulated acini, 42K+ influx was largely mediated by the Na+-K+ pump, and the residual influxes were mediated by a bumetanide-sensitive component (cotransport system) and by K+ channels. Efflux of 42K+ was largely mediated by a bumetanide-sensitive component and by K+ channels. In the unstimulated state, the cotransport system was mediating K+-K+ exchange without contributing to the net uptake of K+. Within 10 s after stimulation, a approximately 10-fold increase in the acinar K+ conductance (gK) occurred, resulting in a rapid net efflux of K+ that amounted to approximately 3.8 mmol.l cells-1.s-1. Measurements of 42K+ fluxes as a function of the external K+ concentration revealed that in the stimulated state gK increases when external K+ is raised from 0.7 to 10 mM, consistent with an activation of acinar gK by the binding of external K+ to the channel. 42K+ flux ratios as well as the effect of the K+ channel inhibitor from scorpion venom (LQV) suggest that approximately 90% of K+ transport in the stimulated state is mediated by "maxi" K+ channels.

Membrane potential of primitive red cells from chick embryo is a proton potential

The membrane potential of primitive red cells from 4- and 6-day old chick embryos has been determined using the fluorescent dye Dis-C3-(5). At day 4 the membrane potential Em was -44 mV for pH 7.4 and 20 degrees C and -36 mV at day 6. Both values are far removed from the equilibrium potential for chloride, which is about -14 mV at day 6. Changes in the external potassium, sodium or chloride concentration were without effect on the membrane potential, except at very high potassium concentrations, where a small but significant depolarization was observed at day 6. The measurements gave the same results in the absence or presence of the anion exchange blocking agent DIDS. Three pieces of evidence indicate that the membrane potential of primitive red cells is primarily caused by an electrogenic H+ conductance: 1) The measured membrane potential of -36 mV at day 6 is close to the previously determined proton equilibrium potential (Baumann and Haller, 1983) EH + of -36 mV. 2) Addition of the electrosilent Cl-/OH- exchanger tributyltin causes a significant depolarization of about 20 mV at day 4 and about 14 mV at day 6. 3) Measurement of hydrogen ion fluxes demonstrate a potential dependent proton conductance, which increases with depolarization. These results indicate that large qualitative differences exist with regard to the mechanisms involved in the generation of membrane potential and hydrogen distribution between red cell and plasma of embryonic and adult chicken.

Relationship between transmembrane potential and activation of motility in rainbow trout (Salmo gairdneri)

A hypothesis is developed that activation of motility in rainbow trout spermatozoa is a result of membrane hyperpolarization. This hypothesis was developed to explain experimental observations of a relationship between membrane potential and motility as revealed by the use of voltage sensitive fluorescent dyes. The results lead to the following conclusions: a) Transmembrane potential hyperpolarizes with decreasing KCl concentration in 100 mM NaCl. b) Transmembrane potential hyperpolarizes with decreasing NaCl concentration. c) NaCl is three time less effective in changing transmembrane potential and two orders of magnitude less effective in inhibiting activation of motility than KCl. d) Chloride ions have little effect on transmembrane potential or motility. e) Increases in osmotic pressure with the non-ionic molecule sucrose increased the amount of KCl required to inhibit activation. f) The major effect of Na(+) on K(+) inhibition may be osmotic.It is suggested that while sperm cells are in the seminal plasma in the reproductive tract of the male rainbow trout their transmembrane potential is maintained above an activation threshold, probably through Na/K pumps which are found in almost all animal cells. Since K(+) is the most important ion in determining the transmembrane potential, hyperpolarization of the plasma membrane below an activation threshold occurs when the sperm cells are diluted, during spawning, into the low K(+) environment of freshwater.

Cholinergic-induced electrolyte transport in rat parotid acini

Secretory responses of parotid acini occurring within 10 sec following cholinergic stimulation were characterized. 1. Measurement of membrane potentials by means of the fluorescent dye diSC3-(5) revealed a value of approximately -59 mV, which remained unaffected on stimulation. 2. Stimulation caused a rapid net loss of 42K+ that was strongly inhibited by the "maxi" K+-channel inhibitor "charybdotoxin" present in scorpion venom. 3. It was calculated that the number of open "maxi" K+-channels per cell was approximately 40 in the unstimulated state and approximately 3000 in the stimulated state. 4. Stimulation caused a transient decrease in the acinar ATP content. 5. Intracellular pH (pHi) measured by means of the fluorescent dye, BCECF, was dependent upon the presence of extracellular HCO3- as well as Na+. Under physiological conditions pHi was 7.27 and stimulation caused a transient decrease of 0.1 pH units due to HCO3- efflux. The decrease was followed by pHi recovery mediated by a Na+/H+ exchange mechanism.

Pontentiometric cyanine dyes are sensitive probes for mitochondria in intact plant cells : kinetin enhances mitochondrial fluorescence

Selected fluorescent dyes were tested for uptake by mitochrondria in intact cells of barley, maize, and onion. The cationic cyanine dye 3,3'-diheptyloxacarbocyanine iodide [DiOC(7)(3)] accumulated in mitochondria within 15 to 30 minutes without appreciable staining of other protoplasmic constituents. The number, shape, and movement of the fluorescent mitochondria could be seen readily, and the fluorescence intensity of the mitochondria could be monitored with a microscope photometer. Fluorescence was eliminated in 1 to 5 minutes by the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) indicating that maintenance of dye concentration was dependent on the inside-negative transmembrane potential maintained by functional mitochondria. Fluorescence of prestained mitochondria was enhanced within 5 to 10 minutes after addition of 0.1 millimolar kinetin to cells. The fluorescence in kinetintreated cells was dissipated by CCCP. These results suggest that kinetin interacted with respiratory processes resulting in higher potential across the mitochondrial membrane.

The effects of mitochondrial energetics inhibitors on the fluorescence of potential-sensitive dyes rhodamine 123 and diS-C3-(5) in lymphocyte suspensions

The effects of uncouplers (FCCP, DNF), oligomycin, and rotenone on the fluorescence of potential-sensitive dyes, rhodamine 123 and diS-C3-(5), in lymphocyte suspensions were compared. The fluorescence of these optical probes gradually increased at higher FCCP concentrations. The dependences of fluorescence intensities and FCCP concentrations were similar for both dyes, and only diS-C3-(5) fluorescence started increasing at lower FCCP concentrations. Rotenone (1 microM) significantly increased rhodamine 123 fluorescence. TMPD-induced and uncoupler-induced diS-C3-(5) fluorescence changes increased 1.5- to 2-fold if the incubation mixture was supplemented with oligomycin (0.1-0.2 microgram/ml). The fluorescence responses of the dyes in the lymphocyte suspension correlate with the effects of mitochondrial energetics inhibitors on delta psi m in isolated mitochondria. The results suggest the possibility of using these dyes for estimating the direction of the delta psi m changes in the lymphocyte suspension.

Potential-sensitive response mechanism of diS-C3-(5) in biological membranes

The potential-sensitive response mechanism of 3,3'-dipropylthiodicarbocyanine iodide (diS-C3-(5] was examined based on our previous model of diS-C3-(5) interaction with brush border membrane vesicles (BBMV) in the absence of a membrane potential. The model contained binding (6 msec), reorientation (30 msec), dimerization (less than 10 nsec), and translocation (1 sec) reaction steps (Cabrini & Verkman, 1986. J. Membrane Biol. 90:163-175). Transmembrane potentials (psi) were induced in BBMV by K+ gradients and valinomycin. Steady-state diS-C3-(5) fluorescence (excitation 622 nm, emission 670 nm) increased linearly with psi. The reorientation and translocation reaction steps were resolved by the stopped-flow technique as a biexponential decrease in fluorescence following mixture of diS-C3-(5) with BBMV at varying psi. The fractional amplitude of the faster exponential increased from 0.36 to 0.73 with increasing psi (-17 to 87 mV); the time constant for the faster exponential (30-35 msec) was independent of psi. There were single exponential kinetics (0.5-1.5 sec) for diS-C3-(5) fluorescence response to a rapid (less than 2 msec) change in psi in the absence of a diS-C3-(5) concentration gradient. These results, and similar findings in placental brush border vesicles, red cell vesicles, and phosphatidylcholine vesicles, support a translocation mechanism for diS-C3-(5) response, where induced membrane potentials drive diS-C3-(5) redistribution between sites at the inner and outer membrane leaflets, with secondary effects on diS-C3-(5) dimerization and solution/membrane partitioning. Fluorescence lifetime and dynamic depolarization measurements showed no significant change in diS-C3-(5) rotational characteristics or in the polarity of the diS-C3-(5) environment with changes in psi. Based on the experimental results, a mathematical model is developed to explain the quantitative changes in diS-C3-(5) fluorescence which accompany changes in psi at arbitrary dye/lipid ratios.