Transmembrane potential measurements on plant cells using the voltage-sensitive dye ANNINE-6

Advances in pulsed electric stimuli as a physical method for treating liquid foods

There is a need for alternative technologies that can deliver safe and nutritious foods at lower costs as compared to conventional processes. Pulsed electric field (PEF) technology has been utilised for a plethora of different applications in the life and physical sciences, such as gene/drug delivery in medicine and extraction of bioactive compounds in food science and technology. PEF technology for treating liquid foods involves engineering principles to develop the equipment, and quantitative biochemistry and microbiology techniques to validate the process. There are numerous challenges to address for its application in liquid foods such as the 5-log pathogen reduction target in food safety, maintaining the food quality, and scale up of this physical approach for industrial integration. Here, we present the engineering principles associated with pulsed electric fields, related inactivation models of microorganisms, electroporation and electropermeabilization theory, to increase the quality and safety of liquid foods; including water, milk, beer, wine, fruit juices, cider, and liquid eggs. Ultimately, we discuss the outlook of the field and emphasise research gaps.

Fluorescence Imaging of Cell Membrane Potential: From Relative Changes to Absolute Values

Membrane potential is a fundamental property of biological cells. Changes in membrane potential characterize a vast number of vital biological processes, such as the activity of neurons and cardiomyocytes, tumorogenesis, cell-cycle progression, etc. A common strategy to record membrane potential changes that occur in the process of interest is to utilize organic dyes or genetically-encoded voltage indicators with voltage-dependent fluorescence. Sensors are introduced into target cells, and alterations of fluorescence intensity are recorded with optical methods. Techniques that allow recording relative changes of membrane potential and do not take into account fluorescence alterations due to factors other than membrane voltage are already widely used in modern biological and biomedical studies. Such techniques have been reviewed previously in many works. However, in order to investigate a number of processes, especially long-term processes, the measured signal must be corrected to exclude the contribution from voltage-independent factors or even absolute values of cell membrane potential have to be evaluated. Techniques that enable such measurements are the subject of this review.

Plant electrical signals: A multidisciplinary challenge

Plant electrical signals, an early event in the plant-stimulus interaction, rapidly transmit information generated by the stimulus to other organs, and even the whole plant, to promote the corresponding response and trigger a regulatory cascade. In recent years, many promising state-of-the-art technologies applicable to study plant electrophysiology have emerged. Research focused on expression of genes associated with electrical signals has also proliferated. We propose that it is appropriate for plant electrical signals to be considered in the form of a "plant electrophysiological phenotype". This review synthesizes research on plant electrical signals from a novel, interdisciplinary perspective, which is needed to improve the efficient aggregation and use of plant electrical signal data and to expedite interpretation of plant electrical signals.

Nanosecond pulsed electrical fields enhance product recovery in plant cell fermentation

The potential of pharmacologically active secondary plant metabolites is limited by the low yield from often rare plants, and the lack of economically feasible chemical synthesis of these complex compounds. Plant cell fermentation offers an alternative strategy to overcome these constraints. However, the efficiency of this approach is limited by intracellular sequestration of the products, such that continuous bioprocessing is not possible. As a precondition for such a, more attractive, continuous process, it is of great importance to stimulate the export of the product into the medium without impairing viability and, thus, the productivity of the cells. Using nicotine alkaloids of tobacco as a case study, an alternative strategy is explored, where nanosecond pulsed electric fields (nsPEFs) are applied for the efficient downstream recovery of the products. To maintain cell viability and allow for the further use of biomass, cells were exposed to strong (1-20 kV·cm^-1), but very short (10-100 ns) electric pulses, which leads to a temporary permeabilisation of cell membranes. Using two transgenic cell lines, where two key genes involved in the metabolism of the anti-Alzheimer compound nornicotine were overexpressed, we could show that this nsPEF treatment improved the partitioning of some nicotine alkaloids to the culture medium without impairing viability, nor the synthesis of alkaloids. However, this release was only partial and did not work for nornicotine. Thus, nsPEFs produced a fractionation of alkaloids. We explain this electrofractionation by a working model considering the differential intracellular compartmentalization of nicotineic alkaloids.

Nanosecond pulsed electric fields modulate the expression of the astaxanthin biosynthesis genes psy, crtR-b and bkt 1 in Haematococcus pluvialis

Nanosecond pulsed electric fields (nsPEFs) have been extensively studied with respect to cellular responses. Whether nsPEFs can regulate gene expression and to modulate the synthesis of valuable compounds, has so far been only tested in the context of apoptosis in cancer cells. We used the unicellular algae Haematococcus pluvialis as system to test, whether nsPEFs could alter gene expression and to promote the biosynthesis of astaxanthin. We find that nsPEFs induce a mild, but significant increase of mortality up to about 20%, accompanied by a moderate increase of astaxanthin accumulation. Steady-state transcript levels of three key genes psy, crtR-b and bkt 1 were seen to increase with a maximum at 3 d after PEF treatment at 50 ns. Pulsing at 25 ns reduce the transcripts of psy, crtR-b from around day 2 after the pulse, while those of bkt 1 remain unchanged. By blocking the membrane-located NADPH oxidase RboH, diphenylene iodonium by itself increased both, the levels of astaxanthin and transcripts of all three biosynthetic genes, and this increase was added up to that produced by nsPEFs. Artificial calcium influx by an ionophore did not induce major changes in the accumulation of astaxanthin, nor in the transcript levels, but amplified the response of crtR-b to nsPEFs at 25 ns, while decreased in 50 ns treatment. When Ca²⁺ influx was inhibited by GdCl₃, the transcript of psy and bkt 1 were decreased for both 25 ns and 50 ns treatments, while crtR-b exhibited an obvious increase for the 25 ns treatment. We interpret these data in a working model, where nsPEFs permeabilise plasma and chloroplast membrane depending on pulse duration leading to a differential release of plastid retrograde signaling to the nucleus.

Plasma membrane H+-ATPases sustain pollen tube growth and fertilization

Pollen tubes are highly polarized tip-growing cells that depend on cytosolic pH gradients for signaling and growth. Autoinhibited plasma membrane proton (H+) ATPases (AHAs) have been proposed to energize pollen tube growth and underlie cell polarity, however, mechanistic evidence for this is lacking. Here we report that the combined loss of AHA6, AHA8, and AHA9 in Arabidopsis thaliana delays pollen germination and causes pollen tube growth defects, leading to drastically reduced fertility. Pollen tubes of aha mutants had reduced extracellular proton (H+) and anion fluxes, reduced cytosolic pH, reduced tip-to-shank proton gradients, and defects in actin organization. Furthermore, mutant pollen tubes had less negative membrane potentials, substantiating a mechanistic role for AHAs in pollen tube growth through plasma membrane hyperpolarization. Our findings define AHAs as energy transducers that sustain the ionic circuit defining the spatial and temporal profiles of cytosolic pH, thereby controlling downstream pH-dependent mechanisms essential for pollen tube elongation, and thus plant fertility.

Gene transfer to plants by electroporation: methods and applications

Developing gene transfer technologies enables the genetic manipulation of the living organisms more efficiently. The methods used for gene transfer fall into two main categories; natural and artificial transformation. The natural methods include the conjugation, transposition, bacterial transformation as well as phage and retroviral transductions, contain the physical methods whereas the artificial methods can physically alter and transfer genes from one to another organisms' cell using, for instance, biolistic transformation, micro- and macroinjection, and protoplast fusion etc. The artificial gene transformation can also be conducted through chemical methods which include calcium phosphate-mediated, polyethylene glycol-mediated, DEAE-Dextran, and liposome-mediated transfers. Electrical methods are also artificial ways to transfer genes that can be done by electroporation and electrofusion. Comparatively, among all the above-mentioned methods, electroporation is being widely used owing to its high efficiency and broader applicability. Electroporation is an electrical transformation method by which transient electropores are produced in the cell membranes. Based on the applications, process can be either reversible where electropores in membrane are resealable and cells preserve the vitality or irreversible where membrane is not able to reseal, and cell eventually dies. This problem can be minimized by developing numerical models to iteratively optimize the field homogeneity considering the cell size, shape, number, and electrode positions supplemented by real-time measurements. In modern biotechnology, numerical methods have been used in electrotransformation, electroporation-based inactivation, electroextraction, and electroporative biomass drying. Moreover, current applications of electroporation also point to some other uncovered potentials for various exploitations in future.

Primer to Voltage Imaging With ANNINE Dyes and Two-Photon Microscopy

ANNINE-6 and ANNINE-6plus are voltage-sensitive dyes that when combined with two-photon microscopy are ideal for recording of neuronal voltages in vivo, in both bulk loaded tissue and the dendrites of single neurons. Here, we describe in detail but for a broad audience the voltage sensing mechanism of fast voltage-sensitive dyes, with a focus on ANNINE dyes, and how voltage imaging can be optimized with one-photon and two-photon excitation. Under optimized imaging conditions the key strengths of ANNINE dyes are their high sensitivity (0.5%/mV), neglectable bleaching and phototoxicity, a linear response to membrane potential, and a temporal resolution which is faster than the optical imaging devices currently used in neurobiology (order of nanoseconds). ANNINE dyes in combination with two-photon microscopy allow depth-resolved voltage imaging in bulk loaded tissue to study average membrane voltage oscillations and sensory responses. Alternatively, if ANNINE-6plus is applied internally, supra and sub threshold voltage changes can be recorded from dendrites of single neurons in awake animals. Interestingly, in our experience ANNINE-6plus labeling is impressively stable in vivo, such that voltage imaging from single Purkinje neuron dendrites can be performed for 2 weeks after a single electroporation of the neuron. Finally, to maximize their potential for neuroscience studies, voltage imaging with ANNINE dyes and two-photon microscopy can be combined with electrophysiological recording, calcium imaging, and/or pharmacology, even in awake animals.

Characterization of Cell Membrane Permeability In Vitro Part II: Computational Model of Electroporation-Mediated Membrane Transport

Electroporation is the process by which applied electric fields generate nanoscale defects in biological membranes to more efficiently deliver drugs and other small molecules into the cells. Due to the complexity of the process, computational models of cellular electroporation are difficult to validate against quantitative molecular uptake data. In part I of this two-part report, we describe a novel method for quantitatively determining cell membrane permeability and molecular membrane transport using fluorescence microscopy. Here, in part II, we use the data from part I to develop a two-stage ordinary differential equation model of cellular electroporation. We fit our model using experimental data from cells immersed in three buffer solutions and exposed to electric field strengths of 170 to 400 kV/m and pulse durations of 1 to 1000 μs. We report that a low-conductivity 4-(2-hydroxyethyl)-1 piperazineethanesulfonic acid buffer enables molecular transport into the cell to increase more rapidly than with phosphate-buffered saline or culture medium-based buffer. For multipulse schemes, our model suggests that the interpulse delay between two opposite polarity electric field pulses does not play an appreciable role in the resultant molecular uptake for delays up to 100 μs. Our model also predicts the per-pulse permeability enhancement decreases as a function of the pulse number. This is the first report of an ordinary differential equation model of electroporation to be validated with quantitative molecular uptake data and consider both membrane permeability and charging.

Activation of respiratory Complex I from Escherichia coli studied by fluorescent probes

Respiratory Complex I from E. coli may exist in two interconverting forms: resting (R) and active (A). The R/A transition of purified, solubilized Complex I occurring upon turnover was studied employing two different fluorescent probes, Annine 6+, and NDB-acetogenin. NADH-induced fluorescent changes of both dyes bound to solubilized Complex I from E. coli were characterized as a function of the protein:dye ratio, temperature, ubiquinone redox state and the enzyme activity. Analysis of this data combined with time-resolved optical measurements of Complex I activity and spectral changes indicated two ubiquinone-binding sites; a possibility of reduction of the tightly-bound quinone in the resting state and reduction of the loosely-bound quinone in the active state is discussed. The results also indicate that upon the activation Complex I undergoes conformational changes which can be mapped to the junction of the hydrophilic and membrane domains in the region of the assumed acetogenin-binding site.

Electroporation-based applications in biotechnology

Electroporation is already an established technique in several areas of medicine, but many of its biotechnological applications have only started to emerge; we review here some of the most promising. We outline electroporation as a phenomenon and then proceed to applications, first outlining the best established - the use of reversible electroporation for heritable genetic modification of microorganisms (electrotransformation), and then explore recent advances in applying electroporation for inactivation of microorganisms, extraction of biomolecules, and fast drying of biomass. Although these applications often aim to upscale to the industrial and/or clinical level, we also outline some important chip-scale applications of electroporation. We conclude our review with a discussion of the main challenges and future perspectives.

Transport, resealing, and re-poration dynamics of two-pulse electroporation-mediated molecular delivery

Electroporation is of interest for many drug-delivery and gene-therapy applications. Prior studies have shown that a two-pulse-electroporation protocol consisting of a short-duration, high-voltage first pulse followed by a longer, low-voltage second pulse can increase delivery efficiency and preserve viability. In this work the effects of the field strength of the first and second pulses and the inter-pulse delay time on the delivery of two different-sized Fluorescein-Dextran (FD) conjugates are investigated. A series of two-pulse-electroporation experiments were performed on 3T3-mouse fibroblast cells, with an alternating-current first pulse to permeabilize the cell, followed by a direct-current second pulse. The protocols were rationally designed to best separate the mechanisms of permeabilization and electrophoretic transport. The results showed that the delivery of FD varied strongly with the strength of the first pulse and the size of the target molecule. The delivered FD concentration also decreased linearly with the logarithm of the inter-pulse delay. The data indicate that membrane resealing after electropermeabilization occurs rapidly, but that a non-negligible fraction of the pores can be reopened by the second pulse for delay times on the order of hundreds of seconds. The role of the second pulse is hypothesized to be more than just electrophoresis, with a minimum threshold field strength required to reopen nano-sized pores or defects remaining from the first pulse. These results suggest that membrane electroporation, sealing, and re-poration is a complex process that has both short-term and long-term components, which may in part explain the wide variation in membrane-resealing times reported in the literature.

Basic features of a cell electroporation model: illustrative behavior for two very different pulses

Science increasingly involves complex modeling. Here we describe a model for cell electroporation in which membrane properties are dynamically modified by poration. Spatial scales range from cell membrane thickness (5 nm) to a typical mammalian cell radius (10 μm), and can be used with idealized and experimental pulse waveforms. The model consists of traditional passive components and additional active components representing nonequilibrium processes. Model responses include measurable quantities: transmembrane voltage, membrane electrical conductance, and solute transport rates and amounts for the representative "long" and "short" pulses. The long pulse--1.5 kV/cm, 100 μs--evolves two pore subpopulations with a valley at ~5 nm, which separates the subpopulations that have peaks at ~1.5 and ~12 nm radius. Such pulses are widely used in biological research, biotechnology, and medicine, including cancer therapy by drug delivery and nonthermal physical tumor ablation by causing necrosis. The short pulse--40 kV/cm, 10 ns--creates 80-fold more pores, all small (<3 nm; ~1 nm peak). These nanosecond pulses ablate tumors by apoptosis. We demonstrate the model's responses by illustrative electrical and poration behavior, and transport of calcein and propidium. We then identify extensions for expanding modeling capability. Structure-function results from MD can allow extrapolations that bring response specificity to cell membranes based on their lipid composition. After a pulse, changes in pore energy landscape can be included over seconds to minutes, by mechanisms such as cell swelling and pulse-induced chemical reactions that slowly alter pore behavior.

Image processing for non-ratiometric measurement of membrane voltage using fluorescent reporters and pulsed laser illumination

The measurement of transmembrane voltages induced by pulsed electric field exposure can be achieved by using fluorescent dyes like ANNINE-6. Such approach requires a quantitative determination of the fluorescence intensity along the cell's membrane by image processing. When high temporal resolution is required, the illumination source is frequently a dye-laser which causes high fluctuations in the intensity of illumination which in turn affects the fluorescence intensity and thus the quality of the results. We propose an image processing technique that allows to overcome the fluctuations and to produce quantitative data. It uses the optical background noise as a correcting factor. Standard deviation in the fluctuations is thus efficiently reduced by at least a factor of 2.5. Additionally we draw attention to the fact that the parasitic component of the laser radiation (ASE) can also suppress fluctuations although it deteriorates wavelength precision.

Plasma membrane lipid-protein interactions affect signaling processes in sterol-biosynthesis mutants in Arabidopsis thaliana

The plasma membrane is an important organelle providing structure, signaling and transport as major biological functions. Being composed of lipids and proteins with different physicochemical properties, the biological functions of membranes depend on specific protein-protein and protein-lipid interactions. Interactions of proteins with their specific sterol and lipid environment were shown to be important factors for protein recruitment into sub-compartmental structures of the plasma membrane. System-wide implications of altered endogenous sterol levels for membrane functions in living cells were not studied in higher plant cells. In particular, little is known how alterations in membrane sterol composition affect protein and lipid organization and interaction within membranes. Here, we conducted a comparative analysis of the plasma membrane protein and lipid composition in Arabidopsis sterol-biosynthesis mutants smt1 and ugt80A2;B1. smt1 shows general alterations in sterol composition while ugt80A2;B1 is significantly impaired in sterol glycosylation. By systematically analyzing different cellular fractions and combining proteomic with lipidomic data we were able to reveal contrasting alterations in lipid-protein interactions in both mutants, with resulting differential changes in plasma membrane signaling status.

Plasma membrane lipid-protein interactions affect signaling processes in sterol-biosynthesis mutants in Arabidopsis thaliana

The plasma membrane is an important organelle providing structure, signaling and transport as major biological functions. Being composed of lipids and proteins with different physicochemical properties, the biological functions of membranes depend on specific protein-protein and protein-lipid interactions. Interactions of proteins with their specific sterol and lipid environment were shown to be important factors for protein recruitment into sub-compartmental structures of the plasma membrane. System-wide implications of altered endogenous sterol levels for membrane functions in living cells were not studied in higher plant cells. In particular, little is known how alterations in membrane sterol composition affect protein and lipid organization and interaction within membranes. Here, we conducted a comparative analysis of the plasma membrane protein and lipid composition in Arabidopsis sterol-biosynthesis mutants smt1 and ugt80A2;B1. smt1 shows general alterations in sterol composition while ugt80A2;B1 is significantly impaired in sterol glycosylation. By systematically analyzing different cellular fractions and combining proteomic with lipidomic data we were able to reveal contrasting alterations in lipid-protein interactions in both mutants, with resulting differential changes in plasma membrane signaling status.

Nanosecond electric pulses affect a plant-specific kinesin at the plasma membrane

Electric pulses with high field strength and durations in the nanosecond range (nsPEFs) are of considerable interest for biotechnological and medical applications. However, their actual cellular site of action is still under debate--due to their extremely short rise times, nsPEFs are thought to act mainly in the cell interior rather than at the plasma membrane. On the other hand, nsPEFs can induce membrane permeability. We have revisited this issue using plant cells as a model. By mapping the cellular responses to nsPEFs of different field strength and duration in the tobacco BY-2 cell line, we could define a treatment that does not impinge on short-term viability, such that the physiological responses to the treatment can be followed. We observe, for these conditions, a mild disintegration of the cytoskeleton, impaired membrane localization of the PIN1 auxin-efflux transporter and a delayed premitotic nuclear positioning followed by a transient mitotic arrest. To address the target site of nsPEFs, we made use of the plant-specific KCH kinesin, which can assume two different states with different localization (either near the nucleus or at the cell membrane) driving different cellular functions. We show that nsPEFs reduce cell expansion in nontransformed cells but promote expansion in a line overexpressing KCH. Since cell elongation and cell widening are linked to the KCH localized at the cell membrane, the inverted response in the KCH overexpressor provides evidence for a direct action of nsPEFs, also at the cell membrane.

A critical evaluation of whole cell patch clamp studies on electroporation using the voltage sensitive dye ANNINE-6

The patch clamp technique in the whole cell configuration is potentially a powerful tool to investigate electroporation (electric-field-induced membrane permeabilization). Membrane polarization beyond certain threshold voltages leads to a steep conductance increase either indicating field-induced pore formation or being due to patch clamp artifacts (seal resistance breakdown). Protoplasts derived from tobacco culture cell lines (Bright Yellow-2, BY-2; Virginia bright Italian-0, VBI-0) were stained with the voltage-sensitive dye ANNINE-6. After establishing the whole cell patch clamp configuration 50-ms command voltage (Ucomm) steps ranging from -500 mV to +500 mV were applied while simultaneously exposing protoplasts to light at 475 nm wavelength. Pulse-induced currents and fluorescence intensity (known to be linearly related to the trans-membrane voltage, Um) were recorded. Plotting fluorescence intensity against Ucomm revealed saturation of the curve at values<-300 mV and >+300 mV and close correlation with theoretical Um values calculated on the basis of membrane pore formation. For BY-2 and VBI-0 protoplasts ANNINE-6 voltage sensitivity was calculated to be -0.0014 mV(-1) and -0.0012 mV(-1), respectively. Voltage ramp experiments revealed cation-selectivity of field-induced pores. Anions are conducted poorly independent of their size. In conclusion, the patch clamp technique is validated as a useful tool in electroporation research.

Inactivation of Pseudomonas putida by pulsed electric field treatment: a study on the correlation of treatment parameters and inactivation efficiency in the short-pulse range

An important issue for an economic application of the pulsed electric field treatment for bacterial decontamination of wastewater is the specific treatment energy needed for effective reduction of bacterial populations. The present experimental study performed in a field amplitude range of 40 > E > 200 kV/cm and for a suspension conductivity of 0.01 = κ(e) > 0.2 S/m focusses on the application of short pulses, 25 ns > T > 10 μs, of rectangular, bipolar and exponential shape and was made on Pseudomonas putida, which is a typical and widespread wastewater microorganism. The comparison of inactivation results with calculations of the temporal and azimuthal membrane charging dynamics using the model of Pauly and Schwan revealed that for efficient inactivation, membrane segments at the cell equator have to be charged quickly and to a sufficiently high value, on the order of 0.5 V. After fulfilling this basic condition by an appropriate choice of pulse field strength and duration, the log rate of inactivation for a given suspension conductivity of 0.2 S/m was found to be independent of the duration of individual pulses for constant treatment energy expenditure. Moreover, experimental results suggest that even pulse shape plays a minor role in inactivation efficiency. The variation of the suspension conductivity resulted in comparable inactivation performance of identical pulse parameters if the product of pulse duration and number of pulses was the same, i.e., required treatment energy can be linearly downscaled for lower conductivities, provided that pulse amplitude and duration are selected for entire membrane surface permeabilization.

Aspects of plant plasmalemma charging induced by external electric field pulses

The charging of the plasmalemma is a necessary condition for permeabilization of the plasma membrane (electroporation) in response to external electric field exposure. Common theories explain this permeabilization by formation of pores in the lipid bilayer. Using pulsed laser fluorescence microscopy, we measured the charging process of the membrane during the application of an external electric field with a temporal resolution of 5 ns. Visualization of the charging process of protoplasts plasma membrane (Nicotiana tabacum Bright Yellow 2) was achieved by staining of the plasma membrane with the voltage-sensitive fluorescent dye ANNINE-6. Measurements on membranes exhibiting negligible membrane permeabilization confirm the sine-shaped azimuthal distribution of the membrane voltage predicted by the relation of Cole. At higher membrane voltages, enhanced pore formation allows for the exchange of charge carriers, leading to deviations from the sine-shaped curve progression, i.e., a saturation of the membrane voltage at membrane segments facing the electrodes. Additionally, measurements on protoplasts exposed to multiple successive pulses indicate that the recovery of the membrane seems to be a fast process, occurring within seconds after termination of the external electric field pulse.

A patch clamp study on the electro-permeabilization of higher plant cells: Supra-physiological voltages induce a high-conductance, K+ selective state of the plasma membrane

Permeabilization of biological membranes by pulsed electric fields ("electroporation") is frequently used as a tool in biotechnology. However, the electrical properties of cellular membranes at supra-physiological voltages are still a topic of intensive research efforts. Here, the patch clamp technique in the whole cell and the outside out configuration was employed to monitor current-voltage relations of protoplasts derived from the tobacco culture cell line "Bright yellow-2". Cells were exposed to a sequence of voltage pulses including supra-physiological voltages. A transition from a low-conductance (~0.1 nS/pF) to a high-conductance state (~5 nS/pF) was observed when the membrane was either hyperpolarized or depolarized beyond threshold values of around -250 to -300 mV and +200 to +250 mV, respectively. Current-voltage curves obtained with ramp protocols revealed that the electro-permeabilized membrane was 5-10 times more permeable to K+ than to gluconate. The K+ channel blocker tetraethylammonium (25 mM) did not affect currents elicited by 10 ms-pulses, suggesting that the electro-permeabilization was not caused by a non-physiological activation of K+ channels. Supra-physiological voltage pulses even reduced "regular" K+ channel activity, probably due to an increase of cytosolic Ca2+ that is known to inhibit outward-rectifying K+ channels in Bright yellow-2 cells. Our data are consistent with a reversible formation of aqueous membrane pores at supra-physiological voltages.