Evidence of voltage-induced channel opening in Na/K ATPase of human erythrocyte membrane

Cellular excitability and ns-pulsed electric fields: Potential involvement of lipid oxidation in the action potential activation

Recent studies showed that nanosecond pulsed electric fields (nsPEFs) can activate voltage-gated ion channels (VGICs) and trigger action potentials (APs) in excitable cells. Under physiological conditions, VGICs' activation takes place on time scales of the order 10-100 µs. These time scales are considerably longer than the applied pulse duration, thus activation of VGICs by nsPEFs remains puzzling and there is no clear consensus on the mechanisms involved. Here we propose that changes in local electrical properties of the cell membrane due to lipid oxidation might be implicated in AP activation. We first use MD simulations of model lipid bilayers with increasing concentration of primary and secondary lipid oxidation products and demonstrate that oxidation not only increases the bilayer conductance, but also the bilayer capacitance. Equipped with MD-based characterization of electrical properties of oxidized bilayers, we then resort to AP modelling at the cell level with Hodgkin-Huxley-type models. We confirm that a local change in membrane properties, particularly the increase in membrane conductance, due to formation of oxidized membrane lesions can be high enough to trigger an AP, even when no external stimulus is applied. However, excessive accumulation of oxidized lesions (or other conductive defects) can lead to altered cell excitability.

Genetically engineered HEK cells as a valuable tool for studying electroporation in excitable cells

Electric pulses used in electroporation-based treatments have been shown to affect the excitability of muscle and neuronal cells. However, understanding the interplay between electroporation and electrophysiological response of excitable cells is complex, since both ion channel gating and electroporation depend on dynamic changes in the transmembrane voltage (TMV). In this study, a genetically engineered human embryonic kidney cells expressing NaV1.5 and Kir2.1, a minimal complementary channels required for excitability (named S-HEK), was characterized as a simple cell model used for studying the effects of electroporation in excitable cells. S-HEK cells and their non-excitable counterparts (NS-HEK) were exposed to 100 µs pulses of increasing electric field strength. Changes in TMV, plasma membrane permeability, and intracellular Ca2+ were monitored with fluorescence microscopy. We found that a very mild electroporation, undetectable with the classical propidium assay but associated with a transient increase in intracellular Ca2+, can already have a profound effect on excitability close to the electrostimulation threshold, as corroborated by multiscale computational modelling. These results are of great relevance for understanding the effects of pulse delivery on cell excitability observed in context of the rapidly developing cardiac pulsed field ablation as well as other electroporation-based treatments in excitable tissues.

Silencing of ATP1A1 attenuates cell membrane disruption by nanosecond electric pulses

This study explored the role of the Na/K-ATPase (NKA) in membrane permeabilization induced by nanosecond electric pulses. Using CRISPR/Cas9 and shRNA, we silenced the ATP1A1 gene, which encodes α1 NKA subunit in U937 human monocytes. Silencing reduced the rate and the cumulative uptake of YoPro-1 dye after electroporation by 300-ns, 7-10 kV/cm pulses, while ouabain, a specific NKA inhibitor, enhanced YoPro-1 entry. We conclude that the α1 subunit supports the electropermeabilized membrane state, by forming or stabilizing electropores or by hindering repair mechanisms, and this role is independent of NKA's ion pump function.

Mechanism on the microbial salt tolerance enhancement by electrical stimulation

The application of biological methods in industrial saline wastewater treatment is limited, since the activities of microorganisms are strongly inhibited by the highly concentrated salts. Acclimatized halotolerant and halophilic microorganisms are of high importance since they can resist the environmental stresses of high salinity. The acclimation to salinity can be passive or active based on whether external simulation is used. However, there is a need for development of economic, efficient and reliable active biological stimulation technologies to accelerate salinity acclimation. Recent studies have shown that electrical stimulation can effectively enhance microbial salt tolerance and pollutant removal ability. However, there have been no comprehensive reviews of the mechanisms involved. Therefore, this mini-review described the mechanisms of electrical stimulation that can significantly improve microbial bioactivity and biodiversity. These mechanisms include regulation of Na⁺ and K⁺ transporters by changing membranepotential and promoting ATP production, as well as regulation of extracellular polymer substances through enhanced release of low molecular weight EPS and quorum sensing molecules. The information provided herein will facilitate the application of biological high-salinity wastewater treatment.

Nanosecond Pulsed Electric Field (nsPEF): Opening the Biotechnological Pandora’s Box

Nanosecond Pulsed Electric Field (nsPEF) is an electrostimulation technique first developed in 1995; nsPEF requires the delivery of a series of pulses of high electric fields in the order of nanoseconds into biological tissues or cells. They primary effects in cells is the formation of membrane nanopores and the activation of ionic channels, leading to an incremental increase in cytoplasmic Ca2+ concentration, which triggers a signaling cascade producing a variety of effects: from apoptosis up to cell differentiation and proliferation. Further, nsPEF may affect organelles, making nsPEF a unique tool to manipulate and study cells. This technique is exploited in a broad spectrum of applications, such as: sterilization in the food industry, seed germination, anti-parasitic effects, wound healing, increased immune response, activation of neurons and myocites, cell proliferation, cellular phenotype manipulation, modulation of gene expression, and as a novel cancer treatment. This review thoroughly explores both nsPEF’s history and applications, with emphasis on the cellular effects from a biophysics perspective, highlighting the role of ionic channels as a mechanistic driver of the increase in cytoplasmic Ca2+ concentration.

Reducing ischemic kidney injury through application of a synchronization modulation electric field to maintain Na+/K+-ATPase functions

Renal ischemia-reperfusion injury is an important contributor to the development of delayed graft function after transplantation, which is associated with higher rejection rates and poorer long-term outcomes. One of the earliest impairments during ischemia is Na⁺/K⁺-ATPase (Na/K pump) dysfunction due to insufficient ATP supply, resulting in subsequent cellular damage. Therefore, strategies that preserve ATP or maintain Na/K pump function may limit the extent of renal injury during ischemia-reperfusion. Here, we applied a synchronization modulation electric field to activate Na/K pumps, thereby maintaining cellular functions under ATP-insufficient conditions. We tested the effectiveness of this technique in two models of ischemic renal injury: an in situ renal ischemia-reperfusion injury model (predominantly warm ischemia) and a kidney transplantation model (predominantly cold ischemia). Application of the synchronization modulation electric field to a renal ischemia-reperfusion injury mouse model preserved Na/K pump activity, thereby reducing kidney injury, as reflected by 40% lower plasma creatinine (1.17 ± 0.03 mg/dl) in the electric field-treated group as compared to the untreated control group (1.89 ± 0.06 mg/dl). In a mouse kidney transplantation model, renal graft function was improved by more than 50% with the application of the synchronization modulation electric field according to glomerular filtration rate measurements (85.40 ± 12.18 μl/min in the untreated group versus 142.80 ± 11.65 μl/min in the electric field-treated group). This technique for preserving Na/K pump function may have therapeutic potential not only for ischemic kidney injury but also for other diseases associated with Na/K pump dysfunction due to inadequate ATP supply.

Modifications of Plasma Membrane Organization in Cancer Cells for Targeted Therapy

Modifications of the composition or organization of the cancer cell membrane seem to be a promising targeted therapy. This approach can significantly enhance drug uptake or intensify the response of cancer cells to chemotherapeutics. There are several methods enabling lipid bilayer modifications, e.g., pharmacological, physical, and mechanical. It is crucial to keep in mind the significance of drug resistance phenomenon, ion channel and specific receptor impact, and lipid bilayer organization in planning the cell membrane-targeted treatment. In this review, strategies based on cell membrane modulation or reorganization are presented as an alternative tool for future therapeutic protocols.

Membrane Electroporation and Electropermeabilization: Mechanisms and Models

Exposure of biological cells to high-voltage, short-duration electric pulses causes a transient increase in their plasma membrane permeability, allowing transmembrane transport of otherwise impermeant molecules. In recent years, large steps were made in the understanding of underlying events. Formation of aqueous pores in the lipid bilayer is now a widely recognized mechanism, but evidence is growing that changes to individual membrane lipids and proteins also contribute, substantiating the need for terminological distinction between electroporation and electropermeabilization. We first revisit experimental evidence for electrically induced membrane permeability, its correlation with transmembrane voltage, and continuum models of electropermeabilization that disregard the molecular-level structure and events. We then present insights from molecular-level modeling, particularly atomistic simulations that enhance understanding of pore formation, and evidence of chemical modifications of membrane lipids and functional modulation of membrane proteins affecting membrane permeability. Finally, we discuss the remaining challenges to our full understanding of electroporation and electropermeabilization.

Lipid vesicles in pulsed electric fields: Fundamental principles of the membrane response and its biomedical applications

The present review focuses on the effects of pulsed electric fields on lipid vesicles ranging from giant unilamellar vesicles (GUVs) to small unilamellar vesicles (SUVs), from both fundamental and applicative perspectives. Lipid vesicles are the most popular model membrane systems for studying biophysical and biological processes in living cells. Furthermore, as vesicles are made from biocompatible and biodegradable materials, they provide a strategy to create safe and functionalized drug delivery systems in health-care applications. Exposure of lipid vesicles to pulsed electric fields is a common physical method to transiently increase the permeability of the lipid membrane. This method, termed electroporation, has shown many advantages for delivering exogenous molecules including drugs and genetic material into vesicles and living cells. In addition, electroporation can be applied to induce fusion between vesicles and/or cells. First, we discuss in detail how research on cell-size GUVs as model cell systems has provided novel insight into the basic mechanisms of cell electroporation and associated phenomena. Afterwards, we continue with a thorough overview how electroporation and electrofusion have been used as versatile methods to manipulate vesicles of all sizes in different biomedical applications. We conclude by summarizing the open questions in the field of electroporation and possible future directions for vesicles in the biomedical field.

Effect of Cooling On Cell Volume and Viability After Nanoelectroporation

Electric pulses of nanosecond duration (nsEP) are emerging as a new modality for tissue ablation. Plasma membrane permeabilization by nsEP may cause osmotic imbalance, water uptake, cell swelling, and eventual membrane rupture. The present study was aimed to increase the cytotoxicity of nsEP by fostering water uptake and cell swelling. This aim was accomplished by lowering temperature after nsEP application, which delayed the membrane resealing and/or suppressed the cell volume mechanisms. The cell diameter in U-937 monocytes exposed to a train of 50, 300-ns pulses (100 Hz, 7 kV/cm) at room temperature and then incubated on ice for 30 min increased by 5.6 +/- 0.7 μm (40-50%), which contrasted little or no changes (1 +/- 0.3 μm, <10%) if the incubation was at 37 °C. Neither this nsEP dose nor the 30-min cooling caused cell death when applied separately; however, their combination reduced cell survival to about 60% in 1.5-3 h. Isosmotic addition of a pore-impermeable solute (sucrose) to the extracellular medium blocked cell swelling and rescued the cells, thereby pointing to swelling as a primary cause of membrane rupture and cell death. Cooling after nsEP exposure can potentially be employed in medical practice to assist tissue and tumor ablation.

The cytotoxic synergy of nanosecond electric pulses and low temperature leads to apoptosis

Electroporation by nanosecond electric pulses (nsEP) is an emerging modality for tumor ablation. Here we show the efficient induction of apoptosis even by a non-toxic nsEP exposure when it is followed by a 30-min chilling on ice. This chilling itself had no impact on the survival of U-937 or HPAF-II cells, but caused more than 75% lethality in nsEP-treated cells (300 ns, 1.8-7 kV/cm, 50-700 pulses). The cell death was largely delayed by 5-23 hr and was accompanied by a 5-fold activation of caspase 3/7 (compared to nsEP without chilling) and more than 60% cleavage of poly-ADP ribose polymerase (compared to less than 5% in controls or after nsEP or chilling applied separately). When nsEP caused a transient permeabilization of 83% of cells to propidium iodide, cells placed at 37 °C resealed in 10 min, whereas 60% of cells placed on ice remained propidium-permeable even in 30 min. The delayed membrane resealing caused cell swelling, which could be blocked by an isosmotic addition of a pore-impermeable solute (sucrose). However, the block of swelling did not prevent the delayed cell death by apoptosis. The potent enhancement of nsEP cytotoxicity by subsequent non-damaging chilling may find applications in tumor ablation therapies.

Ultrastructural changes in Staphylococcus aureus treated with pulsed electric fields / Cambios ultraestructurales en Staphylococcus aureus sometida a campos eléctricos pulsantes

Early stationary phase cells of Staphylococcus aureus were inoculated into a model food, simulated milk ultrafiltrate (SMUF) and subjected to 16, 32, and 64 pulses at electric field intensities of 20, 40 and 60 kV/cm at 13 °C. In addition temperatures of 20, 25 and 30 °C were also tested with 32 pulses and an electric field of 60 kV/cm. The temperature of the SMUF increased by 1-2 ° C at the end of the 64 pulses. Cells subjected to 64 pulses at 20, 40 and 60 kV/cm were observed for ultrastructural changes using scanning and transmission electron microscopy techniques. The cell surface was rough after treatment with electric field when observed by scanning electron microscopy (SEM). The cell wall was broken, and the cytoplasmic contents were leaking out of the cell after exposure to 64 pulses at 60 kV/cm when observed by transmission electron microscopy (TEM). The breaking of the cell wall is an indication of electro-mechanical breakdown of the cell. The increase in inactivation with an increase in the electric field strength can be related to the increase in the damage to the cells. Cells subjected to 32 pulses at 60 kV/cm and 13, 20 or 25 °C were compared microscopically with the untreated control cells. Cells subjected to heat treat ment (10 min, at 66 °C) were compared with electric field-treated and untreated control cells. Although important changes were observed in the protoplast, no cell wall breakdown was observed in heat-treated cells when compared to the electric field-treated cells. This result indi cates a different mechanism of inactivation of cells with heat treatment.

Electropermeabilization of the cell membrane

Membrane electropermeabilization is the observation that the permeability of a cell membrane can be transiently increased when a micro-millisecond external electric field pulse is applied on a cell suspension or on a tissue. Applicative aspects for the transfer of foreign molecules (macromolecules) into the cytoplasm are routinely used. But only a limited knowledge about what is really occurring in the cell and its membranes at the molecular levels is available. This chapter is a critical attempt to report the present state of the art and to point out some of the still open problems. The experimental facts associated to membrane electropermeabilization are firstly reported. They are valid on biological and model systems. Secondly, soft matter approaches give access to the bioelectrochemical description of the thermodynamical constraints supporting the destabilization of simplified models of the biological membrane. It is indeed described as a thin dielectric leaflet, where a molecular transport takes place by electrophoresis and then diffusion. This naïve approach is due to the lack of details on the structural aspects affecting the living systems as shown in a third part. Membranes are part of the cell machinery. The critical property of cells as being an open system from the thermodynamical point of view is almost never present. Computer simulations are now contributing to our knowledge on electropermeabilization. The last part of this chapter is a (very) critical report of all the efforts that have been performed. The final conclusion remains that we still do not know all the details on the reversible structural and dynamical alterations of the cell membrane (and cytoplasm) supporting its electropermeabilization. We have a long way in basic and translational researches to reach a pertinent description.

Synchronization modulation increases transepithelial potentials in MDCK monolayers through Na/K pumps

Transepithelial potential (TEP) is the voltage across a polarized epithelium. In epithelia that have active transport functions, the force for transmembrane flux of an ion is dictated by the electrochemical gradient in which TEP plays an essential role. In epithelial injury, disruption of the epithelial barrier collapses the TEP at the wound edge, resulting in the establishment of an endogenous wound electric field (∼100 mV/mm) that is directed towards the center of the wound. This endogenous electric field is implicated to enhance wound healing by guiding cell migration. We thus seek techniques to enhance the TEP, which may increase the wound electric fields and enhance wound healing. We report a novel technique, termed synchronization modulation (SM) using a train of electric pulses to synchronize the Na/K pump activity, and then modulating the pumping cycles to increase the efficiency of the Na/K pumps. Kidney epithelial monolayers (MDCK cells) maintain a stable TEP and transepithelial resistance (TER). SM significantly increased TEP over four fold. Either ouabain or digoxin, which block Na/K pump, abolished SM-induced TEP increases. In addition to the pump activity, basolateral distribution of Na/K pumps is essential for an increase in TEP. Our study for the first time developed an electrical approach to significantly increase the TEP. This technique targeting the Na/K pump may be used to modulate TEP, and may have implication in wound healing and in diseases where TEP needs to be modulated.

Na,K-ATPase as A Brownian Motor: Electric Field-InducedConformational Fluctuation Leads to Uphill Pumping of Cation inthe Absence of ATP

Na,K-ATPase uses chemical bond energy of ATP to pump K(+) into, andNa(+) out of a cell. Both are uphill transports. During the catalyticcycle the enzyme alternates between two conformational states, E(1) andE(2). This communication describes an experiment, which employs electricfield to drive oscillation or fluctuation of enzyme conformation betweenthe E(1) and the E(2) states. It is shown that the field-inducedconformational oscillation or fluctuation leads to uphill pumping of thecation by the enzyme without consumption of ATP. Biochemical specificityof the catalysis is preserved. Data indicate that Na,K-ATPase can harvestenergy from the applied electric field to perform chemical work, and aratchet mechanism is inherent in this energy transduction process. ATheory of Electroconformational Coupling (TEC) that embodies essentialfeatures of the Brownian Ratchet successfully simulates the field-frequencyand field-amplitude optima and other features of the ion pumping activity.A four-state TEC motor can achieve high efficiency of the energytransduction, asymptotically reaching 100% under the optimal condition.Pumping by ion rectification fails to reach high efficiency. The TECconcept is also mused to understand other biological motors and engines.

Quick and effective hyperpolarization of the membrane potential in intact smooth muscle cells of blood vessels by synchronization modulation electric field

Blood vessel dilation starts from activation of the Na/K pumps and inward rectifier K channels in the vessel smooth muscle cells, which hyperpolarizes the cell membrane potential and closes the Ca channels. As a result, the intracellular Ca concentration reduces, and the smooth muscle cells relax and the blood vessel dilates. Activation of the Na/K pumps and the membrane potential hyperpolarization plays a critical role in blood vessel functions. Previously, we developed a new technique, synchronization modulation, to control the pump functions by electrically entraining the pump molecules. We have applied the synchronization modulation electric field noninvasively to various intact cells and demonstrated the field-induced membrane potential hyperpolarization. We further applied the electric field to blood vessels and investigated the field induced functional changes of the vessels. In this paper, we report the results in a study of the membrane potential change in the smooth muscle cells of mesenteric blood vessels in response to the oscillating electric field. We found that the synchronization modulation electric field can effectively hyperpolarize the muscle membrane potential quickly in seconds under physiological conditions.

Hyperpolarization of the membrane potential in cardiomyocyte tissue slices by the synchronization modulation electric field

Our previous studies have shown that a specially designed, so-called synchronization modulation electric field can entrain active transporter Na/K pumps in the cell membrane. This approach was previously developed in a study of single cells using a voltage clamp to monitor the pump currents. We are now expanding our study from isolated single cells to aggregated cells in a 3-dimensional cell matrix, through the use of a tissue slice from the rat heart. The slice is about 150 μm in thickness, meaning the slices contain many cell layers, resulting in a simplified 3-dimensional system. A fluorescent probe was used to identify the membrane potential and the ionic concentration gradients across the cell membrane. In spite of intrinsic cell-to-cell interactions and the difficulty in stimulating cell aggregation in the tissue slice, the oscillating electric field increased the intracellular fluorescent intensity, indicating elevation of the cell ionic concentration and hyperpolarization of the cell membrane. Blockage of these changes by ouabain confirmed that the results are directly related to Na/K pumps. These results along with the backward modulation indicate that the synchronization modulation electric field can influence the Na/K pumps in tissue cells of a 3-dimensional matrix and therefore hyperpolarize the cell membrane.

Membrane potential hyperpolarization in Mammalian cardiac cells by synchronization modulation of Na/K pumps

In previously reported work, we developed a new technique, synchronization modulation, to electrically activate Na/K pump molecules. The fundamental mechanism involved in this technique is a dynamic entrainment procedure of the pump molecules, carried out in a stepwise pattern. The entrainment procedure consists of two steps: synchronization and modulation. We theoretically predicted that the pump functions can be activated exponentially as a function of the membrane potential. We have experimentally demonstrated synchronization of the Na/K pump molecules and acceleration of their pumping rates by many fold through use of voltage-clamp techniques, directly monitoring the pump currents. We further applied this technique to intact skeletal muscle fibers from amphibians and found significant effects on the membrane resting potential. Here, we extend our study to intact mammalian cardiomyocytes. We employed a noninvasive confocal microscopic fluorescent imaging technique to monitor electric field-induced changes in ionic concentration gradient and membrane resting potential. Our results further confirm that the well-designed synchronization modulation electric field can effectively accelerate the Na/K pumping rate, increasing the ionic concentration gradient across the cell membrane and hyperpolarizing the membrane resting potential.

Importance of intermediary transitions and waveform in the enzyme-electric field interaction

The current theory of enzymes and electric field interaction does not account for all the observed data since we could not observe non-linear behavior of cell suspensions as anticipated by other authors. In our case, we used a pure sinusoidal source, however the experiments that do account for responses used a sum of a central sinusoidal and its harmonics frequencies. As a result, we suggest that the enzyme and electric interaction are favored when the field has a non-strictly sinusoidal waveform, and this behavior is related to the true intermediate transitions of the enzyme during its catalytic cycle. Therefore, we developed an iterative model of the interaction process based on previous models and actual trends. The model well verified all the previous simulations and showed that, for a non-symmetrical enzyme, the energy can harvest its maximal for non sinusoidal fields.

Entrainment of Na/K pumps by a synchronization modulation electric field

We studied entrainment of the catalytic cycle of the Na/K pumps by an imposed external AC electric field. Our results show that a well designed dichotomous oscillating electric field with a frequency close to the pumps' natural turnover rate can synchronize the pump molecules. Characteristics of the synchronized pumps include: (1) outward pump currents responding to Na-extrusion and inward pump currents responding to K-pumping in are separated; (2) magnitude of the outward pump currents can be up to three times higher than that of the randomly paced pump currents; (3) magnitude ratio of the outward over inward pump currents reveals the 3:2 stoichiometry of the pumps. We, further, gradually increased the field oscillating frequency in a stepwise pattern and kept pump synchronization in each step. We found that the pumps' turnover rate could be modulated up as the field frequency increased. Consequently, the pump currents significantly increased by many fold. In summary, these results show that the catalytic cycle of Na/K pumps can be synchronized and modulated by a well designed oscillating electric field resulting in activation of the pump functions.

Membrane perturbation by an external electric field: a mechanism to permit molecular uptake

Electropermeabilisation is a well established physical method, based on the application of electric pulses, which induces the transient permeabilisation of the cell membrane. External molecules, otherwise nonpermeant, can enter the cell. Electropermeabilisation is now in use for the delivery of a large variety of molecules, as drugs and nucleic acids. Therefore, the method has great potential in the fields of cancer treatment and gene therapy. However many open questions about the underlying physical mechanisms involved remain to be answered or fully elucidated. In particular, the induced changes by the effects of the applied field on the membrane structure are still far from being fully understood. The present review focuses on questions related to the current theories, i.e. the basic physical processes responsible for the electropermeabilisation of lipid membranes. It also addresses recent findings using molecular dynamics simulations as well as experimental studies of the effect of the field on membrane components.

Electrical Activation of Na/K Pumps Can Increase Ionic Concentration Gradient and Membrane Resting Potential

It has been previously demonstrated by our group that our specifically designed synchronization modulation electric field can dynamically entrain the Na/K ATPase molecules, effectively accelerating the pumping action of these molecules. The ATPase molecules are first synchronized by the field, and subsequently their pumping rates are gradually modulated in a stepwise pattern to progressively higher and higher levels. Here, we present results obtained on application of the field to intact twitch skeletal muscle fibers. The ionic concentration gradient across the cell membrane was monitored, with the membrane potential extrapolated using a slow fluorescent probe with a confocal microimaging technique. The applied synchronization-modulation electric field is able to slowly but consistently increase the ionic concentration gradient across the membrane and, hence, hyperpolarize the membrane potential. All of these results were fully eliminated if ouabain was applied to the bathing solution, indicating a correlation with the action of the Na/K pump molecules. These results in combination with our previous results into the entrainment of the pump molecules show that the synchronization-modulation electric field-induced activation of the Na/K pump functions can effectively increase the ionic concentration gradient and the membrane potential.

Synchronization modulation of Na/K pump molecules can hyperpolarize the membrane resting potential in intact fibers

Previously, we have theoretically studied the possibility of electrical rhythmic entrainment of carrier-mediated ion transporters, and experimentally realized synchronization and acceleration of the Na/K pumping rate in the cell membrane of skeletal muscle fibers by a specially designed synchronization modulation electric field. In these studies we either used cut fibers under a voltage clamp or intact fibers, but in the presence of ion channels blockers. A question remained as to whether the field-induced activation observed in the pump molecules could effectively increase the intracellular ionic concentration and the membrane potential at physiological conditions. In this paper, we studied the effects of the field on intact fibers without any channel blockers. We monitored the field-induced changes in the ionic concentration gradient across the cell membrane and the membrane potential non-invasively by using a fluorescent probe and confocal microscopic imaging techniques. The results clearly show that the entrainment of the pump molecules by the synchronization modulation electric field can effectively increase the ionic concentration gradient, and hence, hyperpolarize the membrane potential.

Synchronization of Na/K pump molecules by a train of squared pulses

We experimentally studied the Na/K pump currents evoked by a train of squared pulses whose pulse-duration is about the time course of Na-extrusion at physiological conditions. The magnitude of the measured pump current can be as much as three-fold of that induced by the traditional single pulse measurement. The increase in the pump current is directly dependent on the number of pre-pulses. The larger the number of the pre-pulses is, the higher the current magnitude can be obtained. At a particular number of pre-pulses, the pump current becomes saturated. These results suggest that a large number of pre-pulses may synchronize the pump molecules to work at the same pace. As a result, the pump molecules may extrude Na ions at the same time corresponding to the stimulation pulses, and pump in K ions at the same time during the pulse intervals. Therefore, the measured pump current is three-fold of that measured by a single pulse where the outward and inward pump currents are canceled each other.

Electric field-induced changes in membrane proteins charge movement currents

Our previous study showed that thermal effects induced by Joule heating did not play the pivotal role in damage of membrane proteins when cell membranes were shocked by a pulsed membrane potential up to 500 mV. Our analytical study of ion channel currents further indicated that a brief electric shock may cause protein conformational damage in the channel gating system, resulting in a reduction in the number of limiting gating charge particles. In this paper, we present the results of our study into electric shock-induced changes in the intramembrane charge movement currents. We found that a brief electric shock may significantly alter the characteristics of the charge movement currents of the membrane proteins, including reducing the magnitudes of two components Q(beta) and Q(gamma), broadening the hump shape of Q(gamma), and increasing its time delay. This study suggests that a brief intensive electric shock may cause proteins to structurally alter, reducing the amount of movable charge particles and therefore decreasing the protein functions. These results indicate that electro-coupled structural damage in membrane proteins is an important mechanism involved in electrical injury, especially in a field range not sufficient to cause thermal damage.

Supramembrane potential-induced electroconformational changes in sodium channel proteins: A potential mechanism involved in electric injury

Effects of imposed large supraphysiological transmembrane potential (TP) pulses on channel proteins, particularly on the voltage-gated Na channels, were investigated. Voltage clamp techniques were used to deliver both shock and stimulation pulses, and to monitor changes in the channel functions. Our experimental results indicated that more than one 4 ms duration TP shock of -450 mV resulted in electroconformational denature of voltage-gated Na channels. This resulted in functional reductions in muscle cells' excitability. We quantified the TP shock-induced decrease in the Na channel currents, compared the pre- and post-shocked Na channel currents' voltage dependency, and studied the reversibility of the electroconformationally denatured ion channel proteins. These observations are particularly relevant to the problem of explaining the neuromuscular damage following high voltage electrical shock injuries despite no evidence of a thermal injury component.

Cell Injury by Electric Forces

The molecular architecture of biological systems is heavily influenced by the highly polar interactions of water. Thus, macromolecules such as proteins that are highly water soluble must be electrically polar. Energy generation methods needed to support cell metabolic processes depend on compartmentalizing mobile ions and thus require electrical ion transport barriers such as membranes. One consequence of these biological design constraints is vulnerability to injury by electrical forces. Supraphysiological electric forces cause damage to cells and tissues by disrupting cell membranes and altering the conformation of biomolecules. In addition, prolonged passage of electrical current leads to damage by thermal mechanisms. This review will focus on the non-thermal effects.

Electroconformational Denaturation of Membrane Proteins

Because of high electrical impedance of cell membrane, when living cells are exposed to an external electric field, the field-induced voltage drops will mainly occur on the cell membrane. In addition to Joule heating damage and electroporation of the cell membrane, the electric field-induced supraphysiological transmembrane potential may inevitably damage the membrane proteins, especially the voltage-dependent membrane proteins. That is because the charged particles in the amino acid of the membrane proteins and, in particular, the voltage-sensors in the voltage-dependent membrane proteins are vulnerable to the membrane potential. An intensive, brief electric shock may induce electroconformational damage or denaturation in the membrane proteins. As a result, the cell functions are significantly reduced. This electric field-induced denaturation in the membrane proteins strongly suggests a new underlying mechanism involved in electrical injury.

Supra-physiological membrane potential induced conformational changes in K+ channel conducting system of skeletal muscle fibers

The effects of a supra-physiological membrane potential shock on the conducting system of the delayed rectifier K(+) channels in the skeletal muscle fibers of frogs were studied. An improved double Vaseline gap voltage clamp technique was used to deliver stimulation pulses and to measure changes in the channel currents. Our results showed that a single 4 ms, -400 mV pulsed shock can cause a reduction in the K(+) channel conductance and a negative-shift of the channel open-threshold. Following the Boltzmann theory of channel voltage-dependence, we analyzed the shock-induced changes in the channel open-probability by employing both two-state and multi-state models. The results indicate a reduction in the number of channel gating particles after the electric shock, which imply possible conformational changes at domains that gate the channels proteins. This study provides further evidence supporting our hypothesis that high intensity electric fields can cause conformational changes in membrane proteins, most likely in the channel gating system. These structural changes in membrane proteins, and therefore their dysfunctions, may be involved in the mechanisms underlying electrical injury.

Do clinically relevant transthoracic defibrillation energies cause myocardial damage and dysfunction?

Sufficiently strong defibrillation shocks will cause temporary or permanent damage to the heart. Weak defibrillation shocks do not cause any damage to the heart but also do not defibrillate. A relevant and practical question is what range of shock energies is most likely to defibrillate while not causing damage to the heart. This question is most difficult to answer in the pre-hospital defibrillation setting where the patients' size and shape vary, placement of the defibrillation patches vary, and the etiology of their arrhythmia varies. Unlike internal defibrillators, which are tested at implantation, efficacy of an external defibrillator is determined only once, when it is most needed. This review discusses shock damage and dysfunction caused by monophasic waveforms as well as biphasic waveforms. Evidence is presented suggesting that for perfused hearts, the threshold for damage is well above any shock size delivered clinically. For non-perfused hearts, both in humans and animals, evidence is presented that monophasic shocks of up to 5 J/kg do not cause any more cardiac damage/dysfunction than that associated with smaller shocks and that much of this damage is caused by the ischemic period itself rather than the shock. Although many patients can be defibrillated with 150 J (2.2 J/kg) biphasic shocks, some patients may require biphasic shocks up to 360 J (5 J/kg) to be defibrillated. Studies still need to be performed comparing the efficacy and damaging effects of 360 J biphasic shocks to 150 J biphasic shocks. Until those studies are completed, it seems reasonable to use the same 360 J (5 J/kg) energy limit for biphasic shocks as for monophasic shocks.

ELF magnetic fields increase amino acid uptake into Vicia faba L. roots and alter ion movement across the plasma membrane

Vicia faba seedlings, subjected to a 10 microT 50 Hz square wave magnetic field for 40 min together with a radioactive pulse, showed a marked increase in amino acid uptake into intact roots. A more modest increase was observed with a 100 microT 50 Hz square wave. An increase in media conductivity at low field intensities from 10 microT 50 Hz square wave, 100 microT 50 Hz sine wave, and 100 microT 60 Hz square wave fields, indicated an alteration in the movement of ions across the plasma membrane, most likely due to an increase in net outflow of ions from the root cells. Similarly, marked elevation in media pH, indicating increased alkalinity, was observed at 10 and 100 microT for both square and sine waves at both 50 and 60 Hz. Our data would indicate that low magnetic field intensities of 10 and 100 microT at 50 or 60 Hz can alter membrane transport processes in root tips.

The asymmetric, rectifier-like I-V curve of the Na/K pump transient currents in frog skeletal muscle fibers

The Na/K pump transient currents in skeletal muscle fiber were identified using an improved double Vaseline gap voltage clamp technique. The asymmetric characteristics of the pump current-voltage relationship were studied. The definition of the Na/K pump currents was the ouabain-sensitive currents, where ouabain is a specific Na/K ATPase inhibitor. Membrane potential was held at -90 mV, the membrane resting potential. A series of stimulation pulse-pairs symmetric to the membrane resting potential were applied to the cell membrane. The summation of the currents responding to the two pulses in each pair indicates the asymmetry of the pump currents with respect to the membrane resting potential.The voltage dependence of the Na/K pump transient currents from skeletal muscle is similar to the steady-state I-V curve from either skeletal muscle fibers or cardiac muscles. It is a sigmoidal-shaped, asymmetric curve with respect to the membrane resting potential. This asymmetric, rectifier-like voltage dependence indicates that a symmetric oscillating membrane potential may generate a net, outward pump current. In other words, the Na/K pump molecules may be activated by an oscillating membrane potential.

Activation of signal-transduction mechanisms may underlie the therapeutic effects of an applied electric field

Successful treatment of various medical complaints with an applied electric field has been reported over the years. The identities of the cellular mechanisms that are influenced by this type of treatment and facilitate the positive effects, remain elusive. A study of many in vitro and in vivo reports revealed that the beneficial effects can be attributed to the activation of membrane proteins, and specifically proteins involved in signal-transduction mechanisms. Not only may the proteins be affected but it is now well established that enhanced Ca(2+)influx, observed to follow electric stimulation of cells, also contributes to many calcium-dependent cellular processes which can be linked to the therapeutic effects discussed in this paper. An hypothesis of the physical changes caused by an applied, relatively small (10(3)to 10(4)V m(-1)range), electric field with low to moderate frequency (below 150 Hz), is postulated.

Control by membrane order of voltage-induced permeabilization, loading and gene transfer in mammalian cells

Cells can be transiently permeabilized by application of electric pulses. A direct consequence of this treatment is to create a new state in the membrane leading to DNA and protein transfers. A key step, in the interaction between macromolecules and the electropermeabilized membrane, is involved. We previously reported that membrane and DNA associated hydration and undulation forces appeared to be involved in this process by studying the effects of osmotic pressure. Effects of ethanol (EtOH) and L-alpha-lysophosphatidylcholine (lyso-PC), molecules known to affect membrane order and therefore undulation forces, were investigated on Chinese hamster ovary (CHO) cells. We used millisecond square wave pulses, conditions giving high efficiency for gene transfer. No effect was observed on cell permeabilization for small sized molecules. Only little change on electroloading of proteins such as R-phycoerythrin was obtained in presence of EtOH. But, a decrease (increase) in electrotransfection was observed for cells treated with EtOH (lyso-PC). Under our conditions, no additional effects of the chemical treatment were observed on cell viability and on membrane resealing. These results tentatively explained in terms of the effect of membrane order on membrane organization and interaction between molecules and membrane supports the existence of the plasmid-membrane interaction in the mechanism of electrically mediated gene transfer.

The generation of reactive-oxygen species associated with long-lasting pulse-induced electropermeabilisation of mammalian cells is based on a non-destructive alteration of the plasma membrane

Chinese hamster ovary (CHO) cells in suspension were subjected to pulsed electric fields suitable for electrically mediated gene transfer (pulse duration longer than 1 ms). Using the chemiluminescence probe lucigenin, we showed that a generation of reactive-oxygen species (oxidative jump) was present when the cells were electropermeabilised using millisecond pulses. The oxidative jump yield was controlled by the extent of alterations allowing permeabilisation within the electrically affected cell area, but showed a saturating dependence on the pulse duration over 1 ms. Cell electropulsation induced reversible and irreversible alterations of the membrane assembly. The oxidative stress was only present when the membrane permeabilisation was reversible. Irreversible electrical membrane disruption inhibited the oxidative jump. The oxidative jump was not a simple feedback effect of membrane electropermeabilisation. It strongly controlled long-term cell survival. This had to be associated with the cell-damaging action of reactive-oxygen species. However, for millisecond-cumulated pulse duration, an accumulation of a large number of short pulses (microsecond) was extremely lethal for cells, while no correlation with an increased oxidative jump was found. Cell responses, such as the production of free radicals, were present during electropermeabilisation of living cells and controlled partially the long-term behaviour of the pulsed cell.

Time courses of mammalian cell electropermeabilization observed by millisecond imaging of membrane property changes during the pulse

Time courses of electropermeabilization were analyzed during the electric field application using a rapid fluorescent imaging system. Exchanges of calcium ions through electropermeabilized membrane of Chinese hamster ovary cells were found to be asymmetrical. Entry of calcium ions during a millisecond pulse occurred on the anode-facing cell hemisphere. Entry through the region facing the cathode was observed only after the pulse. Leakage of intracellular calcium ions from electropermeabilized cell in low-calcium content medium was observed only from the anode-facing side. The exchanges during the pulse were mostly due to diffusion-driven processes, i.e., governed by the concentration gradient. Interaction of propidium iodide, a dye sensitive to the structural alteration of membrane, with cell membrane was asymmetrical during electropermeabilization. Localized enhancement of the dye fluorescence was observed during and after the pulsation on the cell surface. Specific staining of a limited anode-facing part of the membrane was observed as soon as the pulse was applied. The membrane fluorescence level increased during and immediately after the pulse whereas the geometry of the staining was unchanged. The membrane regions stained by propidium iodide were the same as those where calcium exchanges occurred. The fraction of the membrane on which structural alterations occurred was defined by the field strength. The density of defects was governed by the pulse duration. Electropermeabilization is a localized but asymmetrical process. The membrane defects are created unequally on the two cell sides during the pulse, implying a vectorial effect of the electric field on the membrane.

Chinese hamster ovary cells sensitivity to localized electrical stresses

Application of an external electric field on a cell suspension induces an alteration in the membrane structure giving free access to the cell cytoplasm. Under mild pulsation conditions, permeabilization is a reversible process which weakly affects cell viability while drastic electrical conditions lead to cell death. The field pulse must be considered as a complex stress applied on the cell assembly. This study is a systematic investigation of the stress effects of field strength, pulse duration and number of pulses, at given joule energy. The loss in cell viability is not related to the energy delivered to the system. At a given joule energy, a strong field during a short cumulated pulse duration affects more viability than using a weak field associated with a long cumulated pulsation. At a given field strength and for a given cumulated pulse duration an accumulation of short pulses is also observed to be very damaging for cells. A control by the delay between the pulses suggests a memory effect. The field effect appears also to be vectorial in line with the known asymmetry of the membrane organization. These results suggest that processes at a cellular level are involved, either an activation of cell death or damage in cellular functions.

Electropermeabilization of mammalian cells to macromolecules: control by pulse duration

Membrane electropermeabilization to small molecules depends on several physical parameters (pulse intensity, number, and duration). In agreement with a previous study quantifying this phenomenon in terms of flow (Rols and Teissié, Biophys. J. 58:1089-1098, 1990), we report here that electric field intensity is the deciding parameter inducing membrane permeabilization and controls the extent of the cell surface where the transfer can take place. An increase in the number of pulses enhances the rate of permeabilization. The pulse duration parameter is shown to be crucial for the penetration of macromolecules into Chinese hamster ovary cells under conditions where cell viability is preserved. Cumulative effects are observed when repeated pulses are applied. At a constant number of pulses/pulse duration product, transfer of molecules is strongly affected by the time between pulses. The resealing process appears to be first-order with a decay time linearly related to the pulse duration. Transfer of macromolecules to the cytoplasm can take place only if they are present during the pulse. No direct transfer is observed with a postpulse addition. The mechanism of transfer of macromolecules into cells by electric field treatment is much more complex than the simple diffusion of small molecules through the electropermeabilized plasma membrane.

Electric field-induced functional reductions in the K+ channels mainly resulted from supramembrane potential-mediated electroconformational changes

The goal of this study is to distinguish the supramembrane potential difference-induced electroconformational changes from the huge transmembrane current-induced thermal damages in the delayed rectifier K+ channels. A double Vaseline-gap voltage clamp was used to deliver shock pulses and to monitor the channel currents. Three pairs of 4-ms shock pulses were used to mimic the electric shock by a power-line frequency electric field. Each pair consists of two pulses with the same magnitude, starting from 350 to 500 mV, but with opposite polarities. The shock pulse-generated transmembrane ion flux and the responding electric energy, the Joule heating, consumed in the cell membrane, as well as the effects on the K+ channel currents, were obtained. Results showed that huge transmembrane currents are not necessary to cause damages in the K+ channel proteins. In contrast, reductions in the K+ channel currents are directly related to the field-induced supramembrane potential differences. By a comparison with the shock field-induced Joule heating effects on cell membranes, the field-induced supramembrane potential difference plays a dominant role in damaging the K+ channels, resulting in electroconformational changes in the membrane proteins. In contrast, the shock field-induced huge transmembrane currents, therefore the thermal effects, play a secondary, trivial role.

Dose-dependent reduction of cardiac transmembrane potential by high-intensity electrical shocks

Cardiac tissue dysfunction can result from high-intensity electrical shocks and is manifested as changes in transmembrane potential (Vm). Ten-millisecond shock pulses (SPs) of varying intensity and polarity were applied to frog ventricle in diastole, and Vm was quantified directly under the stimulating electrode by an optical method using voltage-sensitive dye. As SP intensities were increased, the shock-induced action potential (AP) plateau and AP amplitude (APAs) decreased sigmoidally toward 75-85% of the control AP amplitude (APAc) and zero, respectively. APAs was shifted toward lower current densities for anodal compared with cathodal SPs (half-maximal values 185 and 238 mA/cm2, respectively; P = 0.02). Recovery of APAs was marginally significant 1 s after SP delivery (P = 0.063). The peak change in Vm during SP (across all intensity levels) was -200% APAc for anodal and +125% APAc for cathodal pulses. In conclusion, we show that SP reduces APA in a sigmoidal fashion at strengths > 10-20 x diastolic threshold and is more deleterious for anodal polarities.

Transbilayer reorientation of phospholipid probes in the human erythrocyte membrane. Lessons from studies on electroporated and resealed cells

In order to characterize in more detail the previously observed (Dressler et al. (1983) Biochim. Biophys. Acta 732, 304-307) increases in transbilayer mobility of phospholipids in the erythrocyte membrane following electroporation at 0 degrees C and subsequent resealing at 37 degrees C of the cells, we have studied rates of flip and flop as well as steady state distributions of the fluorescent N-(NBD)-aminohexanoyl-analogues of the four major membrane phospholipids. Measurements comprised the passive non-mediated components as well as those mediated by specific translocases (flippase and floppase). The major new findings and insights can be summarized as follows. (1) The enhancement of passive transbilayer mobility which increases with the strength, duration, and number of field pulses at 0 degrees C, cannot be fully reversed by subsequent resealing at 37 degrees C. Flip-flop remains considerably elevated relative to the original values.(2) Enhanced mobilities induced by electroporation differ for the probes studied in the sequence SM <<< PS << PC < PE. Other membrane perturbations going along with enhanced flip-flop share only in part this pattern. (3) Mediated, ATP-dependent components of flip and flop of the probes are suppressed in electroporated/resealed cells, partly due to loss of cellular Mg2+, partly - in case of flippase - due to competition by externalized endogenous PS. (4) Electroporated/resealed cells provide an elegant means to demonstrate the contribution of various components of flip and flop to the steady state transbilayer distribution of phospholipids, in particular the role of passive mobility. The new, detailed information on the displacements of phospholipid between the two leaflets of the membrane bilayer in porated/resealed cells will help to understand erythrocyte shape changes following poration and during resealing (Henszen et al. (1993) Biol. Chem. Hoppe-Seyler 374, 114).

Nonthermal pasteurization of liquid foods using high-intensity pulsed electric fields

Processing foods with high-intensity pulsed electric fields (PEF) is a new technology to inactivate microorganisms and enzymes with only a small increase in food temperature. The appearance and quality of fresh foods are not altered by the application of PEF, while microbial inactivation is caused by irreversible pore formation and destruction of the semipermeable barrier of the cell membrane. High-intensity PEF provides an excellent alternative to conventional thermal methods, where the inactivation of the microorganisms implies the loss of valuable nutrients and sensory attributes. This article presents recent advances in the PEF technology, including microbial and enzyme inactivation, generation of pulsed high voltage, processing chambers, and batch and continuous systems, as well as the theory and its application to food pasteurization. PEF technology has the potential to improve economical and efficient use of energy, as well as provide consumers with minimally processed, microbiologically safe, nutritious and freshlike food products.

Electroporation of the photosynthetic membrane: structural changes in protein and lipid-protein domains

A biological membrane undergoes a reversible permeability increase through structural changes in the lipid domain when exposed to high external electric fields. The present study shows the occurrence of electric field-induced changes in the conductance of the proton channel of the H(+)-ATPase as well as electric field-induced structural changes in the lipid-protein domain of photosystem (PS) II in the photosynthetic membrane. The study was carried out by analyzing the electric field-stimulated delayed luminescence (EPL), which originates from charge recombination in the protein complexes of PS I and II of photosynthetic vesicles. We established that a small fraction of the total electric field-induced conductance change was abolished by N,N'-dicyclohexylcarbodiimide (DCCD), an inhibitor of the H(+)-ATPase. This reversible electric field-induced conductance change has characteristics of a small channel and possesses a lifetime < or = 1 ms. To detect electric field-induced changes in the lipid-protein domains of PS II, we examined the effects of phospholipase A2 (PLA2) on EPL. Higher values of EPL were observed from vesicles that were exposed in the presence of PLA2 to an electroporating electric field than to a nonelectroporating electric field. The effect of the electroporating field was a long-lived one, lasting for a period > or = 2 min. This effect was attributed to long-lived electric field-induced structural changes in the lipid-protein domains of PS II.