Dependence of the electrical breakdown voltage on the charging time inValonia utricularis

Analysis of Current Pulses in HeLa-Cell Permeabilization Due to High Voltage DC Corona Discharge

Corona discharges are commonly utilized for numerous practical applications, including bio-technological ones. The corona induced transfer of normally impermeant molecules into the interior of biological cells has recently been successfully demonstrated. The exact nature of the interaction of the corona discharge with a cell membrane is still unknown, however, previous studies have suggested that it is either the electric fields produced by ions or the chemical interaction of the reactive species that result in the disruption of the cell membrane. This disruption of the cell membrane allows molecules to permeate into the cell. Corona discharge current constitutes a series of pulses, and it is during these pulses that the ions and reactive species are produced. It stands to reason, therefore, that the nature of these corona pulses would have an influence on the level of cell permeabilization and cell destruction. In this investigation, an analysis of the width, rise-time, characteristic frequencies, magnitude, and repetition rate of the nanosecond pulses was carried out in order to establish the relationship between these factors and the levels of cell membrane permeabilization and cell destruction. Results obtained are presented and discussed.

Effects of cell orientation and electric field frequency on the transmembrane potential induced in ellipsoidal cells

The transmembrane potential (Deltaphi) induced by external electric fields is important both in biotech applications and in new medical therapies. We analyzed the effects of AC field frequency and cell orientation for cells of a general ellipsoidal shape. Simplified equations were derived for the membrane surface points where the maximum Deltaphi is induced. The theoretical results were confirmed in experiments with three-axial chicken red blood cells (a:b:c=6.66 microm:4.17 microm:1.43 microm). Propidium iodide (PI) staining and cell lysis were detected after an AC electropermeabilization (EP) pulse. The critical field strength for both effects increased when the shorter axis of a cell was parallel to the field, as well as at higher field frequency and for shorter pulse durations. Nevertheless, data analysis based on our theoretical description revealed that the Deltaphi required is lower for the shorter axis, i.e. for smaller membrane curvatures. The critical Deltaphi was independent of the field frequency for a given axis, i.e. the field strength had to be increased with frequency to compensate for the membrane dispersion effect. Comparison of the critical field strengths of PI staining in a linear field aligned along semi-axis a (142 kV m(-1)) and a field rotating in the a-b plane (115 kV m(-1)) revealed the higher EP efficiency of rotating fields.

Cell Lysis in SWLA-2 Hybridomas due to 1 kHz AC Electric Fields

This paper describes results involving the percentage cell lysis of SWLA-2 murine hybridomas produced by AC electric field pulses at 1 kHz with pulse widths ranging from 1 ms to 1 second. Cells that had been exposed to the electric fields were cultured and replicate samples were examined at 48 hours to determine the number of viable cells.

An examination of the effect of an AC pulsed electric field on cell mortality in SWLA-2 hybridomas

This work describes the initial experimental setup and results involving the percentage cell lysis in SWLA-2 murine hybridomas produced by AC electric field pulses of varying amplitudes and pulse widths. Cells were cultured and separate samples examined at 24 hours. The frequency, pulse width and peak-to-peak voltage were varied. AC electric fields producing at least 1 V across the cell membrane appear to be more effective in producing cell lysis than similar fields producing lower membrane voltages. Additionally, higher frequencies, in the 10 kHz range, appear to be more effective than lower frequencies at membrane voltages above 1 V in producing cell lysis.

An examination of the effect of decaying exponential pulse electric fields on cell mortality in murine spleenocytes, hybridomas, and human natural killer cells

This work describes the percentage cell lysis produced by exponentially decaying electric field pulses of varying amplitudes and time constants. Three different cell types were examined: murine spleenocytes, hybridomas, and human natural killer. Cells were cultured and separate samples examined at 24 hours and 48 hours. Two sets of experiments were performed for each cell type. At 0.3 kV, the spleenocytes exhibited a mortality of roughly 50% twenty-four hours after exposure to the pulse; while at forty-eight hours the spleenocyte cell count had reduced to roughly 25% viable cells. All other cell types showed mortality consistently in excess of 80% at field pulse strengths of about 0.3 V/m.

Inactivation of foodborne viruses of significance by high pressure and other processes

The overall safety of a food product is an important component in the mix of considerations for processing, distribution, and sale. With constant commercial demand for superior food products to sustain consumer interest, nonthermal processing technologies have drawn considerable attention for their ability to assist development of new products with improved quality attributes for the marketplace. This review focuses primarily on the nonthermal processing technology high-pressure processing (HPP) and examines current status of its use in the control and elimination of pathogenic human viruses in food products. There is particular emphasis on noroviruses and hepatitis A virus with regard to the consumption of raw oysters, because noroviruses and hepatitis A virus are the two predominant types of viruses that cause foodborne illness. Also, application of HPP to whole-shell oysters carries multiple benefits that increase the popularity of HPP usage for these foods. Viruses have demonstrated a wide range of sensitivities in response to high hydrostatic pressure. Viral inactivation by pressure has not always been predictable based on nomenclature and morphology of the virus. Studies have been complicated in part from the inherent difficulties of working with human infectious viruses. Consequently, continued study of viral inactivation by HPP is warranted.

Effect of electrical charges and fields on injury and viability of airborne bacteria

In this study, the effects of the electric charges and fields on the viability of airborne microorganisms were investigated. The electric charges of different magnitude and polarity were imparted on airborne microbial cells by a means of induction charging. The airborne microorganisms carrying different electric charge levels were then extracted by an electric mobility analyzer and collected using a microbial sampler. It was found that the viability of Pseudomonas fluorescens bacteria, used as a model for sensitive bacteria, carrying a net charge from 4100 negative to 30 positive elementary charges ranged between 40% and 60%; the viability of the cells carrying >2700 positive charges was below 1.5%. In contrast, the viability of the stress-resistant spores of Bacillus subtilis var. niger (used as simulant of anthrax-causing Bacillus anthracis spores when testing bioaerosol sensors in various studies), was not affected by the amount of electric charges on the spores. Because bacterial cells depend on their membrane potential for basic metabolic activities, drastic changes occurring in the membrane potential during aerosolization and the local electric fields induced by the imposed charges appeared to affect the sensitive cells' viability. These findings facilitate applications of electric charging for environmental control purposes involving sterilization of bacterial cells by imposing high electric charges on them. The findings from this study can also be used in the development of new bioaerosol sampling methods based on electrostatic principles.

Killing of microorganisms by pulsed electric fields

Lethal effects of pulsed electric fields (PEF) on suspensions of various bacteria, yeast, and spores in buffer solutions and liquid foodstuffs were examined. Living-cell counts of vegetative cell types were reduced by PEF treatment by up to more than four orders of magnitude (> 99.99%). On the other hand, endo- and ascospores were not inactivated or killed to any great extent. The killing of vegetative cell types depends on the electrical field strength of the pulses and on the treatment time (the product of the pulse number and the decay time constant of the pulses). For each cell type, a specific critical electric field strength (Ec) and a specific critical treatment time (tc) were determined. Above these critical values, the fractions of surviving cells were reduced drastically. The "limits" Ec and tc depend on the cell characteristics as well as on the type of medium in which the cells are suspended. Especially in acid media living-cell counts were sufficiently decreased at very low energy inputs. In addition to the inactivation of microorganisms, the effect of PEF on food components such as whey proteins, enzymes and vitamins, and on the taste of foodstuffs was studied. The degree of destruction of these food components by PEF was very low or negligible. Moreover, no significant deterioration of the taste of foodstuffs was detected after PEF treatment. Disintegration of cells by PEF treatment in order to harvest intracellular products was also studied. Yeast cells, suspended in buffer solution, were not disintegrated by electric pulses. Hence, PEF treatment is an excellent process for inactivation of microorganisms in acid and in thermosensive media, but not for complete disintegration of microbial cells.

Fusion of fibroblasts with differentiated and nondifferentiated leukemia cells resulting in blockage of DNA synthesis

We have investigated the regulation of DNA synthesis in the heterokaryons of HL60 human myelomonocytic leukemia cells and NIH3T3 mouse fibroblasts to examine if the differentiated leukemia cells contained a replication inhibiting activity. Cell fusions were performed either by exposing a suspension of mixed cells to an electric pulse or by the polyethylene glycol method. To identify the origin of the nuclei in a heterokaryon, one set of partner cells was prelabeled with [3H]thymidine before fusion. DNA synthetic activity after fusion was then revealed immunohistochemically by bromodeoxyuridine incorporation. DNA synthesis in the nuclei of 3T3 was inhibited in the heterokaryons of 3T3 and in either one of the two differentiated forms of HL60, i.e., the macrophage-like or the granulocyte-like. The result supports that a negative regulator of DNA synthesis exists in the differentiated HL60. Surprisingly, we have also found that DNA synthesis was inhibited in the nuclei of both 3T3 and nondifferentiated, proliferating HL60 when these two cells were fused. When unfused, proliferating cells were eliminated with cytosine arabinoside; these nonreplicating heterokaryons survived for at least 5 days, and 15% of them showed alpha-naphthylacetate esterase activity, a trait of the macrophage differentiation. The blockage of DNA synthesis in both partner nuclei was also observed in the heterokaryons of NIH3T3 cells and nondifferentiated human promonocytic leukemia cells U937, and in nondifferentiated HL60 and human diploid fibroblasts WI38. However, this effect was not found in the heterokaryons of NIH3T3 cells and human B lymphoma WI-729-HF2 cells. This is the first demonstration of the inhibition of DNA synthesis upon fusion of two proliferating cells.

Biophysical and anatomical considerations for safe and efficacious catheter ablation of arrhythmias

The development of catheter ablation techniques for therapy of cardiac arrhythmias continues to evolve. Although many patients have benefited from catheter ablation procedures, failure to ablate the arrhythmogenic substrate and complications from the pulse used in these procedures remain too frequent occurrences. The purpose of this review is to focus on these problems of inefficacy and safety with attention directed to the role various direct current and radiofrequency pulses have had in the genesis of these difficulties.

Mobile charges in the cell membranes ofHalicystis parvula

Charge-pulse experiments were performed on cells of the giant marine algaHalicystis parvula. At normal pH (8.2), the voltage decay following a charge-pulse of 500 ns duration fed to the vacuole could be described by summing two exponential relaxations. The amplitudes and time constants of these relaxations were widely separated. The parameters of the two relaxation processes were found to be pH-dependent. Reduction of the external pH value from pH 8.2 to 5 resulted in a complete change of the two relaxation processes within a few minutes. Only one relaxation process could be observed at pH 5, within the time resolution of our instrumentation. The experimental data could not be explained by a two-membrane model with reasonable values for the specific capacitances of tonoplast and plasmalemma. The results of the charge-pulse relaxations were found to be consistent with the assumption that both membranes have very similar electrical properties and that both contain mobile charges with a total surface concentration of about 30 nmol·m(-2) and a translocation-rate constant of about 500·s(-1). The mobile charges became neutralized at pH 5 hhich led to a decrease of the apparent specific capacitance of the algal cells. They are presumably either part of a transport system for cations or connected with the chloride pump ofHalicystis parvula.

Loss of functional sperm entry into Xenopus eggs after activation correlates with a reduction in surface adhesivity

In Xenopus, the plasma membrane of the unactivated egg is receptive to sperm only in the animal hemisphere (R. Grey, M. Bastiani, D. Webb, and E. Schertel, 1982, Dev. Biol. 89, 475-487). The reinsemination experiments of investment-free eggs reported in this paper demonstrate that functional sperm entry is lost after activation. Supernumerary sperm were excluded even though the fertilization envelope was absent and the membrane potential had returned to the level found in the unfertilized egg. Even when the electrical block to polyspermy was suppressed by 40 mM NaI (which reduces the membrane potential), polyspermy could be induced only if denuded eggs were initially inseminated in this medium. We estimate that the loss of functional sperm entry, independent of the electrical block, occurs during the first 10 min following fertilization. Sperm readily adhere to the surface of the animal hemisphere of unactivated eggs divested of their extracellular coats, but they do not adhere to the surface of activated eggs. Denuded eggs also adhere to each other, with the surface of the animal hemisphere of unactivated eggs exhibiting the greatest degree of adhesivity. We used electric field-induced fusion (EFIF), without prior dielectrophoresis, to quantify the regional and temporal adhesiveness of eggs. At electric field strengths greater than 8 V/cm, the probability of fusion during EFIF is highest with the animal hemisphere of unactivated eggs, moderate with both the vegetal hemisphere of unactivated eggs and the animal hemisphere of activated eggs, and lowest with the vegetal hemisphere of activated eggs. When pairs of eggs are constructed with different hemispheres in contact, the fusion characteristics of the pair are similar to the more adhesive member of the pair. The regional and temporal differences in the adhesiveness of the Xenopus egg surface correlate with its receptivity to sperm and could possibly account for the plasma membrane's activation-induced loss of functional sperm entry.

Membrane stability

Rupture and buckling of artificial and biological membranes is an important part of many biological processes. In this review, we present some of the main experimental facts and their analysis. Recent theoretical work, in particular thin film models and nucleation mechanisms of membrane instability, are discussed in detail. Possible applications to membrane adhesion and fusion are pointed out. Attempts are made to explain biological phenomena and experimental results for biological membranes based on a rigorous physicochemical approach developed previously for thin films in colloid systems.

Rotation of cells in an alternating electric field: theory and experimental proof

Protoplasts of Avena sativa rotate in an alternating electric field provided that at least two cells are located close to each other. An optimum frequency range (20 to 30 kHz) exists where rotation of all cells exposed to the field is observed. Below and above this frequency range, rotation of some cells is only occasionally observed. The angular velocity of rotation depends on the square of the electric field strength. At field strengths above the value leading to electrical breakdown of the cell membrane, rotation is no longer observed due to deterioration of the cells. The absolute value of the angular velocity of rotation at a given field strength depends on the arrangement of the cells in the electric field. A maximum value is obtained if the angle between the field direction and the line connecting the two cells is 45 degrees. With increasing distance between the two cells the rotation speed decreases. Furthermore, if two cells of different radii are positioned close to each other the cell with the smaller radius will rotate with a higher speed than the larger one. Rotation of cells in an alternating electric field is described theoretically by interaction between induced dipoles in adjacent cells. The optimum frequency range for rotation is related to the relaxation of the polarization process in the cell. The quadratic dependence of the angular velocity of rotation on the field strength results from the fact that the torque is the product of the external field and the induced dipole moment which is itself proportional to the external field. The theoretical and experimental results may be relevant for cyclosis (rotational streaming of cytoplasm) in living cells.

Water relations of immobilized giant algal cells

Cells of Halicystis parvula, Acetabularia mediterranea, and Valonia utricularis were immobilized in a cross-linked alginate matrix (4-6% w/w) in order to simulate water-relation experiments in individual cells of higher plant tissues. The immobilization of these cells did not lead to an increase in the mechanical stability of the cell walls. This was demonstrated by measuring the volumetric elastic modulus of the cell wall and its dependence on turgor pressure with the aid of the non-miniaturized pressure probe. In immobilized cells, no changes in the absolute value of the elastic modulus of the cell wall could be detected for any given pressure. At the maximum turgor pressure at which non-immobilized cells normally burst (about 3-7 bar for V. utricularis; depending on cell size, 3 bar for A. mediterranea and 0.9 bar for H. parvula) reversible decreases in the pressure are observed which are succeeded by corresponding pressure increases. This obvervation indicates that coating the cells with the cross-linked matrix protects them from rapid water and turgor pressure loss. Turgor pressure relaxation processes in immobilized cells, which could be induced hydrostatically by means of the pressure probe, yielded accurate values for the half-times of water exchange and for the hydraulic conductivity of the cell membrane. The results demonstrate that the water transport equations derived for single cells in a large surrouding medium are valid for immobilized cells, so that any influence exerted by the unstirred layer which is caused by the presence of the cross-linked matrix can be ignored in the calculations. On the other hand, the evaluation of the half-times of water exchange and the hydraulic conductivity from turgor pressure relaxation processes, which have been induced osmotically, only yields correct values under certain circumstances. The model experiments presented here show, therefore, that the correct Lp-value for an individual cell in a higher plant tissue can probably only be obtained presently by using the pressure probe technique rather than the osmotic method. The results are also discussed in relation to the possible applications of immobilized cells and particularly of immobilized micro-organisms in catalytic reaction runs on an industrial scale.

Reversible electrical breakdown of squid giant axon membrane

Charge pulse relaxation experiments were performed on squid giant axon. In the low voltage range, the initial voltage across squid axon membrane was a linear function of the injected charge. For voltages of the order of 1 V this relationship between injected charge and voltage across the membrane changes abruptly. Because of a high conductance state caused by these large electric fields the voltage across the membrane cannot be made large enough to exceed a critical value, Vc, defined as the breakdown voltage, Vc has for squid axon membrane a value of 1.1 V at 12 degrees C. During breakdown the specific membrane conductance exceeds 1 S. cm-2. Electrical breakdown produced by charge pulses of few microseconds duration have no influence on the excitability of the squid axon membrane. The resealing process of the membrane is so fast that a depolarizing breakdown is followed by the falling phase of a normal action potential. Thus, membrane voltages close to Vc open the sodium channels in few microseconds, but do not produce a decrease of the time constant of potassium activation large enough to cause the opening of a significant percentage of channels in a time of about 10 mus. It is probable that the reversible electrical breakdown is mainly caused by mechanical instability produced by electrostriction of the membrane (electrochemical model), but the decrease in the Born energy for ion injection into the membrane, accompanying the decrease in membrane thickness, may play also an important role. Because of the high conductance of the membrane during breakdown it seems very likely that this results in pore formation.

High electric field effects on the cell membranes of Halicystis parvula : A charge pulse study

The electrical breakdown behavior of the giant algal cell Halicystis parvula was studied in order to predict the optimum conditions for electrically induced cell-to-cell fusion. Using the charge pulse technique, the membranes were charged at different pulse lengths to the maximum voltage Vc. Because of a reversible, high-conductance state of the membrane (electrical breakdown), it was not possible to exceed the critical membrane breakdown potential. The breakdown voltage exhibited a strong dependence on the charging time (pulse length) between 10 μs and 100 μs. Below 10 μs the breakdown voltage of the two membranes, tonoplast and plasmalemma, assumed a constant value of about 1.9 V, whereas above a pulse length of about 100 μs the breakdown voltage was nearly constant with a value of about 0.6 V. The extreme values for the breakdown voltage at very short and at very long charging times agree fairly well with results which have been obtained on cells of Valonia utricularis and planar lipid bilayer membranes. However, the pulse length dependence of the breakdown voltage was found to be quite different in H. parvula. In addition, the membrane conductance increase during breakdown in H. parvula cells is much more pronounced than in membranes of V. utricularis, but similar to lipid bilayer membranes. From this result it is suggested that the membrane structure of H. parvula is quite different from V. utricularis (larger lipid domains).

A new electrical method for the determination of the cell membrane area in plant cells

The membrane are of giant algal cells of Valonia utricularis was determined electrically by using the charge-pulse technique. The membrane was charged to low voltages between 2 and 20 mV by injecting charge pulses of defined amplitude and very short duration (about 100 ns). The injected charge was calculated by measuring the current increment via a potential drop across a 10 Ω resistance in the outer circuit and by considering the preselected charging time. The initial voltage across the membrane was calculated by extrapolation to time zero (=end of the charge pulse). From the values of the injected charge and the voltage built up initially across the membrane, the capacitance of the membrane could be calculated. Assuming that the specific capacity of the two membranes, tonoplast and plasmalemma, arranged in series was 0.5 μF cm(-2), the membrane area could be derived from the membrane capacity. The electrically determined membrane area agrees with the geometrically determined one to within 10%.

Electric field induced transient pores in phospholipid bilayer vesicles

A study of the voltage induction of transient pores in phospholipid bilayer vesicles is reported. Unilamellar vesicles (dipalmitoylphosphatidylcholine), with a size distribution of 100 +/- 30 nm, were prepared by the method of Enoch & Strittmatter [Enoch, H., & Strittmatter, P. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 145]. The vesicles loaded with [14C]sucrose and suspended in a mixture of 150 mM NaCl and 272 mM sucrose (both are the isotonic solvent for erythrocytes) were exposed to an intense electric field in the range of 20--40 kV/cm, with a field decay time of 5--15 micro second. A transient leakage of sucrose label was detected when the field strength exceeded 30 kV/cm. After the field was removed, no slow leakage of the tracer molecules occurred during a 65-h incubation period at the room temperature (23 +/- 2 degrees C). The leakage is attributed to the field-induced transmembrane potential, but not other effects such as the Joule heating or the shock wave associated with the voltage discharge. When the potential exceeded a threshold value of 200 mV, corresponding to an applied field strength of 30 kV/cm, there was a dielectric breakdown of the bilayer structure. Pores which allowed passage of sucrose were formed, transiently. Experiments show that these pores were fully reversible, and no global and permanent damages to the vesicle bilayer were detected. The implication of this membrane potential triggered conducting state of lipid bilayers to biological functions of cells is discussed.

High frequency fusion of plant protoplasts by electric fields

Mesophyll cell protoplasts of Vicia faba were collected by dielectrophoresis in a highly inhomogeneous alternating electric field (sine wave, 5 to 10 V peak-to-peak value, 500 kHz, electrode distance 200 μm). Under these conditions, the cells formed aggregates of two or three on the electrodes or bridges consisting of 4 to 6 protoplasts between the electrodes. This "pearl chain" arrangement of the cells was only stable for the duration of the applied field. By the additional application of a high single field pulse (square wave, 15 V, 50 μs), it was possible to induce cell fusion within the aggregates or bridges. This electrically stimulated fusion of cells proceeded at room temperature and under physiological pH-conditions, without the use of chemical reagents, and gave a high yield. Smaller fused aggregates formed spheres within a few minutes. During the dielectrophoretically induced adhesion of the protoplasts to one another, the field strength must be chosen such that dielectric breakdown of the membrane is avoided, but at the same time, the strength of the subsequently applied single field pulse must be high enough to induce dielectric breakdown at the sites of contact between the protoplast membranes. From these results, one can conclude that in addition to close contact between membranes, the prerequisite for electrically stimulated cell fusion is dielectric breakdown which leads to changes in the membrane conductance, permeability, and probably fluidity.