Membrane stability

Cell-attached patch clamp study of the electropermeabilization of amphibian cardiac cells

Potential gradients imposed across cell or lipid membranes break down the insulating properties of these barriers if an intensity and time-dependent threshold is exceeded. Potential gradients of this magnitude may occur throughout the body, and in particular in cardiac tissue, during clinical defibrillation, ablation, and electrocution trauma. To study the dynamics of membrane electropermeabilization a cell-attached patch clamp technique was used to directly control the potential across membrane patches of single ventricular cells enzymatically isolated from frog (Rana pipiens) hearts. Ramp waveshapes were used to reveal rapid membrane conductance changes that may have otherwise been obscured using rectangular waveshapes. We observed a step increase (delta t less than 30 microseconds) or breakdown in membrane conductance at transmembrane potential thresholds of 0.6-1.1 V in response to 0.1-1.0 kV/s voltage ramps. Conductance kinetics on a sub-millisecond time scale indicate that breakdown is preceded by a period of instability during which the noise and amplitude of the membrane conductance begin to increase. In some cells membrane breakdown was observed to be fully reversible when using an intershock interval of 1 min (20-23 degrees C). These findings support energetic models of membrane electropermeabilization which describe the formation of membrane pores (or growth of existing pores) to a conducting state (instability), followed by a rapid expansion of these pores when the energy barrier for the formation of hydrophilic pores is overcome (breakdown).

Phospholipids at the oil/water interface: adsorption and interfacial phenomena in an electric field

Interfacial effects produced in an immiscible liquid system by the action of an external electric field have been considered. The addition of small amounts of neutral phospholipids to the nonaqueous phase has been shown to result in a marked increase in the sensitivity of the interfacial boundary to the voltage applied, which is manifested by: (i) an accelerated decrease of the interfacial tension after the two immiscible liquid phases have been brought into contact; (ii) reduced interfacial tension, by 20-30 mN/m, at the oil/water interface at field strengths of 1-10 kV/m (the interfacial tension drop in the absence of phospholipids does not exceed 5 mN/m); (iii) development of electrohydrodynamic instability at the planar dividing surface between phases; and (iv) dispersion of water into the nonaqueous phase at smaller field strengths by a factor of about 100 as compared to those normally required in the absence of phospholipids. In order to gain a deeper insight into the mechanisms of interfacial phenomena, mainly exemplified by the n-heptane/water system containing phosphatidylcholine, three major issues have been considered: (1) Kinetics of the adsorption of phospholipid at the oil/water interface from the nonaqueous phase, and effects produced by exposure to an external electric field; also, the adsorption under equilibrium conditions, and the structure of the adsorption layer formed. (2) Interactions between neutral phospholipid and inorganic or organic ions at the interfacial boundary under the voltage applied. (3) Conditions for the occurrence of electrohydrodynamic instability at the dividing surface between oil and water and the formation of a water-in-oil emulsion; also aggregation and gelation processes induced in the nonaqueous phospholipid solution bulk by the action of a weak external electric field. Throughout the present paper, an attempt has been made to relate the microscopic behaviour of phospholipids under an external electric field to macroscopically observable properties at the movable interfacial boundaries. The adsorption studies have shown that phosphatidylcholine is prone to self-organization into a liquid-crystalline state at an immiscible liquid interface. The disintegration of the interfacial lipid film thus formed by the action of a weak electric field has been explained as due to an enhanced electrohydrodynamic instability of liquid crystals. This results in the formation of either an emulsion, or a microemulsion in the nonaqueous solution bulk. The formation of a microemulsion is manifested by the appearance of an optically anisotropic gel, stable only under an external applied electric field, in the nonaqueous solution bulk.(ABSTRACT TRUNCATED AT 400 WORDS)

A delay in membrane fusion: lag times observed by fluorescence microscopy of individual fusion events induced by an electric field pulse

Low light level video microscopy of the fusion of DiI- (1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) labeled rabbit erythrocyte ghosts with unlabeled rabbit erythrocyte ghosts, held in stable apposition by dielectrophoresis in sodium phosphate buffers, showed reproducible time intervals (delays) between the application of a single fusogenic electric pulse and the earliest detection of fluorescence in the unlabeled adjacent membranes. The delay increased over the range 0.3-4 s with a decrease in (i) the electric field strength of the fusion-inducing pulse from 1000 to 250 V/mm, (ii) the decay half-time of the fusogenic pulse in the range 1.8-0.073 ms, and (iii) the dielectrophoretic force which brings the membranes into close apposition. A change in the buffer viscosity from 1.8 to 10 mP.s caused the delay to increase from 0.36 to 3.7 s (in glycerol solutions) or to 5.2 s (in sucrose solutions). The delay decreased 2-3 times with an increase in temperature from 21 to 37 degrees C. It did not differ significantly for "white" ghosts [0.013 mM hemoglobin (Hb)] or "red" ghosts (0.15 mM Hb) or buffer strength over the range 5-60 mM (sodium phosphate, pH 8.5). The calculated activation energy, 17 kcal/mol, does not depend on the field strength. The yield of fused cells was high when the delay was short. The delay in electrofusion resembles the delays in pH-dependent fusion of vesicular stomatitis viruses with erythrocyte ghosts [Clague, M. J., Schoch, C., Zech, L., & Blumenthal, R. (1990) Biochemistry 29, 1303-1308] and of fibroblasts expressing influenza hemagglutinin and red blood cells [Morris, S. J., Sarkar, D.P., White, J. M., & Blumenthal, R. (1989) J. Biol. Chem. 264, 3972-3978].(ABSTRACT TRUNCATED AT 250 WORDS)

Cell fusion of Saccharomyces cerevisiae fragile mutants

Fragile mutants of Saccharomyces cerevisiae are defective in the structure of the cell wall and plasma membrane. The mutant cells lyse in hypotonic solutions but grow exponentially when osmotic stabilizer is included in the medium. These mutants display a general increase in the permeability of the plasma membrane. We show here that fragile yeast cells of the same mating type can fuse without protoplast formation. The frequency of cell x cell fusion is lower than that observed for protoplast x protoplast fusion and can be significantly increased if the cells of one partner are converted to protoplasts. Microscopic observations and genetic analysis demonstrate that the hybrids obtained are fusion products. The fusion between fragile cells is explained in terms of the existence of local defects on their surface where the cell wall is thinner (or even missing), thus allowing a direct contact of cells by means of their plasma membranes.

Attraction, deformation and contact of membranes induced by low frequency electric fields

The force of attraction between erythrocyte ghosts induced by low frequency electric fields (60 Hz) was measured as a function of the intermembrane separation. It varied from 10(-14) N for separation of the order of the cell diameter to 10(-12) N for close approach and contact in 20 mM sodium phosphate buffers (conductivity 260 mS/m, pH 8.5). For large separations the interaction force followed a dependence on separation as predicted for dipole-dipole interactions. For small separation an empirical formula was obtained. The membranes deformed at close approach (less than 1 microns) before making contact. The contact area increased with time until reaching the final equilibrium state. The ghosts separated reversibly after switching off the electric field. The membrane tension induced by the ghost interaction at contact was estimated to be of the order of 0.1 mN/m. These first quantitative measurements of the force/separation dependence for intermembrane interactions induced by low frequency electric fields indicate that attractive forces, membrane deformation and contact area of cells depend strongly on intermembrane separation and field strength. The quantitative relationship between them are important for measuring membrane surface and mechanical properties, intermembrane forces and understanding mechanisms of membrane adhesion, instability and fusion in electric fields and in general.

Membrane electroporation--fast molecular exchange by electroosmosis

Human and rabbit erythrocyte ghosts loaded with FITC-dextran (mol. mass = 10 kDa) and NBD-glucosamine (mol. mass = 342 Da) in buffers of different ionic strength and composition were subjected to electric pulses (intensity 0.7 kV/mm and decay half-time 1 ms) at 7-10 degrees C and 20-24 degrees C. The transfer of the fluorescent dyes from the interior of the ghosts through the electropores was observed by low light level video microscopy. The pulses caused the fluorescence to appear outside the membranes as a transient cylindrical cloud directed toward the negative electrode during the first video frame (17 ms). It was similar in both rabbit and human erythrocyte ghosts and at both temperatures but differs for the two dyes, the fluorescence cylinder is long and tall for the FITC-dextran and relatively short and thick for the NBD-glucosamine. The molecular exchange was 2-3 orders of magnitude faster within the first 17 ms after the pulse than the diffusional exchange. It decreased with increasing ionic strength. Formulae for the transfer of molecules by electroosmotic flow through the pores are in agreement with these observations. They allow estimation of the total area of pores with radii larger than that of the fluorescent dye during the pulse. The major conclusion is that electroosmosis is the dominating mechanism of molecular exchange in electroporation of erythrocyte ghosts.

Membrane-membrane interactions: parallel membranes or patterned discrete contacts

Theoretical and experimental studies of thin liquid films show that, under certain conditions, the film thickness can undergo a sudden transition which gives a stable narrower film or ends in film rupture at spatially periodic points. Theoretical analysis have also indicated that similar transitions might arise in the thin aqueous layer separating interacting membranes. Experiments described here show spatially periodic intermembrane contact points and suggest that spontaneous rapid growth of fluctuations can occur on an intermembrane water layer. Normal and pronase pretreated erythrocytes were exposed to 2% Dextran (450,000 Mr) and the resultant aggregates were examined by light and transmission electron microscopy. Cell electrophoresis measurements were used as an index of pronase modification of the glycocalyx. Erythrocytes exposed to dextran revealed a uniform intercellular separation of parallel membranes. This equilibrium between attractive and repulsive intermembrane forces is consistent with the established Derjaguin, Landau, Verwey, Overbeek (DLVO) model for colloidal particle interaction. In contrast to the above uniform separation a spatial pattern of discrete contact regions was observed in cells coming together in dextran following pronase pretreatment. The lateral contact separation distance was 3.0 microns for mild pronase pretreatment and decreased to 0.85 micron for more extensive pronase pretreatments. The system examined here is seen as a useful experimental model in which to study the principles involved in producing either uniform separation or point contacts between interacting membranes.

Vesicle formation in the Golgi apparatus

In this paper we examine the mechanics of vesicle budding from the Golgi apparatus. We propose a model for this process based on the notion that molecular surfactants can release the elastic energy stored in the lipid bilayer. The same physical process may drive other vesiculation processes, including coated vesicle formation and budding of enveloped viruses from the plasma membrane.

Membrane-membrane contact: involvement of interfacial instability in the generation of discrete contacts

The classical approach to understanding the closeness of approach of two membranes has developed from consideration of the net effect of an attractive van der Waals force and a repulsive electrostatic force. The repulsive role of hydration forces and stereorepulsion glycocalyx forces have been recently recognized and an analysis of the effect of crosslinking molecules has been developed. Implicit in these approaches is the idea of an intercellular water layer of uniform thickness which narrows but retains a uniform thickness as the cells move towards an equilibrium separation distance. Most recently an attempt has been made to develop a physical chemical approach to contact which accommodates the widespread occurrence of localized spatially separated point contacts between interacting cells and membranes. It is based on ideas drawn from analysis of the conditions required to destabilize thin liquid films so that thickness fluctuations develop spontaneously and grow as interfacial instabilities to give spatially periodic contact. Examples of plasma membrane behaviour which are consistent with the interfacial instability approach are discussed and experiments involving polycation, polyethylene glycol, dextran and lectin adhesion and agglutination of erythrocytes are reviewed.

Cytoskeletal reorganization during electric-field-induced fusion of Chinese hamster ovary cells grown in monolayers

Mammalian cells were shown to fuse after direct electric pulsation of the plated cells in culture. The extent of fusion was controlled by the duration of the post-pulse incubation. Formation of polynucleated cells was slow, even at 37 degrees C. Pre-pulse incubation with colchicine increased the fusion yield slightly. Cytoskeletal organization during the post-pulse incubation was observed using immunofluorescence techniques. Microfilaments were unaffected, but microtubules disappeared during the first minutes following the pulse, and then reformed on subsequent incubation.

Electro-mechanical permeabilization of lipid vesicles. Role of membrane tension and compressibility

A simple micropipet technique was used to determine the critical electric field strength for membrane breakdown as a function of the applied membrane tension for three different reconstituted membranes: stearoyloleoylphosphatidylcholine (SOPC), red blood cell (RBC) lipid extract, and SOPC cholesterol (CHOL), 1:1. For these membranes the elastic area expansivity modulus increases from approximately 200 to 600 dyn/cm, and the tension at lysis increases from 5.7 to 13.2 dyn/cm, i.e., the membranes become more cohesive with increasing cholesterol content. The critical membrane voltage, Vc, required for breakdown was also found to increase with increasing cholesterol from 1.1 to 1.8 V at zero membrane tension. We have modeled the behavior in terms of the bilayer expansivity. Membrane area can be increased by either tensile or electrocompressive stresses. Both can store elastic energy in the membrane and eventually cause breakdown at a critical area dilation or critical energy. The model predicts a relation between tension and voltage at breakdown and this relation is verified experimentally for the three reconstituted membrane systems studied here.

Bioelectrorheological model of the cell. 1. Analysis of stresses and deformations

An electrorheological model of a cell in alternating electric field is proposed. The model relates changes in the spherical cell's shape to the field conditions, electric parameters of cytoplasm, cell membrane and external medium, and to the rheological parameters of the membrane. Stresses were determined using Maxwell's stress tensor for isotropic media. Shear stresses in the cell membrane were analyzed. Predictions of the model for variations of shear stress in cellular membranes subjected to an external periodic electric field are presented and related to the conditions prevailing in electrobiological research.

Biomechanical interactions of cancer cells with the microvasculature during metastasis

Metastasis is a major, life-threatening complication of cancer. The bloodstream is the most important disseminative route for cancer cells liberated from their parent tumors. Single circulating cancer cells are arrested in the microvasculature, where the vast majority are killed by rapid or slow processes, and the relatively few survivors grow into micrometastases. We review the underlying causes of one type of rapid cancer cell death in the microcirculation, namely, that caused by biomechanical interactions of cancer cells with microvessel walls, which may result in cell surface membrane expansion and lethal rupture. These lethal interactions appear to be important rate-regulators in hematogenous metastasis, and to dictate some aspects of metastatic patterns. Although these are not the only interactions involving cancer cells, in contrast to others involving cellular and humoral defense mechanisms, they have received comparatively little attention.

Fusion events and nonfusion contents mixing events induced in erythrocyte ghosts by an electric pulse

The mechanism of membrane fusion was studied by using human erythrocyte ghosts held in close contact by alternating current-induced dielectrophoresis and inducing fusion with a single electric field pulse. Individual fusion events were followed visually using either 1,1'-dihexadecyl-3,3,3',3'-tetramethylindo carbocyanine perchlorate as a membrane-mixing label or 10-kD fluorescein isothiocyanate-dextran as a contents-mixing label. However, over a range of variables, the number of contents-mixing events usually considerably exceeded the number of membrane-mixing events, although the discrepancy was less at higher ionic strength. However, when the dielectrophoretic force holding the membranes in contact was turned off after the pulse, Brownian motion caused some of the groups of ghosts in which contents mixing occurred to eventually separate from one another, showing that they could not represent fusion events. Separate experiments showed, conversely, that fusion did occur in the groups that did not separate after the dielectrophoresis was turned off.

Spatially periodic discrete contact regions in polylysine-induced erythrocyte-yeast adhesion

Cell-cell adhesion occurs when human erythrocytes and yeast cells are suspended together in suprathreshold concentrations of polylysine in saline. The threshold polycation concentration for adhesion depends on cell concentration and decreases with increasing polycation molecular weight. The threshold concentration was similar for erythrocyte-erythrocyte adhesion and for yeast-erythrocyte adhesion. Transmission electron micrographs show that the erythrocytes adhere to yeast as if to engulf the cell. The regions of close contact between the erythrocyte membrane and the yeast cell walls are spatially discrete. The contact separation distance for the asymmetric erythrocyte-yeast adhesion is very similar to that (0.83 micron) observed when polylysine-induced adhesion occurs in the symmetrical erythrocyte-erythrocyte system. The spacing is attributed to the growth of a squeezing wave as an interfacial instability, on the intercellular aqueous layer. Freeze-fracture electron microscopy of cells that were not fixed during preparation for microscopy confirms the discrete nature of contacts between polylysine treated erythrocytes.

Membrane fusion: overcoming of the hydration barrier and local restructuring

The early stages of membrane fusion have been investigated theoretically. It has been shown that the hydration repulsion, operating between apposed membranes, is overcome locally under the action of out-of-plane thermal fluctuations of the bilayers. The fluctuations lead to the formation of close (less than 0.5 nm) contact between the membranes within a small area (approximately 10 nm2). Increasing hydration repulsion between apposed polar heads of lipid molecules in this area causes the rupture of interacting monolayers. The rupture results in monolayer fusion of the membranes, i.e. in the formation of a bridge connecting the monolayers, which is usually named the monolayer stalk. The influence of degree of hydration of the monolayers and their spontaneous curvature on conditions of monolayer fusion have been analysed. The proposed mechanism of early stages of fusion process can proceed without preliminary formation of tight dehydrated contact between the membranes and even without any dehydration.

The long-lived fusogenic state induced in erythrocyte ghosts by electric pulses is not laterally mobile

The long-lived fusogenic state induced in spherical-shaped erythrocyte ghosts by electric field pulses (Sowers, A.E. 1984. J. Cell Biol. 99:1989-1996; Sowers, A.E. 1986. J. Cell Biol. 102:1358-1362) was studied in terms of how the fusion yield depended on both (a) the location where membrane-membrane contact took place with respect to the orientation of the electric pulse and (b) the time interval between the pulse treatment and membrane-membrane contact. Fusion yields were greater for membrane-membrane contact locations closer to where the pulse-induced transmembrane voltage was expected to be greatest and showed a time interval-dependent accelerating decay. The portion of the membrane that became fusogenic included the area up to a latitude of approximately 38 degrees of arc towards the equators of the membranes. A time interval-dependent increase or decrease in rate of decay in the fusion yield for membrane-membrane contacts induced closer to the equator of the membranes did not occur showing that the pulse-induced fusogenic state is immobile in the early 5-45-s interval after induction and has a rate of decay, which does not permit long time interval changes in lateral position to be measured.

Sterol and phospholipid acyl chain alterations in Saccharomyces cerevisiae secretion mutants as a function of temperature stress

Analyses of free sterol, steryl ester and fatty acid components from yeast secretion mutants indicated that free and esterified sterol remained relatively constant over a growth range of 24 C to 34 C. The saturated fatty acid components (16:0 and 18:0) increased while the unsaturated fatty acids (16:1 and 18:1) decreased as the growth temperature increased. In secretory mutants, fatty acid composition changes are more pronounced than in the wild-type strain. A shift toward increased saturated and decreased unsaturated fatty acid was observed when cells were subjected to a 2-hr temperature upshift to 37 C. Steady-state fluorescence anisotropy data indicated that modifications to the lipid component of yeast plasma membrane produced lipid thermotropic transitions that were 3 C to 6 C higher in yeast cells subjected to thermal stress.

The plasma membrane of yeast protoplasts exposed to hypotonicity becomes porous but does not disintegrate in the presence of protons or polyvalent cations

Protoplasts of Saccharomyces cerevisiae swelled, lysed and disintegrated when exposed to hypotonic solutions at neutral pH. At pH 4.5 or lower the hypotonically treated protoplasts did not disintegrate and they retained their intracellular proteins, nucleic acids and nucleotides. However, they became leaky for K+ and Ca2+, indicating that pores had been created in the surface membrane, relaxing the osmotic stress. Upon readjustment of pH to neutral, the hypotonically treated protoplasts released the intracellular content and disintegrated. Also, at low pH, protoplasts did not swell in isotonic ammonium acetate and were refractory to the permeabilizing effect of nystatin and to lysis with low concentrations of detergents. Protoplasts were similarly protected against lysis and disintegration by hypotonic treatment or by detergents, even at neutral pH, if the incubation media contained polyvalent cations, especially Zn2+, La3+, spermine, and Ca2+ chelated with EDTA. The protoplasts exposed to hypotonic stress at low pH did not respire and could not regenerate into viable cells. Effects of H+ and polyvalent cations on intramembrane forces acting between molecules of membrane phospholipids are considered along with possible changes in interactions between membrane proteins.

Model of cell electrofusion. Membrane electroporation, pore coalescence and percolation

High electric field impulses (1-20 kV/cm, 1-20 microseconds) may trigger fusion between adhering cells or lipid vesicles (electrofusion). In this paper a qualitative model of electrofusion is proposed consistent with both electron and light microscopic data. Electrofusion is considered as a multistep process comprising tight membrane-contact formation, membrane electroporation as well as an alternating series of subsequent fast collective and slow diffusive fusion stages. The following sequence of steps is suggested: The electric field pulse enforces (via polarization) a tight contact between the membranes of the cells or vesicles to be fused. During tight-contact formation between the opposing membrane surfaces the membrane-adherent water layers are partially squeezed out from the intermembraneous space. Pores are formed in the double membrane contact area (electroporation) involving lateral diffusion and rotation of the lipid molecules in both adhering membrane parts. With increasing pore density, pore-pore interactions lead to short-range coalescence of double membrane pores resulting in ramified cracks; especially small tongues and loops are formed. At supercritical pore density long-range coalescence of the pores occurs (percolation) producing one large double membrane loop (or tongue) and subsequently one large hole in the contact area. After switching off the electric field, the smaller pores, tongues and loops reseal and water flows back into the intermembraneous space of the double membrane in the contact area. As a consequence of the increasing membrane-membrane separation due to water backflow, cooperative rounding of the edges of remaining larger tongues and holes occurs. This results in the formation of an intercellular cytoplasm bridge (channel) concomitant with the disappearance of the contact line between the fusing cells. The membrane parts surrounded by continuous loop-like cracks may separate from the system and may finally form vesicles. Our electrofusion model comprises a strong linkage between the membrane pore formation by high electric fields (electroporation) and the process of electrofusion. Additionally, both pore-pore interactions as well as protein-protein interactions in the contact area of the fusing cells are explicitly introduced. The model provides a qualitative molecular description of basic experimental observations such as the production of membrane fragments, of smaller inside-out vesicles and the formation of larger intercellular cytoplasm bridges.

The osmotic response of large unilamellar vesicles studied by quasielastic light scattering

Large unilamellar vesicles of two phosphatidylcholines, one saturated (DMPC) and the other unsaturated (DOPC), prepared by the reverse-phase evaporation method were studied using the quasielastic light scattering technique. The accurate sizing obtained by this technique showed an osmotic response for the two kinds of vesicles when the salinity of the external medium was diluted. The elastic moduli of lipid vesicles bilayers in the liquid phase were then estimated according to the elasticity theory of spherical shells taking into account salt leakage data known from the literature.

Electropore diameters, lifetimes, numbers, and locations in individual erythrocyte ghosts

Low light level video microscopy was used to study the diameter, lifetime, number, and location characteristics of electric field-induced pores (electropores) in erythrocyte ghosts. The diameter of electropores was probed by following the efflux of soluble fluorescent-tagged molecules out of the resealed ghost cytoplasmic compartments. After reaching a peak radius of at least 8.4 nm the electropores resealed within 200 ms to a radius of about 0.5 nm and stayed at that radius thereafter. Video sequences clearly show that pores are induced preferentially in the cathodal hemisphere. Pores induced in the hemisphere facing the positive electrode were either never greater than 0.5 nm in radius, much smaller in number if they were greater than 0.5 nm in radius, or shorter lived. Calculations indicated that an upper limit of 700 electropores were induced per membrane.

Mechanical aspects of the lungs as cancer cell-killing organs during hematogenous metastasis

A substantial proportion of many different types of circulating cancer cells appear to be killed during their interactions with the pulmonary microcirculation. Different tensions exist during respiration within alveolar units, and hence the pulmonary capillaries. We have calculated the effects of these tensions on the entry and subsequent fate of circulating cancer cells. Our calculations indicate that during expiration, when tension in the capillary walls is low, cancer cells can enter and travel along the capillaries without damage, because the vessels are deformed by the cells and the hydrodynamic field surrounding them. During normal inspiration when the alveoli are stretched, the increased tension within the capillary walls serves to compress the contained cancer cells. This compression, together with previously calculated blood pressure differentials between the ends of the cells, is thought in some cases, to increase their membrane tensions above the critical level for rupture, resulting in cytolysis, in accord with experimental observations. In deep inspiration, when a very substantial increase in capillary wall tension occurs, cancer cells already within the capillaries, entering them and in transit along them are expected to develop membrane tensions greatly exceeding the critical values for rupture. It is suggested that these respiration-induced effects may act as an important rate-regulating step in the metastatic process, where the development of pulmonary metastases plays a central role. Furthermore, induced deep inspiration may conceivably be utilized in the inhibition of pulmonary metastasis.

A long-lived fusogenic state is induced in erythrocyte ghosts by electric pulses

Treatment of erythrocyte ghosts in random positions in a suspension with membrane fusion-inducing direct current electric field pulses causes the membranes to become fusogenic. Significant fusion yields are observed if the membranes are dielectrophoretically aligned into membrane-membrane contact with a weak alternating electric field as much as 5 min after the application of the pulses. This demonstrates that a long-lived membrane structural alteration is involved in this fusion mechanism. Other experiments indicate that the areas on the membrane which become fusogenic after treatment with the pulses may be very highly localized. The locations of these fusogenic areas coincide with where the trans-membrane electric field strength was greatest during the pulse. The fusogenic membrane alteration, or components thereof, in these areas laterally diffuses very slowly or not at all, or, to be fusogenic, must be present at concentrations in the membrane above a certain threshold. The loss of soluble 0.9-3-nm-diameter fluorescent probes from resealed cytoplasmic compartments of randomly positioned erythrocyte ghosts occurs through electric field pulse-induced pores only during a pulse but not between pulses or after a train of pulses if the probe diameter is 1.2 nm or greater. For a given pulse treatment of membranes in random positions in suspensions, an increase in ionic strength of the medium results in (a) a decrease in loss during the pulse, (b) no difference in loss between pulses, and (c) an increase in fusion yield when membrane-membrane contact is established. The latter two results (b and c) are incompatible with a fusion mechanism that proposes a simple relationship between electric field-induced pores and fusion.

Kinetics of ultrastructural changes during electrically induced fusion of human erythrocytes

The sequence of events during the electrically induced fusion of human erythrocytes was studied by rapid quench freeze-fracture electron microscopy. A single electric field pulse was used to induce fusion of human erythrocytes treated with pronase and closely positioned by dielectrophoresis. The electronic circuit was coupled to a rapid freezing mechanism so that ultrastructural changes of the membrane could be preserved at given time points. Pronase treatment enabled adjacent cells to approach each other within 15 nm during dielectrophoresis. The pulse caused a brief disruption of the aqueous boundaries which separated the cells. Within 100 msec following pulse application, the fracture faces exhibited discontinuous areas which were predominantly free of intramembranous particles. At 2 sec after the pulse, transient point defects attributed to intercellular contact appeared in the same membrane areas and replaced the discontinuous areas as the predominant membrane perturbation. At 10 sec after the pulse, the majority of the discontinuous areas and point defects disappeared as the intercellular distance returned to approximately 15 to 25 nm, except at sites of cytoplasmic bridge formation. Intramembranous particle clearing was observed at 60 sec following pulse application in discrete zones of membrane fusion.

Determination of intracellular conductivity from electrical breakdown measurements

The intracellular resistivity (conductivity) of cells can be easily calculated with high accuracy from electrical membrane breakdown measurements. The method is based on the determination of the size distribution of a cell suspension as a function of the electrical field strength in the orifice of a particle volume analyser (Coulter counter). The underestimation of the size distribution observed beyond the critical external field strength leading to membrane breakdown represents a direct access to the intracellular resistivity as shown by the theoretical analysis of the data. The potential and the accuracy of the method is demonstrated for red blood cells and for ghost cells prepared by electrical haemolysis. The average value of 180 omega X cm for the intracellular resistivity of intact red blood cells is consistent with the literature.

The hemodynamic destruction of intravascular cancer cells in relation to myocardial metastasis

A variety of observations in humans and experimental animals indicate that large numbers of circulating cancer cells are killed in the microvasculature. It is suggested that this occurs when friction or adhesion between individual cancer cells and capillary walls results in an increase of tension in the cancer cell peripheries above a critical level because of (blood) pressure differentials between their free ends. Hemodynamic and anatomic data relating to the myocardial circulation and deformability measurements on four types of rat cancer cells have been reported previously by others. Novel calculations based on these data suggest that the increased tension at the peripheries of cancer cells passing through the myocardial capillaries will exceed the critical levels for rupture. Analysis of autopsy data for solid tumors reveals a low (less than 3%) incidence of myocardial metastases in the absence of lung metastases and a higher (15%) incidence in their presence. One explanation for these observations is that, in the absence of lung metastases, relatively few of the cancer cells enter the coronary arteries from primary tumors with systemic venous drainage because many are retained or destroyed in transit through the pulmonary vasculature, and most of those delivered to the myocardium then suffer hemodynamic destruction. In the presence of pulmonary metastases, large numbers of viable cancer cells are liberated directly into the pulmonary venules and subsequently are delivered to the myocardium without prior exposure to the arterial side of the microcirculation. The combined effects of increased delivery and the protective effects of arrested cells on those preceding them in files along the capillaries account for the higher incidence of myocardial metastases. It is proposed that hemodynamic destruction of circulating cancer cells may be an important underlying cause of metastatic inefficiency, together with other cytocidal mechanisms.