Conduction onset criteria for transient aqueous pores and reversible electrical breakdown in bilayer membranes

Improved energy model for membrane electroporation in biological cells subjected to electrical pulses

A self-consistent model analysis of electroporation in biological cells has been carried out based on an improved energy model. The simple energy model used in the literature is somewhat incorrect and unphysical for a variety of reasons. Our model for the pore formation energy E(r) includes a dependence on pore population and density. It also allows for variable surface tension, incorporates the effects of finite conductivity on the electrostatic correction term, and is dynamic in nature. Self-consistent calculations, based on a coupled scheme involving the Smoluchowski equation and the improved energy model, are presented. It is shown that E(r) becomes self-adjusting with variations in its magnitude and profile, in response to pore population, and inhibits uncontrolled pore growth and expansion. This theory can be augmented to include pore-pore interactions to move beyond the independent pore picture.

Electroporation in symmetric and asymmetric membranes

We present results of electrical measurements performed both on symmetric and asymmetric membranes in current-clamp conditions. The current-voltage characteristic curve of the membranes shows a reversible conductance transition to a higher level above a critical potential Vc. The experimental results are interpreted in the light of the electroporation theory, which allows estimates of the line tension to be made. These estimates are compared to previous experimental findings or theoretical calculations. The behaviour of symmetric membranes of different chain lengths or consisting of mixtures of short and long chains indicates a strong dependence of Vc on the chain composition and on the presence of charges on the polar head. The electroporation process is also analyzed in asymmetric bilayers consisting of a charged and an uncharged monolayer, a condition which mimics that of natural membranes. Therefore it is possible to analyze the electrical forces acting on the uncharged monolayer due to the presence of charges on the other one, under several ionic-strength conditions. It is shown that the instability arises in the uncharged monolayer, while the coupling between the two monolayers triggers the electroporation process.

The number of molecules taken up by electroporated cells: quantitative determination

Fluorescent and fluorescent-labeled molecules were used with calibrated flow cytometric fluorescence measurements of electrically pulsed cells (intact yeast: Saccharomyces cerevisiae) to demonstrate a method for determining the net number of molecules transported into electroporated cells. For the conditions used, a single pulse of width 50 microseconds and magnitude 8.0 +/- 0.5 kV/cm resulted in an average net molecular uptake which is large, n = 1.4 x 10(5) molecules of 70 kDa FITC-dextran (supplied extracellular concentration of 500 microM), and n = 1.0 x 10(8) molecules of 660 Da propidium iodide (PI; 80 microM). Both molecules were present in pulsed cells at less than equilibrium values, consistent with a transient uptake mechanism. Intracellular FITC-dextran is present in soluble form, while PI is predominantly bound to nucleic acids. A broad, statistically significant distribution of molecular uptake was also observed. Such quantitative determinations should be important for guiding applications of electroporation, and for testing models of electroporation mechanisms. Further, the use of PI, which is well established as a membrane exclusion dye, provides additional support for the interpretation that both PI and FITC-dextran were internalized as a result of an electrical pulse.

Passive ion permeability of lipid membranes modelled via lipid-domain interfacial area

A microscopic interaction model of the gel-to-fluid chain-melting phase transition of fully hydrated lipid bilayer membranes is used as a basis for modelling the temperature dependence of passive transmembrane permeability of small ions, e.g. Na+. Computer simulation of the model shows that the phase transition is accompanied by strong lateral density fluctuations which manifest themselves in the formation of inhomogeneous equilibrium structures of coexisting gel and fluid domains. The interfaces of these domains are found to be dominated by intermediate lipid-chain conformations. The interfacial area is shown to have a pronounced peak at the phase transition. By imposing a simple model for ion diffusion through membranes which assigns a high relative permeation rate to the domain interfaces, the interfacial area is then identified as a membrane property which has the proper temperature variation to account for the peculiar experimental observation of a strongly enhanced passive ion permeability at the phase transition. The excellent agreement with the experimental data for Na+-permeation, taken together with recent experimental results for the phase transition kinetics, provides new insight into the microphysical mechanisms of reversible electric breakdown. This insight indicates that there is no need for aqueous pore-formation to explain the experimental observation of a dramatic increase in ion conductance subsequent to electric pulses.

Electroporation: high frequency of occurrence of a transient high-permeability state in erythrocytes and intact yeast

We present the first determinations of population distributions of macromolecule uptake due to electroporation, the percentage of cells which participate and, for the yeast, the subpopulation of cells whose membranes exhibit significant recovery following macromolecule uptake. Flow cytometry is used to measure the uptake of a first test molecule (green fluorescence, FITC-dextran; 70 kDa) and also, for the yeast, the subsequent uptake of a second, much smaller, test molecule (red fluorescence, propidium iodide; 660 Da), which provides a measure of membrane recovery. A dramatic 20% (erythrocytes) to 75% (intact Schizosaccharomyces pombe) of cells can take up the first test molecule within 5 min of a pulse.