The Cell in the Electric Field

Abstract

An exposure of a cell to an external electric field results in the induced transmembrane voltage (ΔΨm) that superimposes to the resting voltage. This can have a range of effects, from modification of the activity of voltage-gated channels to membrane electroporation, and accurate knowledge of spatial distribution and time course of ΔΨm is important for the understanding of these effects. In this chapter, we present the analytical, numerical, and experimental methods of determination of ΔΨm, and combine them with the monitoring of electroporation-induced transmembrane molecular transport (TMT) in Chinese Hamster Ovary (CHO) cells. Potentiometric measurements are performed using di-8-ANEPPS, and TMT is monitored using propidium iodide. In isolated cells, we combine analytical derivation (for spherical cells) and numerical computation of ΔΨm (for irregularly shaped cells) with potentiometric measurements to show that the latter are accurate and reliable. Monitoring of TMT in these same cells shows that it is confined to the regions with the highest |ΔΨm|. We then review other parameters influencing electroporation of isolated cells, and proceed, through the intermediate case of dense suspensions, to cells in direct contact with each other. We use the scrape-loading test to show that the CHO cells in a monolayer are interconnected, and then study ΔΨm and TMT in a cluster of four such cells. With low pulse amplitudes, the cluster behaves as one big cell, with ΔΨm continuous along its outer boundary, reflecting the interconnections. With interconnections inhibited, the cells start to behave as individual entities, with ΔΨm continuous along the plasma membrane of each cell. With the cluster exposed to porating (higher amplitude) pulses, TMT occurs in the membrane regions for which computations predict the highest |ΔΨm| if the cells are modeled as insulated, suggesting that the interconnections are blocked by supraphysiological ΔΨm, either directly by voltage gating or indirectly through changes in ionic concentrations caused by electroporation.