Effects of external electrical fields on cell membranes

Abstract

Dielectric breakdown of the cell membrane is observed when the membrane potential is taken rapidly (in μs) to a value of the order of 1 V. In giant algal cells a potential differences across the membrane of the order of 1 V can be built up by injecting current pulses of increasing amplitude into the cell via intra- and extra-cellular electrodes. The strong nonlinear pulsed current-voltage curves obtained with the marine algae Valonia utricularis show a dramatic clear-cut discontinuous increase in the membrane conductance at 0.85 V (20°C) due to dielectric breakdown. The mechanism of dielectric breakdown and the non-linear I vs. U characteristics can be explained by an electro-mechanical model. On the basis of this model it is assumed that the electrical compressive force and mechanical forces [ i.e. the elastic restoring force and external pressure (turgor)] in the membrane are in dimensional equilibrium. Therefore, for a sufficiently large compression of the membrane the electric compressive force can increase more rapidly than the elastic restoring force with decreasing membrane thickness. An instability point is reached leading to an electro-mechanical collapse (i.e. dielectric breakdown) of the membrane. Evidence for this model is given by the experimental finding that the breakdown voltage in Valonia utricularis decreases with increasing turgor pressure as predicted theoretically by this model. In microscopic cells such as red blood cells and bacteria, dielectric breakdown of the cell membranes can be induced by application of high electric field strengths (10 3 to 10 4 V cm −1) to the cell suspension either by using a hydrodynamic focusing COULTER counter or a discharge chamber which is part of a high voltage circuit. In the COULTER counter system dielectric breakdown of the cell membrane is observed by an apparent underestimation of the sizes beyond a critical voltage (or current) across the orifice. The critical breakdown voltage can be calculated from the integrated LAPLACE equation if the shape factor of the cell and the electric field strength in the orifice are known. The inherent problems involved in the estimation of the electric field strength in the