Schwan Equation and transmembrane potential caused by alternating electric field
The transmembrane potential generated by an alternating electric field (ac) depends strongly on the frequency of the field and can be calculated using the Schwan Equation. We have measured the critical electric breakdown potential, Δψ(crit), of the plasma membrane of murine myeloma cell linte (Tib9) using ac fields, by monitoring the entry of a fluorescence probe, propidium iodide, into the cells. This dye is weakly fluorescent in solution but becomes strongly fluorescent when it binds to DNA. Experiments were done under a microscopy by direct visual examination of single cells or by examining photographic prints. When an ac field reached the intensity, E(crit), that generated a maximal membrane potential Δψ(max), equal to or greater than the Δψ(crit), the membrane was perforated at the two loci facing the electrodes. The dye diffusion into the cell, giving rise to two bright, narrow bands, which expand to the whole cell in 1-3 min. Δψ(crit)'s were measured in three media of different resistivities, ρ(ext), (52,600, 7,050, and 2,380 Ω cm), over the range of 0.1-300 kHz, with the field duration of 200 ms. Regression analysis based on the Schwan Equation showed that in a medium of given resistivity, the Δψ(crit) was constant over the frequency range studied. When the capacitance of the membrane, C(membr), was taken to be 0.9 μF cm-2, the resistivity of the cytoplasmic medium, ρ(int), was determined to be 910-1,100 Ω cm. The Δψ(crit) were 0.33, 0.48, and 0.53 V, respectively, for the three media in decreasing resistivities. The good fit of these data to the curves calculated using the Schwan Equation indicates that the equation may be used to describe the transmembrane potential of a living cell generated by an oscillating electric field.
Marszalek, P; Liu, DS; Tsong, TY
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