Cell membrane electropermeabilization by symmetrical bipolar rectangular pulses. Part I. Increased efficiency of permeabilization.

The paper presents a comparative study of electropermeabilization of cells in suspension by unipolar and symmetrical bipolar rectangular electric pulses. While the parameters of electropermeabilization by unipolar pulses have been investigated extensively both in cell suspensions and in tissues, studies using bipolar pulses have been rare, partly due to the lack of commercially available bipolar pulse generators with pulse parameters suitable for electropermeabilization. We have developed a high-frequency amplifier and coupled it to a function generator to deliver high-voltage pulses of programmable shapes. With symmetrical bipolar pulses, the pulse amplitude required for the permeabilization of 50% of the cells was found to be approximately 20% lower than with unipolar pulses, while no statistically significant difference was detected between the pulse amplitudes causing the death of 50% of the cells. Bipolar pulses also led to more than 20% increase in the uptake of lucifer yellow. We show that these results have a theoretical background, because bipolar pulses (i) counterbalance the asymmetry of the permeabilized areas at the poles of the cell which is introduced by the resting transmembrane voltage, and (ii) increase the odds of permeabilization of cells having a nonspherical shape or a nonhomogeneous membrane. If similar results are also obtained in tissues, bipolar pulse generators could in due course gain a wide, or even a predominant use in cell membrane electropermeabilization.