Electropermeabilization by uni- or bipolar nanosecond electric pulses: The impact of extracellular conductivity

Cellular effects caused by nanosecond electric pulses (nsEP) can be reduced by an electric field reversal, a phenomenon known as bipolar cancellation. The reason for this cancellation effect remains unknown. We hypothesized that assisted membrane discharge is the mechanism for bipolar cancellation. CHO-K1 cells bathed in high (16.1 mS/cm; HCS) or low (1.8 mS/cm; LCS) conductivity solutions were exposed to either one unipolar (300-ns) or two opposite polarity (300 + 300-ns; bipolar) nsEP (4-40 kV/cm) with increasing interpulse intervals (0.1-50 μs). Time-lapse YO-PRO-1 (YP) uptake revealed enhanced membrane permeabilization in LCS compared to HCS at all tested voltages. The time-dependence of bipolar cancellation was similar in both solutions, using either identical (22 kV/cm) or isoeffective nsEP treatments (12 and 32 kV/cm for LCS and HCS, respectively). However, cancellation was significantly stronger in LCS when the bipolar nsEP had no, or very short (< 1 μs), interpulse intervals. Finally, bipolar cancellation was still present with interpulse intervals as long as 50 μs, beyond the time expected for membrane discharge. Our findings do not support assisted membrane discharge as the mechanism for bipolar cancellation. Instead they exemplify the sustained action of nsEP that can be reversed long after the initial stimulus.