Modulation of biological responses to 2 ns electrical stimuli by field reversal

Nanosecond bipolar pulse cancellation, a recently discovered Phenomenon, is modulation of the effects of a unipolar electric pulse exposure by a second pulse of opposite polarity. This attenuation of biological response by reversal of the electric field direction has been reported with pulse durations from 60 ns to 900 ns for a wide range of endpoints, and it is not observed with conventional electroporation pulses of much longer duration (> 100 mu s) where pulses are additive regardless of polarity. The most plausible proposed mechanisms involve the field-driven migration of ions to and from the membrane interface (accelerated membrane discharge). Here we report 2 ns bipolar pulse cancellation, extending the scale of previously published results down to the time required to construct the permeabilizing lipid electropores observed in molecular simulations. We add new cancellation endpoints, and we describe new bipolar pulse effects that are distinct from cancellation. This new data, which includes transport of cationic and anionic permeability indicators, fluorescence of membrane labels, and patterns of entry into permeabilized cells, is not readily explained by the accelerated discharge mechanism. We suggest that multi-step processes that involve first charged species movement and then responses of cellular homeostasis and repair mechanisms are more likely to explain the broad range of reported results.