Biophysics-based critique of the assisted discharge mechanism hypothesis

Abstract

Cell experiments with large, short electric field pulses of opposite polarity reveal a remarkable phenomenon: Bipolar cancellation (BPC). Typical defining experiments involve quantitative observation of tracer molecule influx at times of order 100 s post pulsing. Gowrishankar et al. BBRC 2018 503:1194-1199 shows that long-lived pores and altered partitioning or hindrance due to inserted occluding molecules can account for BPC. In stark contrast, the Assisted Discharge (AD) hypothesis, Pakhomov et al. CellMol- LifeSci 2014 71(22):4431-4441; Fig. 6, only involves early times of a microsecond down to nanoseconds. Further, well established terminology for cell membrane discharge relates to membrane potential decays shortly after pulsing. Discharge is silent on molecular or ionic transport, and does not address the fact that tracer molecule uptake vs time is measure at about 100s after pulsing ceases. Our critique of AD notes that there can be an association of AD with BPC, but associations are only necessary, not sufficient. A BPC mechanism hypothesis must be shown to be causal, able to describe time-dependent molecular influx. The two hypotheses involve very different time-scales (less than a microsecond vs 100 s) and very different quantities (volts/s vs molecules/s). Unlike pore-based hypotheses the AD hypothesis lacks explicit molecular transport mechanisms, and does not address the greatly delayed measured molecular uptake. We conclude that AD is an implausible candidate for explaining BPC.