Inhibition of Voltage-gated Na(+) Current by Nanosecond Pulsed Electric Field (nsPEF) is Not Mediated by Na(+) Influx or Ca(2+) Signaling

In earlier studies, we found that permeabilization of mammalian cells with nsPEF was accompanied by prolonged inhibition of voltage-gated (VG) currents through the plasma membrane. This study explored if the inhibition of VG Na(+) current (I(Na)) resulted from (i) reduction of the transmembrane Na(+) gradient due to its influx via nsPEF-opened pores, and/or (ii) downregulation of the VG channels by a Ca(2+)-dependent mechanism. We found that a single 300 ns electric pulse at 1.6-5.3 kV/cm triggered sustained Na(+) influx in exposed NG108 cells and in primary chromaffin cells, as detected by increased fluorescence of a Sodium Green Dye. In the whole-cell patch clamp configuration, this influx was efficiently buffered by the pipette solution so that the increase in the intracellular concentration of Na(+) ([Na](i)) did not exceed 2-3 mM. [Na](i) increased uniformly over the cell volume and showed no additional peaks immediately below the plasma membrane. Concurrently, nsPEF reduced VG I(Na) by 30-60% (at 4 and 5.3 kV/cm). In control experiments, even a greater increase of the pipette [Na(+)] (by 5 mM) did not attenuate VG I(Na), thereby indicating that the nsPEF-induced Na(+) influx was not the cause of VG I(Na) inhibition. Similarly, adding 20 mM of a fast Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) into the pipette solution did not prevent or attenuate the inhibition of the VG I(Na) by nsPEF. These findings point to possible Ca(2+)-independent downregulation of the VG Na(+) channels (e.g., caused by alteration of the lipid bilayer) or the direct effect of nsPEF on the channel.