The breakdown of cell membranes by electrical and mechanical stress

We attempted to determine whether mechanical tension and electrical stress couple to cause membrane breakdown in cells. Using cell-attached patches from HEK293 cells, we estimated the mechanically produced tension from the applied pressure and geometry of the patch. Voltage pulses of increasing amplitude were applied until we observed a sudden increase in conductance and capacitance. For pulses of 50 mu s duration, breakdown required >0.5 V and was dependent on the tension. For pulses of 50-100 ms duration, breakdown required 0.2-0.4 V and was independent of tension. Apparently two physically different processes can lead to membrane breakdown. We could explain the response to the short, high-voltage pulses if breakdown occurred in the lipid bilayer. The critical electromechanical energy per unit area for breakdown by short pulses was similar to 4 dyne/cm, in agreement with earlier results on bilayers. Our data suggest that, at least in a patch, the bilayer may hold a significant fraction (similar to 40%) of the mean tension, To be compatible with the large, nonlytic area changes of patches, the bilayer appears to be pulled toward the pipette tip, perhaps by hydrophobic forces wetting membrane proteins bound to the glass. Although breakdown voltages for long pulses were in agreement with earlier work on algae, the mechanism(s) for this breakdown remain unclear.