Electroporation and recovery of cardiac cell membrane with rectangular voltage pulses

Electroporation of the cardiac cell membrane may result from intense electric fields applied to cardiac muscle, associated for example with defibrillation and cardioversion. We analyzed the distribution of voltage levels sufficient to cause electroporation in enzymatically isolated frog cardiac cells, using the cell-attached patch-clamp technique with rectangular pulses similar to those used in experimental studies of cardiac defibrillation. Five-millisecond monophasic or ten-millisecond biphasic symmetric (1/1) and asymmetric (1/0.5) rectangular pulses of either polarity were applied to the cell membrane in 100-mV steps from 0.2 to 0.8 V. The membrane conductance was continuously monitored by a low-voltage pulse train. In a total of 77 cells, we observed a step increase in conductance, occurring in 21% of cells at a transmembrane potential of 0.3 V, 52% at 0.4 V, 14% at 0.5 V, and 13% at 0.6-0.8 V. Electroporation occurred with this voltage distribution regardless of pulse shape, polarity, or the presence of all of the following ionic channel blockers: tetrodotoxin, barium, tetraethylammonium, 4-aminopyridine, cadmium, nickel, and gadolinium. The time course of membrane recovery was highly variable. The maintenance of a high membrane conductance after the shock pulse was associated with irreversible cell contracture provided that Ca²⁺ was included in the patch- pipette solution. However, with biphasic asymmetric pulses, the conductance recovered very quickly (≤37 ms). These findings suggest that 5-ms monophasic or 10-ms biphasic pulses permeabilize the cardiac cell membrane at transmembrane potentials of ~0.4 V by a pathway independent of membrane excitation channels and that biphasic asymmetric pulses may have a beneficial effect in membrane recovery after electroporation.