Nonlinear changes of transmembrane potential during electrical shocks - Role of membrane electroporation

Defibrillation shocks induce nonlinear changes of transmembrane potential (DeltaV(m)) that determine the outcome of defibrillation. As shown earlier, strong shocks applied during action potential plateau cause nonmonotonic negative DeltaV(m), where an initial hyperpolarization is followed by V-m shift to a more positive level. The biphasic negative DeltaV(m) can be attributable to (1) an inward ionic current or (2) membrane electroporation. These hypotheses were tested in cell cultures by measuring the effects of ionic channel blockers on DeltaV(m) and measuring uptake of membrane-impermeable dye. Experiments were performed in cell strands (width approximate to0.8 mm) produced using a technique of patterned cell growth. Uniform-field shocks were applied during the action potential plateau, and DeltaV(m) was measured by optical mapping. Shock-induced negative DeltaV(m) exhibited a biphasic shape starting at a shock strength of approximate to15 V/cm when estimated peak DeltaV(m)(-) was approximate to-180 mV; positive DeltaV(m) remained monophasic. Application of a series of shocks with a strength of 23+/-1 V/cm resulted in uptake of membrane-impermeable dye propidium iodide. Dye uptake was restricted to the anodal side of strands with the largest negative DeltaV(m), indicating the occurrence of membrane electroporation at these locations. The occurrence of biphasic negative DeltaV(m) was also paralleled with after-shock elevation of diastolic V-m. Inhibition of I-f and I-K1 currents that are active at large negative potentials by CsCl and BaCl2, respectively, did not affect DeltaV(m), indicating that these currents were not responsible for biphasic DeltaV(m). These results provide evidence that the biphasic shape of DeltaV(m) at sites of shock-induced hyperpolarization is caused by membrane electroporation.