Nonlinear changes of transmembrane potential caused by defibrillation shocks in strands of cultured myocytes

Organization of cardiac tissue into cell strands and layers has been implicated in changes of transmembrane potential (Delta V-m) during defibrillation. To determine the shock-induced Delta V-m in such structures, cell strands of variable width [strand width (SW) = 0.15-2 mm] were grown in culture. Uniform-field shocks with variable strength [shock strength (SS) = 2-50 V/cm] were applied across strands during the action potential (AP) plateau, and Delta V-m were measured optically. Three different types of Delta V-m were observed. Small Delta V-m [<40%AP amplitude (APA)] were linearly dependent on SS and SW and were symmetrically distributed about a strand centerline with maximal positive and negative Delta V-m on opposite strand sides being equal. Intermediate Delta V-m (<200%APA) were strongly asymmetric with negative Delta V-m > positive Delta V-m because of a negative time-dependent shift of V-m at the depolarized side of the strands. For large Delta V-m (>200%APA), a second time-dependent shift of V-m to more positive levels was observed in the hyperpolarized portions of strands, causing reduction of the Delta V-m asymmetry. We conclude that during application of shocks to cell strands during the AP plateau, passive changes of V-m were followed by two voltage- and time-dependent shifts of V-m, possibly reflecting changes of ionic currents or membrane electroporation.