Calcium dynamics in cultured heart cells exposed to defibrillator-type electric shocks

Spatial and temporal changes in intracellular calcium ion concentration and transmembrane voltage were recorded optically from single-isolated cultured chick-embryo heart cells exposed to high-voltage, defibrillator-type shocks. Fluorescence changes were measured during 5 msec electric shocks of field strengths up to 56 volts/cm in single myocytes stained with a Ca(++)-sensitive or voltage-sensitive dye. Shocks caused a reversible period of depolarization, elevated cytosolic Ca++, and refractoriness. Intracellular Ca++ elevation had two temporal phases: first, a Ca++ spike with morphology independent of shock intensity; and second, a prolonged Ca++ elevation with a shock-intensity-dependent magnitude and duration, and with greatest Ca++ elevation at the poles of the cell adjacent to the electrodes. The prolonged elevation (second phase) was initiated earlier at the anode-facing pole of the cell than at the cathode-facing pole. These results suggest that postshock Ca++ entry consists of two parts: early normal entry through excitation channels plus a prolonged elevation which may be related to cellular damage.