Effects of electrical shocks on Cai2+ and Vm in myocyte cultures

Changes in intracellular calcium concentration (DeltaCa(i)2+) induced by electrical shocks may play an important role in defibrillation, but high-resolution DeltaCa(i)2+ measurements in a multicellular cardiac tissue and their relationship to corresponding Vm changes (DeltaVm) are lacking. Here, we measured shock-induced DeltaCa(i)2+ and DeltaV(m) in geometrically defined myocyte cultures. Cell strands (width=0.8 mm) were double-stained with Vm-sensitive dye RH-237 and a low-affinity Ca(i)2+-sensitive dye Fluo-4FF. Shocks (E approximately 5 to 40 V/cm) were applied during the action potential plateau. Shocks caused transient Ca(i)2+ decrease at sites of both negative and positive DeltaV(m). Similar Ca(i)2+ changes were observed in an ionic model of adult rat myocytes. Simulations showed that the Ca(i)2+ decrease at sites of DeltaV+m was caused by the outward flow of I(CaL) and troponin binding; at sites of DeltaV-m it was caused by inactivation of I(CaL) combined with extrusion by Na-Ca exchanger and troponin binding. The important role of I(CaL) was supported by experiments in which application of nifedipine eliminated Ca(i)2+ decrease at DeltaV+m sites. Largest DeltaCa(i)2+ were observed during shocks of approximately 10 V/cm causing simple monophasic DeltaV(m). Shocks stronger than approximately 20 V/cm caused smaller DeltaCa(i)2+ and postshock elevation of diastolic Ca(i)2+. This was paralleled with occurrence of biphasic negative DeltaVm that indicated membrane electroporation. Thus, these data indicate that shocks transiently decrease Ca(i)2+ at sites of both DeltaV-m and DeltaV+m. Outward flow of I(CaL) plays an important role in Ca(i)2+ decrease in the DeltaV+m areas. Very strong shocks caused smaller negative DeltaCa(i)2+ and postshock elevation of diastolic Ca(i)2+, likely caused by membrane electroporation.