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 V-m changes (DeltaV(m)) 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 V-m-sensitive dye RH-237 and a low-affinity Ca-i(2+)-sensitive dye Fluo-4FF. Shocks (Eapproximate to5 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 approximate to10 V/cm causing simple monophasic DeltaV(m). Shocks stronger than approximate to20 V/cm caused smaller DeltaCa(i)(2+) and postshock elevation of diastolic Ca-i(2+). This was paralleled with occurrence of biphasic negative DeltaV(m) 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 Cai(2+), likely caused by membrane electroporation.