Mechanisms of Ca2+ overload induced by extracellular H2O2 in quiescent isolated rat cardiomyocytes

Rat cardiomyocytes were exposed to H2O2 (1-100 mu mol/L) for 10 min with wa shout for 10 min. Intracellular Ca2+ concentration ([Ca2+](i)) was measured using fluo-3. [Ca2+](i) increased with 100 mu mol/L H2O2 and further incre ased during washout, causing irreversible contracture in one-half of the ce lls. The increase in [Ca2+](i) with 10 mu mol/L H2O2 was modest with few ce lls showing irreversible contracture and attenuated by caffeine, and [Ca-2](i) gradually decreased during washout and this decrease was accelerated b y a calcium-free solution, while 1 mu mol/L H2O2 did not have any effects o n [Ca2+](i) or cell viability. Ca2+ overload caused during exposure to 100 mu mol/L H2O2 was attenuated by caffeine with improved cellular viability b ut not by chelerythrine, KB-R7943 or nifedipine. With 100 mu mol/L H2O2 cal cium-free solution attenuated the increase during exposure and washout whil e KB-R7943 or chelerythrine partly attenuated further increase during washo ut but not improved cell viability, but chelerythrine did not have addition al effect on calcium-free treatment. Catalase abolished the effects of H2O2 . We concluded that the increased [Ca2+](i) during exposure to 100 mu mol/L H2O2 was caused both by release of Ca2+ from the intracellular store sites including the sarcoplasmic reticulum and by influx through route(s) other than the voltage-dependent Ca2+ channels or Na+/Ca2+ exchanger, although th e Na+/Ca2+ exchanger or protein kinase C-mediated mechanism was partly resp onsible for a further increase during washout.