Role of oxidative stress and sodium-hydrogen exchanger in calcium-handling abnormalities in hearts subjected to ischemia-reperfusion

Occurrence of oxidative stress and activation of Na+-H + exchanger (NHE) are considered to he the major mechanisms of intracellular Ca2+-overload and subsequent cardiac dysfunction in hearts subjected to ischemia-reperfusion (I/R). This study was undertaken to test the hypothesis that reduction in the degree of oxidative stress and inhibition of NHE results in the beneficial effects in I/R hearts.; Reperfusion of the 30 min ischemic hearts for different periods (5 to 30 min) revealed marked changes in cardiac function, basal [Ca2+] i and isoproterenol (ISO)-induced increase in [Ca2+] i without any alterations in KCl- or S(-)-Bay K8644-induced increase in [Ca2+]i. The I/R-induced alterations in cardiac function, basal [Ca2+]i and ISO-induced increase in [Ca2+]i in cardiomyocytes were attenuated by SOD plus CAT as well as by ischemic preconditioning. The observed changes due to I/R were simulated in hearts perfused with hydrogen peroxide for 30 min. These results suggest that abnormalities in basal [Ca2+] i as well as mobilization of [Ca2+]i in cardiomyocytes upon stimulation with catecholamines occur due to ischemic injury and are mediated through oxidative stress in I/R hearts.; In another series of experiments reperfusion of 30 min ischemic hearts for 5 to 30 min showed a marked depression in ATP-induced increase in [Ca 2+]i in addition to a dramatic increase in basal [Ca 2+]i. The positive inotropic effect of extracellular ATP was attenuated and the maximal binding of [35S]ATPgammaS with crude membranes from hearts undergoing I/R was decreased. I/R-mediated alterations in ATP-induced increase in [Ca2+]i were depressed by treatment with SOD plus CAT as well as by ischemic preconditioning. All the observed changes due to I/R were simulated in hearts perfused with hydrogen peroxide. The results indicate an impairment of extracellular ATP-induced Ca2+ mobilization in I/R hearts, and this defect appears to be mediated through oxidative stress.; To investigate the mechanisms of ISO- and ATP-induced Ca2+-mobilization, cardiomyocytes were treated with different agents, which are known to modify Ca2+-movements. Vanadate, a sarcolemmal (SL) Ca2+-pump inhibitor, produced augmentation of the ATP-induced but depressed the ISO-induced increase in [Ca2+]i. Ryanodine and cyclopiazonic acid, which affect the sarcoplasmic reticulum (SR) function, depressed the ATP-induced increase in [Ca2+]i without affecting the ISO-induced change. SL L-type Ca2+ channel blockers, verapamil and diltiazem, as well as SL Na+-Ca2+ exchange blockers, amiloride, Ni2+ and KB-R7943, attenuated the ATP- and ISO-mediated increase in [Ca2+]i. These data suggest that ATP-induced increase in [Ca2+]i involves SL L-type Ca2+ channels and Na+-Ca2+ exchanger as well as SR Ca2+-channels and Ca 2+-pump whereas catecholamine-induced increase in [Ca2+] i is mediated through L-type Ca2+ channels and Na +-Ca2+ exchanger in cardiomyocytes.; Perfusion of hearts with MIA, an inhibitor of NHE, did not protect the I/R-induced changes in cardiac performance. Likewise, cardiac dysfunction due to intracellular Ca2+-overload, induced by Ca2+ -paradox, was not improved by MIA. Treatment of cardiomyocytes with MIA increased the basal [Ca2+]i and augmented the KCl-induced increase in [Ca2+]i in a concentration dependent manner. The MIA-induced increase in basal [Ca2+] i was unaffected by extracellular Ca2+, inhibitors of SL L-type Ca2+ channels, Na+-Ca2+ exchanger and Ca2+-pump ATPase as well as blockers of mitochondrial Ca2+-uptake. However, the MIA-induced increase in basal [Ca 2+]i was attenuated by inhibitors of SL Na+-K + ATPase and SR Ca2+-transport. MIA-mediated augmentation of the KCl response was dependent on extracellular concentration of Ca 2+ and attenuated by agents, which inhibit SL L-type Ca2+ channels, Na+-Ca2+ exchanger and Na+ -K+ ATPase as well as SR Ca2+-release channels and Ca2+-pump. Both MIA and a decrease in extracellular pH lowered the intracellular pH and increased...