Mechanisms of cardiodepression by an Na+-H+ exchange inhibitor methyl-N-isobutyl amiloride (MIA) on the heart: lack of beneficial effects in ischemia-reperfusion injury

Although Na+-H+ exchange (NHE) inhibitors such as methyl-N-isobutyl amiloride (MIA) are known to depress the cardiac function, the mechanisms of their negative inotropic effect are not completely understood. In this study, isolated rat hearts were perfused with MIA to study its action on cardiac performance, whereas isolated subcellular organelles such as sarcolemma, myofibrils, sarcoplasmic reticulum, and mitochondria were treated with MIA to determine its effect on their function. The effect of MIA on intracellular Ca2+ mobilization was examined in fura-2-AM-loaded cardiomyocytes. MIA was observed to depress cardiac function in a concentration-dependent manner in HCO3- -free buffer. On the other hand, MIA had an initial positive inotropic effect followed by a negative inotropic effect in HCO3-containing buffer. MIA increased the basal concentration of intracellular Ca2+ (Ca2+]i) and augmented the KCl-mediated increase in Ca2+]i. MIA did not show any direct effect on myofibrils, sarcolemma, and sarcoplasmic reticulum ATPase activities; however, this agent was found to decrease the intracellular pH, which reduced the myofibrils Ca2+-stimulated ATPase activity. MIA also increased Ca2+ uptake by mitochondria without having any direct effect on sarcoplasmic reticulum Ca2+ uptake. In addition, MIA did not protect the hearts subjected to mild Ca2+ paradox as well as ischemia-reperfusion-mediated injury. These results suggest that the increase in Ca2+]i in cardiomyocytes may be responsible for the initial positive inotropic effect of MIA, but its negative inotropic action may be due to mitochondrial Ca2+ overloading as well as indirect depression of myofibrillar Ca2+ ATPase activity. Thus the accumulation of H+]i as well as occurrence of intracellular and mitochondrial Ca2+ overload may explain the lack of beneficial effects of MIA in preventing the ischemia-reperfusion-induced myocardial injury.