Mechanisms of cardiodepression by an Na⁺–H⁺exchange inhibitor methyl-N-isobutyl amiloride (MIA) on the heart: lack of beneficial effects in ischemia–reperfusion injuryThis paper is one of a selection of papers published in this Special Issue, entitled Young Investigators' Forum.

Abstract

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.

Inhibition of acid-sensing ion channels in articular chondrocytes by amiloride attenuates articular cartilage destruction in rats with adjuvant arthritis

“…]i overload [24]. Recent evidence has shown that amiloride inhibits ischemic injury of neurons by inhibiting ASICs [25]. It is unclear whether activation of ASICs is involved in acid-induced articular chondrocyte injury.…”

Strategies for the Regulation of Intracellular Calcium in Ischemic Heart Disease

“…Since ischemia produces accumulation of hydrogen, the stimulation of the sarcolemmal Na + -H + exchanger followed by the activation of the sarcolemmal Na + -Ca 2+ exchanger is considered to be a mechanism for the increase of Ca 2+ influx as well as cytoplasmic levels of free Ca 2+ in ischemic myocardium [7,32,51]. However, this mechanism is confounded by the fact that inhibition of the Na + -H + exchanger with agents such as methyl-N-isobutyl amiloride was observed to increase the cytoplasmic concentration of hydrogen, which caused the release of Ca 2+ from the sarcoplasmic reticulum and resulted in an increase in the cytoplasmic concentration of Ca 2+ [32,52]. Thus, inhibitors of the Na + -H + exchanger may not be a suitable therapy for the treatment of ischemic heart disease.…”

“…It should be noted that ischemia-reperfusion and hypoxia-reoxygenation have been shown to produce marked changes in the sarcolemmal Na + -K + ATPase, Na + -Ca 2+ exchange, Ca 2+ -pump ATPase and Ca 2+ channel [7], the sarcoplasmic reticulum Ca 2+ -release channel and Ca 2+ -pump ATPase [7] as well as mitochondrial function [7,40]. Such alterations favoring the development of intracellular Ca 2+ overload appear to be due to the occurrence of oxidative stress since antioxidant therapy and ischemic preconditioning, which reduces oxidative stress, were found to attenuate these changes in ischemic-reperfused hearts [18,52,54,60,63]. Furthermore, these alterations were simulated by interventions that generate oxidative stress.…”