Saini Harjot K, Elimban Vijayan, Ozcelikay A Tanju, Dhalla Naranjan S
Institute of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, and Department of Physiology, Faculty of Medicine, University of Manitoba, 351 Tache Avenue, Winnipeg, MB R2H 2A6, Canada.
Can J Physiol Pharmacol. 2007 Jan;85(1):67-78. doi: 10.1139/Y06-099.
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.
尽管诸如甲基 - N - 异丁基阿米洛利（MIA）等钠氢交换（NHE）抑制剂已知会抑制心脏功能，但其负性肌力作用的机制尚未完全明确。在本研究中，用MIA灌注离体大鼠心脏以研究其对心脏性能的作用，而用MIA处理分离的亚细胞器如肌膜、肌原纤维、肌浆网和线粒体，以确定其对它们功能的影响。在负载fura - 2 - AM的心肌细胞中检测MIA对细胞内Ca2 +动员的影响。在无HCO3 - 的缓冲液中，观察到MIA以浓度依赖性方式抑制心脏功能。另一方面，在含HCO3 - 的缓冲液中，MIA最初具有正性肌力作用，随后出现负性肌力作用。MIA增加了细胞内Ca2 +的基础浓度（[Ca2 +]i），并增强了KCl介导的[Ca2 +]i增加。MIA对肌原纤维、肌膜和肌浆网ATP酶活性未显示任何直接作用；然而，发现该药物会降低细胞内pH，从而降低肌原纤维Ca2 +刺激的ATP酶活性。MIA还增加了线粒体对Ca2 +的摄取，而对肌浆网Ca2 +摄取没有任何直接影响。此外，MIA不能保护心脏免受轻度Ca2 +反常以及缺血 - 再灌注介导的损伤。这些结果表明，心肌细胞中[Ca2 +]i的增加可能是MIA最初正性肌力作用的原因，但其负性肌力作用可能是由于线粒体Ca2 +过载以及肌原纤维Ca2 + ATP酶活性的间接抑制。因此，[H +]i的积累以及细胞内和线粒体Ca2 +过载的发生可能解释了MIA在预防缺血 - 再灌注诱导的心肌损伤方面缺乏有益作用的原因。
