Effects of neutral endopeptidase 24.11 inhibition on myocardial infarct size and ischemic preconditioning in rabbits

This study was designed to determine whether inhibition of neutral endopeptidase 24.11 (NEP) reduces infarct size and enhances protection afforded by ischemic preconditioning (PC) by elevation of the tissue bradykinin (BK) level in the heart in situ. In experiments to determine a dose of thiorphan (Thio) that inhibits NEP activity in the rabbit, infusion of Thio at a rate of 15 mug/kg per min was found to be NEP-selective, since it increased the extent and duration of hypotension after BK injection (50 ng/kg and 100 ng/kg, i.v.) but did not inhibit pressor response to angiotensin 1 (100 ng/kg and 500 ng/kg, i.v.). Infusion of Thio at a rate of 25 mug/kg per min blunted pressor response to angiotensin I by 30%, suggesting this dose partially inhibits angiotensin-converting enzyme activity. In the second series of experiments, myocardial infarction was induced by 30-min coronary occlusion and 3-h reperfusion in rabbits. In untreated controls, infarct size as a percentage of area at risk (%IS/ AR) was 50.1 +/- 4.1%, and infusion of Thio at 15 mug/kg per min and 25 mug/kg per min failed to limit infarct size (54.3 +/- 4.0% and 50.1 +/- 2.8%, respectively). However, these doses of Thio significantly reduced %IS/AR when combined with PC with 2-min ischemia to 25.7 +/- 3.3% and 19.7 +/- 3.1%, respectively, although this submaximal PC protocol alone did not achieve significant cardioprotection (%IS/AR=35.6 +/- 4.0%). This effect of Thio on PC was abolished by pretreatment with icatibant (2 mug/kg), a BK B-2 receptor blocker. The results of the present study suggest that NEP inhibition does not increase anti-infarct tolerance of the myocardium but significantly enhances cardioprotection of PC via a B-2 receptor-mediated mechanism.