Hypoxic Conditioning Suppresses Cytotoxic Nitric Oxide Production Upon Myocardial Reperfusion

Abstract

Background Physiologocal concentrations of nitric oxide (NO) are generally beneficial, but excessive NO formation can injure ischemic myocardium by producing cytotoxic peroxynitrite upon reperfusion. Recently we reported that intermittent hypoxia conditioning (IHC) produced remarkable cardioprotection against infarction and lethal arrhythmias in a canine model of coronary occlusion-reperfusion. Hypothesis IHC suppresses myocardial formation of NO by the endothelial NO synthase isoform (eNOS) upon coronary artery reperfusion. Methods Mongrel dogs were conditioned by a 20 d program of intermittent normobaric hypoxia (FIO2 9.5- 10%; 5–10 min/cycle, 5–8 cycles/d with intervening 4 min normoxia), and compared with nonhypoxic control dogs. One day later, myocardium was sampled for measurement of left ventricular NOS activity (colorimetric assay) and eNOS content (Western blot). In other anesthetized dogs, myocardial nitrite release, an index of NO formation, was measured at baseline and during reperfusion following 1 h occlusion of the left anterior descending coronary artery (LAD). Results IHC lowered left ventricular NOS activity 45%, from 98 ± 10 to 55 ± 10 mU/g protein (P<0.01), and sharply decreased eNOS content (Figure). IHC reduced cumulative NO2− release during the first 5 min reperfusion from 26.4 ± 7.5 to 11.8 ± 1.8 μmol/g (P<0.05), but did not alter hyperemic LAD flow (15.2 ± 2.1 vs. 14.4 ± 2.4 ml/g). Conclusion IHC suppressed myocardial NOS activity and eNOS content. Reperfusion NO release was decreased in IHC myocardium without compromising reactive hyperemia. (supported by NIH grants HL-64785 and HL-71684)