Evidence that mitochondrial respiration Is a source of potentially toxic oxygen free radicals in intact rabbit hearts subjected to ischemia and reflow

Previous in vitro studies have shown that isolated mitochondria can generate oxygen radicals. However, whether a similar phenomenon can also occur in intact organs is unknown. In the present studwy,e tested the hypothesis that resumption of mitochondrial respiration upon reperfusion might be a mechanism of oxygen radical formation in postischemic hearts, and that treatment with inhibitorosf mitochondrial respiration might prevent this phenomenon. Three groups of Langendorff- perfused rabbit hearts were subjected to 30 min of global ischemia at 37 “C, followed by reflow. Throughout ischemia and early reperfusion the hearts received, respectively: (a) 6 mM KC1 (controls), (b) 6 mM sodium amobarbital (Amytal“, which blocks mitochondrial respiration at Site I, at the level of NADH dehydrogenase), and (c) 6 mM potassium cyanide (to block mitochondrial respiration distallya, t the level of cytochrome c oxidase). The hearts were thperonc essed to directly evaluatoex ygen radical generationb y electron paramagnetic resonances pectroscopy, or to measure oxygen radical-induced membrane lipid peroxidation by malonyl dialdehyde (MDA) content of subcellular fractions. Severity of ischemia, as assessed by “P-nuclear magnetic resonance measurements of cardiac ATP, phosphocreatine, and pH, was similar in all groups. Oxygen-centered free radical concentration averaged 3.84 f 0.64 PM in reperfused control hearts, and it was significantly reduced by Amytal treatment (1.98 2 0.26; p < 0.06), but not by KCN (2.68 f 0.96 PM; p = not significant (NS)), consistent with oxygen radicals being formed in them itochondrial respiratory chain at Site I. Membrane lipid peroxidation of reperfused hearts was also reduced by treatment with Amytal, but not with KCN. MDA content of the mitochondrial fraction averaged 0.76 f 0.06 nM/mg protein in controls, 0.72 f 0.06 in KCN-treated hearts, and0 .64groups). Similarly, MDA content of lysosomal membrane fraction was0 .64 f 0.09 nM/mg protein in controls, 0.79 C 0.16 in KCN-treated hearts, and 0.43 2 0.06 in Amytal-treated hearts ( p 0.06 versus both groups). Since the effects of Amytal are known to be reversible, in a second series of experiments we investigated whether transient mitochondrial inhibition during the initial1 0 min of reperfusion wasa lso associated with beneficial effects on subsequent recovery of cardiac function after wash-out of the drug. At the end of the experiment, recoveroyf left ventriculaer nddiastolic and of developed pressure was significantly greater in those hearts that had been treated with Amytal during ischemia and earlyre flow, as compared to untreated hearts. In conclusion, our data demonstrate that in intact hearts electron flow through the respiratory chain may be an important source of oxygen radicals, which may form at the sites of interactions between Fe-S clusters and ubiquinone, and that resumption of mitochondrial respiration upon reoxygenation might contribute to reperfusion injury.