Myocardial reperfusion injury. Role of myocardial hypoxanthine and xanthine in free radical-mediated reperfusion injury

The aim of this study was to differentiate myocardial reperfusion injury from that of ischemia. We assessed the role of the myocardial adenosine 5'-triphosphate (ATP) catabolites, hypoxanthine and xanthine, generated during ischemia and the early phase of reperfusion, in reperfusion injury by modulating adenosine transport and metabolism with specific metabolic inhibitors. This was followed by intracoronary infusion of exogenous hypoxanthine and xanthine. Twenty-four dogs instrumented with minor-axis piezoelectric crystals and intraventricular pressure transducers were subjected to 30 minutes of normothermic global myocardial ischemia and 60 minutes of reperfusion. In Group 1 (n = 7), normal saline was infused into the cardiopulmonary bypass reservoir before ischemia and before reperfusion. Saline solution containing 25 μM p-nitrobenzylthioinosine (NBMPR) and 100 μM erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) was infused in Group 2 (n = 10) dogs. Group 3 (n = 7) dogs were treated exactly like those in Group 2 except, at the end of the ischemic period and immediately before releasing the cross-clamp, a solution of EHNA-NBMPR containing 100 μM hypoxanthine and 100 μM xanthine was infused into the aortic root. Left ventricular performance and myocardial adenine nucleotide pool intermediates were determined before and after ischemia. ATP was depleted by about 50% (p < 0.05 vs. preischemia) in all groups after 30 minutes of ischemia. Inosine was the major ATP catabolite (9.29 ± 1.2 nmol/mg protein) in Group 1, while adenosine (9.91 ± 0.7 nmol/mg protein) was the major metabolite in EHNA-NBMPR-treated dogs (Groups 2 and 3). Hypoxanthine levels were fivefold more in Group 1 compared with Groups 2 and 3 (p < 0.05). Left ventricular performance in Group 1 decreased from 76.8 ± 7.6 to 42.9 ± 9.8 and 52.3 ± 8.4 dynes/cm² x 10³ (p < 0.05), while myocardial ATP decreased from 30.9 ± 2.2 to 17.2 ± 1.0 and 16.5 ± 1.0 nmol/mg protein during 30 and 60 minutes of reperfusion, respectively (p < 0.05 vs. preischemia). Ventricular function in Group 2 dogs completely recovered within 30 minutes of reperfusion, and myocardial ATP recovered to the preischemic level at 60 minutes of reperfusion. In Group 3, left ventricular performance was depressed by 39% and 30% during 30 and 60 minutes of reperfusion (p < 0.05), respectively, and myocardial ATP did not recover during reperfusion despite a significant intramyocardial adenosine accumulation. Modulation of adenosine transport and metabolism by EHNA-NBMPR did not prevent ATP depletion but did significantly limit the formation and release of free radical substrates and their precursor inosine and allowed ATP repletion during reperfusion. In addition, intracoronary infusion of xanthine oxidase substrates resulted in metabolic derangement and ventricular dysfunction. These data clearly demonstrate that myocardial ATP catabolites play a crucial role in reperfusion injury and postischemic recovery.