Is adenosine 5′-triphosphate derangement or free-radical-mediated injury the major cause of ventricular dysfunction during reperfusion? role of adenine nucleoside transport in myocardial reperfusion injury

The aim of this study was to determine the dual role of ATP as an energy substrate and as a major source of oxygen-derived free-radical-mediated reperfusion injury by using adenine nucleoside blocker, p-nitrobenzylthioinosine (NBMPR), and adenosine deaminase inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA). In a randomized study, 16 dogs were instrumented with minor-axis LTZ-piezoelectric crystals and intraventricular pressure transducers to monitor, off bypass, left ventricular performance by using a sensitive and load-independent index of contractility (slope of the stroke work-end-diastolic length relation). Hearts were subjected to 60 minutes of normothermic global ischemia and 120 minutes of reperfusion. Normal saline without (Group 1, n=8) or with (Group 2, n=8) NBMPR and EHNA was infused in three boluses into the cardiopulmonary bypass reservoir before ischemia and reperfusion. Transmural serial biopsies were obtained before and during ischemia and reperfusion and analyzed for myocardial adenine nucleotide pool intermediates by using high-performance liquid chromatography. In the control group, three hearts developed ischemic contracture and another three hearts exhibited cardiogenic shock during reperfusion. In the EHNA/NBMPR-treated group, left ventricular performance recovered within 30 minutes of reperfusion (p<0.05 vs. control). Myocardial ATP was depleted to 20% of normal in both groups by the end of ischemia (p<0.05). Intramyocardial adenosine in the EHNA/NBMPR-treated group was 12-fold greater (15.09±1.6 nmol/mg protein) than the control group at the end of the ischemic period (p<0.05). Inosine was about fourfold higher in the control group (19.07±1.50 nmol/mg protein) compared with the drug-treated group (p<0.05). During reperfusion, myocardial ATP levels increased to approximately 50% of normal in the EHNA/ NBMPR group while remaining depressed (20% of normal) in the control group. Thus, despite the dramatic loss of myocardial ATP during ischemia, complete recovery of ventricular performance and significant repletion of ATP during reperfusion were observed when adenosine transport and deamination were modulated during ischemia and reperfusion. These results suggest that 1) the myocardium may have more ATP than is needed for basic cardiac functions and 2) washout of ATP diffusible catabolites is detrimental to ventricular performance during reperfusion. Specific blockade of nucleoside transport resulted in complete functional recovery despite low but critical ATP levels. It is concluded that adenine nucleoside transport regulates the release of free radical substrate precursors, thereby preventing ventricular dysfunction during reperfusion.