BackgroundWe have previously reported that ischemic preconditioning increases 5′-nucleotidase activity and adenosine release during ischemia and reperfusion. However, its direct cause-and-effect relation has not been proven. To test the idea that the infarct size-limiting effect of ischemic preconditioning is blunted by inhibition of ectosolic 5′-nucleotidase activity, we assessed 5′-nucleotidase activity, adenosine release, and infarct size caused by sustained ischemia with and without an exposure to α,β,-methylene adenosine 5′-diphosphate (AOPCP) in the ischemia-preconditioned myocardium.Methods and ResultsIn 67 open-chest dogs, the left anterior descending coronary artery was cannulated and perfused with an extracorporeal bypass tube from the carotid artery. After hemodynamic stabilization, the coronary artery was occluded four times for 5 minutes separated by 5 minutes of reperfusion (ischemic preconditioning, n=10). After this procedure, the coronary artery was occluded for 90 minutes followed by 6 hours of reperfusion. Infarct size normalized by the risk area was smaller than the control group (n=8, 41.0±2.6% versus 6.8±1.9%), although there were no significant differences in the endomyocardial collateral flow measured at 80 minutes of ischemia (8.5±1.1 versus 9.4±1.0 mL/100 g per minute). Ectosolic and cytosolic 5′-nucleotidase activity and adenosine release were increased during reperfusion in the ischemic preconditioning group compared with the control group, and the activity of ectosolic 5′-nucleotidase was markedly reduced by AOPCP (n=10). AOPCP affected neither adenosine-induced coronary vasodilation nor increases in myocardial oxygen consumption during an intracoronary infusion of isoproterenol n= 10). To test whether the increase in 5′-nucleotidase activity decreases infarct size, we infused AOPCP 10 minutes before the ischemic preconditioning procedure and continued for 60 minutes after the onset of reperfusion (n=8). AOPCP blunted the infarct size–limiting effect (infarct size, 38.8±4.9%). AOPCP without ischemic preconditioning did not increase infarct size n=9). Furthermore, when AOPCP was infused during the ischemic preconditioning procedure (n=6) or during 60 minutes of reperfusion (n=6), the infarct size-limiting effect was partially blunted (infarct size, 21.3±2.5% and 19.5±2.4%, respectively).ConclusionsIncreases in ectosolic 5′-nucleotidase activity and adenosine release are primarily responsible for the infarct size–limiting effect of ischemic preconditioning. Exposures to adenosine during the ischemic preconditioning procedure and enhanced release of adenosine during reperfusion synergistically contribute to the infarct size–limiting effects.