Augmentation of endogenous adenosine attenuates myocardial 'stunning' independently of coronary flow or hemodynamic effects.

BACKGROUND Mounting evidence suggests a protective effect of exogenous adenosine in myocardial ischemia and reperfusion. We tested the hypothesis that augmentation of endogenous adenosine levels, achieved by inhibiting adenosine catabolism and washout, is beneficial in postischemic myocardial dysfunction ("stunning"). METHODS AND RESULTS In phase I of the study, open-chest dogs undergoing a 15-minute coronary artery occlusion and 4 hours of reperfusion received an intracoronary infusion of either saline (controls, n = 23) or 6-(4-nitrobenzyl)-mercapto: purine ribonucleoside (NBMPR, a selective nucleoside transport inhibitor) combined with erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, a potent adenosine deaminase inhibitor) (EHNA + NBMPR, n = 15) starting 15 minutes before coronary occlusion and ending 15 minutes after the initiation of reflow. Regional myocardial function (assessed as systolic wall thickening) was similar in control and treated groups at baseline and during ischemia. After reperfusion, however, the dogs treated with EHNA + NBMPR exhibited a significant improvement in the recovery of function, which was evident as early as 30 minutes after restoration of flow and was sustained throughout the rest of the reperfusion phase. The enhanced recovery effected by EHNA + NBMPR could not be attributed to nonspecific factors such as differences in collateral flow during occlusion, coronary flow after reperfusion, arterial pressure, heart rate, or other hemodynamic variables. In phase II of the study, the myocardial content of adenine nucleotides and nucleosides was measured by high performance liquid chromatography in myocardial biopsies obtained serially from open-chest dogs undergoing the same protocol used in phase I. There were no significant differences between control (n = 8) and treated (n = 9) dogs with respect to myocardial levels of adenosine triphosphate (ATP) at 30 and 60 minutes after reperfusion, indicating that the beneficial effects of EHNA + NBMPR cannot be ascribed to repletion of ATP stores. Compared with controls, dogs treated with EHNA + NBMPR exhibited a much larger increase in myocardial adenosine (6.07 +/- 1.47 vs 1.03 +/- 0.16 nmol/mg protein, P < .05) and a much smaller increase in inosine (0.52 +/- 0.27 vs 3.04 +/- 0.54 nmol/mg protein, P < .05) at the end of ischemia, such that the inosine-to-adenosine ratio noted in controls was completely reversed (approximately 6:1 vs approximately 1:6, respectively). In the treated group, adenosine levels remained markedly increased compared with controls up to 1 hour after reperfusion. CONCLUSIONS This study demonstrates that (1) administration of an adenosine deaminase inhibitor plus a nucleoside transport blocker is remarkably effective in augmenting myocardial adenosine levels during regional ischemia and subsequent reperfusion in vivo, (2) this augmentation of adenosine results in a significant and sustained attenuation of myocardial stunning, and (3) the attenuation of stunning is not due to ATP repletion or to nonspecific actions on hemodynamic variables or coronary flow. These findings suggest that endogenous adenosine production during ischemia serves as an important pathophysiological mechanism that protects against myocardial stunning. The results also suggest that augmentation of endogenous adenosine (without exogenous adenosine administration) represents an effective therapeutic approach to the alleviation of reversible postischemic dysfunction.