Adenosine. The signal of life?

In this issue of Circulation,' Hori and colleagues report the results of using three antioxidant agents in an attempt to reduce ischemic injury in a coronary microembolization model. Repeated injections of 15-,um microspheres resulted in progressive microvascular obstruction, decreased function , and edema until the extent of embolization decreased flow to nearly zero (less than 0.08 ml/min/ g). In this model, adenosine release from ischemic regions induces hyperemia in nonobstructed vessels. With successive injections, progressively smaller border zones surround areas of increasing microvascular obstruction until the extent of embolization occludes See p 828 the entire bed. Pretreatment with superoxide dismu-tase, allopurinol, or CV3611 resulted in less edema, increased number of microspheres required for complete stoppage of flow, relatively less dysfunction during progressive embolization, and, surprisingly, increased adenosine release. Augmented adenosine release is the subject of this editorial comment; it suggests possible mechanisms for the action of an-tioxidants that should be explored. Free Radicals and Ischemia/Reperfusion Injury The beneficial effects of the three antioxidants in this nonreperfusion model add further evidence that free radicals exacerbate ischemic injury. The pioneering studies of McCord2 and Granger et a13 and subsequent investigations from many laboratories suggest that free radical production contributes to the progression of ischemia and reperfusion injury at certain critical time points. In the original hypothesis, the principal source of superoxide production during reperfusion was the metabolism of hypoxanthine to uric acid by endothelial xanthine oxidase, which was converted from xanthine dehydrogenase to xanthine oxidase by a calcium-activated protease. Under nonischemic conditions, metabolism from hypoxanthine to uric acid occurs without The opinions expressed in this editorial comment are not necessarily those of the editors or of the American Heart Association. formation of a free radical, but the xanthine dehydro-genase-to-xanthine oxidase conversion changes the electron acceptor from nicotinamide-adenine dinucle-otide to molecular oxygen. Subsequent studies have confirmed that xanthine oxidase inhibition by allopuri-nol, oxypurinol, or tungsten feeding may be protective, but the conclusions are controversial.45 A recent study found that a non-purine-based agent that also inhibits xanthine oxidase failed to limit reperfusion injury in a cardiovascular model, whereas oxypurinol reduced the injury under identical circumstances. This raises the question of the role of xanthine oxidase inhibition in the mechanism of protection6; perhaps oxypurinol and allopurinol have additional mechanisms of protection. Inflammatory Reaction to Ischemia Reperfusion Two lines of investigation suggest a role for neu-trophils and perhaps other inflammatory cells in ischemia/reperfusion injury. Neutropenia as …