Characterization of oxygen radical formation mechanism at early cardiac ischemia

Myocardial ischemia-reperfusion (I/R) causes severe cardiac damage. Although the primary function of oxymyoglobin (Mb) has been considered to be cellular O-2 storage and supply, previous research has suggested that Mb is a potentially protective element against I/R injury. However, the mechanism of its protective action is still largely unknown. With a real-time fluorescent technique, we observed that at the onset of ischemia, there was a small burst of superoxide (O-2(center dot-)) release, as visualized in an isolated rat heart. Thus, we hypothesize that the formation of O-2(center dot-) correlates to Mb due to a decrease in oxygen tension in the myocardium. Measurement of O-2(center dot-) production in a Langendorff apparatus was performed using surface fluorometry. An increase in fluorescence was observed during the onset of ischemia in hearts perfused with a solution of hydroethidine, a fluorescent dye sensitive to intracellular O-2(center dot-). The increase of fluorescence in the ischemic heart was abolished by a superoxide dismutase mimic, carbon monoxide, or by Mb-knockout gene technology. Furthermore, we identified that O-2(center dot-) was not generated from the intracellular endothelium but from the myocytes, which are a rich source of Mb. These results suggest that during the onset of ischemia, Mb is responsible for generating O-2(center dot-). This novel mechanism may shed light on the protective role of Mb in I/R injury.