The levels of adenosine and its metabolites in the guinea pig and rat brain during complete ischemia-in vivo study

Changes in levels of adenosine, inosine, hypoxanthine and ATP during complete ischemia after decapitation were determined in various areas of the guinea pig and rat brain using an HPLC method. These results were compared with levels in brains fixed by microwave irradiation. The levels of adenosine during 60 min of complete ischemia were extremely high and unevenly distributed while levels in the microwaved brains were very low and evenly distributed. The ratios of inosine plus hypoxanthine levels to adenosine which indicate the rate of metabolic degradation from adenosine into inosine and hypoxanthine, were also unevenly distributed during complete ischemia in the cerebellum, superior colliculus, cerebral cortex and hippocampus of the guinea pig and rat, and the highest ratio was observed in the cerebellum of the guinea pig and the superior colliculus of the rat. The activities of adenosine deaminase (ADA), one of the enzymes involved in adenosine metabolism, were measured in the four regions of the guinea pig. The ADA activities were unevenly distributed and the highest ADA activity was found in the cerebellum. These regional differences in ADA activities are in good agreement with the regional differences in the ratio of inosine plus hypoxanthine levels to adenosine during complete ischemia. Furthermore, the administration of EHNA [erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride] (10 mg/kg, i.p.), an ADA inhibitor, caused a significant increase of adenosine and decrease of inosine formation in all four regions and a drastic effect on the cerebellum with high ADA activity compared with the other regions in the guinea pig brain. These results indicate that the changes in concentrations of adenosine and its metabolites (inosine and hypoxanthine) during complete ischemia depend on ADA activity in each brain region.