Appearance of Superoxide Anion Radical in Cerebral Extracellular Space during Increased Prostaglandin Synthesis in Cats

When increased prostaglandin synthesis was induced in anesthetized cats equipped with cranial windows by topical application of arachidonate (200 μg/ml) or bradykinin (20 μg/ml), there was reduction of nitroblue terrazolium, resulting in deposition of the reduced insoluble form of this dye on the brain surface. The amount of reduced nitroblue terrazolium deposited on the brain surface was measured spectrophotometrically after fixation of the brain by perfusion with aldehydes to eliminate interference from hemoglobin. Topical application of 56 U/ml superoxide dismutase or 20 μg/ml indomethatin inhibited nitroblue terrazolium reduction by 76.5%–82.5% and by 78%–85.5%, respectively. These results show that most of the nitroblue terrazolium reduction was accounted for by superoxide anion radical generated in the course of arachidonate metabolism via the cyclooxygenase pathway. No superoxide production could be detected in the absence of arachidonate or bradykinin. Histological examination showed no evidence of parenchymal cellular damage or vascular damage and no accumulation of leukocytes. Pronounced leukocyte accumulation occurred 24 hours after topical arachidonate in rabbits with chronically implanted cranial windows. Superoxide appearance was reduced severely by 4, 4′-diisothiocyano-2, 2′-stilbene disulfonate and phenylglyoxal, two specific inhibitors of the anion channel. The most likely explanation for these findings is that increased metabolism of exogenous or endogenous arachidonate via cyclooxygenase results in the appearance of superoxide anion radical in cerebral extracellular space. Superoxide crosses the membrane of undamaged cells via the anion channel.