REDOX BEHAVIOR OF CYTOCHROME-OXIDASE IN THE RAT-BRAIN MEASURED BY NEAR-INFRARED SPECTROSCOPY

Using near-infrared spectroscopy,we developed a new approach for measu ring the redox state of cytochrome oxidase in the brain under normal b lood-circulation conditions. Our algorithm does not require the absorp tion coefficient of cytochrome oxidase, which differs from study to st udy. We employed this method for evaluation of effects of changes in o xygen delivery on cerebral oxygenation in rats. When fractional inspir ed oxygen was decreased in a stepwise manner from 100 to <10%, at whic h point the concentration of oxygenated hemoglobin ([HbO(2)]) decrease d by similar to 60%, cytochrome oxidase started to be reduced. Increas es in arterial Po-2 under hyperoxic conditions caused an increase in [ HbO(2)], whereas further oxidation of cytochrome oxidase was not obser ved. The dissociation of the responses of hemogloblin and cytochrome o xidase was also clearly observed after the injection of epinephrine un der severely hypoxic conditions; that is, cytochrome oxidase was reoxi dized with increasing blood pressure, whereas hemoglobin oxygenation w as not changed. These data indicated that oxygen-dependent redox chang es in cytochrome oxidase occur only when oxygen delivery is extremely impaired. This is consistent with the in vitro data of our previous st udy.