Effects of respiratory gases on cytochrome A in intact cerebral cortex: is there a critical Po2?

Changes in the redox level of cytochrome a and in the amount of oxygenated hemoglobin were measured by dual wavelength reflectance spectrophotometry in the intact cerebral cortex of cats (cerveau isolé preparation) and in unanesthetized rabbits with chronically implanted cranial windows. Increases in inspired oxygen were accompanied by an increase in the oxidation level of cytochrome a and an increase in the amount of oxygenated hemoglobin in the optical field. These changes were larger in the presence of 5% CO2. Reduction of the inspired oxygen concentration produced a decrease in the oxidation/reduction ratio of cytochrome a and a disoxygenation of hemoglobin. The presence of CO2 at these lower oxygen levels diminished the reduction of cytochrome a and the disoxygenation of hemoglobin. These data indicate that, in the resting subject, the reduction levels of cytochrome a are well above the low values seen in isolated mitochondria. They also indicate that the blood supply to the cerebral cortex is regulated at a level of slight hypoxia.