Local tissue oxygen tension-cytochrome a,a3 redox relationships in rat cerebral cortex in vivo

Simultaneous measurements were made from rat cerebral cortex, in situ, of focal changes in both tissue oxygen tension (ptO2) and the reduction/oxidation ratio of cytochrome c oxidase (cytochrome a,a3) in order to study relationships between oxygen supply and consumption in small regions of tissue. Local ptO2 was measured with polarographic microelectrodes and the redox state of cytochrome a,a3 with a dual wavelength reflectance spectrophotometer. Increased ptO2, produced by respiration of gas mixtures with elevated O2 and/or CO2 content, was accompanied by increased oxidation of cytochrome a,a3. This confirms that cytochrome oxidase is not fully oxidized in focal brain tissue regions in vivo, as it is in mitochondria isolated in vitro. Decreased ptO2 was accompanied by cytochrome a,a3 reduction. The oxidative changes of cytochrome a,a3 with increases in ptO2 were smaller than the reductive changes associated with decreases in ptO2. Curves relating cytochrome a,a3 redox state to ptO2 were qualitatively alike, regardless of the initial ptO2 value from which they were generated. Thus, the reduction level of cytochrome a,a3 varied with ptO2 on a continuum. This consistent relationship demonstrates that changes in mitochondrial redox state provide an index of relative changes in tissue oxygenation in intact neocortex. The results suggest also that local rates of cerebral oxidative metabolism may not always be constant with changes in local ptO2.