Local tissue oxygen tension - cytochrome a,a 3 redox relationships in rat cerebral cortex in vivo

Abstract

Simultaneous measurements were made from rat cerebral cortex, in situ, of focal changes in both tissue oxygen tension (p tO 2) and the reduction/oxidation ratio of cytochrome c oxidase (cytochrome a,a 3) in order to study relationships between oxygen supply and consumption in small regions of tissue. Local p tO 2 was measured with polarographic microelectrodes and the redox state of cytochrome a,a 3 with a dual wavelength reflectance spectrophotometer. Increased P tO 2, produced by respiration of gas mixtures with elevated 0 2 and/or C 02 content, was accompanied by increased oxidation of cytochrome a,a 3. This confirms that cytochrome oxidase is not fully oxidized in focal brain tissue regions in vivo, as it is in mitochrondria isolated in vitro. Decreased p tO 2 was accompanied by cytochrome a,a 3 reduction. The oxidative changes of cytochrome a,a 3 with increases in p tO 2 were smaller than the reductive changes associated with decreases in p tO 2. Curves relating cytochrome a,a 3 redox state to p tO 2 were qualitatively alike, regardless of the initial p tO 2 value from which they were generated. Thus, the reduction level of cytochrome a,a 3 varied with p tO 2 on a continuum. This consistent relationship demonstrates that changes in mitochondrial redox state provide an index ofrelative 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 p tO 2.