Reversal of carbon monoxide-cytochrome c oxidase binding by hyperbaric oxygen in vivo

Cytochrome a,a3 redox state of the parietal cortex of pentobarbital anesthetized rats was continuously monitored through intact skull with four wavelength differential spectrophotometry during exposure to 90% O2 plus either 1.0 or 0.5% CO at 1 and 3 (ATA). The formation of HbCO was monitored in the brain by absorbance differences between 569 and 586 nm and correlated positively in graded 0.25 to 1% CO exposures with measured HbCO levels. Exposure to 90% O2, 1% or 0.5% CO (balance N2) decreased mean arterial pressure (MAP), calculated arterial O2 content and cytochrome a,a3 oxidation measured at 605 nm relative to 620 nm while HbCO rose. After compression to 3 ATA, rats breathing CO mixtures increased MAP and O2 content with reoxidation of cytochrome a,a3 while HbCO remained constant. Further treatment of both groups with 90% O2 at 3 ATA recovered the above parameters to at least control values except small persistent elevations of HbCO. Difference spectra recorded from 568 to 620 nm in parallel experiments showed twin absorbance peaks at 588 to 592 nm and 600 to 605 nm in response to CO. These absorbance maxima were consistent with formation of the cytochrome a3-CO complex and cytochrome a reduction respectively. These studies indicate that CO binds to reduced cytochrome a3 in blood circulated rat cortex in CO hypoxia and this effect can be reversed by increasing dissolved arterial O2 content at 3 ATA.