Mitochondrial oxygenation in the brain
Department of Radiology, Dartmouth, USA
Functional magnetic resonance imaging uses the blood oxygenation level dependent (BOLD) signal to non-invasively map neuronal activation. The technique has revolutionized the study of the working human brain. The oxygen limitation model is the prevailing model for the physiology behind the BOLD signal. It assumes that mitochondrial oxygen tension (PmO 2) is zero or near zero and constant, and that there are large oxygen gradients between the vasculature and the mitochondria so that large increases in blood flow (CBF) are necessary to support a small increase in oxygen consumption (CMRO 2).
We have developed and validated an optical spectroscopy system that can measure the oxidation state of mitochondrial cytochrome c (Cytc) as well as the absolute hemoglobin concentration and saturation (SmcO 2) from the rat cerebral cortex. We have integrated this system with laser Doppler flowmetry to simultaneously measure CBF and CMRO 2 as a fraction of baseline. These measurements provide a complete picture of cerebral oxygenation because SmcO 2 is a measure of vasculature oxygen tension (PcO 2), Cytc is sensitive to PmO 2 whereas CMRO 2 generates the oxygen gradient between capillary bed and mitochondria.
We have applied this system to the study of cerebral oxygenation during cortical spreading depression and forepaw activation in the rat. Cortical spreading depression generates large and dynamic changes in CBF and CMRO 2 from which PmO 2 and the gradient in the oxygen tension between capillary bed and mitochondria can be estimated and compared to the predictions of the oxygen limitation model. We have performed forepaw activation studies at normoxia and mild hypoxia and have used the Cytc signal to compare changes in PmO 2 during activation and particularly during the “early dip”, to the predictions of the oxygen limitation model.