Increased oxygen consumption following activation of brain: Theoretical footnotes using spectroscopic data from barrel cortex

Optical imaging spectroscopy (OIS) and laser Doppler flowmetry (LDF) data s equences from anesthetized rats were used to determine the relationship bet ween changes in oxy-and deoxygenated hemoglobin concentration and changes i n blood volume and flow in the presence and absence of stimulation. The dat a from Jones et al. (accompanying paper) were used to explore the differenc es between two theoretical models of flow activation coupling. The essentia l difference between the two models is the extension of the model of Burton and Frank by Hyder et al, (1998, J, Appl. Physiol. 85: 554-564) to incorpo rate change in capillary diffusivity coupled to flow, In both models activa tion-increased flow changes increase oxygen transport from the capillary; h owever, in Hyder et at's model the diffusivity of the capillary itself is i ncreased. Hyder ct at proposed a parameter (Omega), a scaling "constant" li nking increased blood flow and oxygen "diffusivity" in the capillary bed. T hus, in Burton and Frank's theory, Omega = 9; i,e,, there are no changes in diffusivity, In Hyder et al's theory, 9 < Omega < 1, and changes in diffus ivity are assumed to be linearly related to flow changes. We elaborate the theoretical position of both models to show that, in principle, the differe nt predictions from the two theories can be evaluated using optical imaging spectroscopy data. We find that both theoretical positions have limitation s when applied to data from brief stimulation and when applied to data from mild hypercapnia. In summary, the analysis showed that although Hyder et a l.'s proposal that diffusivity increased during activation did occur; it wa s shown to arise from an implementation of Burton and Frank's theory under episodes of brief stimulation. The results also showed that the scaling par ameter Omega is not a constant as the Hyder et al, model entails but in fac t varies over the time course of the flow changes. Data from experiments in which mild hypercapnia was administered also indicated changes in the diff usivity of the capillary bed, but in this case the changes were negative; i ,e,, oxygen transport from the capillary decreased relative to baseline und er hypercapnia, Neither of the models could account for the differences bet ween the hypercapnia and activation data when matched for equivalent flow c hanges. A modification to the models to allow non-null tissue oxygen concen trations that can be moderated by changes due to increased metabolic demand following increased neural activity is proposed. This modification would a llow modulation of oxygen transport from the capillary bed (e.g., changes i n diffusivity) by tissue oxygen tension and would allow a degree of decoupl ing of flow and oxygen delivery, which can encompass both the data from sti mulation and from hypercapnia. (C) 2001 Academic Press.