Experimental and theoretical studies of oxygen gradients in rat pial microvessels

Using modified oxygen needle microelectrodes and intravital videomicroscopy, measurements were made of tissue oxygen tension (PO ₂) profiles near cortical arterioles and transmural PO₂ gradients in the pial arterioles of the rat. Under control conditions, the transmural PO₂ gradient averaged 1.17±0.06 mm Hg/μm (mean±s.e., n=40). Local arteriolar dilation resulted in a marked decrease in the transmural PO₂ gradient to 0.68±0.04 mm Hg/μm (P<0.001, n=38). The major finding of this study is a dependence of the transmural PO₂ gradient on the vascular tone of the pial arterioles. Using a model of oxygen transport in an arteriole and experimental PO₂ profiles, values of radial perivascular and intravascular O ₂ fluxes were estimated. Our theoretical estimates show that oxygen flux values at the outer surface of the arteriolar wall are approximately 10^-5 mL O₂/cm² per sec, independent of the values of the arteriolar wall O₂ consumption within a wide range of consumption values. This also means that PO₂ transmural gradients for cerebral arterioles are within the limits of 1 to 2 mm Hg/μm. The data lead to the conclusion that O₂ consumption of the arteriolar wall is within the range for the surrounding tissue and O₂ consumption of the endothelial layer appears to have no substantial impact on the transmural PO₂ gradient.