Arteriolar oxygen reactivity: where is the sensor?

The hypothesis that arterioles are intrinsically sensitive to oxygen was tested by comparing arteriolar diameter responses with local and global PO2 changes in superfused hamster cheek pouch preparations. Local PO2 changes were produced by microapplication of fluid onto the surface of occluded or unoccluded aparenchymal arterioles or by cannulation and perfusion of arterioles in situ. Global changes refer to PO2 changes in the superfusate flowing over the entire preparation. Local, effective PO2 changes had no significant effect on arteriolar diameters. In contrast, global PO2 changes produced significant, reproducible changes in diameter. These observations do not support the hypothesis that arterioles are intrinsically oxygen sensitive, unless the oxygen-sensitive sites are distributed sparsely along the arteriolar tree. The data are consistent with oxygen sensors located either in vessels downstream from 15-micron arterioles (in terminal arterioles, capillaries, or venules) or in the parenchyma. The data also suggest that these sensors detect changes in PO2 and then initiate responses that can be conducted along the vasculature to an arteriole distant from the sensor.