Nonlinear regulation of capillary perfusion in relation to ambient pO(2) changes in skeletal muscle

To study the process of O(2) transport to tissue, we investigated how capillary perfusion is controlled in response to changes in tissue O(2) levels in skeletal muscle. Capillary red blood cell (RBC) velocity and perfused capillary recruitment were measured in rabbit tenuissimus muscle at various ambient oxygen tensions (pO(2)) by intravital microscopy. Both RBC velocity and capillary recruitment significantly decreased as the pO(2) level of the suffusate was increased, and the relationship between capillary perfusion, calculated from the velocity and recruitment data, and the pO(2) level of the suffusate clearly yielded a nonlinear correlation that fitted a sigmoidal curve. Capillary perfusion dramatically decreases or increases above or below a suffusate pO(2) level of around 40 Torr, where the O(2) dissociation curve of hemoglobin changes slope. These findings support the hypothesis that microvasculature possesses an intrinsic, effective flow-control mechanism by sensing the metabolic demands of tissue, intimately related to the O(2) saturation of hemoglobin.