Effects of norepinephrine and sympathetic stimulation on intestinal oxygen extraction

Previous studies have shown that stimulation of the sympathetic nerves may constrict precapillary sphincters and thereby reduce the density of the perfused intestinal capillary bed. A computer model of the intestinal circulation (Physiologist 15:264, 1972) predicted that the diffusion parameters altered by precapillary sphincter closure (e.g. capillary surface area and mean capillary to cell diffusion distance) would be sufficient to affect intestinal O₂ extraction. To test this prediction in an animal model, the authors made continuous measurements of arteriovenous oxygen difference (A-V ΔO₂) and perfusion pressure during constant flow perfusion of isolated loops of canine small bowel. Intra arterial infusion of norepinephrine (NE) or stimulation of peri arterial nerves (SS) for a 7 min period produced sustained increases in vascular resistance (R) and sustained reductions in A-V ΔO₂. Maximal changes in R and A-V ΔO₂ occurred by the 2nd min of NE or SS, and subsequently both parameters 'escaped' somewhat. The maximal and steady state values of these parameters were dose or frequency dependent. In other experiments, SS caused synchronous decreases in A-V ΔO₂ and Rb⁸⁶ extraction. The extractions of Rb and O₂ were significantly correlated. These results support the view that NE and SS cause sustained reductions in the density of the perfused capillary bed which are sufficient to reduce intestinal extraction of Rb and O₂ despite constant flow perfusion.