Regulation of perfused capillary density in canine intestinal mucosa during endotoxemia

When O₂ delivery (blood flow x arterial O₂ content) is reduced, many tissues respond by increasing perfused capillary density. This facilitates the increase in O₂ extraction required to maintain tissue O₂ consumption in the face of limited O₂ supply. In a previous study of isolated canine small intestine (J. Appl. Physiol. 64: 2410-2419, 1988), endotoxin administration was associated with an impaired ability to increase O₂ extraction in response to progressive reductions in O₂ delivery. The aim of the present study was to determine whether reductions in perfused capillary density occur after endotoxin administration. Fourteen male dogs were anesthetized with chloralose (150 mg/kg iv) and urethan (750 mg/kg iv), and a segment of small intestine was exteriorized through a midline laparotomy. The segment was isolated vascularly, autoperfused, and maintained at body temperature. Escherichia coli endotoxin (5 mg/kg) or sham challenge was administered, and the animals were allowed to stabilize. Blood flow and arterial and gut venous blood O₂ contents were measured after 3 h. Perfused vessels were then labeled by injecting colloidal carbon (<0.8 μm) through the arterial cannula and clamping the artery and vein as the bolus passed through the tissue. In some of the experiments a second gut segment was successfully obtained within 1 h of the first, yielding a total of 14 gut segments in nine endotoxin animals and nine segments in five control animals. Morphological analysis of capillary surface density in mucosal villi and crypts showed a significantly higher perfused capillary density in control tissue blocks (77.8 ± 9.2%) than in blocks from endotoxin-treated animals (68.8 ± 8.0%, P < 0.04). Regression analyses yielded highly significant relationships between perfused capillary density and indexes of tissue O₂ delivery in control segments but no significant relationships among endotoxin-treated segments. These results suggest that gut adjustments in perfused capillary density in response to changes in O₂ delivery are impaired after endotoxin administration. This may contribute to an impaired O₂ extraction ability in endotoxemia.