Slowing of intestinal transit by fat depends on naloxone-blockable efferent, opioid pathway

Slowing of transit through the proximal small intestine by fat in the distal gut is termed the ileal brake. Intravenous naloxone, an opioid receptor antagonist, abolished the fat-induced ileal brake, suggesting that an endogenous opioid pathway may be involved in this response. To test the hypothesis that slowing of intestinal transit by fat in the distal half of the gut depends on an opioid pathway located on the efferent limb of this response, we compared intestinal transit in dogs equipped with duodenal and midgut fistulas while naloxone was either compartmentalized with oleate to the distal half of the gut or with buffer to the proximal half of the gut. We found that intestinal transit depended on the perfusion conditions (P < 0.00001). Specifically, compared with ileal brake (marker recovery of 35.7 +/- 7.4%), intestinal transit was accelerated when naloxone was delivered into the proximal half of the gut (76.2 +/- 5.2%) (P < 0.005) but not the distal half of the gut (29.4 +/- 5.4%). We conclude that slowing of intestinal transit by fat in the distal half of the gut depends on an opioid pathway located on the efferent limb of the ileal brake.