The intestinal phase of enterohepatic circulation, such as site and state of bile acid absorption, along the length of the intestinal tract has been speculated but not directly quantitated. In order to gain insight into the actual state of intestinal absorption of bile acid, the bile acid composition of portal blood from various segments of the intestinal tract was studied in dogs after loading endogenous bile acid by injection of caerulein. Total and unconjugated bile acids were determined with and without enzymatic hydrolysis, respectively. The amount of conjugated bile acids was calculated by subtracting unconjugated from total bile acids. Quantitation of cholic, chenodeoxycholic, deoxycholic and lithocholic acids and their conjugates was carried out by gas chromatography/mass spectrometry/selected ion monitoring with deuterated bile acids as internal standards. The major site of absorption of taurine-conjugated bile acid, a major conjugate form in the dog, was the distal small intestine. In addition, a considerable amount of cholic acid was found to be absorbed from the distal large intestine, the majority of which was still in the conjugated form. The pronounced absorption of the unconjugated secondary bile acid from the large intestine suggests the very active formation of the secondary bile acid in situ.

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