Intestinal Absorption of Bile Pigments. III. The Enterohepatic Circulation of Urobilinogen in the Rat*

Abstract

A method is described for the preparation of crystalline mesobilirubinogen-C14 by sodium amalgam reduction of bilirubin-C14. Radiochemical purity of the chromogen was established by spectroscopic and chromatographic means, by melting point determination, and by recrystallization to constant specific activity. After intravenous injection of mesobilirubinogen-C14 into rats, 85 to 92% of the dose was excreted in the bile within 4 hours. When labeled chromogen was infused into the intestine, a significant fraction was absorbed intact and reexcreted in the bile, with insignificant amounts of isotope appearing in the urine. The rate and magnitude of absorption from the small bowel were much larger than the values obtained when labeled chromogen was injected into the large bowel. In the presence of complete biliary obstruction, the absorbed meso-bilirubinogen-C14 was excreted in the urine. The findings suggest that a limited enterohepatic circulation of urobilinogen occurs under physiologic conditions.