MATERIALS AND METHODS

In the following studies the term" absorption" is used to describe unidirectional absorptive flux of bilirubin, rather than net intestinal absorption (flux from gut to blood minus flux from blood to gut). In interpreting the data it should be noted that transfer of unconjugated bilirubin from the body pool to the gut lumen has been demonstrated in rats with congenital hyperbilirubinemia (23).

Preparation of radioactive pigments. Crystalline unconjugated bilirubin-C" 4 with specific activities ranging from 300 to 2,000 dpm per fg (disintegrations per minute per microgram) was prepared by biosynthetic techniques described previously (19). In preliminary studies of intestinal absorption, unconjugated bilirubin-C" was administered into theduodenum dissolved in rat bile, in 5% human albumin solution, or in aqueous solutions of chromatographically pure 1 (24), or commercially available taurocholate. Since in all instances the pat-terns of absorption were similar, in subsequent experi-ments unconjugated bilirubin-C" 4 was administered dissolved in 5 to 10 mM commercial taurocholate. In order to prepare conjugated bilirubin-C", unconjugated bilirubin-C'4 was infused intravenously into Sprague-Dawley rats with an externalbiliary fistula. During the ensuing 3 hours, virtually all injected labeled pigment was excreted in the bile as conjugated bilirubin-C"(25). Bile specimens were collected in the dark at 40 C andstored at-300 C for brief intervals before use. In studies of absorption, conjugated bilirubin-C4" was administered in native bile without further purification. Preparation of experimeental animals and administration of radioactive pignment. Fasting male Sprague-Dawley rats weighing 300 to 450 g were anesthetized by intraperitoneal administration of 2 mg per 100 g pentobarbital, and anesthesia was then maintained by intermittent inhalation of ethyl ether. An 8-X 8-cm cellophane square was sutured around the anus, permitting separate collection of urine and feces. Laparatomy was performed