Circadian distribution of bile acids in the enterohepatic circulatory system in rats

The bile acid pool was first determined in six adult male rats to be 12.8 +/- 0.7 mg/100 g by comparing the total radioactivity of tritiated bile acid drained through the bile fistula and the initial bile acid specific activity. The distribution of bile acids in the enterohepatic circulatory system at various times of the day was then studied in 24 additional rats, each received a single dose of tritiated taurocholate intraperitoneally and was sacrificed 24 h later. The nearly complete recovery of the administered radioactivity from the serum, liver intestinal wall and content, and 24-h feces indicated the confinement of bile acids to the enterohepatic circulation. A remarkable circadian fluctuation of the bile acid content was observed in serum, liver, and intestinal contents. The patterns of such rhythmic change varied from each other in various segments of the intestinal tract but seemed to correlate with the time sequence of movement of bowel content and absorption of bile acids. The circadian rhythm of hepatic synthesis of bile acids but not cholesterol observed by others might be, in part, directly related to the circadian fluctuation of the amount of bile acids in the liver.