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LIVER AND BILIARY TRACT

Increased cholesterol 7-hydroxylase expression and size of the bile acid pool in the lactating rat

¹Graduate Center for Toxicology, University of Kentucky College of Medicine, Lexington, Kentucky; ²Division of Gastroenterology, Brigham and Women's Hospital and Harvard-Massachusetts Institute of Technology, Division of Health Sciences and Technology, Boston, Massachusetts

Submitted 11 January 2008 ; accepted in final form 15 February 2008

Maximal bile acid secretory rates and expression of bile acid transporters in liver and ileum are increased in lactation, possibly to facilitate increased enterohepatic recirculation of bile acids. We determined changes in the size and composition of the bile acid pool and key enzymes of the bile acid synthetic pathway [cholesterol 7-hydroxylase (Cyp7a1), sterol 27-hydroxylase (Cyp27a1), and sterol 12-hydroxylase (Cyp8b1)] in lactating rats relative to female virgin controls. The bile acid pool increased 1.9 to 2.5-fold [postpartum (PP) days 10, 14, and 19–23], compared with controls. A 1.5-fold increase in cholic acids and a 14 to 20% decrease in muricholic acids in lactation significantly increased the hydrophobicity index. In contrast, the hepatic concentration of bile acids and small heterodimer partner mRNA were unchanged in lactation. A 2.8-fold increase in Cyp7a1 mRNA expression at 16 h (10 h of light) demonstrated a shift in the diurnal rhythm at day 10 PP; Cyp7a1 protein expression and cholesterol 7-hydroxylase activity were significantly increased at this time and remained elevated at day 14 PP but decreased to control levels by day 21 PP. There was an overall decrease in Cyp27a1 mRNA expression and a 20% decrease in Cyp27a1 protein expression, but there was no change in Cyp8b1 mRNA or protein expression at day 10 PP. The increase in Cyp7a1 expression PP provides a mechanism for the increase in the bile acid pool.

postpartum; circadian rhythm; lipid absorption; Cyp7a1; Cyp27a1; Cyp8b1