Bile acid pool sizes and excretion rates in wild-type and Cyp8b1^–/– mice. (a) The mass of bile acid in the enterohepatic circulation of normal and mutant male and female mice (n = 6 per group) was determined by extraction, purification, and quantitation by HPLC. Values (mean ± SEM) determined in the Cyp8b1^–/– mice were significantly different (P = 0.0037 for males; P = 0.04 for females by two-tailed Student t test) from those of normal controls. (b) Fecal bile acid excretion rates, a measure of hepatic bile acid synthesis, were determined as described in Methods. Excretion in Cyp8b1^–/– mice was significantly greater (P = 0.003 for males; P = 0.0002 for females) than that in wild-type mice. (c) Radiolabeled cholesterol and a nonabsorbable recovery standard (sitostanol) were administered by gavage to male and female mice (n = 6 per group) of the indicated genotypes. The percentage of cholesterol absorbed was determined after extraction of sterols from the feces excreted from the animals over a 72-hour period. Absorption values measured in mutant mice were significantly smaller (P = 0.02 for males; P = 0.0007 for females) than those in wild-type controls. (d) Fecal neutral sterols (cholesterol, coprostanol, and cholestanone) were quantitated by gas chromatography. Reduced absorption (see c) led to a significantly increased sterol excretion rate in Cyp8b1^–/– mice (P < 0.0003 for males and females). Bw, body weight.