Dietary psyllium increases expression of ileal apical sodium-dependent bile acid transporter mRNA coordinately with dose-responsive changes in bile acid metabolism in rats

Psyllium (PSY), a type of dietary fiber containing mainly soluble component s, has been shown to decrease serum cholesterol concentrations in several s pecies; however, mechanisms involved are not clearly defined. Four groups o f 10 rats were fed sem purified diets containing 10% dietary fiber from cel lulose and/or PSY for 21 d. Increasing levels of PSY were fed (0,3.33, 6.67 and 10% PSY) with the remaining 10% made up with cellulose. Liver choleste rol, cholesterol 7 alpha-hydroxylase (CYP7A) activity and mRNA, 3-hydroxy-3 -methylglutaryl CoA reductase (HMGR) mRNA, ileal apical sodium-dependent bi le acid transporter (ASBT) mRNA, fecal bile acids and total steroids, and i ntestinal bile acid content were measured. All variables responded in a dos e-dependent manner to PSY in the diet. Total liver cholesterol content was significantly reduced in all groups fed PSY compared to cellulose-fed contr ols [138(a), 105(b), 105(b) ard 93(c) mu mol (SEM = 4.2) for 0, 3.33, 6.67 and 10% PSY, respectively]. Activity of CYP7A was significantly greater in all groups fed PSY compared to the cellulose-fed controls [6.36(c), 16.92(b ), 15.28(b) and 20.37(a) pmol . min(-1) . mg protein(-1) (SEM = 3.19) for 0 , 3.33, 6.67 and 10% PSY, respectively]. These differences in CYP7A activit y were similar to differences in CYP7A, HMGR and ASBT mRNA levels. Fecal bi le acid and total steroid excretion as well as total intestinal bile acids were significantly greater in rats fed PSY-containing diets compared to 0% PSY-fed rats. These results suggest that the reduction in liver cholesterol involves modulating the size and composition of the bile acid pool via reg ulation of ileal ASBT, CYP7A and HMGR mRNA levels.