GLUCOCORTICOIDS UP-REGULATE TAUROCHOLATE TRANSPORT BY ILEAL BRUSH-BORDER MEMBRANE

The regulation of the enterohepatic circulation of bile acids has not been fully elucidated. Substrate availability has been shown to have a regulatory role on the ileal uptake of taurocholate (TC) by a positiv e feedback mechanism. Other mechanisms are likely to be involved in re gulating ileal bile acid uptake. The present study was designed to tes t the hypothesis that the ileal bile acid transporter (iBAT) is glucoc orticoid sensitive and that changes in expression are mediated by chan ges in iBAT synthesis. Adult Sprague-Dawley rats (300-400 g) received intraperitoneal injections with either corticosterone (5 mg/100 g body weight) or an equivalent vehicle (control) daily for 3 days. On day 4 , ileal brush-border membrane vesicles (BBMV) and hepatic basolateral membrane vesicles (BLMV) were prepared, and TC transport was performed using the rapid filtration technique. Initial velocity was measured a t selected time points, and kinetics were calculated over a range of T C concentrations. Ileal RNA was isolated, and Northern analysis of ste ady-state iBAT mRNA levels was determined. Western blot analysis was p erformed to quantitate the level of the 48-kDa iBAT protein. The initi al velocity of Na+-dependent TC uptake at 30 s by ileal BBMV was highe r in treated animals (264.3 +/- 64.6 pmol/mg protein) compared with co ntrol animals (148.3 +/- 41.1 pmol/mg protein; P = 0.07). The maximal velocity of uptake (V-max) was significantly higher: in treated vs. co ntrol animals (1,091 +/- 62.7 vs. 689.1 +/- 55.0 pmol.min(-1).mg prote in(-1), respectively; P = 0.002), whereas there was no significant dif ference in the Michaelis constant (K-m) between the control and treate d animals (43.3 +/- 7.2 vs. 35.3 +/- 8.7 mu M, respectively; P = not s ignificant). Steady-state iBAT mRNA levels were increased twofold in t he treated vs. control groups. Western blot analysis showed that the a bundance of the 48-kDa iBAT protein was eightfold higher in the treate d animals compared with control. Kinetic analysis of hepatic Na+-depen dent TC uptake revealed nearly identical V-max and K-m between the stu dy and control animals. Therefore, we conclude that TC transport by il eal BBMV is upregulated by administration of glucocorticoids. The incr ease in BBMV transport V-max corresponds to an increase in both iBAT t ranscript and protein.