Ontogenic regulation of components of ileal bile acid absorption.

The apical sodium-dependent bile acid cotransporter (ASBT) and the ileal bile acid binding protein (IBABP) are two components of ileal bile acid absorption. During the third postnatal week of the rat, there is a dramatic increase in ASBT and IBABP expression. The goals of this study were to examine the role of hormones on the ontogenic expression of ASBT mRNA and the role of weaning for both ASBT and IBABP mRNA. Administration of various doses of dexamethasone during the second postnatal week induced ASBT mRNA levels, and this effect was significantly increased with concomitant thyroxine treatment. Early weaning and weaning prevention were utilized to investigate the influence of dietary factors. ASBT and IBABP mRNA levels were significantly elevated by early weaning and were decreased by weaning prevention compared with littermate controls. Thus, glucocorticoids and thyroxine appear to play a role in the ontogenic expression of ASBT mRNA and weaning appears to participate in both ASBT and IBABP expression.