Weanling, but not adult, rabbit colon absorbs bile acids: flux is linked to expression of putative bile acid transporters

Abstract

Intestinal handling of bile acids is age dependent; adult, but not newborn, ileum absorbs bile acids, and adult, but not weanling or newborn, distal colon secretes Cl−in response to bile acids. Bile acid transport involving the apical Na+-dependent bile acid transporter (Asbt) and lipid-binding protein (LBP) is well characterized in the ileum, but little is known about colonic bile acid transport. We investigated colonic bile acid transport and the nature of the underlying transporters and receptors. Colon from adult, weanling, and newborn rabbits was screened by semiquantitative RT-PCR for Asbt, its truncated variant t-Asbt, LBP, multidrug resistance-associated protein 3, organic solute transporter-α, and farnesoid X receptor. Asbt and LBP showed maximal expression in weanling and significantly less expression in adult and newborn rabbits. The ileum, but not the colon, expressed t-Asbt. Asbt, LBP, and farnesoid X receptor mRNA expression in weanling colon parallel the profile in adult ileum, a tissue designed for high bile acid absorption. To examine their functional role, transepithelial [3H]taurocholate transport was measured in weanling and adult colon and ileum. Under short-circuit conditions, weanling colon and ileum and adult ileum showed net bile acid absorption: 1.23 ± 0.62, 5.53 ± 1.20, and 11.41 ± 3.45 nmol·cm−2·h−1, respectively. However, adult colon secreted bile acids (−1.39 ± 0.47 nmol·cm−2·h−1). We demonstrate for the first time that weanling, but not adult, distal colon shows net bile acid absorption. Thus increased expression of Asbt and LBP in weanling colon, which is associated with parallel increases in taurocholate absorption, has relevance in enterohepatic conservation of bile acids when ileal bile acid recycling is not fully developed.