Submitted on April 8, 2005
Accepted on September 10, 2005
Weanling, but not adult, rabbit colon absorbs bile acids: Flux is linked to expression of putative bile acid transporters
Dirk Weihrauch1, Jainuch Kanchanapoo2, Mei Ao3, Roli Prasad3, Pawinee Piyachaturawat4, and Mrinalini C. Rao3*
1 Department of Physiology & Biophysics, University of Illinois at Chicago, Chicago, IL, USA; Department of Animal Physiology, University of Osnabruck, Osnabruck, Germany
2 Department of Physiology & Biophysics, University of Illinois at Chicago, Chicago, IL, USA; Department of Physiology, Faculty of Science, Mahidol University, Bangkok, Thailand; Division of Biopharmacy, Faculty of Pharmaceutical Sciences, Ubon Ratchathani University, Ubol Ratchathani, Thailand
3 Department of Physiology & Biophysics, University of Illinois at Chicago, Chicago, IL, USA
4 Department of Physiology, Faculty of Science, Mahidol University, Bangkok, Thailand
* To whom correspondence should be addressed. E-mail: meenarao{at}uic.edu.
Intestinal handling of bile acids is age-dependent; adult, but not newborn, ileum
absorbs bile acids, and adult, but not weanling/newborn, distal colon secretes chloride
in response to bile acids. Bile acid transport involving the apical sodium-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/receptors. Colon from adult,
weanling and newborn rabbits were screened by semi-quantitative RT-PCR for Asbt, its
truncated variant t-Asbt, LBP, multidrug resistance protein 3 (Mrp3), organic solute
transporter 
( Ost) and farnesoid X receptor (FXR). Asbt and LBP show maximal
expression in weanlings and significantly less expression in the adults and newborns.
The ileum, but not the colon, expresses t-Asbt. The mRNA expression of Asbt, LBP
and FXR in the weanling colon parallels the profile in the adult ileum, a tissue designed
for high bile acid absorption. To examine their functional role, transepithelial 3H-taurocholate
transport in weanling and adult colon and ileum was measured. Under
short-circuit conditions the weanling colon (1.23 ± 0.62 nmol/hr/cm2) and ileum (5.53 ±
1.20 nmol/hr/cm2) and the adult ileum (11.41 ± 3.45 nmol/hr/cm2) show net bile acids
absorption. However, the adult colon secretes bile acids (-1.39 ± 0.47 nmol/hr/cm2).
We demonstrate for the first time that the weanling, but not adult, distal colon shows
net bile acid absorption. Thus, the increased expression of Asbt and LBP in the
weanling colon is associated with parallel increases in taurocholate absorption. This
has relevance in the enterohepatic conservation of bile acids at a stage when ileal bile
acid recycling is not fully developed.
