Carrier-mediated jejunal absorption of conjugated bile acids in the guinea pig

Background & Aims: Conjugated bile acid absorption is known to occur in the jejunum in mammals, but the mechanism has not been well defined. The aim of this study was to define the mechanisms by which conjugated bile acids are absorbed from the jejunum. Methods: The steady-state absorption of eight conjugated bile acids from a perfused jejunal segment was measured in the anesthetized biliary fistula guinea pig. Experiments defined the effect of bile acid structure, tested for competitive inhibition and saturation of transport, and compared the absorption rate of taurine conjugates with that of glycine conjugates at pH 7.6 or 5.0. Results: Dihydroxy conjugates were absorbed twice as rapidly as cholyl conjugates from the perfused jejunum; glycine and taurine conjugates of a given bile acid were absorbed at a similar rate. Absorption of ursodeoxycholate taurine showed saturability and competitive inhibition by other conjugated bile acids. When intraluminal pH was decreased to pH 5.0, the absorption rate of glycine (but not taurine) conjugates increased, indicating passive absorption of the protonated species of glycine-conjugated bile acids. Conclusions: Uptake of glycine- or taurine-conjugated bile acids by the guinea pig jejunum occurs by at least two mechanisms: carrier-mediated transport (dihydroxy conjugates greater than trihydroxy conjugates) and passive absorption in protonated (uncharged) form of glycine conjugates.