GIARDIA-LAMBLIA - EVIDENCE FOR CARRIER-MEDIATED UPTAKE AND RELEASE OFCONJUGATED BILE-ACIDS

Giardia lamblia trophozoites colonize the human small intestine, where they are exposed to high concentrations of conjugated bile acids. Pre vious work has shown that bile acids enhance trophozoite survival, mul tiplication, and differentiation into the cyst stage, Therefore, exper iments were performed to test whether carrier-mediated uptake of conju gated bile acids is present in this primitive parasite. Uptake of both cholyltaurine (C-tau) and cholylglycine (C-gly) was increased manyfol d after culturing trophozoites in medium lacking bile acids. Absence o f uptake at 4 degrees C and inhibition by other conjugated bile acids provided additional evidence for carrier-mediated uptake. Uptake of C- tau was greater than that of C-gly under all experimental conditions a nd appeared to be mediated by a different carrier. The major evidence for different carriers is that C-tau uptake was Na+-dependent, while C -gly uptake was not. In addition, C-tau uptake was more strongly inhib ited by DTNB and several organic anions than C-gly uptake. Radiolabele d C-tau and C-gly were each released rapidly from trophozoites at 37 d egrees C but not at 4 degrees C, suggesting that release of conjugated bile acids was also carrier-mediated, These findings are consistent w ith the notion that multiple transporters for conjugated bile acids ar e present in a lower eukaryote. We speculate that intracellular bile a cids may facilitate lipid trafficking and membrane biosynthesis. (C) 1 997 Academic Press.