Characterization of bile salt uptake by Giardia lamblia

We have shown previously that Giardia lamblia takes up conjugated bile salt in vitro, and have now investigated the mechanism by which this occurs. Uptake of sodium taurocholate (TC) and glycocholate (GC) with respect to time had an initial exponential component followed by a linear component, consistent with a combination of both active and passive transport processes. The presence of an active transport process was further supported by experiments which showed that bile salt uptake: (i) was concentration dependent (apparent Km's for TC and GC were 0.21 and 0.63 mM, respectively); (ii) was competitively inhibitable; (iii) was reduced by the metabolic inhibitor sodium fluoride (50 mM) and low temperature (4 degrees C). Bile salt was not taken up by glutaraldehyde-fixed parasites, indicating that bile salt was not merely being adsorbed on to the parasite surface. Differential centrifugation of lyzed parasites following exposure to radiolabelled GC, showed that the majority of bile salt was located in the cytosol fraction (76%) with a relatively minor component associated with cell membrane, indicating that bile salt had been internalized. Bile salt analysis of extracts of parasites and culture medium indicated that GC had not been metabolized by Giardia. Thus, like the mammalian ileum, Giardia appears to take up conjugated bile salts by active and passive transport processes. Conjugated bile salts are known to promote encystation and thus these uptake mechanisms may constitute an important survival mechanism for the parasite enabling it to complete its life cycle.