INTERACTION BETWEEN CHOLEPHILIC ANIONS AND BILE-ACID TRANSPORT ACROSS BASAL MEMBRANE OF HUMAN TROPHOBLAST

The sensitivity of radiolabeled bile acid (BA) binding and transport by basal plasma membrane (BPM) vesicles of human trophoblast to cholephilic organic anions (COAs) was studied by a rapid filtration technique. Glycocholate (GC) efflux from preloaded (15 muM GC) vesicles was investigated in the presence of 300 muM COAs at the trans-side of the membrane. Bilirubin (BR) diglucuronide and rose bengal induced a very strong transstimulating effect, whereas phalloidin and phenol red showed a negligible effect. This effect was from strong to moderate for indocyanine green > bromosulfophthalein (BSP) greater-than-or-equal-to fusidic acid greater-than-or-equal-to phenolphthalein greater-than-or-equal-to BR ditaurate greater-than-or-equal-to rifamycin SV greater-than-or-equal-to rifampicin. BSP-induced transstimulation was not additive to the ''velocity effect'' previously reported for bicarbonate. At the cis-side, BSP reduced the saturable component of taurocholate (TC) binding to BPM vesicles. BSP also induced a partial and mixed type of inhibition both in TC uptake [inhibitor constant (K(i)) 227 muM] and efflux (K(i) 209 muM). Two binding sites with overlapping specificity for BAs and other COAs are proposed in this carrier, the site for non-BA COA presumably corresponding to that for bicarbonate. In summary, the results indicate that several COAs can act as potential substrates for the BA carrier located at the BPM of human trophoblast. This stresses the ''biliary-like'' role of the placenta and suggests the possibility of developing new functional tests for this organ on the basis of fetal-maternal transfer of nontoxic cholephilic dyes.