Evidence for Carrier-mediated Transport of Unconjugated Bilirubin Across Plasma Membrane Vesicles from Human Placental Trophoblast

Abstract

Unconjugated bilirubin (UCB) is currently believed to cross the placenta only by passive diffusion. To assess whether carrier-mediated transport might be involved, the uptake of [³H]-UCB by basal (bTPM) and apical (aTPM) plasma membrane vesicles from human placental trophoblast at term was investigated. In both types of vesicles, the uptake of [³H]-UCB into an osmotically sensitive space was temperature-dependent, independent of the presence of Na⁺, and not affected by changes in membrane potential. The uptake of [³H]-UCB by aTPM, but not bTPM, was activated by ATP hydrolysis and inhibited by vanadate. Thus, the exact contribution of both inside out and right-side out bTPM to UCB uptake could not be distinguished, while only inverted aTPM were expected to carry out ATP-dependent UCB uptake. In bTPM and aTPM, uptake of free (unbound) [³H]-UCB (B_f) consisted of a dominant, saturable, presumably carrier-mediated process and a diffusional component that became predominant only at B_f near or above aqueous solubility limit for UCB (70 n m ). For bTPM, K_m=7.2 n m ; V_max=9.8 pmol/20 s/mg protein; and diffusion coefficient (K_D)=0.14 ml/20 s/mg protein. For aTPM in the presence of 9.5 m m ATP, K_m=18 n m ; V_max=131 pmol/20 s/mg protein; and K_D=0.47 ml/20 s/mg protein. The uptake of [³H]-UCB by bTPM was cis-inhibited by estrone-3-sulfate and estradiol-17β-glucuronide and trans-stimulated by unlabelled UCB and bromosulphopthalein. ATP-dependent UCB uptake by aTPM was cis-inhibited by doxorubicin, cholic acid, methotrexate and pronenecid. These findings suggest the presence of distinct transporters in the two domains of human placental trophoblast that could cooperate to transfer UCB from the foetus to the maternal circulation.