Bilitranslocase is Involved in the Uptake of Bromosulfophthalein in Rat and Human Liver

Michela      Terdoslavich; Inge   A.M.   de Graaf; Johannes   H.   Proost; Alessandra      Cocolo; Sabina      Passamonti; Geny   M.M.   Groothuis

Abstract

Hepatic disposition of bromosulfophthalein (BSP), bilirubin and bile salts partially overlap, as these anions share both uptake and excretion mechanisms. Multiple organic anion transporters mediate hepatic BSP uptake, i.e. members of the SLCO and SLC22 gene families and bilitranslocase (TCDB #2.A.65.1.1). This study aimed at evaluating the relative contribution of bilitranslocase in BSP uptake in precision-cut human and rat liver slices. To this purpose, two different anti-sequence bilitranslocase antibodies were used as specific, functional inhibitors of bilitranslocase. The intact liver physiology was accurately reproduced in this BSP uptake assay, since uptake was strongly temperature-dependent and inhibited by hepatotropic organic anions, such as 50 nM bilirubin, 1 μM nicotinic acid, 2 μM digoxin, 5 μM indocyanine green and 100 μM taurocholate. The bilitranslocase antibodies inhibited BSP uptake both in rat and human liver slices. The combined use of bilitranslocase antibodies and taurocholate caused additive-type inhibition, confirming that bilitranslocase is not a bile salt transporter; by contrast, bilirubin caused no additive-type inhibition. In conclusion this data, indicate the role of the bilirubin transporter bilitranslocase as one of the transporters involved in the uptake of anions like BSP in parallel with other organic anion carriers. Moreover this data indicate the value of precision-cut liver slices for phenotypic drug uptake studies.

Keywords: Bilirubin, bilitranslocase, bromosulfophthalein, digoxin, indocyanine green, liver slices, membrane transport, taurocholate.