Vol. 290, Issue 3, 1107-1115, September 1999

Carrier-Mediated Hepatic Uptake of Peptidic Endothelin Antagonists in Rats¹

Sharif Akhteruzzaman, Yukio Kato, Hirokazu Kouzuki, Hiroshi Suzuki, Akihiro Hisaka, Bruno Stieger, Peter J. Meier and Yuichi Sugiyama

Graduate School of Pharmaceutical Sciences, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan (S.A., Y.K., H.K., H.S., Y.S.); Tsukuba Research Institute, Banyu Pharmaceutical Co., Ltd., Okubo, Tsukuba, Japan (A.H.); and Department of Medicine, Division of Clinical Pharmacology and Toxicology, University Hospital, Zurich, Switzerland (B.S., P.M.)

The endothelin antagonist BQ-123, an anionic cyclopentapeptide, is taken up by rat hepatocytes through active transport systems. Here, we have examined the hepatocellular uptake mechanism for several BQ-123 derivatives with anionic charges using isolated rat hepatocytes. BQ-485, a linear peptide, BQ-518, a cyclic peptide, and compound A, a cyclic peptide with a cationic moiety, were taken up by hepatocytes in a concentration-dependent manner. The uptake of BQ-485 was most efficient, whereas compound A showed comparable uptake with BQ-123. The uptake of these peptides was Na⁺- and energy-dependent, suggesting that active transport mechanisms are involved in their uptake into hepatocytes. BQ-485, BQ-518, and compound A can almost completely inhibit both the Na⁺-dependent and -independent uptake of [³H]BQ-123, with inhibition constants (K_i) that are comparable to the Michaelis-Menten constants (K_m) for their Na⁺-dependent and -independent uptake, respectively. Inhibition by BQ-485 was competitive, and the uptake of BQ-485 can be inhibited by BQ-123, with K_i values that are comparable with the K_m values for BQ-123 uptake. The uptake of BQ-123 by COS-7 cells transfected with either Na⁺-dependent taurocholate-cotransporting polypeptide (Ntcp) or Na⁺-independent basolateral organic anion-transporting polypeptide (oatp1) was minimal. Thus, these three peptides share the transporters that also recognize BQ-123 but appear to differ from Ntcp and oatp1.