Multiple carriers for uptake of [3H]estradiol-17 beta(beta-D-glucuronide) in isolated rat hepatocytes

Abstract

The transport of the cholestatic steroid glucuronide, 3H-estradiol-17 beta-(beta-D-glucuronide) (E2 17G), was examined in isolated female rat hepatocytes over a broad substrate concentration range (0.1-100 microM). Two different carrier systems were identified with the following kinetic parameters: Km1 = 4.54 microM; Vmax1 = 0.149 nmol/min/mg protein; Km2 = 149 microM; Vmax2 = 0.641 nmol/min/mg protein. Taurocholate and testosterone glucuronide selectively and competitively inhibited [3H]-E2 17G uptake at the high affinity site. Ki values calculated for taurocholate (43 microM) and testosterone glucuronide (28 microM) indicated that these two inhibitors were relatively weak competitors for this E2 17G transport site. Conversely, E2 17G inhibited [3H]taurocholate uptake into isolated hepatocytes (Ki = 43 microM). Bromosulfophthalein (10 microM) inhibited uptake of 0.5-50 microM [3H]-E2 17G by 55-85%, whereas morphine glucuronide (100 microM) had no significant effect on uptake of [3H]-E2 17G at these concentrations. The effects of taurocholate, testosterone glucuronide, bromosulfophthalein, and morphine glucuronide on [3H]-E2 17G uptake into isolated rat hepatocytes correlated with the ability of these agents to inhibit binding of [3H]-E2 17G to specific sites in rat liver plasma membranes. These data support the postulate that the two [3H]-E2 17G binding sites identified in female rat liver plasma membranes represent two distinct organic anion carriers and indicate that the high affinity site for [3H]-E2 17G represents a carrier that is shared by organic anions and bile acids.