Effect of 17 beta-estradiol-D-17 beta-glucuronide on the rat organic anion transporting polypeptide 2-mediated transport differs depending on substrates.

Rat organic anion transporting polypeptide 2 (rOatp2) is a member of the OATP family. It exhibits broad substrate specificity and accepts amphipathic organic anions, cardiac glycosides (digoxin and ouabain; a neutral compound), and organic cations (rocuronium and N-(4,4-azo-n-pentyl)-21-deoxyajamalinium). In the present study, kinetic analyses were carried out to investigate whether taurocholate (TCA), digoxin, and 17beta-estradiol-D-17beta-glucuronide (E(2)17betaG) share the same recognition site on rOatp2 for their transport. The transport of TCA and digoxin was mutually inhibited, and the K(i) values of digoxin and TCA for the transport of TCA and digoxin were 0.58 and 160 microM, respectively. The K(m) and V(max) values of TCA and digoxin were 190 microM and 140 pmol/min/mg of protein and 1.1 microM and 6.6 pmol/min/mg of protein, respectively. The K(m) and K(i) values were consistent. In addition, digoxin (1 microM) and TCA (100 microM) increased the K(m) values of TCA and digoxin, respectively, but they did not affect the V(max) values, suggesting that their inhibition is competitive. The transport of digoxin via rOatp2 was inhibited slightly by E(2)17betaG, whereas the uptake of TCA was stimulated by E(2)17betaG in a concentration-dependent manner. These results suggest that rOatp2 has at least two substrate recognition sites, one for TCA and digoxin and the other for E(2)17betaG.