CHARACTERIZATION OF THE HEPATIC CANALICULAR MEMBRANE-TRANSPORT OF A MODEL OLIGOPEPTIDE - DITEKIREN

Many small oligopeptides are rapidly excreted unchanged into bile, whi ch requires vectorial transport across the hepatocyte. To characterize the involved carrier system(s) at the canalicular membrane, studies w ere undertaken with vesicle preparations from the rat and the model ps eudohexapeptide ditekiren. The initial uptake rate into inside-out-ori ented vesicles was found to be ATP- and temperature-dependent and satu rable. Kinetic analysis indicated the involvement of three processes: (1) an ATF-dependent carrier-mediated process (K-m = 19.1 +/- 4.26 mu M; mean +/- S.E.M.), V-max = 140 +/- 29.4 pmol/mg of protein/15 sec), (2) an ATF-independent carrier-mediated transporter (K-m = 17.2 +/- 9. 58 mu M, V-max = 62.9 +/- 24.5 pmol/mg of protein/15 sec) and (3) a no nsaturable component. ATF-dependent uptake was inhibited by several ot her oligopeptides, which in the case of EMD 51921 was competitive. Cis -inhibition studies with known substrates for the canalicular bile sal t (taurocholate), multispecific organic anion (glutathione disulfide) and P-glycoprotein (daunomycin, nicardipine, cyclosporin A) transporte rs indicated a major role for the latter carrier system. Inhibition of the initial uptake rate of ditekiren by daunomycin was found to be co mpetitive in nature (K-i = 16 mu M). These findings indicate that the biliary excretion of ditekiren and possibly other hydrophobic oligopep tides is mediated, in part, by P-glycoprotein and suggest a possible p hysiological role for this hepatic transporter.