Search

We have proposed previously that the evaluation of transcellular transport across the double-transfected Madin-Darby canine kidney II (MDCK II) monolayer that expresses both human organic anion transporting polypeptide 4 (OATP2/SLC21A6) and multidrug resistance associated protein 2 (MRP2/ABCC2) on the basal and apical membranes, respectively, may be useful in characterizing human biliary excretion (J Biol Chem 277: 6497-6503, 2002). However, to demonstrate that this in vitro system represents in vivo biliary excretion, it is essential to compare in vitro data with in vivo biliary excretion. The problem is that we cannot determine the human biliary excretion for many ligands. In the present study, we have established a double-transfected MDCK II monolayer that expresses both rat Oatp4/Slc21a10 and Mrp2/Abcc2 on the basal and apical membranes, respectively, for the purpose of quantitatively comparing the clearance for transcellular transport with that for in vivo biliary excretion. The basal-to-apical transport of 17beta-estradiol-17beta-d-glucuronide, pravastatin, leukotriene C(4), cyclo-[D-Asp-Pro-d-Val-Leu-d-Trp] (BQ123), temocaprilat, and taurolithocholate 3-sulfate was significantly higher than that in the opposite direction in the double transfectant. Kinetic analysis suggested that that the rate-determining step of these compounds is the uptake process. The extent of the transcellular transport across the rat double-transfectant correlated well with that across the double-transfectant for human OATP2/SLC21A6 and MRP2/ABCC2. Moreover, considering the scaling factor, the clearance values for in vitro transcellular transport correlated well with those for in vivo biliary clearance. The double-transfected MDCK II monolayer may be useful in analyzing the hepatic vectorial transport of organic anions and in predicting in vivo biliary clearance.

Our previous kinetic analyses have shown that the transporter responsible for the renal uptake of pravastatin, an HMG-CoA reductase inhibitor, differs from that involved in its hepatic uptake. Although organic anion transporting polypeptides are now known to be responsible for the hepatic uptake of pravastatin, the renal uptake mechanism has not been clarified yet. In the present study, the involvement of rat organic anion transporter 3 (rOat3; Slc22a8) in the renal uptake of pravastatin was investigated. Immunohistochemical staining indicates the basolateral localization of rOat3 in the kidney. rOat1- and rOat3-expressed LLC-PK1 cells exhibited specific uptake of p-aminohippurate (PAH) and pravastatin, respectively, with the Michaelis-Menten constants (Km values) of 60 microM for rOat1-mediated PAH uptake and 13 microM for rOat3-mediated pravastatin uptake. Saturable uptake of PAH and pravastatin was observed in kidney slices with Km values of 69 and 11 microM, respectively. The difference in the potency of PAH and pravastatin in inhibiting uptake by kidney slices suggests that different transporters are responsible for their renal uptake. This was also supported by the difference in the degree of inhibition by benzylpenicillin, a relatively selective inhibitor of rOat3, for the uptake of PAH and pravastatin by kidney slices. These results suggest that rOat1 and rOat3 are mainly responsible for the renal uptake of PAH and pravastatin, respectively.