Characterization of uremic toxin transport by organic anion transporters in the kidney

Background. Harmful uremic toxins, such as indoxyl sulfate (IS), 3-carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF), indoleacetate (IA), and hippurate (HA), accumulate to a high degree in uremic plasma. IS has been shown to be a substrate of rat organic anion transporter 1 (rOat1) and rOat3. However, the contribution of rOat1 and rOat3 to the renal uptake transport process of IS and other uremic toxins in the kidney remains unknown. Methods. The cellular uptake of uremic toxins was determined using stable transfectants of rOat1/hOAT1 and rOat3/ hOAT3 cells. Also, the uptake of uremic toxins by rat kidney slices was characterized to evaluate the contribution of rOat1 and rOat3 to the total uptake by kidney slices using inhibitors of rOat1 (p-aminohippurate) and rOat3 (pravastatin and benzylpenicillin). Results. Saturable uptake of IS, CMPF, IA, and HAby rOat1 was observed with K-m values of 18, 154, 47, and 28 mumol/L, respectively, whereas significant uptake of IS and CMPF, but not of IA or HA, was observed in rOat3- expressing cells with Km values of 174 and 11 mumol/L, respectively. Similar parameters were obtained for human OAT1 and OAT3. Kinetic analysis of the IS uptake by kidney slices revealed involvement of two saturable components with K-m1 (24 mumol/L) and K-m2 (196 mumol/L) values that were comparable with those of rOat1 and rOat3. The K-m value of CMPF uptake by kidney slices (22 mumol/L) was comparable with that of rOat3, while the corresponding values of IA and HA (42 and 33 mumol/L, respectively) were similar to those of rOat1. PAH preferentially inhibited the uptake of IA and HAby kidney slices, while pravastatin and benzylpenicillin preferentially inhibited the uptake of CMPF. The effect of these inhibitors on the uptake of IS by kidney slices was partial. Conclusion. rOat1/hOAT1 and rOat3/ hOAT3 play major roles in the renal uptake of uremic toxins on the basolateral membrane of the proximal tubules. Both OAT1 and OAT3 contribute almost equally to the renal uptake of IS. OAT3 mainly accounts for CMPF uptake by the kidney, while OAT1 mainly accounts for IA and HA uptake.