In the kidneys, the secretion of amphiphilic organic anions (OA) takes place in proximal tubular cells. The first step in this process is the uptake of OAs from the blood across the basolateral membrane into these cells. Two organic anion transporters, OAT1 and OAT3, have been identified to be involved in this step. Both transporters function as OA-dicarboxylate exchangers and are coupled to the human sodium dicarboxylate cotransporter, hNaDC3. The prototypical substrates for OAT1, OAT3, and hNaDC3 are p-aminohippurate (PAH), estrogen sulfate (ES), and succinate, respectively. The present study provides a systematic approach of the respective IC₅₀ values for the inhibition of OAT1, OAT3, and hNaDC3 by various dicarboxylates. Uptakes of PAH and ES into HEK 293 cells transfected with OAT1 and OAT3 were inhibited by glutarate, a-ketoglutarate, adipate, pimelate, and suberate with IC₅₀ values of 3.3, 4.7, 6.2, 18.6, and 19.3 mM, and 79, 93, 136, 634, and 232 mM, respectively. Fumarate, succinate, and maleate had a much lower affinity to OAT1 and OAT3. A double-logarithmic plot of these data revealed a linear correlation between the IC₅₀ values for OAT1 and OAT3 with OAT1 having a 20-fold higher affinity towards these dicarboxylates. Succinate uptake by hNaDC3 was inhibited by a-ketoglutarate, fumarate, and glutarate with IC₅₀s of 84.4, 95.2, and 147 mM, respectively. There was no correlation to the IC₅₀ values obtained with OAT1 and OAT3. Thus, the selectivity of OAT1 and 3 on one side and hNaDC3 on the other for dicarboxylates is distinctly. Supported by DFG BU 998/5-1