Numerous drugs and endogenous compounds are efficiently excreted from the renal proximal tubule via carrier-mediated pathways. Transepithelial excretion of organic anions occurs via their accumulative transport from the blood into the proximal tubule cells across the basolateral membrane and subsequent secretion into the urine through the apical membrane. Here we report on the isolation of a novel complementary DNA from rat kidney that encodes a 551-amino acid residue protein (OAT1) with 12 putative membrane-spanning domains. When expressed in Xenopus laevis oocytes, OAT1 mediated sodium-independent para-aminohippurate (PAH) uptake (K m ‫؍‬ 14.3 ؎ 2.9 M). The uptake rate of PAH was increased by the outwardly directed dicarboxylate gradient, consisting with the idea that OAT1 is an organic anion/dicarboxylate exchanger. OAT1 displayed remarkably wide substrate selectivity, covering endogenous substrates such as cyclic nucleotides, a prostaglandin and uric acid, and a variety of drugs with different structures (e.g. antibiotics, a nonsteroidal antiinflammatory drug, diuretics, an antineoplastic drug, and a uricosuric drug). The Northern blot analysis and in situ hybridization revealed that OAT1 is exclusively expressed in the particular segment of the proximal tubule in the kidney. These data suggest that OAT1 is a multispecific organic anion transporter at the basolateral membrane of the proximal tubule. Isolation of OAT1 will facilitate elucidation of the molecular basis of drug kinetics and the development of new drugs lacking unwanted side effects.The kidney plays an essential role in the elimination of numerous organic anions including endogenous compounds, xenobiotics, and their metabolites (1-3). The proximal tubule cells actively secrete them into the urine. The first step of this secretion is the extraction of organic anion from the peritubular blood plasma by the proximal tubule cells through the basolateral membrane. This basolateral uptake of organic anions has been extensively investigated using para-aminohippuric acid (PAH) 1 as a test substrate. The most striking feature of this organic anion transport system is its extremely wide substrate selectivity, covering not only endogenous anionic substrates but also a number of clinically important drugs (1, 3). Because of its importance in renal physiology and pharmacology, cloning of the organic anion transporter has been attempted by many investigators using different approaches; however, the molecular structure of the responsible transporter has not yet determined.For the last decade it has been proposed that the basolateral uptake of organic anion is mediated by organic anion/dicarboxylate exchanger (1, 4). According to this model, outwardly directed dicarboxylate gradient is essential to express the transport activity of this exchanger. In the present study, we isolated first rat sodium dicarboxylate transporter (rNaDC-1) and then co-expressed it together with rat kidney poly(A) ϩ RNA in Xenopus oocytes to energize organic anion transport in oocyte...

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These results suggest that direct bilirubin, GCDCA-S, and coproporphyrin I are promising surrogate probes for the quantitative assessment of potential OATP1B-mediated DDI.

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We report here the isolation, functional characterization, tissue distribution, and membrane localization of rat renal Na+-dicarboxylate transporter (rNaDC-1). rNaDC-1 consists of 2,245 nucleotides, and the deduced amino acid sequence showed 73% and 75% identity to rabbit and human NaDC-1, respectively. When expressed in Xenopus laevis oocytes, rNaDC-1 mediated sodium-dependent uptake of di- and tricarboxylates. Substrates of rNaDC-1 evoked inward currents in oocytes expressed with rNaDC-1; succinate, α-ketoglutarate, and glutarate were relatively high-affinity substrates, and citrate was a low-affinity substrate of rNaDC-1. The coupling ratio of citrate to charge was determined to be 1:1 at pH 7.4; influx of one positive charge per citrate molecule suggests a symport of three Na+with a divalent citrate. Expression of rNaDC-1 mRNA was detected in the kidney and the small and large intestines. Immunohistochemistry using polyclonal antibodies raised against the 14 amino acids at the COOH terminus of rNaDC-1 revealed that rNaDC-1 is localized exclusively in the luminal membrane of S2 and S3.