Selective availability of protein bound estrogen and estrogen conjugates to the rat kidney

Differential contributions of rOat1 (Slc22a6) and rOat3 (Slc22a8) to the in vivo renal uptake of uremic toxins in rats

PURPOSE

Evidence suggests that uremic toxins such as hippurate (HA), indoleacetate (IA), indoxyl sulfate (IS), and 3-carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF) promote the progression of renal failure by damaging tubular cells via rat organic anion transporter 1 (rOat1) and rOat3 on the basolateral membrane of the proximal tubules. The purpose of the current study is to evaluate the in vivo transport mechanism responsible for their renal uptake.

METHODS

We investigated the uremic toxins transport mechanism using the abdominal aorta injection technique [i.e., kidney uptake index (KUI) method], assuming minimal mixing of the bolus with serum protein from circulating serum.

RESULTS

Maximum mixing was estimated to be 5.8% of rat serum by measuring estrone sulfate extraction after addition of 0-90% rat serum to the arterial injection solution. Saturable renal uptake of p-aminohippurate (PAH, K(m) = 408 microM) and benzylpenicillin (PCG, K(m) = 346 microM) was observed, respectively. The uptake of PAH and PCG was inhibited in a dose-dependent manner by unlabeled PCG (IC(50) = 47.3 mM) and PAH (IC(50) = 512 microM), respectively, suggesting that different transporters are responsible for their uptake. A number of uremic toxins inhibited the renal uptake of PAH and PCG. Excess PAH, which could inhibit rOat1 and rOat3, completely inhibited the saturable uptake of IA, IS, and CMPF by the kidney, and by 85% for HA uptake. PCG inhibited the total saturable uptake of HA, IA, IS, and CMPF by 10%, 10%, 45%, and 65%, respectively, at the concentration selective for rOat3.

CONCLUSIONS

rOat1 could be the primary mediator of the renal uptake of HA and IA, accounting for approximately 75% and 90% of their transport, respectively. rOat1 and rOat3 contributed equally to the renal uptake of IS. rOat3 could account for about 65% of the uptake of CMPF under in vivo physiologic conditions. These results suggest that rOat1 and rOat3 play an important role in the renal uptake of uremic toxins and the induction of their nephrotoxicity.

Transporter-mediated renal handling of nafamostat mesilate

Nafamostat mesilate (NM) is a serine-protease inhibitor that is rapidly eliminated from the circulation and accumulated in the kidney. This study was conducted to characterize the mechanism of NM transport in the kidney because a serious side effect of NM-induced hyperkalemia may be related to accumulation of NM in the kidney. Measurements of uptake of NM in vivo by the kidney uptake index (KUI) method and of transport in an in vitro-cultured LLC-PK1 cell system suggested the involvement of an organic cation transporter (OCT). To clarify the involvement of OCTs located in the basolateral membrane of proximal tubules, we evaluated NM transport by OCTs expressed in Xenopus laevis oocytes. The IC(50) values of NM on [(14)C]TEA ([(14)C]tetraethylammonium) uptake by rOCT1, rOCT2, and hOCT2 were 50, 0.5, and 20 microM, respectively, and NM was concluded to be a substrate of OCTs. To investigate the transport of NM across the brush-border membrane, we examined the uptake of NM into brush-border membrane vesicles (BBMVs) isolated from rat renal cortex. NM was taken up into the BBMVs, and the uptake was decreased by unlabeled NM and temperature, implying that a transporter(s) is also involved in NM transport across the apical membrane. NM was not a substrate of hOCTN1, hOCTN2, or P-gp, implying the involvement of some unknown transporter(s). Thus, renal accumulation of NM can be explained by the involvement of the basolateral OCTs, though the influence of the apical membrane transporter remains to be clarified.

Major role of organic anion transporter 3 in the transport of indoxyl sulfate in the kidney

BACKGROUND

Indoxyl sulfate is a uremic toxin that accumulates in the body because of the patient's inability to excrete it and it induces a number of uremic symptoms and leads to chronic renal failure. The functional failure of the excretion system for indoxyl sulfate causes its accumulation in blood. The purpose of the present study was to characterize the transport mechanism responsible for the renal excretion of indoxyl sulfate.

METHODS

The [3H]indoxyl sulfate transport mechanism was investigated using an in vivo tissue-sampling single-injection technique, the kidney uptake index (KUI) method. Rat organic anion transporter 3 (rOAT3)-expressing Xenopus laevis oocyte system was used for measuring [3H]indoxyl sulfate uptake activity.

RESULTS

Probenecid showed a concentration-dependent inhibitory effect on the uptake of [3H]indoxyl sulfate using the KUI method, and uptake was inhibited by organic anions such as para-aminohippuric acid (PAH) and benzylpenicillin, by weak base such as cimetidine, and by uremic toxins, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) and hippuric acid (HA). However, salicylic acid, indomethacin, 3,5,3'-triiodo-l-thyronine and indole acetic acid (IA) had no effect on the uptake. rOAT3-expressing oocytes exhibited uptake of [3H]indoxyl sulfate by rOAT3 (Km = 158 micromol/L). Moreover, a number of uremic toxins inhibited the uptake of [3H]indoxyl sulfate by rOAT3.

CONCLUSIONS

These results suggest that rOAT3 is responsible for the renal uptake of indoxyl sulfate, and uremic toxins share the transport mechanism for indoxyl sulfate. Mutual inhibition of these uremic toxins via OAT3 may accelerate their accumulation in the body and, thereby, the progression of nephrotoxicity in uremia.