Transport of Kynurenic Acid by Rat Organic Anion Transporters rOAT1 and rOAT3: Species Difference between Human and Rat in OAT1

Abstract

A tryptophan catabolite, kynurenic acid, is involved in schizophrenia and uremia; there is little information on the mechanism of its disposition. Recently, our laboratory showed that kynurenic acid is a good substrate of human organic anion transporters hOAT1 and hOAT3. In this study, we performed uptake experiment using Xenopus laevis oocytes to characterize the transport of kynurenic acid by rat homologs of the transporters, rOAT1, and rOAT3. These transporters stimulated the uptake of kynurenic acid into oocytes, and transport by rOAT3 was marked. The Km values of the transport were estimated to be 8.46 μM for rOAT1 and 4.81 μM for rOAT3, and these values are comparable to their human homologs. The transport activity of kynurenic acid by rOAT1 was about one quarter of that of p-aminohippurate, although they were at the similar levels in hOAT1. A comparative experiment with hOAT1 was added in this study, showing that uptake amounts of kynurenic acid by hOAT1-expressing oocytes were 4 times greater than rOAT1-expressing oocytes. rOAT3 transported kynurenic acid as efficiently as estrone sulfate; this phenomenon was also observed in hOAT3. In conclusion, transport of kynurenic acid by rOAT1 and rOAT3 was shown. The characteristics of rOAT3 were similar to hOAT3, but low transport activity of kynurenic acid by rOAT1 was exhibited compared with hOAT1.

Keywords: OAT1; OAT3; kynurenic acid; species difference; transport.

Figure 1

Time-dependent uptake of kynurenic acid by rOAT1 and rOAT3. Notes: Oocytes injected with water (open circle), rOAT1 cRNA (closed circle), or rOAT3 cRNA (open triangle) were incubated with 20 nM [³H] kynurenic acid for the indicated periods. The uptake amounts of [³H] kynurenic acid in each oocyte were determined. Each point represents the mean ± S.E.M. of 29 to 30 oocytes from 3 experiments. When an error bar is not shown, it is smaller than the symbol.

Figure 2

Concentration-dependent uptake of kynurenic acid by rOAT1 (A) and rOAT3 (B). (A) Oocytes injected with rOAT1 cRNA were incubated with [³H]kynurenic acid at various concentrations for 2 hours. rOAT1-mediated uptake of [³H]kynurenic acid was determined by subtracting its uptake amount in water-injected oocytes from that in oocytes injected with rOAT1 cRNA. (B) Oocytes injected with rOAT3 cRNA were incubated with [³H]kynurenic acid at various concentrations for 1 hour. Notes: rOAT3-mediated uptake of [³H]kynurenic acid was determined by subtracting its uptake amount in water-injected oocytes from that in oocytes injected with rOAT3 cRNA. Each point represents the mean ± S.E.M. of 27 to 30 oocytes from 3 experiments. When an error bar is not shown, it is smaller than the symbol.

Figure 3

Uptake of p-aminohippurate, aminopterin and kynurenic acid by hOAT1 and rOAT1. Notes: Oocytes injected with water (open column), hOAT1 cRNA (hatched column) or rOAT1 cRNA (closed column) were incubated with 221 nM [³H]p-aminohippurate, 28.5 nM [³H]aminopterin or 20 nM [³H] kynurenic acid for 1 hr. The uptake amounts of the radiolabeled compounds in each oocyte were determined and divided by their concentrations in uptake buffer. Each column represents the mean ± S.E.M. of 26 to 30 oocytes from 3 experiments. ***P < 0.001, significantly different; *P < 0.05, significantly different.