Abstract
The dicarboxylate, α-ketoglutarate (αKG), has been identified as the most likely physiological anion involved in renal proximal tubule basolateral membrane (BLM) dicarboxylate/organic anion exchange. In the present study, we characterized the uptake of αKG in BLM and brush-border membrane (BBM) vesicles isolated from rat kidney. In both membrane preparations, αKG uptake was Na+-dependent, saturable, electrogenic and inhibited by Li+. The initial rate of αKG (5 μM) uptake in BLM vesicles was twice that in BBM vesicles (258 ± 8.2 vs. 126 ± 3.9 pmol/mg/5 sec). The BLM transporter had a high affinity for αKG (apparentK m = 15.2 μM), but a relatively low transport capacity (V max = 386 pmol/mg/5 sec). In contrast, the BBM transporter had characteristics of a low-affinity (K m = 158 μM), high-capacity (V max = 1106 pmol/mg/5 sec) system. Other dicarboxylates such as succinate, malate, fumarate and glutarate at a concentration of 1 mM inhibited αKG uptake into BLM and BBM vesicles to the same extent (>90%). The tricarboxylate, citrate, also inhibited αKG uptake (70–80%). However, of these Krebs’ cycle intermediates, only αKG and glutarate were able to affectp-aminohippurate (PAH) uptake into BLM vesicles. These results lend further support for a BLM PAH/αKG exchanger. Furthermore, if extracellular αKG plays a role in the operation of the PAH/αKG exchanger, the high-affinity Na+-dependent αKG transporter located in the BLM is the likely source of the organic anion.
Footnotes
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Send reprint requests to: Richard Edwards, Ph.D., SmithKline Beecham, Dept. of Renal Pharmacology, UW2521, P.O. Box 1539, King of Prussia, PA 19406-0939.
- Abbreviations:
- BBM
- brush border membrane
- BLM
- basolateral membrane
- PAH
- p-aminohippurate
- αKG
- α-ketoglutarate
- HEPES
- N-2-hydroxyethylpiperazine-N′-ethanesulfonic acid
- Received November 20, 1996.
- Accepted February 21, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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