alpha-Ketoglutarate transport in rat renal brush-border and basolateral membrane vesicles

The dicarboxylate, alpha-ketoglutarate (alphaKG), 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 alphaKG in BLM and brush-border membrane (BBM) vesicles isolated from rat kidney. In both membrane preparations, alphaKG uptake was Na+-dependent, saturable, electrogenic and inhibited by Li+. The initial rate of alphaKG (5 microM) 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 alphaKG (apparent Km = 15.2 microM), but a relatively low transport capacity (Vmax = 386 pmol/mg/5 sec). In contrast, the BBM transporter had characteristics of a low-affinity (Km = 158 microM), high-capacity (Vmax = 1106 pmol/mg/5 sec) system. Other dicarboxylates such as succinate, malate, fumarate and glutarate at a concentration of 1 mM inhibited alphaKG uptake into BLM and BBM vesicles to the same extent (>90%). The tricarboxylate, citrate, also inhibited alphaKG uptake (70-80%). However, of these Krebs' cycle intermediates, only alphaKG and glutarate were able to affect p-aminohippurate (PAH) uptake into BLM vesicles. These results lend further support for a BLM PAH/alphaKG exchanger. Furthermore, if extracellular alphaKG plays a role in the operation of the PAH/alphaKG exchanger, the high-affinity Na+-dependent alphaKG transporter located in the BLM is the likely source of the organic anion.