Effect of alpha-KG in lumen on PAH transport by isolated perfused rabbit renal proximal tubules

To determine whether dicarboxylate taken up at the luminal membrane could function in the p-aminohippurate (PAH) countertransport at the basolateral membrane, we examined the effect of adding alpha-ketoglutarate (alpha-KG) or glutarate (a nonmetabolized dicarboxylate that is countertransported for PAH at the basolateral membrane) to the luminal perfusate on net secretion of radiolabeled PAH in isolated perfused S2 segments of rabbit proximal tubules. Addition of 100 mu M alpha-KG or glutarate to the luminal perfusate in tubules perfused and bathed with HEPES-buffered medium (in the absence of bicarbonate, glycine, lactate, malate, and citrate) produced a reversible twofold stimulation of net PAH transepithelial secretion. Addition of 4 mM LiCl (an inhibitor of Na-dicarboxylate transport that does not directly affect PAH transport) to the luminal perfusate along with alpha-KG eliminated stimulation of net PAH secretion. Addition of 100 mu M or 1 mM alpha-KG or glutarate to the luminal perfusate in tubules perfused and bathed with bicarbonate-buffered medium containing glycine, lactate, malate, and citrate had no effect on net PAH transport from bath to lumen. These data indicate that alpha-KG (or glutarate) that enters the tubule cells via the luminal Na-dicarboxylate cotransporter can stimulate net PAH secretion, apparently via countertransport at the basolateral membrane, but only when tubules are not in an optimal metabolic state to produce intracellular alpha-KG.