Real-time assessment of α-ketoglutarate effect on organic anion secretion in perfused rabbit proximal tubules

To determine the quantitative roles of the basolateral and luminal Na(+)-dicarboxylate (Na-DC) cotransporters in establishing and maintaining the alpha-ketoglutarate (alpha KG) gradient required for renal tubular secretion of organic anions, we measured net steady-state transepithelial secretion of fluorescein (FL) in real time in isolated, perfused S2 segments of rabbit renal proximal tubules. Net "basal" FL secretion in the absence of exogenous alpha KG had a K(t) of similar to 4 mu M and a maximal transepithelial secretion rate (J(max)) of similar to 380 fmol.min(-1).mm(-1) (where K(t) is the FL concentration that produces one-half the J(max)). It could be almost completely inhibited by basolateral p-aminohippurate (PAH). Selective inhibition of the basolateral Na-DC cotransporter indicated that recycling via this transporter of alpha KG that had been exchanged for FL supports similar to 25% of the "basal" FL secretion. Physiological alpha KG concentrations of 10 mu M in the bath or 50 PM in the perfusate stimulated net secretion of FL by similar to 30 or similar to 20%, respectively. These data indicate that the basolateral Na-DC cotransporter supports similar to 42% of the net FL secretion. The luminal and basolateral effects of physiological concentrations of alpha KG were additive, indicating that the combined function of the luminal and basolateral Na-DC cotransporters can support similar to 50% of the net FL secretion. This apparently occurs by their establishing and maintaining similar to 50% of the outwardly directed alpha KG gradient that is responsible for driving basolateral FL/alpha KG exchange. The remaining similar to 50% would be maintained by metabolic production of alpha KG in the cells.