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

To determine the quantitative roles of the basolateral and luminal Na+-dica rboxylate (Na-DC) cotransporters in establishing and maintaining the alpha- ketoglutarate (alpha KG) gradient required for renal tubular secretion of o rganic anions, we measured net steady-state transepithelial secretion of fl uorescein (FL) in real time in isolated, perfused S2 segments of rabbit ren al proximal tubules. Net "basal" FL secretion in the absence of exogenous a lpha KG had a K-t of similar to 4 mu M and a maximal transepithelial secret ion rate (J(max)) of similar to 380 fmol.min(-1).mm(-1) (where K-t is the F L concentration that produces one-half the J(max)). It could be almost comp letely inhibited by basolateral p-aminohippurate (PAH). Selective inhibitio n 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 1 0 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 concentrati ons 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 mai ntaining similar to 50% of the outwardly directed alpha KG gradient that is responsible for driving basolateral FL/alpha KG exchange. The remaining si milar to 50% would be maintained by metabolic production of alpha KG in the cells.