MODULATION BY ANIONS OF P-AMINOHIPPURATE TRANSPORT IN BOVINE RENAL BASOLATERAL MEMBRANE-VESICLES

In the presence of 10 mu M 2-oxoglutarate (2-OG) and of an inward Na+ gradient, uphill [H-3]p-aminohippurate (PAH) uptake occurs due to cooperation of the PAH/2-OG exchanger and the Na+-coupled 2-OG transporter in bovine renal basolateral membrane vesicles. Uphill PAH uptake is observed with Cl-, but not with gluconate as the bulk anion. To determine specificity and nature of this anion effect [H-3]PAH uptake was measured in the presence of several anions without and with ionophores to distinguish indirect from direct effects on the PAH transporter. Na+-gradient plus 2-OG-stimulated [H-3]PAH uptake is fast with Cl-, intermediate with F-, Br-, I-, NO3- and SCN-, and slow in the presence of gluconate, SO42- and HPO42- . Stimulation by Cl- (as compared to gluconate) is attenuated, but not abolished, by clamping electrical potential and pH differences to zero, suggesting a partial effect through charge compensation and a major effect of anions on the PAH transporter itself. Indeed, [H-3]PAH/2-OG and [H-3]PAH/PAH exchange rates under voltage- and pH-clamped condition depend on bulk anions although the anion effects are less pronounced than with Na+-gradient plus 2-OG-stimulated [H-3]PAH uptake. Since an inward Cl- gradient does not drive [H-3]PAH above or below equilibrium distribution, Cl- ions are most probably not translocated by the PAH transporter. We propose that anions modulate the PAH transporter by interacting with a site not directly related to anion transport.