Transport of PAH, urate, TEA, and fluid by isolated perfused and nonperfused avian renal proximal tubules

Transport of organic anions [p-aminohippurate (PAH) and urate] and organic cations [tetraethylammonium (TEA)] and reabsorption of fluid were studied for the first time in individual renal proximal tubules isolated from avian kidneys. In isolated nonperfused tubules, PAH and urate uptake occurred against electrochemical gradients, whereas TEA uptake appeared to result from the electrical gradient. Radiolabeled PAH uptake and radiolabeled urate uptake were inhibited to an equal extent by high concentrations of unlabeled PAH and probenecid, suggesting that they might share the same transport system. However, the rate of uptake of radiolabeled PAH was significantly stimulated by preloading with α-ketoglutarate (α-KG), suggesting PAH/α-KG countertransport as in mammals and reptiles, whereas uptake of radiolabeled urate was not clearly stimulated. In isolated perfused tubules, net fluid reabsorption averaged ~2 nl · min^-1 · mm^-1 and was inhibited by ouabain with or without bicarbonate in the perfusate and bathing medium. In these perfused tubules, the unidirectional bath-to-lumen fluxes of PAH and urate exceeded the unidirectional lumen-to-bath fluxes, indicating net secretion of both compounds. During the bath-to-lumen fluxes the uptake across the basolateral membrane was against an electrochemical gradient for both compounds. However, for PAH the steady-state intracellular concentration was about half that observed in nonperfused tubules, as generally expected during net secretion, whereas for urate the steady-state intracellular concentration was about twice that observed in nonperfused tubules, suggesting stimulation of uptake during net secretion. During the PAH lumen- to-bath flux, the steady-state intracellular concentration was significantly above that in the perfusate, suggesting that this flux involved transport into the cells from the lumen against an electrochemical gradient. However, during the urate lumen-to-bath flux, there was no urate in the cells, suggesting that this flux, as in reptiles, occurred by a paracellular route.