The transport mechanism of urate and p-aminohippurate (PAH) was evaluated in microvillus membrane vesicles isolated from the renal cortex of the mongrel dog. Imposition of a transmembrane pH gradient (pHo less than pH1) markedly accelerated the uptake of [14C]urate and [3H]PAH and caused the transient accumulation ("overshoot") of each anion above its final level of uptake. The transport of urate and PAH under both stimulated (pHo less than pHi) and basal (pHo = pHi) conditions was insensitive to valinomycin-induced K+ diffusion potentials. The pH gradient-stimulated uptake of 25 microM [14C]urate and 1.0 microM [3H]PAH was significantly inhibited by 1.2 mM PAH, urate, furosemide, salicylate, or probenecid. The effect of each inhibitor on [14C]urate transport was identical to the effect of the same inhibitor on [3H]PAH flux. We conclude that the transport of urate and PAH in dog renal microvillus membrane vesicles occurs via a pH gradient-stimulated electroneutral carrier-mediated process such as 1:1 H+-anion cotransport or OH-anion exchange. Such a transport mechanism may possibly play a role in effecting uphill urate reabsorption in the proximal tubule. Moreover, this study demonstrates that secondary active solute transport in epithelial membranes may be coupled to the electrochemical gradient of an ion other than Na+.
J W Blomstedt, P S Aronson