Mechanism of urate and p-aminohippurate transport in rat renal microvillus membrane vesicles

Abstract

The transport of urate and p-aminohipurate (PAH) was evaluated in brush border membrane vesicles from the rat renal cortex. The binding of urate to the membranes was 6% of total uptake and no conversion of urate to allantoin was detected. The binding of PAH to the membranes was 24% of total uptake. In the presence of an outwardly directed OH gradient(pHi = 7.5, pH0 = 6.0), the uptake of urate and PAH was stimulated relative to the absence of a OH- gradient (pHi = pH0 = 7.5) and the influx of urate resulted in a transient overshoot of the equilibrium value. The OH- gradient-stimulated uptake of urate and PAH was not solely due to a change in membrane potential. Probenecid, DIDS [4,4''-diisothiocyano-2,2''-disulfonic stilbene], furosemide and pyrazinoate inhibited the hydroxyl ion gradient-stimulated uptake of urate and PAH in a dose-dependent manner. The uptake of [14C]urate and [3H]PAH could be cis-inhibited and trans-stimulated by either unlabeled urate or PAH. In the presence of an outwardly directed HCO3- gradient and 10% CO2 (outside HCO3- = 5.4 mM, inside HCO3- = 54 nM, pH0 = 6.5, pHi = 7.5), the initial rate of urate uptake was faster and the initial rate of urate efflux was slower compared with vesicles that had the same pH gradient without HCO3- or CO2. The effects of HCO3- gradients on organic anion transport were not dependent on diffusion potentials. Extravesicular Na+, K+, Li+, or Cs+ (100 mM) did not affect urate or PAH uptake. Brush border membrane vesicles from the rat kidney may contain an anion exchange transport system with affinity for urate, PAH, OH- and HCO3-. There was no evidence for a Na-urate or Na-PAH cotransport mechanism in these membranes.