Effects of inhibitors in lumen on PAH and urate transport by isolated renal tubules

Effects of the presence of unlabeled p-aminohippurate (PAH) or urate, probenecid, and phenol red in the lumen on labeled PAH or urate transport by isolated, perfused snake (Thamnophis spp.) proximal renal tubules were studied. Net secretion of labeled urate and luminal membrane permeability to urate were unaffected by the presence of unlabeled urate (up to 0.1 mM) or probenecid (up to 1.0 mM) in lumen only. The data are compatible with movement of urate from cells to lumen during urate secretion by a simple passive process. Net secretion of labeled PAH was rapidly and reversibly depressed to about 25-35% of control when unlabeled PAH (0.05 mM), phenol red (0.05 mM), or probenecid (0.1 mM) was added to the lumen only. During maximum depression of PAH transport, luminal membrane permeability to PAH was reduced by 60-70%. The data suggest that movement of PAH from cells to lumen down an electrochemical gradient during PAH secretion occurs by a readily inhibited, mediated process.