Renal transepithelial secretion of ochratoxin A in the non-filtering toad kidney

Renal excretion is an important way of elimination for the nephrotoxin ochratoxin A (OTA). Because binding to proteins hinders filtration, excretion is mainly due to proximal tubular secretion. The goal of this study was to investigate the kinetics of secretion as well as the extent of urine and tissue accumulation in situ using the non-filtering amphibian kidney model. Transepithelial secretion is a saturable process (K(m) = 0.63.10(-6) mol/l) and leads to a concentration-dependent accumulation of OTA in the tubular lumen and in renal tissue. Maximum accumulation achieved is approximately 14-fold as compared to the perfusate concentration. There was no accumulation in the tubular lumen as compared to renal tissue (lumen-to-tissue concentration ratio approximately 1). Tissue and tubular lumen accumulation were reduced to approximately 40% of control in the presence of 10(-3) mol/l p-aminohippurate (PAH). Addition of 10(-3) mol/l alpha-ketoglutarate (KG) to PAH-containing perfusate did not lead to a further reduction of secretion. By contrast, addition of 10(-2) mol/l L-phenylalanine (L-Phe) reduced secretion further to approximately 25% of control. In the presence of 10(-3) mol/l probenecid tissue accumulation was reduced to 7% and tubular lumen accumulation to 1% of control. Lumen-to-tissue concentration ratio decreased to 0.15 in the presence of probenecid, indicating an inhibitory action at the luminal membrane. Addition of albumin to the perfusate, reduced secretion to only 50% of control, whereas the concentration of free OTA was reduced below 1% as compared to control. The results of this study show that transepithelial secretion is an effective way for accumulation of OTA in the tubular lumen and thus its urinary excretion. Transport via the basolateral organic anion and a basolateral amino acid carrier are the active steps in transepithelial secretion. Luminal exit of OTA is a passive process. Furthermore, tissue accumulation by the active transport across the basolateral membrane supports the toxic action of OTA on proximal tubular cells. Due to the qualitative similarity of organic anion transport our findings should also apply for the mammalian kidney.