Apical uptake of radiolabelled ochratoxin A into Madin-Darby canine kidneycells

Uptake of ochratoxin A (OTA) across the apical cell membrane of collecting duct cells is the first step in reabsorption and partly mediated by proton- dipeptide cotransport. As the remaining part of apical OTA uptake remained unclear, we studied the characteristics of apical uptake of tritium-labelle d OTA (H-3-OTA) in MDCK-C11 cells in detail. Uptake of H-3-OTA was pH- and temperature-dependent and led to intracellular accumulation of OTA. Lowerin g pH led to an increase and lowering temperature (4 degrees C) to a decreas e of OTA uptake. Besides dipeptides, the beta-lactam antibiotics cephalexin and ceftibuten inhibited the H-3-OTA uptake also confirming the role of th e proton-dipeptide cotransporter. In addition, substrates of organic anion transporter, taurocholate and methotrexate, inhibited H-3-OTA uptake in par t. Aspartylphenylalanine methyl ester (aspartame) had no inhibitory effect on H-3-OTA uptake. Uptake of OTA was not dependent on sodium. Sixty minutes of preincubation with phorbol 12-myristate 13-acetate (PMA) led to increas ed apical uptake of OTA. The PMA effects were inhibited by ethylisopropylam ilorid (EIPA). We conclude that apical uptake of OTA occurs by Na+-independ ent transport. One part of the uptake is mediated by proton-dipeptide cotra nsport (30%, dipeptide-inhibitable), by organic anion transporter (20%, tau rocholate-inhibitable) and by diffusion (20%, responsible for uptake at 4 d egrees C). The remaining part occurs by as yet unidentified but pH-dependen t transport mechanisms. An acidic urine in distal parts of the nephron prov ides thus the main risk for OTA uptake leading to its reabsorption and cons equently alkalinisation of the urine should help to prevent this reabsorpti on. (C) 1998 Elsevier Science Ireland Ltd. Ail rights reserved.