Peritubular transport of ochratoxin A in rabbit renal proximal tubules

The transport of the nephrotoxic mycotoxin ochratoxin A across the renal peritubular membrane was examined in suspensions of rabbit renal proximal tubules. Ochratoxin A transport across the peritubular membrane was a high- affinity, low-capacity carrier-mediated process with a J(max) value of 0.12 ± 0.4 nmol/mg of protein/min and a K(m) value of 1.4 ± 0.1 μM. The apparent Michaelis constants for inhibition of [³H]para-aminohippurate (PAH) uptake by ochratoxin A inhibition was 1.5 μM, which is similar to the K(m) value for ochratoxin A uptake in tubule suspensions and suggests that ochratoxin A could be a substrate for the organic anion pathway. The capacity and affinity for peritubular ochratoxin A transport were 40-fold lower and >100-fold greater, respectively, than those measured for the peritubular uptake of [³H]PAH in tubule suspensions. A concentration of 2.5 mM PAH, which reduced the uptake of [³H]PAH by 90%, reduced ochratoxin A uptake by only 40% to 50%, whereas probenecid concentrations of 0.6 to 2 mM reduced ochratoxin A accumulation in tubule suspensions up to ≃80% to 90%. This probenecid-sensitive, PAH-insensitive uptake of ochratoxin A suggested that at least one mediated pathway other than the organic anion transporter was involved in the peritubular uptake of this mycotoxin. A 2 mM concentration of the fatty acid octanoate and 1.5 mM concentration of the nonsteroidal anti- inflammatory agent piroxicam were as effective as probenecid in blocking ochratoxin A uptake. The apparent K(l) values for inhibition of ochratoxin A uptake by probenecid, piroxicam and octanoate were 30.5 ± 7.9, 23.2 ± 10.4 and 81.5 ± 8.7 μM, respectively. The ability of octanoic acid to inhibit ochratoxin A transport to the same extent as probenecid and a greater extent than PAH suggests that a separate fatty acid transport pathway may be involved in the accumulation of ochratoxin A by suspensions of rabbit renal proximal tubules.