Peritubular transport of ochratoxin a by single rabbit renal proximal tubules

Epifluorescence microscopy was used to study peritubular transport of the fluorescent mycotoxin ochratoxin A (OTA) into single proximal tubule segments of the rabbit. Initial rates of OTA uptake into S2 segments were saturable and adequately described by Michaelis-Menten kinetics, with an apparent K(m) of 2.2 ± 0.3 μM (SEM). Several lines of evidence indicated that peritubular uptake of OTA in S2 segments was effectively limited to the 'classical' organic anion transporter. First, 5 mM p-aminohippurate (PAH) cis-inhibited the uptake of 1 μM OTA into tubules by 96%. Kinetic analysis of the inhibition of OTA uptake by PAH (100 μM to 5 mM) yielded an apparent K(i) of 164 μM, similar to the 100 to 200 μM range of K(m) values previously reported for the peritubular uptake of PAH. Second, efflux of OTA from tubules was trans-stimulated 3.2-fold by the presence of 2.5 mM PAH in the uptake medium. Third, 100 μM α-ketoglutarate (αKG) trans-stimulated the uptake rate of 1 μM OTA by 1.8-fold. Fourth, besides PAH, other organic anions effectively cis-inhibited the uptake of 1 μM OTA into tubules (inhibitor, % inhibition): 1.5 mM αKG, 80%; 1 mM probenecid, 100%; 1 mM piroxicam, 100%; 1 mM octanoate, 100%. In contrast, 1.5 mM tetraethylammonium, an organic cation, blocked uptake of 1 μM OTA by only 7%. The inhibition of OTA uptake into S1 and S3 segments of the proximal tubule was qualitatively similar: 5 mM PAH cis-inhibited the uptake of 1 μM OTA by approximately 95% in both S1 and S3 segments. Thus, peritubular OTA uptake into all segments of the proximal tubule appears to be dominated by its interaction with the classical organic anion transporter. The high- affinity and relatively high capacity of this pathway for OTA suggest that peritubular uptake may be a significant avenue for the entry of this toxin into proximal tubule cells.