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 Km of 2.2+/-0.3 microM (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 microM OTA into tubules by 96%. Kinetic analysis of the inhibition of OTA uptake by PAH (100 microM to 5 mM) yielded an apparent Ki of 164 microM, similar to the 100 to 200 microM range of Km 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 microM alpha-ketoglutarate (alphaKG) trans-stimulated the uptake rate of 1 microM OTA by 1.8-fold. Fourth, besides PAH, other organic anions effectively cis-inhibited the uptake of 1 microM OTA into tubules (inhibitor, % inhibition): 1.5 mM alphaKG, 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 microM 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 microM 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.
A Novel Human Organic Anion Transporter NPT4 Mediates the Transport of Ochratoxin A
