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Vol. 284, Issue 3, 943-948, March 1998
Department of Physiological Sciences (C.E.G., M.M.), College of
Veterinary Medicine, University of Florida, Gainesville, Florida; and
Department of Physiology, College of Medicine, University of Arizona,
Tucson, Arizona (S.H.W.)
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 Jmax value of 0.12 ± 0.4 nmol/mg of protein/min and a Km
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 Km 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 Ki 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.
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