OCHRATOXIN-A IMPAIRS POSTPROXIMAL NEPHRON FUNCTION IN-VIVO AND BLOCKS PLASMA-MEMBRANE ANION CONDUCTANCE IN MADIN-DARBY CANINE KIDNEY-CELLS IN-VITRO

Ochratoxin A (OTA) is a widespread nephrotoxin which causes porcine nephropathy and is supposed to have caused the human Balkan endemic nephropathy. We performed experiments in viveo and in vitro to eluci date the mechanism of OTA action in renal epithelium. Application of OTA to male Wistar rats [1.25 mu mol/ (kg . day)] for 6 days led to a reduction of glomerular filtration rate (to 63% of control), an increased fractional water (194% of control), Na+ (199% of control), K+ (147% of control) and Cl- (270% of control) excretion and an increased dependence of the osmole clearance on urine flow. Acute application of OTA to rats (3 mu mol/ kg) increased urinary pH from 6.0 +/- 0.2 to 6.6 +/- 0.1 and urinary NaCl excretion, but decreased titratable acid excretion to 47% of control. As these in vivo findings may be the result of an action of OTA beyond the proximal tubule (''postproximal'') we investigated the effect of OTA on cultured Madin-Darby canine kidney (MDCK) cells, regarded as a model of collecting duct epithelium. In confluent monolayers formed by MDCK cells OTA reduced the number of domes in a dose-dependent manner and impaired the formation of a transepithelial Cl- gradient. Electrophysiological measurements in giant MDCK cells revealed that OTA blocks fractional anion conductance of the plasma membrane with an IC50 value of 30 + 5 nmol/l, unmasking OTA as a naturally occurring anion conductance blocker about 20-times more effective than the most potent synthetic blocker 5-nitro-2-(3-phenylpropyl-amino) benzoic acid (NPPB) (IC50 = 600 + 50 nmol/l). The effects of OTA and NPPB are not additive. We conclude that OTA acts acutely on ''postproximal'' parts of the nephron. This effect is probably located in the collecting duct and is due to its inhibitory action on plasma membrane anion conductance.