The mycotoxin ochratoxin A deranges pH homeostasis in Madin-Darby canine kidney cells

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Abstract
Ochratoxin A (OTA) is a nephrotoxin which blocks plasma membrane anion conductance in Madin-Darby canine kidney (MDCK) cells. Added to the culture medium, OTA transforms MDCK cells in a manner similar to exposure to alkaline stress. By means of video-imaging and microelectrode techniques, we investigated whether OTA (1 μmol/liter) affects intracellular pH (pH.), Cl− (Cl − i ) or cell volume of MDCK cells acutely exposed to normal (pHnorm=7.4) and alkaline (pHalk=7.7) conditions. At pHnorm, OTA increased Cl − i by 2.6±0.4 mmol/liter (n=14, P i . At pHalk, application of OTA increased Cl − i by 8.6±2.6 mmol/liter (n=10, P< 0.05) and raised pH i by 0.11±0.03 (n= 8, P−HCO −3 exchange inhibitor DNDS (4,4′-dinitro-stilbene-2, 2′-disulfonate; 10 μmol/liter) eliminated the OTA-induced changes of pH i and Cl − i . OTA did not affect cell volume under both pHnorm and pHalk conditions. We conclude that the OTA-induced blockade of plasma membrane anion conductance increases Cl − i without changing cell volume. The driving force of plasma membrane Cl−/HCO −3 exchange dissipates, leading to a rise of pH i when cells are exposed to an acute alkaline load. Thus, OTA interferes with pH i and Cl − i homeostasis leading to morphological and functional alterations in MDCK cells. The work was supported by the Deutsche Forschungsgemeinschaft (DFG, Si 170/7-1). We thank the Zeiss Company (Oberkochen, Germany) for providing the Attofluor™ video-imaging system for the intracellular Ca2+ measurements.