Proximal tubule-derived opossum kidney (OK) cells are a suitable model to study proximal tubular protein endocytosis by using fluorescein-isothiocyanate-albumin as substrate. We used OK cells to investigate several steps of the endocytotic process in control cells and in ochratoxin A (OTA)-treated cells. OTA is a mycotoxin which causes proteinuria. When OTA was present only during the 15-min period in which uptake was studied, it had no effect on albumin endocytosis. Preincubation of OK cells with OTA (10 mumol/l) for 24 hr led to a reduction of transport capacity (Jmax to approximately 50% of control) and of apparent affinity (Km to approximately 200% of control). Specific binding of albumin to the apical cell surface was reduced also. Maximum binding capacity was reduced to 72% of control. By contrast, endocytotic uptake of the fluid-phase marker dextran was not affected by OTA. Preincubation of OK cells for 24 hr with 10 mumol/l of OTA reduced degradation of fluorescein-isothiocyanate-albumin to trichloroacetic acid-soluble fluorescence to 59% of control. We could not detect any difference in endosomal pH (6.13 +/- 0.05 in controls vs. 6.04 +/- 0.10 in OTA-treated cells). Furthermore, the rate of re-exocytosis of albumin taken up was significantly greater in OTA-treated cells. We conclude that: 1) OK cells are a suitable model to study several steps of the endocytotic process separately and thus to investigate the pathophysiology of reduced tubular protein reabsorption and 2) 24-hr exposure to OTA reduces protein uptake because of a decrease of specific binding sites and of enhanced exocytosis.(ABSTRACT TRUNCATED AT 250 WORDS)