CADMIUM INHIBITS VACUOLAR H-ATPASE AND ENDOCYTOSIS IN RAT-KIDNEY CORTEX()

The mechanism of cadmium (Cd)-induced damage in the mammalian proximal tubule that is manifested by defects in reabsorption of various compo unds, is poorly understood. A vacuolar H+-ATPase (V-ATPase) in proxima l tubule (PT) brush border and intracellular vesicles may be affected by Cd, and this may influence intracellular vesicle trafficking and re absorption of the filtered proteins. We studied the effects of Cd on V -ATPase and endocytosis in rat renal PT in vivo and on acidification m echanisms in isolated renal cortical organelles in vitro. The V-ATPase activity in brush border membrane (BBM) from Cd-intoxicated rats was 40% lower compared to that in control animals. Immunofluorescence stud ies in cortical tissue sections and Western blot studies in BBM from C d-treated rats showed a strongly decreased abundance of the 31 kDa and 70 kDa V-ATPase subunits. Functional studies in vivo showed a dramati cally diminished endocytosis of fluorescein-labeled dextran in PT cell s from Cd-treated animals, whereas morphological studies revealed a lo ss of endocytic invaginations and subapical vesicles in the same cells . In studies in vitro, Cd inhibited V-ATPase activity in a concentrati on-and time-dependent manner in both BBM and endocytic vesicles, where as in endocytic vesicles, Cd inhibited ATP-driven intravesicular acidi fication and accelerated the dissipation of transmembrane pH gradients . We conclude that Cd may impair acidification in cell organelles by ( a) causing a loss of V-ATPase protein in their limiting membranes, (b) inhibiting the intrinsic V-ATPase activity, and (c) dissipating the t ransmembrane pH gradient. This may inhibit endocytosis of filtered pro teins and impair vesicle-mediated recycling of some membrane transport ers, thus contributing to the loss of reabsorptive capacity of the PT.