MORPHOLOGICAL ALTERATIONS AND CYTOSKELETAL REORGANIZATION IN OPOSSUM KIDNEY (OK) CELLS DURING OSMOTIC SWELLING AND VOLUME REGULATION

Cells from a variety of tissues regulate their volume when exposed to anisotonic conditions. After exposure of cells to hypotonic conditions , the rapid phase of cell swelling is followed by a slower phase of ce ll shrinkage towards the initial volume. The present study investigate s morphological alterations of adherent and fully spread cells after e xposure to hypotonic conditions and the reorganization of cytoskeletal components such as F-actin, actin-binding proteins, microtubules and intermediate-sized filaments. We used cells of a continuous epithelial cell line from the opossum kidney (OK cells), which were exposed to h ypotonic conditions for a period of 60 min at 25 degrees C. The osmola rity was reduced by 40% from 320 mosmol/l (isotonic conditions) to 192 mosmol/l (hypotonic conditions). The initial swelling after exposure of OK cells to hypotonic conditions caused enhanced ruffling membrane activity, formation of lamellipodia and an extended space between adja cent cells which was caused by a more rounded cell shape. Moreover, th e height of cells located in the centre of cell clusters increased by 32+/-8% (mean value+/-SEM) as checked by morphometric analysis of the vertical distance between the apical and basolateral F-actin domain. A lthough the fluorescence intensity and organization of F-actin in a ho rizontal direction remained unaltered during cell swelling, we observe d a loss of periodicity and irregular distribution of myosin aggregate s and a partial rearrangement of vimentin filaments in the form of sho rt fragments. In all experiments the organization of microtubules was observed to be unaltered. The alterations described above were reversi ble during cell shrinkage towards the initial volume, i.e. at 60 min a fter exposure to hypotonic conditions cell morphology and cytoskeletal organization no longer differed from the corresponding controls which were kept under isotonic conditions for the whole experimental period . The results demonstrate that only certain intracellular cytoskeletal components are actively involved in cell swelling and regulatory volu me decrease.