Volume dynamics in migrating epithelial cells measured with atomic force microscopy

Migration of transformed renal epithelial cells (transformed Madin-Darby ca nine kidney cells, MDCK-F cells) relies on the activity of a Ca2+-sensitive K+ channel (IK channel) that is more active at the rear end of these cells . We have postulated that intermittent IK channel activity induces local ce ll shrinkage at the rear end of migrating MDCK-F cells and thereby supports the cytoskeletal mechanisms of migration. However, due to the complex morp hology of MDCK-F cells we have not yet been able to measure volume changes directly. The aim of the present study was to devise a new technique employ ing atomic force microscopy (AFM) to measure the volume of MDCK-F cells in their physiological environment and to demonstrate its dependence on IK cha nnel activity. The spatial (x, y and z) co-ordinates of each pixel of the t hree-dimensional image of MDCK-F cells allow calculation of the volume of t he column "underneath" a given pixel. Thus, total cell volume is the sum of all pixel-defined columns. The mean volume of 17 MDCK-F cells was 2500+/-3 00 fl. Blockade of the IK channel with the specific inhibitor charybdotoxin (CTX) increased cell volume by 17+/-4%; activation of IK by elevating the intracellular [Ca2+] with the Ca2+ ionophore ionomycin decreased cell volum e by 19+/-3%. Subtraction images (experimental minus control) reveal that s welling and shrinkage occur predominantly at the rear end of MDCK-F cells. In summary, our experiments show that AFM allows the measurement not only o f total cell volume of living cells in their physiological environment but also the tracing of local effects induced by the polarized distribution of K+ channel activity.