Apical membrane permeability of MDCK cells

The osmotic water permeability (Pf) and permeability to nonelectrolytes were determined for the apical membrane of clonal strain Madin-Darby canine kidney (MDCK) C12 cells cultured as cysts with the apical membrane facing the surrounding medium. Pf and solute permeabilities were calculated from the rate of volume change of cysts by digitizing images at 1-s intervals after instantaneous osmotic challenge. Image measurement was fully automated with the use of a program that separated the image of the cyst from the background by using adaptive intensity thresholding and shape analysis. Pf, calculated by curve fitting to the volume loss data, averaged 2.4 +/- 0.1 micron/s and was increased by addition of amphotericin B. The energy of activation for Pf was high (16.3 kcal/mol), and forskolin (50 microM) had no effect on Pf. Two populations of MDCK cysts were studied: those with two to three cells and those that appeared to be composed of only one cell. The Pf of multicell cysts was the same as single cell cysts, suggesting that paracellular water flow is not significant. Solute permeability was measured using paired osmotic challenges (sucrose and test solute) on the same cyst. Urea permeability was not different from zero, whereas the permeabilities of acetamide and formamide were consistent with their relative oil-water partition coefficients. Our data are similar to values from studies on the permeability properties of vesicles of water-tight epithelial apical membrane. The combination of the unique model of MDCK apical-out cysts and fully automated data analysis enabled determination of apical membrane permeability in intact epithelial cells with high precision.