LOW PERMEABILITIES OF MDCK CELL MONOLAYERS - A MODEL BARRIER EPITHELIUM

Barrier epithelia such as the renal collecting duct (in the absence of antidiuretic hormone) and thick ascending limb, as well as the stomac h and mammalian bladder, exhibit extremely low permeabilities to water and small nonelectrolytes. A cell culture model of such epithelia is needed to determine how the structure of barrier apical membranes redu ces permeability and how such membranes may be generated and maintaine d. In the present studies, the transepithelial electrical resistance a nd isotopic water and urea fluxes were measured for Madin-Darby canine kidney (MDCK) type I and type II cells, as well as type I cells expre ssing the mucin protein, MUC1, in their apical membranes. Although ear lier studies had found the unstirred layer effects too great to permit measurement of transepithelial permeabilities, use of ultrathin semip ermeable supports in this study overcame this difficulty. Apical membr ane diffusive water permeabilities were 1.8 +/- 0.4 x 10(-4) cm/s and 3.5 +/- 0.5 x 10(-4) cm/s in MDCK type I and type II cells, respective ly, at 20 degrees C. Urea permeability in type I cells at the same tem perature was 6.0 +/- 0.9 x 10(-6) cm/s. These values resemble those of other barrier epithelial apical membranes, either isolated or in inta ct epithelia, and the water permeability values are far below those of other epithelial cells in culture. Transfection of MDCK type I cells with the major human urinary epithelial mucin, MUC1, led to abundant e xpression of the fully glycosylated form of the protein on immunoblots , and flow cytometry revealed that virtually all the cells expressed t he protein. However, MUC1 had no effect on water or urea permeabilitie s. In conclusion, MDCK cells grown on semipermeable supports form a mo del barrier epithelium. Abundant expression of mucins does not alter t he permeability properties of these cells.