Subtypes of Madin-Darby canine kidney (MDCK) cells defined by immunocytochemistry: Further evidence for properties of renal collecting duct cells

The Madin-Darby canine kidney (MDCK) cell line has been proposed as a model for studying intercalated (IC) cells of the renal cortical collecting duct. The IC cells are characterized by peanut lectin (PNA) binding capacity, carbonic anhydrase (CA) activity and Cl^-–HCO₃^-exchange mediated by a band 3-related protein. It has been suggested that these properties are also expressed in MDCK cells. So far however, the nature of the specific protein involved in Cl^-–HCO₃^-exchange, the type of CA isozyme and the relationship between these two characteristics and PNA binding, have not been investigated in MDCK cells by immunocytochemical methods. Using two antibodies raised against human erythrocyte band 3 protein and two against human erythrocyte CA I and II isozymes, our study provides evidence that a protein related to band 3 is expressed in about 5% of cultured MDCK cells; these band 3-positive cells do not bind PNA and are not reactive for CAI or CAII. About 30% of the MDCK cells bind PNA, two-thirds of which are also CAII-positive. A majority (about 65%) of MDCK cells is not reactive for the three markers used; their density is increased after incubation with aldosterone. These data indicate (i) that the Cl^-–HCO₃^-exchanger of the MDCK cells could be related to human erythrocyte band 3, (ii) that the CA activity of the MDCK cell line bears antigenic identity with the erythrocyte CA II isozyme and (iii) that the latter is always co-localized with PNA binding. These results provide immunocytochemical evidence for the heterogeneity of the MDCK cell line, which might reflect the cellular heterogeneity encountered in the renal cortical collecting duct. Our data also indicate that clonal selection will be required for future functional studies of the MDCK cells.