PARACELLULAR BARRIER AND JUNCTIONAL PROTEIN DISTRIBUTION DEPEND ON BASOLATERAL EXTRACELLULAR CA2+ IN CULTURED EPITHELIA

The polarised nature of the increase in paracellular permeability induced by Ca2+-chelation with EGTA was investigated in several cultured epithelial cell lines. In strain I MDCK cells (canine kidney cells), a marked decrease (> 90%) in transepithelial electrical resistance (R(T)) and increase in mannitol and inulin permeabilities were only observed after addition of EGTA (for 4 h) to either basolateral (basal) or both (apical + basal) bathing solutions; apical Ca2+-chelation resulted in significant smaller changes (similar to 30%) in these variables. The increase in paracellular permeability upon basal EGTA addition was significantly lower than that produced by simultaneous apical and basal addition of 2 mM EGTA. A higher concentration of EGTA (20 mM) did not significantly eliminate this difference in potency between basal and apical + basal Ca2+-chelation. The polarised Ca2+-dependence of the paracellular barrier was associated with polarised effects on the junctional/cytoskeletal protein distribution. Basal or apical + basal EGTA addition induced substantial internalisation of uvomorulin with some cellular redistribution of the perijunctional actin ring and desmosomes and gaps in ZO-1 location between adjacent cells. In addition, polarised Ca2+-dependence of the paracellular barrier (assessed by measuring R(T)) was observed also in strain II MDCK and mio human adenocarcinoma intestinal cell lines, Caco-2 and HCT-8, demonstrating generality of the phenomenon. Therefore, the data show a polarity in the ability of EGTA to enhance epithelial permeability and induce cellular redistribution of cytoskeletal/junctional proteins in several epithelia. The basolateral membrane sensitivity to Ca2+-chelation might be explained by the polarised distribution of uvomorulin.