Development and polarization of the Na+/H+ antiport system during reorganization of LLC-PK1A cells into an epithelial membrane

Changes in Na+/H+ antiport activity and transepithelial electrical resistance were analyzed in a clone of LLC-PK1 cells as the dispersed cells became organized into an epithelial membrane. The clone designated LLC-PK1A showed a 250% increase in Na+/H+ exchange activity as compared with the parent cell line. Na+ influx induced by an outwardly oriented H+ gradient is almost completely abolished during active cell proliferation or after cell dispersion. The activity of the Na+/H+ antiport system increases after plating the cells at high density. This increase precedes the increase in the transepithelial electrical resistance. The increase in the Na+/H+ antiport activity was not observed when the cells were plated at low density in the presence of an antimitotic agent indicating that close cell contact is an absolute requirement for the development of the system. The increase in Na+ influx correlated with an increase in Vmax, while the Km for Na+ remained essentially unchanged. Unidirectional Na+ influx measured from the apical or basolateral side as the dispersed cells became reorganized into an epithelial membrane indicated that the insertion of the Na+/H+ antiporter proteins occurred directly in the apical membrane of the epithelial cells. This finding is consistent with the hypothesis that the sorting of native proteins occurs intracellularly prior to their insertion in the apical membrane of the epithelial cells. The delay in the increase of transepithelial electrical resistance as compared with the increase in Na+ influx indicates that the settlement of the limits between the apical and basolateral membrane (fence function) precedes the closing of the intercellular space (barrier function) during the development of the occluding junctions. Further, the development of the Na+/H+ antiporter was inhibited by cycloheximide but not by actinomycin D, suggesting that the expression of epithelial cell polarization is a translational or posttranslational event.