PLASTICITY IN EPITHELIAL POLARITY OF RENAL INTERCALATED CELLS - TARGETING OF THE H-ATPASE AND BAND-3()

The intercalated cell is an epithelial cell of the renal collecting tu bule that is specialized for H+ and HCO3- transport. These cells exist as two types, alpha and beta. The alpha-cell secretes H+ into the lum en by an apical H+-ATPase and a basolateral Cl-/HCO3-, exchanger that is a form of band 3 protein (AE1). The beta-cell secretes HCO3- into t he lumen by an apical Cl-/HCO3- exchanger and a basolateral H+-ATPase. In a previous study, it was suggested that a reversal in epithelial p olarity of these cells occurs during the response of the kidney to an acid load (G. J. Schwartz, J. Barasch, and Q. Al-Awqati. Nature Lend. 318: 368-371, 1985). Recent studies, however have shown that there are many other subtypes where the distribution of these two proteins does not fit into this neat bipolar classification. This group of investig ators recently generated an immortalized cell line of the beta-interca lated cell and found that the apical Cl-/HCO3- exchanger is also AE1. Furthermore, when these cells were seeded at high densities, the polar ized targeting of the apical band 3 was reversed to the basolateral me mbrane. This was produced by the secretion of extracellular matrix pro teins that by themselves were capable of reversing the polarity of ban d 3 (J. S. van Adelsberg, J. C. Edwards, J. Takito, B. Kiss, and Q. Al -Awqati. Cell 76: 1053-1061, 1995). A large new extracellular matrix p rotein, hensin, was identified and found to be present exclusively in the collecting tubule. The extensive recent literature on the biology of alpha- and beta-intercalated cells is reviewed here and found to be compatible with the idea of the reversal of polarity as a mechanism f or the regulation of H+ secretion by the tubule.