BASOLATERAL LOCALIZATION OF NA-HCO3- COTRANSPORTER ACTIVITY IN ALVEOLAR EPITHELIAL-CELLS()

We investigated the polarized distribution of Na+- and HCO3--dependent recovery from intracellular acidification in alveolar epithelial cell monolayers. Rat alveolar type II cells were grown in primary culture on detachable tissue culture-treated Nuclepore filters. Each filter wa s mounted in a cuvette containing two fluid compartments (apical and b asolateral) separated by the monolayer. Cells were loaded with the pH- sensitive dye BCECF and intracellular pH (pH(i)) measured spectrofluor ometrically. Monolayers were studied at ambient temperature on days 3- 4 in culture, coincident with the development of high tissue resistanc e (R(T) greater than or equal to 1000 Omega cm(2)). After the cells we re acidified by NH3 prepulse, pH(i) recovered to baseline when Na+ was present in the basolateral fluid, but did not recover when Na+ was pr esent only in the apical fluid, This basolateral Na+-dependent pH(i) r ecovery in the presence of HCO3-/CO2 was reduced, but present, in expe riments where dimethylamiloride (DMA, 100 mu M) or the stilbene deriva tive DIDS (500 mu M) was in basolateral fluid. However, recovery was c ompletely inhibited when both DMA and DIDS were present basolaterally. pH(i) recovery was not inhibited under Cl--free conditions, indicatin g that cytoplasmic realkalinization was not effected by Na+ -dependent Cl--HCO3- exchange. These data indicate that alveolar epithelial cell s express a basolateral Na+- and HCO3--dependent, DIDS-sensitive, Cl-- independent pH(i) recovery process that probably represents Na+-HCO3-- cotransport (symport). Basolateral Na+-HCO3- cotransport modulates pH( i) in alveolar epithelial cells, may contribute to regulation of intra cellular volume and osmolarity, and may participate in signal transduc tion by hormones and growth factors.