Basolateral localization of Na(+)-HCO3- cotransporter activity in alveolar epithelial cells

Abstract

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 was mounted in a cuvette containing two fluid compartments (apical and basolateral) separated by the monolayer. Cells were loaded with the pH-sensitive dye BCECF and intracellular pH (pHi) measured spectrofluorometrically. Monolayers were studied at ambient temperature on days 3-4 in culture, coincident with the development of high tissue resistance (RT > or = 1000 omega.cm2). After the cells were acidified by NH3 prepulse, pHi recovered to baseline when Na+ was present in the basolateral fluid, but did not recover when Na+ was present only in the apical fluid. This basolateral Na(+)-dependent pHi recovery in the presence of HCO3-/CO2 was reduced, but present, in experiments where dimethylamiloride (DMA, 100 microM) or the stilbene derivative DIDS (500 microM) was in basolateral fluid. However, recovery was completely inhibited when both DMA and DIDS were present basolaterally. pHi recovery was not inhibited under Cl(-)-free conditions, indicating that cytoplasmic realkalinization was not effected by Na(+)-dependent Cl-HCO3- exchange. These data indicate that alveolar epithelial cells express a basolateral Na(+)- and HCO3(-)-dependent, DIDS-sensitive, Cl(-)-independent pHi recovery process that probably represents Na(+)-HCO3(-)-cotransport (symport). Basolateral Na(+)-HCO3- cotransport modulates pHi in alveolar epithelial cells, may contribute to regulation of intracellular volume and osmolarity, and may participate in signal transduction by hormones and growth factors.