Sodium chloride cotransport at the basolateral membrane of type II pneumocytes

Current evidence indirectly supports the hypothesis that Na+/Cl- cotransport occurs at basolateral membranes of type II pneumocytes. To test this hypothesis, enriched apical and basolateral plasma membrane vesicles were prepared from adult bovine type II pneumocytes. Uptake of 22Na+ or 36Cl- by these vesicle populations was monitored over time. Using enriched basolateral vesicles, substitution of formate- for Cl- nearly eliminated 22Na+ uptake, and substitution of Tris+ for Na+ significantly reduced 36Cl- uptake. These observations of Cl(-)-dependent 22Na+ uptake and Na(+)-dependent 36Cl- uptake demonstrated coupling between Na+ and Cl- absorption at the basolateral membrane. Na+/Cl- cotransport inhibitors, furosemide (1 mM) and bumetanide (0.1 mM), also reduced 22Na+ and 36Cl- uptakes by enriched basolateral vesicles. By contrast, furosemide and bumetanide did not affect 22Na+ and 36Cl- uptakes by enriched apical vesicles. Collectively, these observations support localization of Na+/Cl- cotransport to the basolateral membranes of mature type II pneumocytes.