SODIUM-COUPLED TRANSPORT OF GLUCOSE BY PLASMA-MEMBRANES OF TYPE-II PNEUMOCYTES

Sodium-dependent absorption of alveolar fluid promotes efficient gas e xchange. In animal models, alveolar glucose stimulates phlorizin-sensi tive, Na+-dependent fluid absorption. It is hypothesized that Na+/gluc ose cotransporters are localized to apical membranes of type II pneumo cytes. Enriched apical and basolateral plasma membrane vesicles were i solated from adult bovine type II pneumocytes. Uptakes of Na-22(+) and [H-3]glucose by enriched apical and basolateral vesicles were monitor ed over time. Following addition of external glucose (75 mM), Na-22(+) uptake by mannitol-loaded, apically-enriched vesicles was significant ly increased over controls. Substitution of interior-negative charge g radients for internally directed Na+ gradients increased glucose-depen dent Na+ uptakes even greater. By contrast, external glucose did not s ignificantly promote Na-22(+) uptake by enriched basolateral vesicles. External Na+ (75 mM) significantly increased [H-3]glucose uptakes by enriched apical vesicles with evidence of overshoot. Phlorizin (100 mu M) inhibited both glucose-coupled Na-22(+) uptakes and Na+-coupled [3 H]glucose uptakes. These observations support localization of electrog enic, Na+/glucose cotransporters to enriched apical membranes of matur e type II pneumocytes.