Sodium-coupled transport of glucose by plasma membranes of type II pneumocytes

Abstract

Sodium-dependent absorption of alveolar fluid promotes efficient gas exchange. In animal models, alveolar glucose stimulates phlorizin-sensitive, Na +-dependent fluid absorption. It is hypothesized that Na +/glucose cotransporters are localized to apical membranes of type II pneumocytes. Enriched apical and basolateral plasma membrane vesicles were isolated from adult bovine type II pneumocytes. Uptakes of 22Na + and [ 3H]glucose by enriched apical and basolateral vesicles were monitored over time. Following addition of external glucose (75 mM), 22Na + uptake by mannitol-loaded, apically-enriched vesicles was significantly increased over controls. Substitution of interior-negative charge gradients for internally directed Na + gradients increased glucose-dependent Na + uptakes even greater. By contrast, external glucose did not significantly promote 22Na + uptake by enriched basolateral vesicles. External Na + (75 mM) significantly increased [ 3H]glucose uptakes by enriched apical vesicles with evidence of overshoot. Phlorizin (100 μM) inhibited both glucose-coupled 22Na + uptakes and Na +-coupled [ 3H]glucose uptakes. These observations support localization of electrogenic, Na +/glucose cotransporters to enriched apical membranes of mature type II pneumocytes.