Sodium-coupled transport of glucose by plasma membranes of type II pneumocytes.
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 microM) 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.
Oelberg
D G
DG
Center for Pediatric Research, Eastern Virginia Medical School, Norfolk 23510-1001.
Xu
F
F
Shabarek
F
F
D000818:Animals; D002417:Cattle; D002458:Cell Fractionation; D002462:Cell Membrane / Q000378:metabolism*; D004058:Diffusion; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology; D008168:Lung / Q000378:metabolism*; D008564:Membrane Potentials; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D012979:Sodium Radioisotopes; D014316:Tritium
D009004:Monosaccharide Transport Proteins; D012979:Sodium Radioisotopes; D014316:Tritium; D012964:Sodium; D005947:Glucose