PATHWAYS FOR GLUCOSE-TRANSPORT IN TYPE-II PNEUMOCYTES FRESHLY ISOLATED FROM ADULT GUINEA-PIG LUNG

Previous in vivo studies of sugar transport across the mature pulmonary epithelium have provided evidence for the existence of a specific phlorizin-inhibitable, sodium-dependent transport process for D-glucose, although no direct evidence for the cellular location of this transport system in fresh cells has been shown to date. With the use of elastase digestion and lectin agglutination, a pure preparation of type II alveolar epithelial cells was isolated from adult guinea pig lung. This preparation always contained >90% type II cells and typically showed approximately 85% cell viability 2-3 h after the isolation procedure had begun. At 37-degrees-C, cells showed specific [H-3]phlorizin binding that was attenuated by D-glucose and completely abolished by sodium replacement. Substantial accumulation of the hexose [C-14]methyl alpha-D-glucopyranoside (C-14-labeled AMG), a substrate specific for the sodium-dependent glucose cotransporter was found in the presence of extracellular sodium; this accumulation above equilibrium was abolished on removal of sodium, addition of phlorizin, or in the presence of a saturating concentration (69 mM) of D-glucose. The apparent inhibition constant (K(i)) for glucose inhibition of AMG uptake was 0.4 mM and for phlorizin, 0.5 muM. The Hill plot of sodium activation of AMG uptake gave a coefficient of 2.8, suggesting cooperativness between sodium and AMG transport. 3-O-[C-14]methyl-D-glucose (3-OMG) transport was also blocked by phlorizin. Phloretin, in the presence of phlorizin, slowed the initial rate of entry but did not affect the equilibrium that was attained in the presence of phlorizin alone. These findings are consistent with the existence in freshly isolated type II pneumocytes of two routes for glucose transport. One is sodium dependent, with a high affinity for phlorizin binding, whereas the other is facilitative.