D- and L-glucose transport across the pulmonary epithelium

Previously we observed what appeared to be augmented D-glucose transport across the pulmonary epithelium. To investigate this phenomenon we placed fluid containing L-[³H]glucose and D-[U-¹⁴C]glucose in the alveoli of isolated Ringer-perfused lungs from 4-wk-old rabbits. The appearance of radioactivity in recirculating glucose-free perfusate was measured. ³H appearing in the perfusate was associated with L-glucose. ¹⁴C, however, was associated with three compounds, with approximate molecular weights of 180 (glucose), 300, and 560. The nonglucose species were not identified. This ¹⁴C movement was inhibited by phlorizin, but not phloretin, in the alveolar fluid. A similar pattern of ¹⁴C movement occurred when D-[U-¹⁴C]glucose was replaced with 2-deoxy-D[U¹⁴C]-glucose, but not with methyl-α-D-[U-¹⁴C]glucopyranoside. The activation energy of the ¹⁴C metabolism-transport process was found to be 34 kcal/mol, and L-glucose transport showed an unusual temperature dependence, with maximum conductance at 15°C. It appears that some D-glucose crosses the pulmonary epithelium as does L-glucose. However, most enters epithelial cells and is incorporated into larger molecules which enter the vascular but not the alveolar space.