Abstract

The airway epithelium maintains differential glucose concentrations between the airway surface liquid (ASL, ~0.4mM) and the blood/interstitium (5-6mM) which is important for defence against infection. Glucose primarily moves from the blood to the ASL via paracellular movement, down its concentration gradient, across the tight junctions. However, there is evidence that glucose can move transcellularly across epithelial cells. Using a Förster Resonance Energy Transfer (FRET) sensor for glucose, we investigated intracellular glucose concentrations in airway epithelial cells and the role of hexokinases in regulating intracellular glucose concentrations in normo- and hyperglycaemic conditions. Our findings indicated that in airway epithelial cells (H441 or primary human epithelial cells HBEC) exposed to 5mM glucose (normoglycaemia), intracellular glucose concentration is in the mM range. Inhibition of facilitative glucose transport (GLUT) with Cytochalasin B reduced intracellular glucose concentration. When cells were exposed to 15mM glucose (hyperglycaemia), intracellular glucose concentration reduced. Airway cells expressed hexokinases 1, 2 and 3. Inhibition with 3-bromopyruvate decreased hexokinase activity by ~25% and elevated intracellular glucose concentration but levels remained in the mM range. Exposure to hyperglycaemia increased glycolysis, glycogen and sorbitol. Thus, glucose enters the airway cell via GLUT transporters and is then rapidly processed by hexokinase-dependent and hexokinase-independent metabolic pathways to maintain low intracellular glucose concentrations. We propose this prevents transcellular transport, aids the removal of glucose from the ASL and that the main route of entry for glucose into the ASL is via the paracellular pathway.