Figure 14.

Overview of glucose transport in the lung epithelial cells. In normal state, glucose is transcellularly transported from bloodstream into the ALS by specialized basolateral transporters GLUT, and is controlled by the intracellular hexokinase activity. Glucose is also transported by a passive paracellular diffusion. The latter is limited through tight junctions which link epithelial cells and maintain the tissue cohesion and integrity, rendering the paracellular diffusion extremely low. Reuptake of glucose from ASL occurs through apical GLUT or SGLT1 transporters. The overall glucose transfer results in ALS concentration of glucose 10–12 times lower than in blood. Conversely, in hyperglycemia where glucose concentration in the blood is higher (right panel), the glucose gradient is increased toward ASL leading to augmented transcellular transport. Additionally, defective tight junctions result in higher paracellular permeability. In such condition, the reuptake of glucose and rapid hexokinase metabolism of epithelial cells is then not sufficient to maintain a low concentration of glucose in the ASL, that could dramatically increase. GLUT, glucose transporter; SGLT, sodium-glucose linked transporter; Na⁺, sodium ion.