Glucose Transport

Overview

• Sodium/glucose cotransporter (SGLT)
  • function
    • transport glucose actively across lumen against concentration gradient
      • energy provided by transport of sodium down its concentration gradient
  • location
    • small intestine (SGLT1) 
      • 2:1 Na⁺:Glu
    • proximal tubule of nephron (SGLT2)
      • 1:1 Na⁺:Glu
• Glucose transporters (GLUT)
  • function
    • facilitated diffusion of glucose down concentration gradients  
      • with increasing glucose concentration, rate of transport assumes a hyperbolic curve due to carrier saturation 
    • divided into many subtypes that localize to different tissues
• GLUT-1
  • function
    • basal glucose uptake
      • high affinity
        • transporters saturated at normal blood glucose levels
        • ensures glucose entry to cells
  • location
    • wide distribution in tissues in the body (brain, erythrocytes, endothelial cells, cornea etc.)
• GLUT-2
  • function
    • low affinity glucose uptake
      • in the fasting state glucose does not enter cells
      • mediates glucose surplus storage in liver when blood glucose levels rise
      • facilitates insulin release in β-cells
  • location
    • hepatocytes
    • pancreatic β-cells
    • kidney
    • small intestines
• GLUT-3
  • function 
    • high affinity glucose uptake
      • glucose preferentially accessed by neurons in low-glucose states
  • location
    • brain
    • neurons
• GLUT-4 
  • function
    • insulin-controlled uptake of glucose 
    • basal level of glucose intake without insulin
      • presence of insulin ↑ translocation of transporters to the cell membrane
        • ↑↑↑ glucose uptake
        • also stimulated by exercise
  • location
    • adipocytes
    • myocytes
• cardiomyocytes