Glucose enters the brain parenchyma across the blood-brain barrier primarily by which mechanism?

• A Insulin-dependent recruitment of GLUT4 to the endothelial membrane
• B Simple diffusion down its concentration gradient
• C Secondary active cotransport with sodium via SGLT1
• D Facilitated diffusion through GLUT1 transporters on endothelial cells ✓

Explanation

Cerebral capillary endothelial cells express abundant GLUT1, which mediates bidirectional, saturable, insulin-independent facilitated diffusion of glucose into the brain. Simple diffusion cannot account for the rapid, stereospecific transfer of this water-soluble molecule across lipid membranes. Insulin-dependent GLUT4 translocation governs uptake mainly in muscle and adipose tissue, and brain glucose consumption continues essentially normally in the absence of insulin, which eliminates option A.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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