The Mammalian Brain Glucose Transport System
Glucose is an essential substrate for brain oxidative metabolism (1,2). Circulating glucose crosses the blood-brain barrier and accesses the brain parenchymal cells. Glucose, being a polar substance, crosses lipid bilayers of cell plasma membranes (3,4) by a saturable stereo-specific carrier system (2,5,6,7). This carrier system consists of a family of closely related membrane associated glycoproteins termed the glucose transporters (GTs) (8–15). In most tissues examined, the facilitative type of GTs transport glucose intracellularly. Besides intracellular transport, these GTs are capable of transcellular transport and transport of glucose outside the cell (16). Structurally they consist of 12 transmembraneous domains with the amino and carboxyl termini facing the cytoplasmic surface of the cell, and a glycosylation site on the exofacial domain located between the first and second transmembraneous segments (8,16). While there is considerable primary sequence homology between the different GT isomers, their tissue specific expression and Km varies based on the individual tissue’s glucose needs and function of the specific GT (16). Typically the facilitative GTs are classified into five major types: Glut 1 (erythrocyte/Hep G2/rat brain type; Km - 1–2 mM) (8,9) and Glut 3 (fetal skeletal muscle/brain/placenta type; Km - ?) (10) are the insulin-insensitive types, present almost ubiquitously in most tissues examined and are responsible mainly for the basal transport of glucose. Glut 2 (liver/pancreatic beta islet cell type; Km - 15 mM) is capable of bidirectional transfer of glucose and mainly located in hepatocytes and the pancreatic beta islet cells (11,12).
KeywordsGlial Cell Glucose Transporter Cerebral Glucose Utilization Pancreatic Beta Islet Cell Brain Parenchymal Cell
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