After overnight fasing, levels of glucosetranpoers reduced in
Adipocytes (Ref: Harper 28/e, p138, 171 (26/e p160, 232); Lippincott Biochem 3/e p310; Ganong23/e p320] Overnight fasting causes insulin levels to fall. This decreases the number of glucose transpoers which are insulin dependent. Insulin dependent glucose transpoers are GLUT-4, present in adipocytes, skeletal and cardiac muscles. Rest of the glucose transpoers are not insulin dependent. Harper writes-"Glucose uptake into muscle and adipose tissue is controlled by insulin, which is secreted by the B islet cells of the pancreas in response to an increased concentration of glucose in the poal blood. An early response to insulin in muscle and adipose tissue is the migration of glucose transpoer vesicles to the cell surface, exposing active glucose transpoers (GLUT 4). These insulin-sensitive tissues will only take up glucose from the blood stream to any significant extent in the presence of the hormone. As insulin secretion falls in the starved state, so the transpoers are internalized again, reducing glucose uptake" Glucose Transpoers Glucose enters cells either by facilitated diffusion or by secondary active transpo with No+ (in the intestine and kidneys) Glucose transpoers are a family of membrane proteins that are responsible for facilitated diffusion of glucose across cell membranes. They differ from and have no homology with the sodium-dependent glucose transpoers, SGLT 1 and SGLT 2, responsible for the secondary active transpo of glucose in the intestine and renal tubules. Seven different glucose transpoers, named GLUT 1-7 in order of discovery. have been characterized. Each transpoer appears to have evolved for special tasks. GLUT 4 is the transpoer in muscle and adipose tissue that is stimulated by insulin. Glucose Transpoers in Mammals Function Major Sites of Expression Facilitated diffusion GLUT 1 Basal glucose uptake Brain, kidney, colon, placenta, erythrocytes GLUT 2 B-cell glucose sensor; transpo out of intestinal and renal epithelial cells B cells of pancreatic islets, liver, small intestine, kidneys GLUT 3 Basal glucose uptake Brain, placenta, kidneys GLUT 4 Insulin-stimulated glucose uptake Skeletal and cardiac muscle, adipose tissue GLUT 5 Fructose transpo Jejunum, sperm GLUT 6 None Pseudogene GLUT 7 Glucose 6-phosphate transpoer in endoplasmic reticulurn Liver, ? other tissues Secondary active transpo (Nal? glucose cotranspo) SGLT 1 Absorption of glucose Small intestine, renal tubules SGLT 2 Absorption of glucose Renal tubules
Brain cells
RBCs
Adipocytes
Hepatocytes