| Both the anabolic hormone insulin and contractile activity stimulate the uptake of glucose into mammalian skeletal muscle. We examined the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the stimulation of hexose uptake in response to hormone and contraction. Phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate accumulate in skeletal muscle exposed to insulin but not hypoxia, which mimics stimulation of the contractile-dependent pathway of hexose transport activation. The fungal metabolite wortmannin, an inhibitor of PI 3-kinase, completely blocks the appearance of 3;Insulin stimulates glucose transport by recruiting Glut4 to the plasma membrane from intracellular storage sites in its target tissues. In unstimulated adipocytes, Glut1 resides both on the plasma membrane and in the cytoplasm whereas Glut4 is almost totally excluded from the plasma membrane. To explain the kinetics of glucose transporter trafficking, a model with two intracellular compartments and one plasma membrane compartment is proposed. Kinetic analysis of the model showed that glucose transporter can move back to the plasma membrane from either intracellular compartment.;The very high level of expression of Glut1 in brain microvessels suggests its importance in supplying glucose across the blood-brain barrier. This is supported by studies in patients with intractable seizures who exhibit decreased Glut1 activity. Quantitative Southern blot analysis and DNA polymorphism analysis of the Glut1 gene showed one patient with a single copy of the Glut1 gene.;Overexpressing or deleting a gene are ways to learn about the function of the gene in a physiological system. The knock-out technology allows the creation of mice deficient in specific genes, while DNA injection of zygotes allows overexpression of a gene in mice. Chapter 4 describes experiments in which we attempted to create Glut2 and Glut4 knock-out mice and rat Glut2 overexpressing mice. |
