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Insulin-receptor interaction

1.3.2 Insulin effect on glucose metabolism Insulin enhances entrance of glucose to the cells through its action on the insulin receptors. Insulin receptor complex will stimulates mobilization of glucose carrier protein (GLUT- 4 transporter) from the interior of the cell to the plasma membrane which will transport glucose inside the cell by the process of facilitated diffusion. Furthermore, insulin-receptor complex will activates the storage of some glucose as glycogen while others will be metabolized into pyruvate and then fatty acids which are stored as triglycerides (fat) (Ganong, 2003; Guyton & Hall, 2006). 1.4 Insulin-receptor interaction To initiate insulin effects on target cells, it first binds with and activates a membrane receptor protein. [4] It is the activated receptor, not the insulin that causes the subsequent effects. The combination of insulin with the alpha subunits will induce autophosphorylation of the beta subunits which will activates a local tyrosine kinase [(phosphatidylinositol 3- kinase (PI3-K)] , which in turn begins a cascade of cell phosphorylation that increase or decrease the activity of enzymes, including insulin receptor substrates (IRSs). There are different types of IRSs (IRS-1, IRS-2, and IRS-3) which are expressed in different tissues Pathophysiology of Gestational Diabetes Mellitus: The Past, the Present and the Future 93 which explain the diversity of insulin action, activating or inactivating certain enzymes to produce the desired effect on the cellular carbohydrate, fat, and protein metabolism (Zwick et al., 2001; Pawson, 1995; Hans-Georg,1995; Perz & Torlińska, 2001). Within seconds after insulin binds with its membrane receptors, glucose transporters are moved to the cell membrane to facilitate glucose entry into the cell especially to the muscle and adipose tissues (Guyton & Hall, 2006; Sherwood, 2010). 2. Physiology of pregnancy The endocrinology of human pregnancy involves endocrine and metabolic changes that result from physiological alterations at the boundary between mother and fetus, known as the feto-placental unit (FPU), this interface is a major site of protein and steroid hormone production and secretion. Many of the endocrine and metabolic changes that occur during pregnancy can be directly attributed to hormonal signals originating from the FPU (Ganong, 2003; Guyton & Hall, 2006; Monga & Baker, 2006). During early pregnancy, glucose tolerance is normal or slightly improved and peripheral (muscle) sensitivity to insulin and hepatic basal glucose production is normal (Catalano et al., 1991; Catalano et al., 1992; Catalano et al., 1993). These could be caused by the increased maternal estrogen and progesterone in early pregnancy which increase and promote pancreatic ß-cell hyperplasia (Expansion of beta-cell mass in response to pregnancy) causing an increased insulin release (Carr & Gabbe, 1998; Rieck & Kaestner, 2010). This explains the rapid increase in insulin level in early pregnancy, in response to insulin resistance. In the second and third trimester, the continuous increase in the feto-placental factors will decrease maternal insulin sensitivity, and this will stimulate mother cells to use sources of fuels (energy) other than glucose as free fatty acids, and this will increase supply of glucose to the fetus (Catalano et al., 1991; Catalano et al., 1992; Ryan & Enns, 1988). In the normal physiological conditions, the fetal blood glucose is 10-20% less than maternal blood glucose allowing the transport of glucose in the placenta to the fetal blood by the process of simple diffusion and facilitated transport. Therefore, glucose is the main fuel required by the developing fetus, whether as a source of energy for cellular metabolism or to provide energy for the synthesis of protein, lipids, and glycogen. During pregnancy, the insulin resistance of the whole body is increased to about three times the resistance in the non-pregnant state. In general, the resistance to insulin can be characterized as pre-receptor (insulin antibodies) as in autoimmune diseases, receptor (decreased number of receptors on the cell surface) as in obesity, or post-receptor (defects in the intracellular insulin signaling pathway). In pregnancy, the decreased insulin sensitivity is best characterized by a post-receptor defect resulting in the decreased ability of insulin to bring about SLC2A4 (GLUT4) mobilization from the interior of the cell to the cell surface (Catalano, 2010). This could be due to increase in the plasma levels of one or more of the pregnancy-associated hormone