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effect on glucose metabolism

1.1 Insulin Only about 1-2% of the pancreatic structure is endocrine tissues which are represented by the presence of 1-2 million islets of langerhans. These islets contain four main types of cells (A, B, D, and F cells). Insulin is secreted by B (beta) cells which constitute about 60-70% of the islets www.intechopen.com 92 Gestational Diabetes cells. Insulin is a 51-amino acid polypeptide (small protein) hormone consist of A and B-chains connected together by disulphide bridges (Ganong, 2003; Guyton & Hall, 2006). 1.2 Insulin Receptor (IR) The IR is a large heterotetrameric, transmembrane glycoprotein, having a molecular weight of about 300,000. Each receptor consists of two alpha (┙) subunit that lie outside the cell membrane and two beta (┚) subunits that penetrate the cell membrane protruding into the cytoplasm connected together by disulphide bridges in a ┚-┙- ┙-┚ configuration. IR is assembled from a single polypeptide pro-receptor, by dimerization, proteolytic cleavage, and glycosylation within the cytoplasm and Golgi apparatus, before trafficking of the mature receptor to the plasma membrane. These insulin receptors have also been designated recently as CD220 (cluster of differentiation 220) (Ganong, 2003; Guyton & Hall, 2006; Ward & Lawrence, 2009). 1.3 Insulin action Insulin has many metabolic functions such as enhancing cellular uptake of glucose, fatty acids, amino acids, and potassium ions. It also has an anabolic action by increasing cellular formation of glycogen, lipids, and protein. These physiological functions will be reversed if insulin action is decreased as seen with the increase in insulin resistance during pregnancy. The main function of insulin concerning gestational diabetes mellitus (GDM) is its action on glucose and lipid metabolism. 1.3.1 Insulin effect on lipid metabolism Normally insulin stimulates the synthesis and release of lipoprotein lipase from the endothelial cells of blood vessels causing lipolysis of triglycerides in the blood and release of free fatty acids (FFA). Insulin enhances the transport of FFA to the fatty cells (adipocytes) to be stored as lipids. Furthermore, insulin inhibits lipoprotein lipase in adipose cells preventing lipolysis.