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BCHS-4361-09-DIABETE_38446

BCHS-4361-09-DIABETE_38446 - THE INSULIN SIGNALING NETWORK...

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THE INSULIN SIGNALING NETWORK The full network is complex and can be divided into five levels: Activation of the insulin receptor tyrosine kinase and closely linked events; Phosphorylation of a family of insulin receptor substrate (IRS) proteins; Interaction of the receptor and its substrates with several intermediate signaling molecules via SH2 (src homology 2) and other protein-protein recognition domains; Activation of serine and lipid kinases, resulting in a broad range of phosphorylation-dephosphorylation events; and Regulation of the final biological effectors of insulin action, such as glucose transport, lipid synthesis, gene expression, and mitogenesis.
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GLUCAGON (and INSULIN) Glucagon in many ways antagonizes effects of insulin. The hormone is a 29 amino acid single chain polypeptide. It is released from pancreatic α cells as proglucagon. Its concentration in blood is comparable to insulin. In contrast to the biphasic rate of insulin secretion, glucagon displays a gradual increase in secretion rate in response to a meal. However, in IDDM, glucagon rises to a peak more rapidly. Glucagon mainly acts in liver where it stimulates glycogenolysis, gluconeogenesis and ketogenesis. In contrast, insulin stimulates protein, glycogen and lipid synthesis, and prevents lipolysis and gluconeogenesis. It is therefore an anabolic hormone.
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I. Normal Regulation of Plasma Glucose Under normal conditions, the pancreas is the chief regulator of glucose metabolism, maintaining a balance between glucose production and glucose uptake The cells in the pancreas maintain normal plasma glucose levels by working in concert with the liver and peripheral tissue to control glucose production and glucose clearance. Glucose-stimulated insulin secretion suppresses hepatic glucose production and stimulates glucose uptake by muscle and adipose tissue In the fasting state, basal insulin secretion regulates hepatic glucose production. In the fed state, absorption of glucose from the gastrointestinal tract stimulates the β -cells in the pancreas to secrete more insulin. Meal-stimulated insulin secretion suppresses hepatic glucose production and also stimulates the uptake of glucose by muscle and adipose tissue. Any excess glucose that the body does not use for energy is converted by the liver and muscle to glycogen and stored for future use.
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II. Normal Regulation of Plasma Glucose When blood glucose levels return to normal following a meal, insulin secretion returns to its basal rate. If blood glucose levels fall below normal, the α -cells in the pancreas are stimulated to secrete the hormone glucagon, which signals the liver to convert stored glycogen back into glucose.
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