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GLUCAGON

GLUCAGON. GLUCAGON. Glucagon: is secreted when “Glucose is GONE” Peptide hormone made of 29 amino acids. MW: 3485 Has several functions that are dramatically opposite to Insulin

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GLUCAGON

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  1. GLUCAGON

  2. GLUCAGON • Glucagon: is secreted when “Glucose is GONE” • Peptide hormone made of 29 amino acids. • MW: 3485 • Has several functions that are dramatically opposite to Insulin • One injection of purified glucagon can have profound Hyperglycemic effects! Therefore, it is also called the HYPERGLYCEMIC HORMONE!

  3. GLUCAGON • SYNTHESIS: in the alpha cells of the Islets of Langerhans. Preproglucagon (158 AA) ↓ Glucagon + Major proglucagon fragment (in the alpha cells)

  4. GLUCAGON METABOLISM • Circulates without binding to carrier proteins. • Plasma half-life: 5 minutes • 25% of the Glucagon is destroyed during passage through the Liver. • Glucagon also degraded by the kidneys & the plasma peptides.

  5. ACTIONS OF GLUCAGON The physiological role of Glucagon is to stimulate hepatic production & secretion of glucose. It accomplishes this by: • Glycogenolysis • Increased Gluconeogenesis NOTE: • Muscle DOES NOT respond to Glucagon. • It exerts its effects on the Liver and Adipose tissues.

  6. Mechanism of Glycogenolysis Glucagon activates adenylyl cyclase in the hepatic cell membrane ↓ Formation of cyclic adenosine monophosphate (cAMP) ↓ Protein kinase regulator protein is activated ↓ Protien kinase is activated ↓ Phosphorylase b kinase is activated ↓ Phosphorylase b is converted into phosphorylase a ↓ Promotes degradation of Glycogen into Glucose-1-phosphate ↓ Glucose-1-phosphate is dephosphorylated ↓ Glucose is released from the liver

  7. MECHANISM OF GLUCONEOGENESIS • It increases the rate of amino acid uptake by the liver cells. • It then stimulates the conversion of many amino acids to Glucose. ↓ This is achieved by activating many enzymes required for AA transport & gluconeogenesis.

  8. OTHER EFFECTS: • It activates adipose cell lipase→ increased quantities of fatty acids are made available. • It inhibits storage of TG in the liver → this prevents the liver from removing fatty acids from the blood! • It increases the blood flow in some tissues such as kidneys. • Enhances bile secretion. • Inhibits gastric acid secretion.

  9. REGULATION OF GLUCAGON SECRETION • Increased blood glucose concentration is the most potent factor: It INHIBITS Glucagon secretion • Increased amino acids stimulate Glucagon secretion (same effect as on INSULIN!) • Decrease in fatty acid levels stimulate Glucagon secretion (opposite to the effect on Insulin). • Somatostatin inhibits Glucagon & Insulin secretion • Exhaustive exercise stimulates Glucagon secretion

  10. What happens when you take a protein only diet? • Glucagon release is stimulated by plasma amino acids. This pathway prevents hypoglycemia after ingestion of a pure protein meal. • If a meal contains protein but no carbohydrate, amino acids absorbed from the food cause insulin secretion. Even though no glucose has been absorbed, insulin-stimulated glucose uptake increases, and plasma glucose concentrations fall. • Unless something counteracts this process, the brain’s fuel supply is threatened by hypoglycemia. Co-secretion of glucagon in this situation prevents hypoglycemia by stimulating hepatic glucose output. • As a result, although only amino acids were ingested, both glucose and amino acids are made available to peripheral tissues.

  11. Glucagon Dominates in Fasting State Metabolism Figure 21-14: Endocrine response to hypoglycemia

  12. The Balance b/w insulin & Glucagon • Over-riding concern is glucose homeostasis : – must maintain sufficient levels for use by brain – other tissues adjust to other energy sources as necessary. Insulin is known as the “Hormone of Feasting”, while Glucagon is known as the “Hormone of Fasting”.

  13. Muscle: Fed State Glucose from circulation (Active muscle) Pyruvate Glycogen Glucose (Inactive muscle)

  14. Muscle: Fasting State 1st uses own glycogen stores 2nd absorbs fatty acids and ketone bodies Glycogen Glucose Pyruvate Fatty Acids and Ketone Bodies from Circulation Acetyl CoA

  15. Liver: Fed State Glucose from circulation Pyruvate Glycogen Glucose Fatty Acids released to Circulation Acetyl CoA

  16. Adipose: Fed State Glucose from circulation Pyruvate Glycogen Glucose Fatty Acids Acetyl CoA Triacylglycerol (Fat) Fatty Acids from Circulation

  17. Adipose: Fasting State Pyruvate Glycogen Glucose Fatty Acids and glycerol Acetyl CoA Triacylglycerol (Fat) Fatty Acids and glycerol released to circulation

  18. somatostatin

  19. SOMATOSTATIN • Secreted by the Delta cells of Islets of Langerhans • Polypeptide containing only 14 AA • Extremely short half life of 3 minutes • All matters related to food ingestion increase its secretion: -increased blood Glucose -increased fatty acids -increased amino acids -increased concentrations of GI hormones

  20. ACTIONS OF SOMATOSTATIN • It has paracrine function locally within the Islets of langerhans where it depresses the secretion of both Insulin & Glucagon • It decreases the motility of stomach, duodenum & gall bladder • It decreases both secretion & absorption in the GIT Principal role is to extend the period of time over which the food nutrients are absorbed into the blood by slowing its passage through the GIT!

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