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| | **During fasting, the citric acid cycle turns slowly as oxaloacetate is removed to make glucose. Therefore, acetyl CoA (produced from lipolyis and protein catabolism) builds up and is converted to ketone bodies. [[Ketosis|Ketosis]] may occur if the situation is prolonged. | | **During fasting, the citric acid cycle turns slowly as oxaloacetate is removed to make glucose. Therefore, acetyl CoA (produced from lipolyis and protein catabolism) builds up and is converted to ketone bodies. [[Ketosis|Ketosis]] may occur if the situation is prolonged. |
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| − | ===Insulin=== | + | ===[[Insulin]]=== |
| − | [[Image:Pancreatic Beta Cell.jpg|thumb|right|150px|Pancreatic Beta Cell - © RVC 2008]] | |
| − | [[Image:Insulin Secretion Pattern.jpg|thumb|right|150px|Insulin Secretion Pattern - © RVC 2008]]
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| − | *Insulin secretion is stimulated under conditions of hyperglycaemia and inhibited under conditions of hypoglycaemia.
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| − | *It has an ''anabolic'' effect on energy sources.
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| − | *It is a small polypeptide consisting of two chains; A and B; which are linked by disulphide bonds.
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| − | **It is synthesised inactively as a pre-pro-hormone which undergoes cleavage to produce the active form.
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| − | *Insulin is released when blood glucose levels are high. The process of insulin release occurs as follows:
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| − | **GLUT 2, which only operate at high glucose concentrations, are present in the plasma membranes of β cells.
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| − | **After a meal, glucose concentrations in the blood rise and glucose is taken into the cell.
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| − | **Glucose is metabolised inside the β cell which produces ATP.
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| − | **ATP inhibits potassium channels which depolarises the plasma membrane.
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| − | **Depolarisation causes calcium signalling, which stimulates insulin to be released.
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| − | *Pre-formed granules of insulin are release initially, but this supply soon diminishes. This produces a decrease in plasma insulin levels. Insulin levels are restored as de novo insulin is synthesised and secreted.
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| − | *Insulin exerts it's anabolic effect on the liver, adipocytes and skeletal muscle.
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| − | **It binds to receptors on the cell membrane, which triggers an intracellular signalling cascade.
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| − | **This causes preformed granules containing GLUT 4 receptors to be transported and expressed at the cell membrane.
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| − | **Cells can now take up glucose from the blood and store it as glycogen or fat.
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| − | *Increased glucose levels inside the cell increases the rate of glycogenesis and an increase in the rate of production of triacylglycerides.
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| − | *There is an increase in basal metabolic rate in other tissues.
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| − | **Insulin regulates the reaction in glycolysis where PEP is converted into pyruvate. This reaction is regulated by the enzyme pyruvate kinase. Insulin '''activates''' this enzyme by ''dephosphorylation'' via protein phosphatase 1 (PP1). This increases metabolism.
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| | ===[[DM|Diabetes Mellitus]]=== | | ===[[DM|Diabetes Mellitus]]=== |