Insulin is secreted by the organ pancreas. The physiological actions of insulin can be categorized as rapid, intermediate and delayed. The rapid actions are those which occur within minutes and are mainly, increased transport of glucose, aminoacids and potassium ions into insulin sensitive cells. The intermediate actions are those which are seen within minutes after release of insulin. These are mainly stimulation of protein synthesis, inhibition of protein degradation, activation of glycolytic enzymes and glycogen synthase and inhibition of phosphorylase and gluconeogenic enzymes. The delayed actions are seen after hours of release.
One of the main delayed action is an increase in mRNAs for lipogenic and other enzymes (Ganong 2003). The main actions of insulin are seen in adipose tissue, skeletal, cardiac and smooth muscles and on the liver. In the adipose tissue, the insulin increases glucose entry, fatty acid synthesis, glycerol phosphate synthesis and triglyceride deposition. It activates lipoprotein lipase and inhibits hormone sensitive lipase. It also increases potassium uptake. In the muscle, glucose entry is increased. The hormone also increases glycogen synthesis, amino acid uptake, ketone uptake, potassium uptake and protein synthesis in the ribosomes.
It decreases protein catabolism and release of gluconeogenic aminoacids. In the liver, insulin decreases ketogenesis and glucose output secondary to decreased gluconeogenesis and increased glycolysis. It also increases protein and lipid synthesis (Ganong 2003). Insulin acts by acting on the insulin receptors which are found in many different cells in the body. The insulin secretion is rather regulated by the glucose concentration in blood. It is secreted in primarily in response to elevated blood concentrations of glucose. The normal fasting blood glucose concentration in humans is 80 to 90 mg per 100 ml and is associated with very low levels of insulin secretion.
Other stimuli which increase insulin secretion are some neural stimuli like sight and taste of food and increased blood concentrations of other fuel molecules, including amino acids and fatty acids (Ganong 2003).
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