Tuesday, February 28, 2017

Insulin

Insulin is a polypeptide hormone produced, stored and secreted in Beta cells of the islets of Langerhans, in the pancreas (in a histological section it is seen that they occupy the central part). It is an anabolic hormone that acts at the level of the liver, adipose tissue and with influence in the brain.
This protein has two polypeptide chains, with 21 amino acids in the A chain and 30 in the B chain, joined by disulfide bonds, which gives a greater stability and a correct folding. It begins to be produced in the form of pre-pro-insulin which, by action of the peptidase is cleaved to form the proinsulin. The proteolytic cleavage of peptide C forms the two chain bioactive insulin, which is stored in secretory granules for subsequent insulin secretion.Its main function is to regulate blood glucose levels in a context of hyperglycemia. In this way, glucose acts as a biochemical signal that triggers its secretion. Thus, when carbohydrate-containing foods are absorbed, glucose is metabolized to ATP and this in turn triggers insulin secretion. Protein-protein interactions and phosphorylations are used to transmit the signal. In adipose tissue and muscle, the binding of insulin to membrane receptors triggers the displacement of GLUT4-rich vesicles that fuse with the membrane, increasing cell uptake, being an insulin-dependent transport.
On the other hand, in the liver, insulin activates the enzyme glycokinase, which is responsible for the conversion of glucose into glucose-6-phosphate; Guarantees an intracellular concentration of glucose lower than the extracellular concentration and, therefore, a gradient of glucose concentration favorable to its entry into these cells, through the GLUT-2 transporter, following metabolization by glycolysis, Krebs and the respiratory chain to produce ATP. Thus, after food intake, glucose is absorbed into the intestines and is released into the bloodstream, causing blood concentrations to rise, leading to transient hyperglycemia. The pancreas releases insulin to lower glucose concentration, allowing glucose to be consumed by the cells, as well as stimulating the storage of glucose in the liver in the form of glycogen; The liver also metabolize glucose into triacylglycerols, transported as VLDL to be stored in adipose tissue, which are useful reserves in fasting situations. Signal transmission ceases, at meal time, by dephosphorylation of the insulin receptor by protein tyrosine phosphatase.
In summary, insulin stimulates glycogenesis, fatty acid synthesis and glycolysis and inhibits antagonistic pathways: glycogenolysis, fatty acid degradation and hepatic gluconeogenesis. It also stimulates protein synthesis. It has action on inherent enzymes as well as effects on gene transcription. It also acts on specific receptors in the hypothalamus to inhibit the act of eating, thus regulating feeding and energy conservation.
Inborn errors of beta cell metabolism can produce excessive or defective production of insulin by gene mutations (GCK), Kir 6.2 alterations, or insulin synthesis transcription factors, respectively. Increased glucose leads to increased osmotic pressure, glycation of proteins and formation of reactive oxygen species (EROS).
Diabetes is the metabolic disease characterized by increased blood sugar: It may be Type I - in which the body stops producing insulin by destroying the B cells of the pancreas. It is important to check for symptoms of polydipsia, fruity aroma breathing, blood glucose and blood ketones levels. Essential therapies focus on insulin therapy, fluid replacement, replacement of electrolytes and nourishment. In turn, in Type II diabetes, the cells do not produce enough insulin to lower the concentration of gucose or there is a condition of insulin resistance. Adipocytes, myocytes and hepatocytes do not respond correctly. It presents symptoms similar to type I but more gradual. It is necessary to test for fasting blood glucose and for abnormal levels to continue the investigation for glycemic curve; glycated hemoglobin, control alcohol consumption, etc.
They can lead to complications such as diabetic retinopathy, atherosclerosis, diabetic nephropathy, neuropathy, myocardial infarction/stroke, infections - leucocytes less effective in hyperglycemia, hypertension and oxidation of blood vessels. There are currently several drugs on the market that address problems with insulin, as well as different types of injectable insulin depending on the cause of the disease and the purpose of action.


Text written by:
Denilson Araújo
Prescília Sampa
Solange da Costa
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