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HomeEndocrine PhysiologyInsulin Hormone - secretion, functions, regulation and facts

Insulin Hormone – secretion, functions, regulation and facts

Secretion

Insulin hormone is secreted from the pancreas as an endocrine function. The beta cells of islets of Langerhans of the pancreas release insulin with the stimulus of an increase in glucose.

Different cells of islets of Langerhans cell of the pancreas

There is the secretion of Glucagon by alpha cells, Insulin by beta cells, and somatostatin by delta cells of islets of Langerhans of the pancreas. These secreted hormones have a local effect on the regulation of each other secretion shown in the diagram below.

Glucagon has a stimulatory effect on both beta cells (insulin) and delta cells (somatostatin).

Insulin has an inhibitory effect on alpha cells that secrete glucagon.

Somatostatin has an inhibitory effect on both alpha cells and beta cells.

Relation of glucagon, insulin, and somatostatin in the secretion of each other hormones

Mechanism of insulin secretion from the beta cell of islets of Langerhans

Mechanism of insulin secretion. fig – Guyton and Hall Textbook of Physiology 12th edition

An increase in glucose level is sensed by beta cells – glucose is moved inside the cell by GLUT 2 channel and glycolysis takes place – release of ATP inside the cell that blocks ATP-dependent potassium channels- potassium is accumulated inside the cell increase its potential and depolarization of cell take place- depolarization cause entry of calcium – increase in calcium inside the cell cause release of insulin hormone by exocytosis process.

Function of insulin

It is an anabolic hormone. For the action of the insulin it binds with the tyrosine kinase receptor in the target cell- phosphorylation of enzymes that will cause different functions like fat synthesis, protein synthesis, and glucose synthesis. In simple language, it provides all the raw materials needed for fat synthesis, protein synthesis, and so on.

Mechanism of action of insulin hormone: fig – Guyton and Hall Textbook of Physiology 12th edition

Effects of Insulin on carbohydrate metabolism

After meals increase glucose uptake by peripheral tissues (skeletal muscle & adipose tissue by GLUT 4 channels by the action of insulin). There is the new formation and translocated of already formed GLUT 4 channels in the cell membrane of skeletal muscle and adipose tissue for the entry of glucose inside the cell and decrease the blood glucose level.

Note: In exercise conditions, there is entry of glucose inside the skeletal muscle without the help of insulin. This is the reason, why exercise is important in diabetes mellitus.

The action of insulin in skeletal muscle for GLUT 4 transport.

•In muscle- glucose is stored in glycogen

•In the liver- promote glycogen synthesis

•Excess glucose- converse into FA- packaged as triglyceride- transported & store in adipose tissue

•Inhibit gluconeogenesis- inhibiting gluconeogenic enzymes

Net effect- increase glycogen stored in the liver (100 gms), decrease glucose output

Effects of Insulin on fat metabolism

Increase fat synthesis

In the liver, excess carbohydrates- provide a substrate for fat synthesis.

store fat in adipose tissue

Effect of Insulin on protein metabolism

Transport amino acids into cells translation of mRNA, rate of transcription of selected DNA genetic sequence

Decrease rate of amino acid release from muscles & extrahepatic tissue

Net effect- promotes protein synthesis & inhibits protein catabolism

Regulation of insulin secretion

Secretion of insulin is stimulated by increased blood glucose levels, for example after eating a

meal, and to a lesser extent by parasympathetic stimulation, raised blood amino acid and fatty acid levels, and gastrointestinal hormones, e.g. gastrin, secretin, and cholecystokinin.

Secretion is decreased by sympathetic stimulation, glucagon, adrenaline, cortisol, and somatostatin  which is secreted by the hypothalamus and pancreatic islets.

Factors/Conditions that increase or decrease insulin secretion

Increase Insulin Secretion

•Increased blood glucose**

•Increased blood free fatty acids

•Increased blood amino acids

•GIT hormones – gastrin

•Glucagon, growth hormone, cortisol

•β-Adrenergic stimulation

•Insulin resistance; obesity

•Sulfonylurea drugs (glyburide, tolbutamide)

Decrease Insulin Secretion

•Decreased blood glucose

•Fasting

•Somatostatin

•α-Adrenergic activity

•Leptin

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