Glucagon and insulin are pancreatic hormones. The function of all hormones is the regulation of metabolism in the body. The main function of insulin and glucagon is to provide the body with energy substrates after meals and during fasting. After eating, it is necessary to ensure the flow of glucose into the cells and the storage of its excess. During fasting - to extract glucose from reserves (glycogen) or synthesize it or other energy substrates.
It is widely believed that insulin and glucagon break down carbohydrates. This is not true. Enzymes break down substances. Hormones regulate these processes.
Synthesis of glucagon and insulin
Hormones are produced in the endocrine glands. Insulin and glucagon - in the pancreas: insulin in β-cells, glucagon - in α-cells of the islets of Langerhans. Both hormones are protein in nature and are synthesized from precursors. Insulin and glucagon are secreted in opposite conditions: insulin for hyperglycemia, glucagon for hypoglycemia. The half-life of insulin is 3-4 minutes, its constant varying secretion ensures the maintenance of blood glucose levels within narrow limits.
Insulin effects
Insulin regulates metabolism, especially glucose concentration. It affects the membrane and intracellular processes.
Membrane effects of insulin:
- stimulates the transport of glucose and a number of other monosaccharides,
- stimulates the transport of amino acids (mainly arginine),
- stimulates the transport of fatty acids,
- stimulates the absorption of potassium and magnesium ions by the cell.
Insulin has intracellular effects:
- stimulates the synthesis of DNA and RNA,
- stimulates protein synthesis,
- enhances the stimulation of the enzyme glycogen synthase (provides synthesis of glycogen from glucose - glycogenesis),
- Stimulates glucokinase (an enzyme that promotes the conversion of glucose to glycogen in conditions of its excess),
- inhibits glucose-6-phosphatase (an enzyme that catalyzes the conversion of glucose-6-phosphate into free glucose and, accordingly, increases blood sugar),
- stimulates lipogenesis,
- inhibits lipolysis (due to inhibition of cAMP synthesis),
- stimulates the synthesis of fatty acids,
- activates Na + / K + -ATPase.
The role of insulin in glucose transport to cells
Glucose enters the cells using special transporter proteins (GLUT). Numerous GLUTs are localized in different cells. In the membranes of skeletal and cardiac muscle cells, adipose tissue, white blood cells, and the renal cortex, insulin-dependent transporters GLUT4 work. The transporters of insulin in the membranes of the central nervous system and liver cells are not insulin independent, therefore, the supply of glucose to these cells depends only on its concentration in the blood. In cells of the kidneys, intestines, red blood cells, glucose enters without carriers at all, by passive diffusion. Thus, insulin is necessary for glucose to enter the cells of adipose tissue, skeletal muscle and heart muscle. With a lack of insulin, only a small amount of glucose will fall into the cells of these tissues, insufficient to ensure their metabolic needs, even in conditions of a high concentration of glucose in the blood (hyperglycemia).
The role of insulin in glucose metabolism
Insulin stimulates glucose utilization, including several mechanisms.
- Increases glycogen synthase activity in liver cells, stimulating the synthesis of glycogen from glucose residues.
- Increases glucokinase activity in the liver, stimulating glucose phosphorylation with the formation of glucose-6-phosphate, which "locks" glucose in the cell, because it is not able to pass through the membrane from the cell into the intercellular space.
- It inhibits liver phosphatase, which catalyzes the reverse conversion of glucose-6-phosphate to free glucose.
All of these processes ensure the absorption of glucose by cells of peripheral tissues and a decrease in its synthesis, which leads to a decrease in the concentration of glucose in the blood. In addition, increased glucose utilization by cells retains reserves of other intracellular energy substrates - fats and proteins.
The role of insulin in protein metabolism
Insulin stimulates both the transport of free amino acids into cells and the synthesis of protein in them. Protein synthesis is stimulated in two ways:
- due to activation of mRNA,
- by increasing the flow of amino acids into the cell.
In addition, as mentioned above, the increased use of glucose as an energy substrate by a cell slows down the breakdown of protein in it, which leads to an increase in protein stores. Due to this effect, insulin is involved in the regulation of the development and growth of the body.
The role of insulin in fat metabolism
Membrane and intracellular effects of insulin lead to an increase in fat stores in adipose tissue and liver.
- Insulin provides the penetration of glucose into the cells of adipose tissue and stimulates its oxidation in them.
- Stimulates the formation of lipoprotein lipase in endothelial cells. This type of lipase ferments the hydrolysis of triacylglycerols associated with blood lipoproteins and ensures the receipt of the resulting fatty acids in the cells of adipose tissue.
- It inhibits intracellular lipoprotein lipase, thus inhibiting lipolysis in cells.
Glucagon Functions
Glucagon affects carbohydrate, protein and fat metabolism. We can say that glucagon is an insulin antagonist in terms of its effects. The main result of glucagon is an increase in blood glucose concentration. It is glucagon that ensures the maintenance of the necessary level of energy substrates - glucose, proteins and fats in the blood during fasting.
1. The role of glucagon in carbohydrate metabolism.
Provides glucose synthesis by:
- increased glycogenolysis (breakdown of glycogen to glucose) in the liver,
- enhancing gluconeogenesis (synthesis of glucose from non-carbohydrate precursors) in the liver.
2. The role of glucagon in protein metabolism.
The hormone stimulates the transport of glucagon amino acids to the liver, which contributes to liver cells:
- protein synthesis
- glucose synthesis from amino acids - gluconeogenesis.
3. The role of glucagon in fat metabolism.
The hormone activates lipase in adipose tissue, resulting in increased levels of fatty acids and glycerin in the blood. This ultimately again leads to an increase in the concentration of glucose in the blood:
- glycerin as a non-carbohydrate precursor is included in the process of gluconeogenesis - glucose synthesis;
- fatty acids are converted into ketone bodies, which are used as energy substrates, which preserves glucose reserves.
Hormone relationship
Insulin and glucagon are inextricably linked. Their task is to regulate the concentration of glucose in the blood. Glucagon provides its increase, insulin - a decrease. They do the opposite work. The stimulus for insulin production is to increase the concentration of glucose in the blood, glucagon - a decrease. In addition, insulin production inhibits the secretion of glucagon.
If the synthesis of one of these hormones is disrupted, the other starts to work incorrectly. For example, in diabetes mellitus, the level of insulin in the blood is low, the inhibitory effect of insulin on glucagon is weakened, as a result, the level of glucagon in the blood is too high, which leads to a constant increase in blood glucose, which is what this pathology characterizes.
Errors in nutrition lead to incorrect production of hormones, their incorrect ratio. The abuse of protein foods stimulates the excessive secretion of glucagon, and simple carbohydrates - insulin. The appearance of an imbalance in the level of insulin and glucagon leads to the development of pathologies.