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 that glucose enters the cells and stores its excess. During the fasting period, 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 provide the breakdown of 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 released in opposite states: insulin in hyperglycemia, glucagon in hypoglycemia. The half-life of insulin is 3-4 minutes, its constant varying secretion maintains the level of glucose in the blood in narrowwithin.
Effects of insulin
Insulin regulates metabolism, primarily the concentration of glucose. It affects 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 DNA and RNA synthesis,
- stimulates protein synthesis,
- increases the stimulation of the enzyme glycogen synthase (ensures the synthesis of glycogen from glucose - glycogenesis),
- stimulates glucokinase (enzyme that promotes the conversion of glucose into glycogen in conditions of its excess),
- inhibits glucose-6-phosphatase (an enzyme that catalyzes the conversion of glucose-6-phosphate to free glucose and thus raises blood sugar),
- stimulates lipogenesis,
- inhibits lipolysis (due to inhibition of cAMP synthesis),
- stimulates the synthesis of fatty acids,
- activates Na+/K+-ATP-ase.
The role of insulin in the transport of glucose into cells
Glucose enters the cells with the help of special transporter proteins (GLUT). Numerous GLUTs are localized in different cells. In the cell membranes of skeletal and cardiac muscles, adipose tissue, leukocytes, and the cortical layer of the kidneyswork insulin-dependent transporters - GLUT4. Insulin transporters in the membranes of the cells of the central nervous system and the liver are nsulin-independent; therefore, the supply of cells of these tissues with glucose depends only on its concentration in the blood. In the cells of the kidneys, intestines, erythrocytes, glucose enters without carriers at all, by passive diffusion. Thus, insulin is necessary for the entry of glucose into the cells of adipose tissue, skeletal muscle and cardiac muscle. With a lack of insulin, only a small amount of glucose will enter the cells of these tissues, insufficient to meet their metabolic needs, even in conditions of high blood glucose concentration (hyperglycemia).
The role of insulin in glucose metabolism
Insulin stimulates glucose utilization through several mechanisms.
- Increases the activity of glycogen synthase in liver cells, stimulating the synthesis of glycogen from glucose residues.
- Increases the activity of glucokinase in the liver, stimulating the phosphorylation of glucose 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 to the extracellular space.
- Inhibits liver phosphatase, which catalyzes the reverse conversion of glucose-6-phosphate to free glucose.
All of the above processes ensure the uptake of glucose by the cells of peripheral tissues and reduce its synthesis, which leads to a decrease in the concentration of glucose in the blood. In addition, increased glucose utilization by cells preserves the 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 protein synthesis in them. Protein synthesis is stimulated in two ways:
- due to mRNA activation,
- by increasing the supply of amino acids to the cell.
In addition, as mentioned above, increased use of glucose as an energy substrate by the cell slows down the breakdown of protein in it, which leads to an increase in protein reserves. 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
The membrane and intracellular effects of insulin lead to an increase in fat stores in adipose tissue and the liver.
- Insulin ensures 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 flow of the resulting fatty acids into adipose tissue cells.
- Inhibits intracellular lipoprotein lipase, thus inhibiting lipolysis in cells.
Glucagon functions
Glucagon affects carbohydrate, protein and fat metabolism. It can be said that glucagon is an insulin antagonist in terms of its effects. The main result of the work of glucagon is an increase in the concentration of glucose in the blood. It is glucagon that maintainsthe required level of energy substrates - glucose, proteins and fats in the blood during the fasting period.
1. The role of glucagon in carbohydrate metabolism.
Provides glucose synthesis by:
- enhancement of glycogenolysis (breakdown of glycogen to glucose) in the liver,
- increased 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 the liver cells:
- protein synthesis,
- synthesis of glucose from amino acids – gluconeogenesis.
3. The role of glucagon in fat metabolism.
The hormone activates lipase in adipose tissue, as a result, the level of fatty acids and glycerol in the blood rises. This eventually again leads to an increase in the concentration of glucose in the blood:
- glycerol as a non-carbohydrate precursor is included in the process of gluconeogenesis - glucose synthesis;
- fatty acids are converted to ketone bodies, which are used as energy substrates, thus conserving glucose reserves.
The relationship of hormones
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 job. The stimulus for the production of insulin is an increase in the concentration of glucose in the blood, glucagon - a decrease. In addition, the production of insulin inhibits the secretion of glucagon.
If the synthesis of one of these hormones is disturbed, the other begins 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 characterizes this pathology.
Incorrect production of hormones, their incorrect ratio leads to errors in nutrition. Abuse of protein foods stimulates excess secretion of glucagon, simple carbohydrates - insulin. The appearance of an imbalance in the level of insulin and glucagon leads to the development of pathologies.
