It’s interesting that the growth hormone remains in blood just for few minutes. But this time is quite enough for introduction into the liver, where it’s transformed into the growth factors. The most important one is insulin-like growth factor-1 (IGF-1), also known as somatomedine-C. Exactly IGF-1, not the growth hormone itself, whose level can vary widely throughout the day, is used as a measure of how much hormone is secreted by the body.
Insulin-like growth factor 1 (IGF-1) consists of 70 amino acids that form a chain with intermolecular bridges. It is a peptide that binds to blood plasma proteins, so-called growth factor carriers. They allow somatomedine to keep its activity much longer. It lasts for several hours, while in free form this period is not more than 30 minutes. The hormone is similar to proinsulin, for which it received its name. And in the synthesis of somatomedine, insulin plays an enormous role. After all, it helps the liver to get all the necessary amino acids to start the mechanism of creating an IGF.
IGF in muscle cells exerts a growth-stimulating and insulin-like activity. It catalyzes the synthesis of protein and slows down the process of its destruction. It also shifts metabolism and promotes accelerated fat burning. Insulin-like growth factor 1 is associated with the pituitary and hypothalamus. For example, at a low concentration of it, secretion of somatropin increases. Also the production of somatropin-releasing hormone increases. But with a high level of IGF-1, the secretion of these hormones decreases. A direct relationship between somatostatin and insulin-like growth factor has been established. As one of them increases, the concentration of the second also increases.
The secretion of growth hormone can be stimulated by various physiological factors, such as sleeping, exercises, prolonged hunger, stress, lowering glucose levels and lack of protein in food. The decrease in the secretion of growth hormone is observed in hyperglycemia and an increase in the level of free fatty acids in the blood.
Under natural conditions, growth hormone secretion is carried out in a pulsating way, every 3-5 hours in accordance with the circadian rhythm. The content of STH in the blood, as well as the amount and volume of secretors emissions, fluctuates significantly during different periods of life. Characteristic for the secretion of STH is its significant release in 60-90 minutes from the onset of sleep. The liver is the main place of formation of IGF in healthy animals and human. At the same time, there are data that cells of the pituitary gland, hypothalamus, cerebral cortex, cerebellum, bones, and brain cells in culture also form these biologically active compounds.
Regulation of the secretion of growth hormone is carried out by the CNS through the secretion and release of pituitary hormones – somatostatin and somatoliberin (blocking and stimulating the release of STH). Excess IGF-1 in the body reduces the secretion of GH, suppressing the release of somatoliberin from the cells of the hypothalamus and stimulating the release of somatostatin. Inhibition is also provided with an excess of IGF-1 by the GH itself, which inhibits the release of somatoliberin.
It is noticed that insulin-like growth factor 1 is lowered in old age and in childhood, and the highest in adolescents. But researchers have also found that older people, who have a level of this hormone closer to the upper limit of the norm for their age group, live longer. In addition, they are less prone to heart and vascular disease. Separately it is worth noting that its amount increases during pregnancy. Concentration in the blood during the day is approximately the same. Therefore, it is used to assess abnormalities in the production of somatropin. After all, the concentration of growth hormone in the blood changes throughout the day, the maximum level is determined at night.