The effect of growth hormone on organs and tissues
Growth hormone is called growth hormone for the fact that in children and adolescents, as well as young people with areas of growth not yet closed in the bones, it causes a pronounced acceleration of linear (long) growth, mainly due to the growth of long tubular bones of the extremities. Somatotropin has a powerful anabolic and anti-catabolic effect, enhances protein synthesis and inhibits its breakdown, and also helps to reduce the deposition of subcutaneous fat, increase fat burning and increase the ratio of muscle mass to fat. In addition, somatotropin takes part in the regulation of carbohydrate metabolism - it causes a marked increase in the level of glucose in the blood and is one of the counter-insulin hormones, insulin antagonists on action on carbohydrate metabolism. Its effects on pancreatic islet cells, immunostimulating effect, enhancement of calcium absorption by bone tissue, etc. are described. Many effects of growth hormone cause directly, but a significant part of its effects is mediated by insulin-like growth factors, mainly IGF-1 (formerly called somatomedin C) , which is produced by the action of growth hormone in the liver and stimulates the growth of most internal organs. Additional amounts of IGF-1 (English Insulin-like growth factor) are synthesized in target tissues.
The growth hormone receptor is a transmembrane protein belonging to the superfamily of receptors with tyrosine kinase activity. According to the data of most researchers, when interacting with one molecule of the hormone, two receptor molecules (dimerization) combine, after which the receptor is activated, and its intracellular domain phosphorylates the receptor and the main target protein, Janus kinase (JAK-2). Further signal transmission proceeds in several ways: through the STAT proteins, the Janus kinase activates the transcription of a number of genes through the IRS protein (substrate of the insulin receptor), glucose transport to the cells is influenced, etc. JAK-2 can also directly activate other receptors, for example, the epidermal receptor growth factor, which, apparently, explains the mitogenic effect of growth hormone.
Interaction with other hormones
For the manifestation of the anabolic action of somatotropin on protein synthesis and linear growth of the body, the presence of insulin is necessary - in the absence of insulin or at its lowered level, somatotropin does not have anabolic effect. With respect to protein synthesis, somatotropin and insulin do not act antagonistically, as on carbohydrate metabolism, but synergistically. Apparently, it is the lack or insufficiency of the enhancing effect of insulin on the sensitivity of cells to the anabolic effect of somatotropin due to poor linear growth and a delay in the physical development of children with type I diabetes, especially noticeable in insulin insufficiency (insufficient compensation for diabetes).
For the manifestation of the anabolic and fat burning effect of somatotropin on the cells, in addition, the presence of sex hormones and thyroid hormones is necessary. This explains the delay in linear growth and the lag in the physical development of children and adolescents with hypogonadism (insufficiency of sex hormones) and hypothyroidism (thyroid insufficiency).
An antagonistic action with respect to the influence of somatotropin on protein synthesis, fat burning and on linear growth is exerted by glucocorticoids, in particular cortisol.
