growth hormone is a peptide hormone that stimulates growth and cell reproduction and regeneration, and is very similar between different mammals.
Growth hormone is produced by somatotrophic cells of the anterior pituitary and has a very important role in the development of the organism, being considered a mitogenic agent, a substance that stimulates cell mitosis.
It is often known by the acronym GH (Growth Hormone) or hGH (Human Growth Hormone). Also called somatotropin (STH) although somatotropin is actually the non-proprietary international name for the drug form of the hormone.
Somatotropin is usually obtained by genetic recombination, so it can be found in scientific texts as rhGH (recombinant human growth hormone).
Somatotropin, known as growth hormone, should not be confused with chorionic somatotropin or human placental lactogen (HPL), a hormone produced by the placenta during pregnancy.
HPL and growth hormone have a similar structure and similar effects. Although the action of HPL is weaker, it guarantees fetal nutrition even in states of maternal malnutrition.
HPL, together with growth hormone and prolactin, form a group of hormones with lactogenic activity and growth promoters encoded in related genes.
Growth hormone occurs in several molecular isoforms, the most common being a peptide chain of 191 amino acids with a molecular weight of 22124 Da. Two different genes or alleles are known for the hormone, GH1 and GH2 located at the genetic locus of the q22-24 region of chromosome 17.
Structurally, the peptide chain is arranged in four helices and is very similar between different hominid species. although only human growth hormone and the primate order Catarrhini (“old world monkeys”) are active in humans.
secretion and regulation
Growth hormone is secreted by somatotrophic cells of the lateral wings of the pituitary gland or pituitary gland and its secretion is regulated by the neurosecretory nucleus of the hypothalamus Therefore, growth hormone secretion is considered a branch of the hypothalamic-pituitary axis.
Neurosecretory cells from the hypothalamus release various hormones into the pituitary portal vein, and through this vein they reach the pituitary gland to regulate GH secretion:Growth Hormone Releasing Hormone or GHRH from English Growth Hormone Releasing Hormone, also called somatocrinin: Stimulates GH secretion. Growth hormone inhibitory hormone (GHIH) or somatostatin: inhibits GH secretion.
The balance of these two hormones is what determines whether the pituitary gland releases more or less growth hormone, which is affected by numerous physiological stimuli. For example, physical exercise, nutritional status or sleep stimulate GH secretion, while a large amount of free fatty acids inhibits it.
Growth hormone secretion is markedly pulsatile and higher concentrations occur during sleep. Approximately 50% of GH secretion occurs during the third and fourth stages of NREM sleep.
There is also a peak of secretion approximately 1 hour after falling asleep, with plasma concentrations that can range between 13 and 72 ng/mL. Sleep suppression generally inhibits GH secretion.
During the day, peaks occur every 3-5 hours, although the concentrations reached in the blood are highly variable between people. Between each increase, the GH level can drop by as much as 1-5 ng/mL.
Other factors that affect the amount of GH secreted are sex, age, diet, stress, other hormones and certain exogenous substances. Adolescents, for example, can produce up to 700 μg/day of GH, while adults do not usually exceed 400 μg/day.
growth hormone stimulators:sex hormones: estrogens and androgens Peptide hormones: somatocrinin, ghrelin Drugs: clonidine, L-dopa α4β2 nicotinic agonists: nicotine Deep sleep vitamin B3 (niacin, nicotinic acid) intense exercise
GH secretion inhibitors:Somatostatin High concentrations of GH and IGF-1 (insulin-like growth factor type 1). They act as negative feedback on the hypothalamus. hyperglycemia Endocrine disrupting glucocorticoids
Growth hormone has an eminently anabolic effect on tissues. growth hormone stimulates biosynthesis and the development of cellular components and to exert its action, GH binds to specific receptors on the surface of cells.
Undoubtedly, the best-known function of GH is height gain in childhood and adolescence. This body growth is likely produced by numerous complex mechanisms, including activation of the MAPK/ERK and JAK/STAT pathways:MAPK/ERK Route: The route or pathway of MAP kinases (Mitogen-activated protein kinases) is a signal transduction pathway observed in eukaryotic cells that transports signals from membrane receptors to the cell nucleus, where they modulate gene expression to stimulate cell division and multiplication. Growth hormone follows this mechanism in chondrocytes, cartilage, ligaments, and probably other cells and tissues. JAK/STAT pathway: The JAK/STAT signaling pathway is another signal transduction pathway from membrane receptors to the cell nucleus. It is used by growth hormone and other substances from the cytokine family, interferon and interleukin. Through this pathway, GH stimulates the production of insulin-like growth factor type I (IGF-1) in the liver.
Other effects of growth hormone on the body are:Increases osteogenesis and heuso mineralization Increase muscle mass Stimulates fat catabolism (lipolysis) Stimulates protein synthesis Stimulates hepatic gluconeogenesis Increases the passage of thyroxine (T4) to a more active form, triiodothyronine (T3) In several studies, GH appears to increase cognitive function as well as mental and emotional health, so it would have a significant effect on people’s quality of life.
Clinical importance and medical uses
Growth hormone levels, both in excess and in deficiency, have clinically relevant consequences.
excess growth hormone
Among the most common causes of GH excess are benign pituitary tumors called somatotrophic adenomas.
Sustained excess of GH produces acromegaly in adults and gigantism in babies due to overgrowth of bones, especially of the jaws, hands and feet. It is usually accompanied by pressure on the nerves (eg, carpal tunnel syndrome), muscle weakness, and insulin resistance.
growth hormone deficiency
The effects of growth hormone deficiency are highly variable at different ages. In children, the main effect is stunted growth, short stature and delayed development of sexual characteristics and functions.
In adults, GH deficiency increases osteoclast activity and bone resorption, which in turn increases the risk of osteoporosis and pathological bone fractures (fractures without the need for severe trauma).
However, GH deficiency in adults is relatively rare and almost always occurs as a continuation of a pathology inherited from childhood.
Use of somatotropin as a drug
Somatotropin is used to treat growth hormone deficiency in both childhood and adults. It usually produces severe and frequent side effects, so its use is very limited and subject to monitoring.
Somatotropin is also used to treat other disorders characterized by short stature but not GH deficiency. For example, to treat Turner syndrome, Prader-Will syndrome, or idiopathic short stature.
For its anabolic effect, somatotropin has been used in sports competitions, although it is banned by the International Olympic Committee and is currently considered doping.
The anabolic effect is also used in livestock to stimulate the growth of animals. This use is approved in very few countries, including the United States.