Growth hormone (GH) is a protein-based peptide hormone. It stimulates growth, cell reproduction and regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior pituitary gland.
Somatotropin refers to the growth hormone 1 produced naturally in animals, whereas the term somatropin refers to growth hormone produced by recombinant DNA technology, and is abbreviated "HGH" in humans.
Growth hormone is used in medicine to treat children's growth disorders and adult growth hormone deficiency. In recent years, growth hormone replacement therapies have become popular in the battle against ageing and obesity.
Reported effects on GH-deficient patients (but not on healthy people) include decreased body fat, increased muscle mass, increased bone density, increased energy levels, improved skin tone and texture, increased sexual function, and improved immune system function. At this time, hGH is still considered a very complex hormone, and many of its functions are still unknown.
In its role as an anabolic agent, HGH has been used by competitors in sports since the 1970s, and it has been banned by the IOC and NCAA.
Traditional urine analysis could not detect doping with HGH, so the ban was unenforceable until the early 2000s when blood tests that could distinguish between natural and artificial hGH were starting to be developed.
Blood tests conducted by WADA at the 2004 Olympic Games in Athens primarily targeted HGH. while recently an additional variant of ~ 23-24 kDa has also been reported in post-exercise states at higher proportions.
This variant has not been identified, but it has been suggested to coincide with a 22 kDa glycosilated variant of 23 kDa identified in the pituitary gland.
Furthermore, these variants circulate partially bound to a protein (growth hormone-binding protein, GHBP), which is the truncated part of the growth hormone receptor, and an acid-labile subunit (ALS).
Peptides released by neurosecretory nuclei of the hypothalamus (Growth hormone-releasing hormone/''somatocrinin'' and Growth hormone-inhibiting hormone/''somatostatin'') into the hypophyseal portal venous blood surrounding the pituitary are the major controllers of GH secretion by the somatotropes.
However, although the balance of these stimulating and inhibiting peptides determines GH release, this balance is affected by many physiological stimulators (e.g., exercise, nutrition, sleep) and inhibitors of GH secretion (e.g., Free fatty acids)
''Stimulators'' of HGH secretion include:
- peptide hormones
- Growth hormone-releasing hormone (GHRH) through binding to the growth hormone-releasing hormone receptor (GHRHR)
- ghrelin through binding to growth hormone secretagogue receptors (GHSR)
- sex hormones
- increased androgen secretion during puberty (in males from testis and in females from adrenal cortex)
- clonidine and L-DOPA by stimulating GHRH release
- hypoglycemia, arginine and propranolol by inhibiting somatostatin release
- vigorous exercise
''Inhibitors'' of GH secretion include:
- somatostatin from the periventricular nucleus
- circulating concentrations of GH and IGF-1 (negative feedback on the pituitary and hypothalamus)
In addition to control by endogenous and stimulus processes, a number of foreign compounds (xenobiotics such as drugs and endocrine disruptors) are known to influence GH secretion and function.
HGH is synthesized and secreted from the anterior pituitary gland in a pulsatile manner throughout the day; surges of secretion occur at 3- to 5-hour intervals. The largest and most predictable of these GH peaks occurs about an hour after onset of sleep. Otherwise there is wide variation between days and individuals.
Nearly fifty percent of HGH secretion occurs during the third and fourth REM sleep stages. Between the peaks, basal GH levels are low, usually less than 5 ng/mL for most of the day and night.
A number of factors are known to affect HGH secretion, such as age, gender, diet, exercise, stress, and other hormones.
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