Growth hormone (GH) is a peptide hormone that stimulates growth and cell reproduction in humans and other animals. It is a 191-amino acid, single chain polypeptide hormone which is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior pituitary gland. Somatotrophin refers to the growth hormone produced natively in animals, the term somatropin refers to growth hormone produced by recombinant DNA technology, and is abbreviated "rhGH" in human.

Gene locus

main: Growth hormone 2 The genes for human growth hormone, known as Growth hormone 1 and Growth hormone 2, are localized in the q22-24 region of chromosome 17 and are closely related to human chorionic somatomammotropin (also known as placental lactogen) genes. GH, human chorionic somatomammotropin, and prolactin (PRL) are a group of homologous hormones with growth-promoting and lactogenic activity.

Molecular structures

The major isoform of the human growth hormone is a protein of 191 amino acids and a molecular weight of 22 124 daltons. The structure includes four helices necessary for functional interaction with the GH receptor. GH is structurally and apparently evolutionarily homologous to prolactin and chorionic somatomammotropin. Despite marked structural similarities between growth hormone from different species, only human and primate growth hormones have significant effects in humans.

Secretion

Several molecular isoforms of GH circulate in the plasma. Much of the growth hormone in the circulation is bound to a protein (growth hormone binding protein, GHBP) which is derived from the growth hormone receptor, and an acid label sub unit (ALS).

Regulation

Peptides released by neurosecretory nuclei of the hypothalamus (Growth hormone releasing hormone and somatostatin) into the 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 GH secretion include:

• growth hormone releasing hormone (GHRH) from the arcuate nucleus
• ghrelin
• sleep
• exercise
• low levels of blood sugar (hypoglycemia)
• dietary protein
• increased androgen secretion during puberty (in males from testis and in females from adrenal cortex)
• arginine

Inhibitors of GH secretion include:

• somatostatin from the periventricular nucleus
• circulating concentrations of GH and IGF-1 (negative feedback)
• hyperglycemia
• glucocorticoids
• estradiol or any estrogen