Growth hormone - GH

Growth hormone is a polypeptide hormone synthesised and secreted by the anterior pituitary gland which stimulates growth and cell reproduction in humans and other vertebrate animals.

This article describes human growth hormone physiology, with brief mentions of the diseases of GH deficiency, GH excess (acromegaly and pituitary gigantism), as well as GH treatment, and HGH quackery. Each of these topics is treated more fully in separate articles.

Terminology

Growth hormone (GH) is also called somatropin or somatotropin (British: somatotrophin). HGH refers to human growth hormone and was used as an abbreviation for human GH measured in the blood or extracted from human pituitary glands for therapeutic administration. Since the mid-1990s this older abbreviation has begun to carry paradoxical connotations and now rarely refers to real GH (see article on GH treatment and HGH quackery for a fuller discussion of HGH therapy).

Structure and gene of the human GH molecule

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

Human growth hormone is a protein of 191 amino acids with a molecular weight of about 22,000. 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 hormone is active in humans.

Secretion of GH

GH is secreted into the blood by the somatotrope cells of the anterior pituitary gland, in larger amounts than any other pituitary hormone. The transcription factor PIT-1 stimulates both the development of these cells and their production of GH. Failure of development of these cells, as well as destruction of the anterior pituitary gland, results in GH deficiency.

Peptides released by neurosecretory nuclei of the hypothalamus into the portal venous blood surrounding the pituitary are the major controllers of GH secretion by the somatotropes. Growth hormone releasing hormone (GHRH) from the arcuate nucleus and ghrelin promote GH secretion, and somatostatin from the periventricular nucleus inhibits it.

Although the balance of these stimulating and inhibiting peptides determines GH release, this balance is in turn affected by many physiologic stimulators and inhibitors of GH release. Stimulators of GH release include (among others) sleep, exercise, hypoglycemia, dietary protein, and estradiol. Inhibitors of GH secretion include dietary carbohydrate and glucocorticoids.

Most of the physiologically important GH secretion occurs as several pulses or peaks of GH release each day. The level of GH during these peaks may range from 5 to 30 mg/dl or more. Peaks typically last from 10 to 30 minutes before returning to basal levels. 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. Between the peaks, basal GH levels are quite low, usually less than 3 ng/ml for most of the day and night.

The amount and pattern of GH secretion change throughout life. Basal levels are highest in early childhood. The amplitude and frequency of peaks is greatest during the pubertal growth spurt. Healthy children and adolescents average about 8 peaks per 24 hours. Adults average about 5 peaks. Basal levels and the amplitude and frequency of peaks decline throughout adult life.

Several molecular forms of GH circulate. Much of the growth hormone in the circulation is bound to a protein (growth hormone binding protein, GHBP) which is derived from the GH receptor.

Functions of GH

The effects of growth hormone on the tissues of the body can generally be described as anabolic (building up). Like most other protein hormones GH acts by interacting with a specific receptor on the surface of cells.

Height growth in childhood is the best known effect of GH action, and appears to be stimulated by at least two mechanisms. 1. GH directly stimulates division and multiplication of chondrocytes of cartilage. These are the primary cells in the growing ends (epiphyses) of children's long bones (arms, legs, digits). 2. GH also stimulates production of insulin-like growth factor 1 (IGF1, also known as somatomedin C), a hormone homologous to proinsulin. The liver is a major target organ of GH for this process, and is the principal site of IGF1 production. IGF1 has growth-stimulating effects on a wide variety of tissues. Additionally, some IGF1 is generated within target tissues, making it apparently both an endocrine and an autocrine hormone.

Although height growth is the best known effect of GH, it serves many other metabolic functions as well. GH increases calcium retention, and strengthens and increases the mineralization of bone. It increases muscle mass. It induces protein synthesis and growth of many different organ systems of the body, resulting in a "positive nitrogen balance".

GH stimulates the immune system. It cross reacts with prolactin receptors but a physiologic role for this effect is uncertain.

GH plays a role in fuel homeostasis. GH reduces liver uptake of glucose, an effect that opposes that of insulin. GH also contributes to the maintenance and function of pancreatic islets. It tends to promote lipolysis, which results in some reduction of adipose tissue (body fat) and rising amounts of free fatty acids and glycerol in the blood.