It is a protein hormone of around 190 amino acids synthesized and secreted by cells called somatotrophs in the anterior pituitary.
It is an essential participant in controlling several complex physiological processes, including growth and metabolism.
Growth hormone is also of great interest as a drug used in humans and animals.
Physiological effects of somatotropin
A critical concept to understanding the activity of growth hormone is that it has two different types of effects:
- The direct effects result from the binding of the growth hormone to its receptor in the target cells.
- Fat cells (adipocytes), for example, have growth hormone receptors, and growth hormone stimulates them to break down triglycerides and suppresses their ability to absorb and accumulate circulating lipids.
- Indirect effects are mediated primarily by a growth factor similar to insulin I (IGF-I), a hormone secreted by the liver and other tissues in response to growth hormone.
- Most of the growth promotion effects of growth hormone are due to IGF-I acting on its target cells.
Considering this distinction, we can analyze two main functions of growth hormone and its subaltern IGF-I in physiology.
Effects on growth
Growth is a very complex process and requires the coordinated action of several hormones.
The primary role of growth hormone in stimulating body growth is to promote the liver and other tissues to secrete IGF-I. IGF-I stimulates the proliferation of chondrocytes (cartilage cells), resulting in bone growth.
Growth hormone seems to affect bone growth by stimulating the differentiation of chondrocytes directly.
IGF-I also seems to be the key player in muscle growth. It stimulates both the differentiation and the proliferation of myoblasts. It also enables the absorption of amino acids and the synthesis of proteins in muscles and other tissues.
Metabolic effects of somatotropin
Growth hormone has significant effects on the metabolism of proteins, lipids, and carbohydrates.
In some cases, a direct effect of growth hormone has been demonstrated; in others, IGF-I is thought to be the critical mediator, and in some cases, direct and indirect results appear to be at stake.
- Protein metabolism: in general, growth hormone stimulates protein anabolism in many tissues. This effect reflects a more excellent absorption of amino acids, a more remarkable synthesis of proteins, and lower oxidation of proteins.
- Metabolism of fats: growth hormone improves the utilization of fats by stimulating the degradation and oxidation of triglycerides in adipocytes.
- Carbohydrate metabolism: The growth hormone is one of a battery of hormones that serves to keep blood glucose within a normal range.
Growth hormone is often said to have anti-insulin activity because it suppresses insulin’s ability to stimulate glucose absorption in peripheral tissues and improve the synthesis of glucose in the liver.
In a somewhat paradoxical manner, the administration of growth hormone stimulates insulin secretion, which leads to hyperinsulinemia.
Control of growth hormone secretion
The growth hormone production is modulated by many factors, including stress, exercise, nutrition, sleep, and growth hormone itself. However, its primary controllers are two hypothalamic hormones and one stomach hormone:
- Growth hormone-releasing hormone (GHRH): is a hypothalamic peptide that stimulates both the synthesis and the secretion of growth hormone.
- Somatostatin (SS): is a peptide produced by various body tissues, including the hypothalamus. Somatostatin inhibits growth hormone release in response to GHRH and other stimulating factors such as low blood glucose concentration.
- Ghrelin: is a peptide hormone secreted by the stomach. Ghrelin binds to the somatotroph receptors and potently stimulates the growth hormone secretion.
The secretion of growth hormone is also part of a negative feedback loop that involves IGF-I. Elevated levels of IGF-I in the blood lead to a decrease in growth hormone secretion not only by directly suppressing the somatotroph but also by stimulating the release of somatostatin from the hypothalamus.
Growth hormone is also fed back to inhibit the secretion of GHRH and probably has a direct (autocrine) inhibitory effect on the secretion of the somatotroph.
Integrating all factors that affect the synthesis and secretion of growth hormone leads to a pulsatile pattern of release. The basal concentrations of the growth hormone in the blood are deficient.
In children and young adults, the most intense period of growth hormone release is shortly after the onset of deep sleep.
The states of deficiency and excess of the growth hormone provide evident testimonies of the role of this hormone in normal physiology.
Such disorders may reflect lesions in the hypothalamus, pituitary, or target cells. A deficiency state can result not only from a deficiency in the production of the hormone but in the response of the target cell to the hormone.
Clinically, deficiency in growth hormone or defects in its binding to the recipient is considered stunted or dwarfed.
The manifestation of the growth hormone deficiency depends on the age of onset of the disorder and may be the result of a hereditary or acquired disease.
The effect of excessive secretion of growth hormone also depends a lot on the age of onset and is considered two specific disorders:
Giantism results from excessive secretion of growth hormone that begins in young children or adolescents.
It is a sporadic disorder usually the result of a somatotropic tumor. One of the most famous giants was a man named Robert Wadlow. He weighed 8.5 pounds at birth, but at five years old, he weighed 105 pounds and was 5 feet 4 inches tall.
Robert reached an adult weight of 490 pounds and was 8 feet 11 inches tall. He died at 22 years old.
Acromegaly: is the result of excessive secretion of growth hormone in adults, usually resulting from benign pituitary tumors. The onset of this disorder is generally internal and occurs over several years.
The clinical signs of acromegaly include:
- Overgrowth of the extremities.
- Swelling of the soft tissues.
- Abnormalities in the structure of the jaw.
- Heart disease.
Excessive growth hormone and IGF-I also lead to many metabolic disorders, including hyperglycemia.
Pharmaceutical and biotechnological applications of growth hormone
In past years, purified growth hormone from human cadaver pituitaries was used to treat children with severe growth retardation.
More recently, the virtually unlimited supply of growth hormone produced by recombinant DNA technology has led to several other applications for human and animal populations.
Human growth hormone is commonly used to treat children with pathologically low stature.
There is concern that this practice extends to the treatment of essentially normal children, the so-called “improvement therapy” or growth hormone on demand. Similarly, growth hormone has been used by some to improve athletic performance.
Although growth hormone therapy is generally safe, it is not as safe as any therapy and carries unpredictable health risks. Parents who request growth hormone therapy for children of essentially average height are wrong.
The role of growth hormone in normal aging remains poorly understood, but some of the cosmetic symptoms of aging appear to be susceptible to growth hormone therapy.
This is an active area of research, and additional information and recommendations about the risks and benefits will undoubtedly arise shortly.
The somatotropin in animals
Growth hormone is currently approved and marketed to improve milk production in dairy cattle.
There is no doubt that the administration of bovine somatotropin to lactating cows increases milk production and, depending on the way cows are handled, can be an economically viable therapy.
However, this treatment breeds excellent controversy, even among dairy producers.
One thing that seems clear is that drinking milk from cattle treated with bovine growth hormone does not pose a risk to human health.