High blood levels of IGF-I lead to decreased secretion of growth hormone not only by directly suppressing the somatotroph, but by stimulating release of somatostatin from the hypothalamus. Growth hormone secretion is also part of a negative feedback loop involving IGF-I. Ghrelin binds to receptors on somatotrophs and potently stimulates secretion of growth hormone. Ghrelin is a peptide hormone secreted from the stomach.Somatostatin inhibits growth hormone release in response to GHRH and to other stimulatory factors such as low blood glucose concentration. Somatostatin (SS) is a peptide produced by several tissues in the body, including the hypothalamus.Growth hormone-releasing hormone (GHRH) is a hypothalamic peptide that stimulates both the synthesis and secretion of growth hormone.However, its primary controllers are two hypothalamic hormones and one hormone from the stomach: Production of growth hormone is modulated by many factors, including stress, exercise, nutrition, sleep and growth hormone itself. Somewhat paradoxically, administration of growth hormone stimulates insulin secretion, leading to hyperinsulinemia. Growth hormone is often said to have anti-insulin activity, because it supresses the abilities of insulin to stimulate uptake of glucose in peripheral tissues and enhance glucose synthesis in the liver. Carbohydrate metabolism: Growth hormone is one of a battery of hormones that serves to maintain blood glucose within a normal range.Fat metabolism: Growth hormone enhances the utilization of fat by stimulating triglyceride breakdown and oxidation in adipocytes.This effect reflects increased amino acid uptake, increased protein synthesis and decreased oxidation of proteins. Protein metabolism: In general, growth hormone stimulates protein anabolism in many tissues.In some cases, a direct effect of growth hormone has been clearly demonstrated, in others, IGF-I is thought to be the critical mediator, and some cases it appears that both direct and indirect effects are at play. Growth hormone has important effects on protein, lipid and carbohydrate metabolism. It also stimulates amino acid uptake and protein synthesis in muscle and other tissues. It stimulates both the differentiation and proliferation of myoblasts. IGF-I also appears to be the key player in muscle growth. Growth hormone does seem to have a direct effect on bone growth in stimulating differentiation of chondrocytes. IGF-I stimulates proliferation of chondrocytes (cartilage cells), resulting in bone growth. The major role of growth hormone in stimulating body growth is to stimulate the liver and other tissues to secrete IGF-I. Growth is a very complex process, and requires the coordinated action of several hormones. Keeping this distinction in mind, we can discuss two major roles of growth hormone and its minion IGF-I in physiology. A majority of the growth promoting effects of growth hormone is actually due to IGF-I acting on its target cells. Indirect effects are mediated primarily by a insulin-like growth factor-I (IGF-I), a hormone that is secreted from the liver and other tissues in response to growth hormone.Fat cells (adipocytes), for example, have growth hormone receptors, and growth hormone stimulates them to break down triglyceride and supresses their ability to take up and accumulate circulating lipids. Direct effects are the result of growth hormone binding its receptor on target cells.Growth hormone is also of considerable interest as a drug used in both humans and animals.Ī critical concept in understanding growth hormone activity is that it has two distinct types of effects: It is a major participant in control of several complex physiologic processes, including growth and metabolism. Growth hormone is a protein hormone of about 190 amino acids that is synthesized and secreted by cells called somatotrophs in the anterior pituitary. Endocrine System > Hypothalamus and Pituitary Growth Hormone (Somatotropin)
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