Physiologic Effects of Growth Hormone A critical concept in understanding growth hormone activity is that it has 2 distinct types of effects: Direct effects –due to GH binding its receptor on target cells. Fat cells (adipocytes), for example, have growth hormone receptors (GHR), and growth hormone stimulates them to break down triglyceride and suppresses their ability to take up and accumulate circulating lipids.
Physiologic Effects of Growth Hormone A critical concept in understanding growth hormone activity is that it has 2 distinct types of effects: Indirect effects are mediated primarily by a IGF-1 a hormone that is secreted from the liver and other tissues in response to growth hormone. A majority of the growth promoting effects of growth hormone is actually due to IGF-I acting on its target cells.
Effects on Growth Growth is a very complex process and requires the coordinated action of several hormones. Major role of GH in stimulating body growth is to stimulate the liver and other tissues to secrete IGF-I. IGF-I stimulates proliferation of chondrocytes (cartilage cells), resulting in bone growth. IGF-I also appears to be the key player in muscle growth. It stimulates both the differentiation and proliferation of myoblasts. It also stimulates aa uptake and protein synthesis in muscle and other tissues .
Metabolic Effects GH has important effects on protein, lipid and carbohydrate metabolism. Protein metabolism : GH stimulates protein anabolism in many tissues. This effect reflects increased amino acid uptake, increased protein synthesis and decreased oxidation of proteins. Fat metabolism: GH enhances the utilization of fat by stimulating triglyceride breakdown and oxidation in adipocytes. Carbohydrate metabolism : GH is one of many hormones that serves to maintain blood glucose within a normal range.
Control of Growth Hormone Secretion Production of GH is modulated by many factors, including stress, exercise, nutrition, sleep and GH itself. primary controllers are 2 hypothalamic hormones and one hormone from the stomach:
GHRH is a hypothalamic peptide that stimulates both the synthesis and secretion of growth hormone. Somatostatin (SS) is a peptide produced by several tissues in the body, including the hypothalamus. SS inhibits growth hormone release in response to GHRH and to other stimulatory factors such as low blood glucose concentration. Ghrelin is a peptide hormone secreted from the stomach. It binds to receptors on somatotrophs and potently stimulates secretion of growth hormone.
Growth hormone secretion part of a negative feedback loop involving IGF-I . High blood levels of IGF-I lead to decreased secretion of growth hormone by -directly suppressing the somatotroph -stimulating release of somatostatin from the hypothalamus.
GH also feeds back to inhibit GHRH secretion and probably has a direct (autocrine) inhibitory effect on secretion from the somatotroph.
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- Fall '08
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