Growth Hormone source, chemistry, Blood level and Actions of Growth Hormone

GROWTH HORMONE Source of Secretion Growth hormone is secreted by somatotropes which are the acidophilic cells of anterior pituitary. Chemistry, Blood Level and Daily Output GH is protein in nature, having a singlechain polypeptide with 191 amino acids. Its molecular weight is 21,500. Basal level of GH concentration in blood of normal adult is up to 300 g/dL and in children, it is up to 500 ng/dL. Its daily output in adults is 0.5 to1.0 mg. Transport Growth hormone is transported in blood by GH-binding proteins (GHBPs). Half-life and Metabolism Half-life of circulating growth hormone is about 20 minutes. It is degraded in liver and kidney. Actions of Growth Hormone GH is responsible for the general growth of the body. Hypersecretion of GH causes enormous growth of the body, leading to gigantism. Deficiency of GH in children causes stunted growth, leading to dwarfism. GH is responsible for the growth of almost all tissues of the body, which are capable of growing. It increases the size and number of cells by mitotic division. GH also causes specific differentiation of certain types of cells like bone cells and muscle cells. GH also acts on the metabolism of all the three major types of foodstuffs in the body, viz. proteins, lipids and carbohydrates. 1. On metabolism GH increases the synthesis of proteins, mobilization of lipids and conservation of carbohydrates. a. On protein metabolism GH accelerates the synthesis of proteins by: i. Increasing amino acid transport through cell membrane: The concentration of amino acids in the cells increases and thus, the synthesis of proteins is accelerated. ii. Increasing ribonucleic acid (RNA) translation:GH increases the translation of RNA in the cells Because of this, ribosomes are activated and more proteins are synthesized. GH can increase the RNA translation even without increasing the amino acid transport into the cells. iii. Increasing transcription of DNA to RNA: It also stimulates the transcription of DNA to RNA. RNA, in turn accelerates the synthesis of proteins in the cells iv. Decreasing catabolism of protein: GH inhibits the breakdown of cellular protein. It helps in the building up of tissues. v. Promoting anabolism of proteins indirectly: GH increases the release of insulin (from β-cells of islets in pancreas), which has anabolic effect on proteins. b. On Fat metabolism GH mobilizes fats from adipose tissue. So, the concentration of fatty acids increases in the body fluids. These fatty acids are used for the production of energy by the cells. Thus, the proteins are spared. During the utilization of fatty acids for energy production, lot of acetoacetic acid is produced by liver and is released into the body fluids, leading to ketosis. Sometimes, excess mobilization of fat from the adipose tissue causes accumulation of fat in liver, resulting in fatty liver. c. On carbohydrate metabolism Major action of GH on carbohydrates is the conservation of glucose. Effects of GH on carbohydrate metabolism: i. Decrease in the peripheral utilization of glucose for the production of energy: GH reduces the peripheral utilization of glucose for energy production. It is because of the formation of acetyl-CoA during the metabolism of fat, influenced by GH. The acetyl-CoA inhibits the glycolytic pathway. Moreover, since the GH increases the mobilization of fat, more fatty acid is available for the production of energy. By this way, GH reduces the peripheral utilization of glucose for energy production. ii. Increase in the deposition of glycogen in the cells: Since glucose is not utilized for energy production by the cells, it is converted into glycogen and deposited in the cells. iii. Decrease in the uptake of glucose by the cells: As glycogen deposition increases, the cells become saturated with glycogen. Because of this, no more glucose can enter the cells from blood. So, the blood glucose level increases. iv. Diabetogenic effect of GH: Hypersecretion of GH increases blood glucose level enormously. It causes continuous stimulation of the β-cells in the islets of Langerhans in pancreas and increase in secretion of insulin. In addition to this, the GH also stimulates β-cells directly and causes secretion of insulin. Because of the excess stimulation, β-cells are burnt out at one stage. This causes deficiency of insulin, leading to true diabetes mellitus or full-blown diabetes mellitus. This effect of GH is called the diabetogenic effect. 2. On bones In embryonic stage, GH is responsible for the differentiation and development of bone cells. In later stages, GH increases the growth of the skeleton. It increases both the length as well as the thickness of the bones. In bones, GH increases: i. Synthesis and deposition of proteins by chondrocytes and osteogenic cells ii. Multiplication of chondrocytes and osteogenic cells by enhancing the intestinal calcium absorption iii. Formation of new bones by converting chondrocytes into osteogenic cells iv. Availability of calcium for mineralization of bone matrix. GH increases the length of the bones, until epiphysis fuses with shaft, which occurs at the time of puberty. After the epiphyseal fusion, length of the bones cannot be increased. However, it stimulates the osteoblasts strongly. So, the bone continues to grow in thickness throughout the life. Particularly, the membranous bones such as the jaw bone and the skull bones become thicker under the influence of GH. Hypersecretion of GH before the fusion of epiphysis with the shaft of the bones causes enormous growth of the skeleton, leading to a condition called gigantism. Hypersecretion of GH after the fusion of epiphysis with the shaft of the bones leads to a condition called acromegaly.