Growth hormone

Book recommended: Medical Physiology by Jaypee

1. Growth Hormone Danish Hassan Lecturer, University of Sargodha

2. Hormone A hormone is usually studied as follows: Synthesis & Source of secretion Chemistry Half life Metabolism Actions Mode of action Regulation of secretion Applied physiology

3. Growth Hormone Synthesis & Source of Secretion Synthesized by chromphill cells of anterior pituitary. Secreted by somatotropes which are the acidophilic cells of anterior pituitary. They are in fact regulated by neuro -hormones secreted by hypothalamus

4. Chemistry, Blood Level and Daily Output Protein in nature, having a single-chain polypeptide with 191 amino acids. Its molecular weight is 21,500. Basal level of GH concentration in blood: Normal adult: 300 g/ dL Children:500 ng / dL Its daily output in adults is 0.5 to1.0 mg.

5. Normal Growth Hormones Levels in Body(Random) Men : < 5 ng / mL or < 226 pmol /L Women: < 10 ng / mL or < 452 pmol /L Children: 0-20 ng / mL or 0-904 pmol /L Newborns: 5-40 ng / mL or 226-1808 pmol /L

6. Half-life and Metabolism Half-life of circulating growth hormone is about 20 minutes. It is degraded in liver and kidney Transport Transported in blood by GH-binding proteins (GHBPs)

7. Mode of Action of GH: GH act through somatomedin is a poly-peptide through which growth hormone acts. It is secreted by liver. Somatomedins are of two types: Insulin-like growth factor-I (IGF-I), which is also called somatomedin C , acts on bones and protein metabolism. Insulin-like growth factor-II , plays an important role in growth of fetus

8. GH is transported in blood by loose binding with plasma protein. It is released from plasma protein rapidly. Action also lasts only for a short duration of 20 minutes. But, the somatomedin C binds with plasma proteins very strongly. Because of this, the molecules of somatomedin C are released slowly from the plasma proteins. The action of somatomedin C lasts for about 20 hours.

9. Mode of action of somatomedin C: Somatomedin C acts through the second messenger called cyclic AMP GH receptor is called growth hormone secretagogue GH binds with the receptor situated mainly in liver cells and forms the hormonereceptor complex. Hormone-receptor complex induces various intracellular enzyme pathways, resulting in somatomedin secretion. Somatomedin in turn, executes the actions of growth hormone.

10. Actions of Growth Hormone Responsible for the general growth of the body It increases the size and number of cells by mitotic division. Also causes specific differentiation of certain types of cells like bone cells and muscle cells. GH affects Metabolism of carbohydrates, fats & proteins Bones

11. Metabolism of Proteins: GH accelerates the synthesis of proteins by: Increasing amino acid transport through cell membrane Increasing transcription of DNA to RNA Increasing ribonucleic acid RNA translation Decreasing catabolism of protein Promoting anabolism of proteins indirectly

12. Metabolism of Fats: GH mobilizes fats from adipose tissue. Increases the concentration of fatty acids which are used for the production of energy by the cells. Proteins are spared. During the utilization of fatty acids for energy production, lot of aceto -acetic acid is produced by liver and is released into the body fluids, leading to ketosis.

13. Sometimes, excess mobilization of fat from the adipose tissue causes accumulation of fat in liver, resulting in fatty liver

14. Metabolism of Carbohydrates: Effects of GH on carbohydrate metabolism: Decrease in the peripheral utilization of glucose for the production of energy Increase in the deposition of glycogen in the cells Decrease in the uptake of glucose by the cells Diabeto-genic effect of GH

15. Effects on Bone: 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.

16. In bones, GH increases: Synthesis and deposition of proteins by chondrocytes and osteogenic cells Multiplication of chondrocytes and osteogenic cells by enhancing the intestinal calcium absorption Formation of new bones by converting chondrocytes into osteogenic cells Availability of calcium mineralization of bone matrix.

17. GH increases the length of the bones, until epiphysis fuses with shaft, which occurs at the time of puberty. After the epiphyseal fusion, it stimulates the osteoblasts strongly. 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.

18. Regulation of GH Secretion GH secretion is stimulated by: Hypoglycemia Fasting Starvation Exercise Stress and trauma Initial stages of sleep. GH secretion is inhibited by: Hyperglycemia Increase in free fatty acids in blood Later stages of sleep

19. Regulation of GH: GH secretion is under negative feedback control Hypothalamus releases GHRH and GHRP, which in turn promote the release of GH from anterior pituitary. GH acts on various tissues. It also activates the liver cells to secrete somatomedin C (IGF-I) Somatomedin C increases the release of GHIH from hypothalamus.

20. GHIH, in turn inhibits the release of GH from pituitary. Somatomedin also inhibits release of GHRP from hypothalamus. It acts on pituitary directly and inhibits the secretion of GH