Growth hormone

Growth HormoneDANISH HASSANLECTURER, UNIVERSITY OF SARGODHA

Hormone A hormone is usually studied as follows:

1. Synthesis & Source of secretion2. Chemistry3. Half life4. Metabolism5. Actions6. Mode of action7. Regulation of secretion8. Applied physiology

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

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.

Normal Growth Hormones Levels in Body(Random)1. Men: < 5 ng/mL or < 226 pmol/L2. Women: < 10 ng/mL or < 452 pmol/L3. Children: 0-20 ng/mL or 0-904 pmol/L4. Newborns: 5-40 ng/mL or 226-1808 pmol/L

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)

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:

1. Insulin-like growth factor-I (IGF-I), which is also called somatomedin C, acts on bones and protein metabolism.

2. Insulin-like growth factor-II, plays an important role in growth of fetus

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.

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.

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

1. Metabolism of carbohydrates, fats & proteins2. Bones

Metabolism of Proteins: GH accelerates the synthesis of proteins by:

1. Increasing amino acid transport through cell membrane

2. Increasing transcription of DNA to RNA3. Increasing ribonucleic acid RNA translation4. Decreasing catabolism of protein5. Promoting anabolism of proteins indirectly

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.

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

Metabolism of Carbohydrates: Effects of GH on carbohydrate metabolism:

1. Decrease in the peripheral utilization of glucose for the production of energy

2. Increase in the deposition of glycogen in the cells3. Decrease in the uptake of glucose by the cells4. Diabeto-genic effect of GH

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.

In bones, GH increases:1. Synthesis and deposition of proteins by

chondrocytes and osteogenic cells2. Multiplication of chondrocytes and osteogenic

cells by enhancing the intestinal calcium absorption

3. Formation of new bones by converting chondrocytes into osteogenic cells

4. Availability of calcium 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, 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.

Regulation of GH SecretionGH secretion is stimulated by:1. Hypoglycemia2. Fasting3. Starvation4. Exercise5. Stress and trauma6. Initial stages of sleep.

GH secretion is inhibited by:

1. Hyperglycemia2. Increase in free fatty

acids in blood3. Later stages of sleep

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.

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