Growth Hormone

 

 

Growth Hormone

•    Also called as Somatotropin or somatotropic Hormone (STH)

•    Proteinous hormone produced by somatotropes

–  Specialized group of acidophilic cells in anterior pituitary

Chemistry of GH:

•    Single polypeptide of M. Wt. 21500

•    Consist of 191 aminoacids

–  Two disulfide linkages between adjacent cysteine residues

•    53 & 165 and 182 & 189

•    GH can bring about some action of prolactin (PL) and human placental lactogen (HPL)

–  Due to high degree of amino acid sequence homology

•    Release:

•    Regulated by feed back mechanism through GRH and GRIH (Somatostatin)

–  Several factors influence its production and release

•    Sleep, Stress (Pain, cold etc.), exercise, food intake etc.

– Maximum release occurs after the onset of sleep

•    Physiological Role of Growth Hormone:

•    Act by binding to membrane bounding receptors on its target cells

–  Exact mechanism of action and second messenger is not yet known

•    Promotes growth and normal metabolism of proteins, carbohydrates, lipids and minerals

•    i) Effect on growth:

•    Growth related effects are mediated via insulin like growth factor-I (IGF-I) 

–  Also called somatomedin-C produced by liver

–  Prolong the growth of epiphyseal cartilages 

•    Growth in Epiphyseal plate or Growth plate cause the growth of long bones

•    Promote bone growth by

–  Retention of Ca²⁺ and PO₄^3- that helps in osteogenesis and ossification

–  Enhance incorporation of

•    Hydroxy proline into collagen matrix

•    Amines to glycosaminoglycans (GAGs)

•    Sulfate into matrix proteoglycan like chondroitin sulfate

– Increase synthesis of DNA and RNA in Chondrocytes

•    Hyposecretions may cause Stunted growth

•    Hypersecretion can lead to acromegaly in adults

– Gigantism in children

 

ii) Effect on Protein metabolism:

•    Promotes uptake of aminoacid into tissues

•    Stimulates overall protein synthesis

•    Bring about positive nitrogen balance by retaining nitrogen

– Increased body mass

iii) Effect on Carbohydrate metabolism

•    Growth hormone is diabetogenic hormone

– Antagonizes the effects of insulin

•    Reduces insulin sensitivity

•    It cause hyperglycemia

–  Increase gluconeogenesis

–  Decrease glucose utlilization

–  Impairs glycolysis

• Increased cardiac and smooth muscles glycogen

–  Reduces the glucose uptake by the tissues

• Exhibits glycostatic effects – Increase liver glycogen

iv) Effect on Lipids:

•    Mobilizes fats from fat depots

–  Activates triacylglycerol lipase enzyme

•    Promotes lipolysis in adipose tissue

•    Increase circulating free fatty acid and their oxidation

–  May lead to ketogenesis particularly in diabetes

v) Effect on mineral metabolism:

•    Promotes bone mineralization

–  Increased absorption of Ca2+ from intestine – Retains Na⁺, K⁺, Mg²⁺, Ca²⁺ and PO₄^3- ions

 

•    Tropins:

•    A tropin or tropic hormone is the one which influences the activities of other endocrine gland

•    Important tropin hormones of pituitary gland are

–  Leuteotropic hormone (LTH): also called Prolactin

–  Thyrotropin:  Thyroid Stimulating hormone (TSH)

–  Gonadotropins: FSH, LH

–  Corticotropin: ACTH

•    Prolactin (Leuteotropic Hormone):

•    Also called lactogenic hormone, Leuteotropin, mammotropin

–  Promotes the growth of corpus luteum

–  Stimulate secretion of progesteron

•    Secreted by lactotroph α-cells of anterior pituitary

•    Similar in aminoacid sequence with growth hormone

•    Physiological Function:

•    Act through specific glycoprotein receptors on plasma membrane of mammary gland cells

–  Stimulate mRNA synthesis

•    Increase the level of several enzymes involved in 

–  Carbohydrate metabolism

•    Promotes HMP shunt pathway, stimulate lactose production in mammary glands

–  Lipid metabolism

•    Increase lipid biosynthesis

 

•    Primarily concerned with the initiation and maintenance of lactation in mammals

–  Stimulate mammary growth especially during pregnancy

•    Mammotropic action

–  Secretion of milk

•    lactogenic action

•    Increased milk protein synthesis after birth