Growth hormone

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Growth hormone is a single chain polypeptide comprising 191 amino acids and containing two intramolecular disulphide bonds. It has a molecular weight of 21500. The plasma half-life is 20-50 minutes.

Anterior pituitary hormones

The number of amino acid residues are as follows:

Somatotrophic hormones

GH 191 PRL 198

Corticotrophin-related peptides

ACTH 39

Beta-lipotrophin (LPH) 91 Beta-LPH 61-91 31 (beta-endorphin) Alpha-MSH 13

Glycoprotein hormones

TSH alpha chain 89; beta chain 112 FSH alpha chain 89; beta chain 115 LH alpha chain 89; beta chain 115 The beta subunit is the hormone-specific subunit.

Growth hormone is secreted in an irregular and intermittent pulsatile s fashion by somatotrophs. Secretion shows diurnal variation, peak secretion s occurring during sleep. Secretion is stimulated by GHRH and inhibited by y is somatostatin. This represents dual control by opposing hypothalamic n peptides. Stimuli for GH secretion include increased plasma amino acid concentrations and decreased plasma concentrations of fatty acids and carbohydrates.

Growth hormone stimulates the synthesis and release of somatomedin C (insulin-like growth factor 1), a 70 amino acid peptide, from the liver and kidneys. Somatomedin C mediates most of the effects of GH on target tissues. Somatomedin antagonises GH release by antagonising the effect of GHRH on pituitary somatotrophs and by the stimulation of somatostatin release in the hypothalamus.

Growth hormone actions

Promotes linear growth:

Induces precursor cells to differentiate and secrete IGF-1, which stimulates cell division.

Stimulates protein synthesis: increases amino acid transport into muscle, liver and adipose cells and accelerates transcription and translation of mRNA. Anti-insulin effects: Adipocytes more responsive to lipolytic stimuli; Stimulation of gluconeogenesis; Reduced ability of insulin to stimulate glucose uptake. Muscle: Reduced glucose uptake; Increased amino acid uptake; Increased protein synthesis; Increased lean body mass. Metabolism: Increased plasma glucose; Increased plasma free fatty acids; Reduced plasma amino acids; Reduced plasma urea. Chondrocytes: Increased amino acid uptake; Increased protein synthesis; Increased DNA, RNA synthesis; Increased chondroitin sulphate synthesis;

5 Collagen synthesis; d g Increased cell size and number; i ne Increased linear growth.

y Adipose tissue:

0 Reduced glucose uptake;

gy Increased lipolysis;

Reduced adiposity.

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