Veterinary Physiology-III

Physiological effect of STH

PHYSIOLOGICAL EFFECTS OF STH

• Growth hormone has two distinct types of effects
  • Direct effects are the result of growth hormone binding its receptor on target cells. Fat cells (adipocytes), for example, have growth hormone receptors, and growth hormone stimulates them to break down triglyceride and suppresses their ability to take up and accumulate circulating lipids.
  • Indirect effects are mediated primarily by an insulin-like growth factor-1 (IGF-1), a hormone that is secreted from the liver and other tissues in response to growth hormone. A majority of the growth promoting effects of growth hormone is actually due to IGF-1 acting on its target cells.

Normal Effects in the Body

• The STH does not have a specific target organ
• GH exerts its effect on almost all tissues of the body.
• STH causes both increase in size (hypertrophy) and number of cells (hyperplasia), and promotes growth of all tissues of the body which are capable of growing.
• GH enhances the activities of the visceral organs like liver, kidney, intestine, endocrine glands- parathyroid and pancreas.
• It stimulates cardiac output, glomerular filtration and the metabolic activities in liver, skeletal muscle and heart.
• Increases both the soft and osseous tissues of the body and
• Has a profound effect on lactation.

Effects on Growth

• Growth is a very complex process, and requires the coordinated action of several hormones.
• GH does not act directly on bone and cartilage, but indirectly by causing the liver to produce smaller proteins called somatomedins that act on cartilages and bone to promote their growth.
• Two somatomedins, C and A have been identified which resemble structurally to insulin and hence they are also known as insulin like growth factors I and II (IGF I & II) respectively.
• IGF I is transported in blood bound with specific binding proteins called IGF-binding proteins (IGFBP); this binding of IGF prolongs the half-life of IGF I.
• The receptor for IGF is similar to insulin receptors.
• IGF I promotes skeletal and cartilage growth and IGF II is functional during foetal period.
• The metabolic effects of GH on most of the cells are promoted by stimulating the liver and other tissues to secrete IGF-I or somatomedin C.
• IGF-I stimulates proliferation of chondrocytes (cartilage cells) resulting in bone growth. GH also seems to have a direct effect on bone growth in stimulating differentiation of chondrocytes. Somatomedin production is inhibited by estrogen and cortisol.
• IGF-I also increase muscle growth. It stimulates both the differentiation and proliferation of myoblasts. It also stimulates amino acid uptake and protein synthesis in muscle and other tissues.
• Growth of the long bones continues so long as the epiphyseal lines do not close. In domestic animals, closure of epiphyseal lines soon after puberty signals the cessation of skele­tal growth under normal conditions.