27 Endocrine Control of Growth

27.1 Somatotropic hormone (STH)

• Years ago, we were all taught about growth hormone produced by the anterior pituitary gland. It had a magic control over body size - too little and an animal did not grow, too much and the animal became a giant. But then "growth hormone" was found to have many other properties - such as making energy available for strenuous exercise. Eventually, because of its other functions, "growth hormone" was re-named somatotropic hormone (= somatotrophin) - meaning it has a general function on the body, including growth.
• In comparison with other body cells, skeletal myofibres are unusual because they are multinucleated and because they derive new nuclei from the mitosis of satellite cells. Originally, STH was thought to be the main factor responsible for directly stimulating mitosis. Much of the information on STH levels in meat animals supported this belief, because STH has a strong effect on skeletal muscle.
• Injection of STH into meat animals usually causes increases in live weight gain, feed conversion and leanness.
• Plasma STH often declines with age so it is correlated with average daily gain and feed efficiency.
• STH levels are particularly high at birth or hatching.
• A primary function of STH is to conserve muscle tissue at the expense of fat. Thus, STH tends to act in the opposite direction to insulin, This accounts for the increases in STH production  found in exercised as well as starved animals - see why the name of the hormone had to be changed?
• Anabolic steroids such as diethystilboestrol may act in ruminants by increasing STH secretion, although this is not always the case in all meat animals.
• Breeds of cattle with rapid early growth and a large mature size may secrete more STH than small, slow-growing breeds. However, this is not always the case. Sometimes there may be no relationship between STH levels and rate of growth, or the relationship may be negative.  Many of the exceptions, however, may be explained by the indirect action of STH in maximizing lipid utilization for energy requirements and in maintaining essential protein synthesis under unfavourable circumstances.
• Another factor to take into account is STH may be released in surges throughout the day. Thus, single measurements may be of little value in estimating the overall status of STH activity. Animals also show an enhanced response to exogenous STH if they are injected at intervals of several hours in a manner that simulates the natural pattern of STH release.

27.2 Insulin-like growth factor (IGF)

Sulphation factor

• The importance of STH as a primary growth stimulant has been eclipsed by IGF.
• The restoration of STH to hypophysectomized animals (unable to produce their own STH because of surgical removal of the anterior pituitary gland) causes a growth spurt in cartilagenous growth zones (from epiphyseal plates).
• The growth of cartilage is conveniently monitored by the rate of sulphate incorporation into the matrix. Thus, we have a nice measure of growth.
• Even high levels of STH produce only slight increases in the rate of sulphate incorporation. The action of STH is mediated by IGF from the liver in response to STH.
• In the oldest reports, therefore, IGF is called sulphation factor.

IGF-1 and IGF-2

• Farm mammals have two forms of IGF, 1 and 2, both with appropriate receptors and acting separately in myogenesis, whereas poultry may have only one type of receptor.
• The distribution of receptors in muscle may be quite complex.  In lambs, Type 1 and 2 receptors may differ between muscle and connective tissue, depending on the animals' nutritional state.
• Hormonal IGF in the blood is complexed by binding proteins (IGFBPs), of which several types are known.
• IGFBP modulates the function of IGF by inhibiting or stimulating target tissues.
• IGF-1 = somatomedin C.
• IGF-2 = somatomedin A.
  

Insulin-like properties

• IGF is distinguished from insulin by not being blocked by anti-insulin serum, and by being present in hypophysectomized diabetic rats treated with STH.
• A very high concentration of insulin (relative to normally blood levels) stimulates differentiation in cultured premyoblasts. In this case, the insulin appears to act as an analog of IGF.
• In contrast to this, IGF produces an equivalent stimulation of premyoblast growth and differentiation while at a normal low physiological concentration.
• Porcine serum certainly contains something  stimulating myogenic cells in vitro, and these circulating factors are more effective when isolated from genetically lean pigs than from genetically obese pigs.

27.3  Control of protein synthesis in myofibres

• The hollow cylinder represents a myofibre with just one of many striated myofibrils shown down the axis.
• The arrows show things with a positive effect (sometimes these enhance muscle growth - sometimes they reduced muscle growth).
• The bar blocking the arrows shows things with a negative effect.
• Thyroid hormones increase messenger RNA formation.
• Steroid hormones enhance messenger RNA activity.
• Insulin and STH enhance the transport of amino acid building blocks for the polyribosomal assembly of proteins.
• Cortisol blocks the assembly of proteins (AA-AA-AA-AA etc).
• Insulin blocks proteolysis and the loss of already assembled proteins.
• Cortisol  accelerates the loss of already assembled proteins.
• Insulin blocks the release of already assembled proteins.
• Cortisol facilitates the release of amino acids from the myofibre.
• IGF and insulin increase the uptake of amino acids into the myofibre.
  

  27.4 Cortisol and animal stress

• Cortisol is a powerful glucocorticoid hormone produced by the adrenal gland.
• It is based on cholesterol and its production is regulated  by pituitary ACTH (adrenocorticotropic hormone) itself regulated by CRF (corticotropin releasing factor).
• Cortisol has a negative feedback by inhibiting  ACTH and CRF production.
• Most cortisol in blood plasma is tightly bound to corticosteroid binding globulin (CBG).
• Cortisol acts on intracellular receptors.
• It has many different effects on the body, including glucose metabolism, functioning of the immune system,function, blood vessel diameter and bone metabolism.
• Cortisol is produced by stressed animals in order to help cope with the stress by increasing blood pressure and blood glucose (useful if the animal has to fight or run).
• But farm animals should not be stressed if they are being properly reared.
• You can see in the diagram above how cortisol will prevent muscle growth.

Further information