Species Specificity in the Induction of Ornithine Decarboxylase by Somatotrophin

Direct demonstration of the existence of a growth hormone, which is produced by the pars distalis of the anterior portion of the pituitary gland, dates back to 1927. At this particular time, Evans and Long illustrated that saline extracts of the "anterior lobe" of the pituitary gland of cattle could bring about an acceleration of growth in rats In later years, it was demonstrated by Smith that such extracts could restore growth in animals from which the pituitaries have been removed. In due time it became more evident that a pituitary growth hormone (name somatotrophin now) was secreted throughout the bodies of gnathostome vertebrates. It was also stated that somatotrophin may be secreted in cyclostomes even though this statements has not been proven. (1)In light of the above information regarding the activity of growth hormone, a brief view of its biological roles shall follow. Growth hormone is frequently called somatotrophin or STH to impart the fact that it functions directly upon the peripheral tissues of the body. (2) The characteristic function of somatotrophin is the stimulation of an increase in body size. The most responsive tissues appear to be the skeletal and muscular tissues. Immediately accompanying the growth response to STH is a metabolic retention of nitrogen, which is said to be attributable to the formation of protein in the synthesis of new tissue. Especially sensitive structural targets to somatotrophin are the epiphyseal discs of long bones. Nonetheless, STH stimulates periosteal bone growth and endochondral osteogenesis, as well. Although it is less well-established, the alpha cells of the islets of Langerhane of the pancreas is yet another target of STH action. This action has the effect of releasing glucagon, which could partially explain the "diabetogenic" properties of somatotrophin.(3)For a number of years, it was erroneously assumed that growth hormone exhibited only an effect on general body growth. Whenever extracts which were abundant in STH produced other actions, these actions were ascribed to contaminating factors. However, it is presently known that somatotrophins play a very significant role in the metabolism of proteins, fats and carbohydrates. (4)Because of its diverse effects on various phases of metabolism, STH has often been named the "metabolic" hormone. First of all, it stimulates the anabolism or build-up of protein by encouraging the organism to retain amino acids, which are essential for the synthesis of proteins. The expected results are decreased concentration of amino acids in the blood and elevated serum protein. In regard to the STH effect on fat metabolism, STH mobilizes fat from fat reserves to the periphery with acceleration of fatty acid breakdown, inhibition of the rate of conversion of carbohydrates to fat, and inhibition of the oxidation of acetate. The results of these actions are the changes in fat metabolism, the increase in ketone body production. Such adipokinetic activity of STH has the effect of elevating the concentration of non-esterified fatty acids in the blood.There is also a pronounced effect of somatotrophin on the metabolism of carbohydrates. It can be demonstrated that the hypophysectomized dog utilizes carbohydrates more rapidly than does the normal dog. Thus, by injecting STH into the hypophysectomized dog, a decreased utilization of glucose and increased blood-sugar level occurs. However, the continuous injection of large quantities of STH results in damage to the pancreatic islets of langerhans and a form of permanent diabetes mellitus called, "pituitary diabetes."One of the most devastating effects of somototrophic on mammals is the "disappearance" of insulin sensitivity following the removal of the pituitary gland and the development of diabetes after repeated injections of STH. A reasonable explanation of this action is the eventual destruction of the beta cells of the islets of Langerhans. (2