Modification of animal growth with growth hormone and insulin-like growth factors

Abstract

A brief overview is presented on the effects of growth hormone administration to rats, humans and animals of economic significance with an emphasis on the differences in responsiveness observed between species. Unlike the situation in humans, pigs, cattle and sheep, growth hormone is only active at very high doses in pituitary-complete rats, and it is essentially inactive in poultry. The growth hormone resistance in rats and poultry can be explained by equivalent reductions in the ability of the hormone to elicit increases in circulating IGF-I concentrations. However, the rat is not resistant to IGF-I itself because the administration of this growth factor to both male and female animals leads to marked increases in live weight gain and nitrogen retention as well as a more efficient conversion of feed to live weight. Selective growth of the gut, kidneys and spleen is also observed. To date, no IGF-I growth responses have been demonstrated in animals of economic significance, although results of acute experiments show the expected increased tissue delivery of nutrients. It is suggested that the increased availability of recombinant IGF-I and especially genetically engineered potent variants will lead to extensive growth trials in the next few years. The cc-administration of IGF peptides and growth hormone also provides an interesting opportunity because these two agents exhibit complementary effects that could lead to a synergistic growth response.