Somatotropic function: The somatomedin hypothesis revisited

ABSTRACT The discovery in 1922 that an alkaline extract of the anterior pituitary can increase growth and change carcass composition of rats led to the discovery of growth hormone (somatotropin, ST). Since the early studies, much has been learned about the biological effects of ST. The advent of recombinant DNA technology has led to the commercial development of ST-based products for animal agriculture. Administration of porcine ST (pST) at maximally effective doses (approximately 100 μg•kg BW^–1•d^–1) to growing pigs for 30 to 77 d increases average daily gain approximately 10 to 20%, improves productive efficiency (i.e., the ratio of body weight gain to feed consumed) 13 to 33%, decreases lipid accretion rates by as much as approximately 80%, and stimulates protein deposition (muscle growth) by as much as 70%. These responses are associated with a decrease in feed intake of approximately 10 to 15%. The effects of ST are mediated directly and indirectly. The indirect effects of ST are mediated by the somatomedin (insulin-like growth factor-I). The discovery of somatomedin led to the introduction of the somatomedin hypothesis, which explained the basis of ST action. Since the discovery of the somatomedins, there have been several modifications of the hypothesis developed to accommodate the evolution in understanding of how ST and IGF-I affect a diverse array of biological events. This review will summarize the history of ST and the evolution of the somatomedin hypothesis.

Key Words: Growth Hormone, Insulin-Like Growth Factor-I, Somatomedin Hypothesis, Somatotropin

© 2004, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2004. 82(E. Suppl.):E239-E244

Implications

In science, new discoveries bring clarity to ideas and concepts that, in some instances, are quite entrenched. The somatomedin hypothesis has evolved substantially and is now better framed because we have an improved understanding of the somatotropic axis and how the diversity of biological effects of somatotropin are mediated. With respect to the somatomedin hypothesis, there are effects of somatotropin that are mediated by insulin-like growth factor-I, both circulating (endocrine) and locally produced. In addition, there is compelling evidence that somatotropin has effects that are independent of insulin-like growth factor-I. With respect to animal agriculture, a better understanding of somatotropin and insulin-like growth factor-I biology has led to commercially approved products of biotechnology, such as recombinant bovine somatotropin, that have benefited producers who have adopted them. As is always the case, future research will lead to new biotechnological products that will benefit animal agriculture in ways that are presently not envisioned.