Submitted to: Growth in Ruminants Symposium
Publication Type: Proceedings
Publication Acceptance Date: August 20, 1999
Publication Date: N/A
Interpretive Summary: This brief review shows that 5'-deiodinases, which are responsible for extrathyroidal generation of T3, are present in various tissues of ruminants, although the distribution of 5'-deiodinase types and their specific function may not exactly resemble that found in other laboratory animals. The data suggest that tissue-specific changes in 5'-deiodinase activity could play an important role in the normal and somatotropin-stimulated lactation and in the mechanism of growth acceleration after treatment with somatotropin or estrogenic growth promoters. The data also show that decreased thyroxine to triiodothyronine conversion during the acute phase response to infection can be regarded as a short-term adaptive mechanism to save energy for metabolic purposes of higher priority than growth. However, prolonged decrease in peripheral triiodothyronine generation becomes pathological on it own, and could be involved in the long-term impairment of growth performance in calves. There is a close positive relationship between triiodothyronine, somatotropin and insulin-like growth factor-I synthesis and their physiological action on growth and development. Although much has been learned about function and regulation of 5'-deiodinases, further detailed studies are required in ruminants to establish the importance of these enzymes in normal and stimulated growth, lactation, and metabolic adaptation to environmental and internal challenges.
Technical Abstract: Triiodothyronine (T3) is the active thyroid hormone produced mainly in extrathyroidal tissues by enzymatic 5'-deiodination (5'D) of thyroxine (T4). Two distinct deiodinases, Type I (D1) and type II (D2) catalyze 5'D of T4 to form T3. They differ in tissue distribution, some biochemical properties and physiological role. In ruminants, D1 is present in liver, kidney, skeletal muscle and, during the perinatal period, in brown adipose tissue (BAT). D1 in liver and kidney has been considered responsible for most of the circulating T3. D2, which provides most of the intracellular T3 locally bound to nuclear receptors, has been found in pituitary gland, BAT and in lactating mammary gland. In growing beef cattle, treatment with growth hormone (bGH) increases 5'D activity in liver, kidney and pituitary gland. In lactating cows, increasing lactation intensity by bGH administration results in elevated 5'D activity in mammary tissue, but not in liver and kidney. Implantation of an estrogenic growth promoter Synovex-S in growing beef steers, decreases 5'D activity in liver but stimulates this activity in pituitary gland. This indicates that organ-specific changes in T3generation may be essential to support metabolic demands for growth and lactation. Endotoxin (LPS) challenge in heifers and chronic parasitic infection associated with decreased growth performance in calves reduced circulating concentration of T4 and T3 and hepatic 5'D activity. These results suggest that 5'D may be involved in growth perturbation during infectious disease in cattle. More studies are required to elucidate the importance of 5'D regulation to animal production.