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Title: INSULIN-LIKE GROWTH FACTOR-I AND THYROID HORMONE EFFECT ON MILK LIPID AND PROTEIN SYNTHESIS IN VITRO.

Author
item KEYS JOHN E - 1265-30-00
item KAHL STANISLAW - UNIVERSITY OF MARYLAND

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Bovine somatotropin (bST) increases milk production when administered to dairy cattle. It was thought that this hormone influenced milk synthesis through the release of another hormone, insulin-like growth factor-I (IGF-I) from liver tissue. However, in previous work by this laboratory, we were unable to demonstrate a cause and effect relationship between bST and IGF-I. Others have suggested that triiodothyronine (T3) and thyroxine (T4) modify effects of IGF-I and bST on lipid and protein synthesis by mammary tissue of species other than the bovine. The current research was to determine if the thyroid hormones interact with bST or IGF-I to increase the synthesis of milk products by bovine mammary tissue. IGF-I, but not bST, depressed protein synthesis by mammary tissue across liver treatments as did both of the thyroid hormones. However, there was no evidence of a synergistic relationship between the thyroid hormones and bST or IGF-I in bovine mammary tissue. This suggests that bST affects milk production by a mechanism other than through IGF-I or the thyroid hormones.

Technical Abstract: To determine if thyroid hormones, triiodothyronine (T3) and thyroxine (T4), modify effects of IGF-I and bovine somatotropin (bST) on lipid and protein synthesis by mammary tissue, explants of mammary and adipose tissue from 8 mid-lactation Holstein cows were co-cultured for 24 h in the presence or absence of liver explants. Hormone treatments were control media, 1 ug/ml pituitary bST or 100 ng/ml IGF-I with and without 2 nM T3 or 20 nM T4. Lipid synthesis was not affected by hormone treatment and there were no synergistic relationships between the thyroid hormones and bST or IGF-I. Including liver explants in the culture depressed total lipid (-49.9%) and protein (-25.9%) synthesis by mammary tissue across all hormone treatments as measured by [14C] acetate or amino acid incorporation. IGF-I, but not bST, depressed protein synthesis by mammary tissue across liver treatments. Triiodothyronine and T4 significantly depressed protein synthesis by mammary tissue. Results suggest that physiological concentrations of thyroid hormones do not modify effects of bST or IGF-I on protein or lipid synthesis by mammary tissue. However, independently, both thyroid hormones and IGF-I modify milk protein synthesis.