Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 22, 2011
Publication Date: October 1, 2011
Citation: Capuco, A.V., Binelli, M., Tucker, H.A. 2011. Neither bST nor Growth Hormone Releasing Factor Alter Expression of Thyroid Hormone Receptors in Liver and Mammary Tissues. Journal of Dairy Science. 94:4915-4921.
Interpretive Summary: It has been hypothesized that growth hormone increases milk production by reallocating nutrients toward the mammary gland. Establishing this metabolic priority to support lactation may be supported by the actions of thyroid hormones. Thyroid hormone action in a tissue may be influenced by local alterations in thyroid hormone metabolism or by changes in the number or affinity of thyroid hormone receptors. In the present study, bovine growth hormone or growth hormone releasing factor was administered to lactating cows and the effects on expression of thyroid hormone receptors in liver and mammary gland were studied. Treatments did not alter the total number or affinity of thyroid hormone receptors liver or mammary gland, nor did treatments alter quantity of the alternative forms of thyroid hormone receptor that are expressed in mammary gland.
Physiological effects of thyroid hormones are mediated primarily by binding of triiodothyronine, to specific nuclear receptors. It has been hypothesized that organ-specific changes in production of triiodothyronine from its prohormone, thyroxine, target the action of thyroid hormones to the mammary gland and play a role in mediating or augmenting a galactopoietic response to bST. Additionally, tissue responsiveness to thyroid hormones may be altered by changes in the number or affinity of nuclear receptors for thyroid hormones. In the present study, effects of bST and bovine growth hormone releasing factor on thyroid hormone receptors in liver and mammary gland were studied. Lactating Holstein cows received continuous infusions of bST, or bovine growth hormone releasing factor (bGRF), or served as uninfused controls. After 63 d of treatment, nuclei were isolated and incubated with [125I]-triiodothyronine. Treatments did not alter the capacity or affinity of specific binding sites for triiodothyronine in liver or mammary nuclei. Evaluation of transcript abundance for thyroid hormone receptors showed that isoforms of thyroid hormone receptor or retinoid receptor (which may influence thyroid receptor action) expressed in mammary gland were not altered by bST or bGRF treatment. Data do not support the hypothesis that administration of bST or bGRF alters sensitivity of mammary tissue by altering the expression of thyroid hormone receptors.