Submitted to: Comparative Biochemistry and Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/16/2007
Publication Date: 2/11/2007
Citation: Rosebrough, R.W., Russell, B.A., Richards, M.P. 2007. Responses of chickens subjected to thyroid hormone depletion-repletion. Comparative Biochemistry and Physiology. Part A. 147:543-549. Interpretive Summary: Excess fat production in the modern broiler accounts for an annual loss to the poultry industry of 1000 to 1500 million dollars annually. The original source of this problem relates to selection genetic practices that emphasized rapid growth at the expense of other carcass characteristics. The literature is of limited value in determining methods to depress fat synthesis and allow lean tissue synthesis to remain at an elevated rate. The thyroid axis is more important in regulating intermediary metabolism in birds than in mammals because of a questionable role for insulin in birds. The purpose of this set of experiments was to 1) chemically inhibit thyroid hormone production and 2) examine lipogenic gene expression as a moderator of this inhibition. Although hypothyroidism decreased lipid synthesis in 28-day old chickens, restoration of thyroid function in these birds increased synthetic ability in market-age birds (49 days of age). Assigning a role to the thyroid hormone axis may be difficult because an interplay between protein nutrition and background thyroid status.
Technical Abstract: The purpose of this experiment was to determine the relationship between lipid metabolism and the expression of specific genes in chickens fed methimazole to produce hypothyroidism. Male, broiler chickens growing from 14 to 28 days of age were fed diets containing 18% crude protein and either 0 or 1 g methimazole per kg of diet. At 28 days, these two groups were further subdivided into groups receiving 18% crude protein diets containing either 0 or 1 mg triiodothyronine (T3) per kg. Birds were sampled at intervals from 0 to 120 hr. Measurements taken included in vitro lipogenesis (IVL), malic enzyme (ME), isocitrate dehydrogenase (ICD-NADP), aspartate aminotransferase (AAT) activities and the expression of the genes for ME, fatty acid synthase (FAS), NADP-ICD, AAT and acetyl coenzyme carboxylase (ACC). Gene expression was estimated with real time RT-PCR assays. Expression rates were noted as Ct’s. Dietary methimazole decreased IVL and ME at 28 d of age. T3 and supplementation for 1 d restored both IVL and ME. Paradoxically, continuing T3 replenishment for a longer period decreased IVL without affecting ME activity. Although methimazole decreased ME gene expression, there was only a transitory relationship between enzyme activity and gene expression when plasma T3 was replenished with exogenous T3. These data explain the apparent dichotomies in lipid metabolism elicited by changes in the thyroid state of animals. Most metabolic changes in response to feeding T3 occurred within a short period of time, suggesting that changes in intermediary metabolism preceded morphological changes. Furthermore, the thyroid state of the animal will determine responses to exogenous T3.