Effects of short term triiodothyronine administration to broiler chickens fed methimazole

Abstract

The purposes of these experiments were to determine possible relationships among certain indices of lipid metabolism and specific gene expression in chickens ( Gallus gallus) fed methimazole (MMI) and the subsequent effects of providing supplemental T 3 to relieve the effects of MMI. Male, broiler chickens growing from 14 to 28 days of age were fed diets containing 18% crude protein and either 0 or 1 g MMI/kg of diet. At 28 days, birds received 18% crude protein diets containing either 0 or 1 mg triiodothyronine (T 3)/kg. Birds were sampled at 0, 1, 2 & 4 days post relief from MMI or at 0, 3, 6, 9, 24 & 48 h. Measurements taken in the first experiment included in vitro lipogenesis (IVL), malic enzyme (ME), isocitrate dehydrogenase (ICD-NADP), aspartate aminotransferase (AST) enzyme activities and the expression of the genes for ME, fatty acid synthase (FAS) and acetyl coenzyme carboxylase (ACC), ICD and AST. The same enzyme activities and gene expressions were assayed over the intervals mentioned above. In vitro lipogenesis was eliminated due to constraints imposed by sampling times. Gene expression was estimated with real time RT-PCR assays. Dietary MMI decreased IVL and ME at 28 days of age. T 3 supplementation for 1 day restored both IVL and ME. Continuing T 3 replenishment decreased IVL without affecting ME activity. Although MMI decreased ME gene expression, there was only a transitory relationship between enzyme activity and gene expression when apparent thyroid function was restored with exogenous T 3. Metabolic changes in response to feeding T 3 occurred within a short period, suggesting that changes in intermediary metabolism preceded morphological changes. Furthermore, the thyroid state of the animal will determine responses to exogenous T 3.