Tissue-specific regulation of thyroid hormone receptor mRNA isoforms and target gene proteins in domestic ducks

Skeletal muscles are important target tissues for thyroid hormone action. T he present study examines the influence of thyroid status on muscle growth and tissue-specific expression of thyroid receptor (TR) mRNA isoforms in a commercial strain of the domestic duck (Anas platyrhynchos). Four groups (n = 5) of 1-week-old ducklings were rendered either hypothyroid by treatment with methimazole (6 mg 100 g(-1) body mass or 12 mg 100 g(-1) body mass), or hyperthyroid by treatment with methimazole (6 mg 100 g(-1) body mass) in combination with thyroid hormones (5 mu g thyroxine (T-4) and tri-iodothyr onine (T-3) 100 g(-1) body mass or 10 mu g T-4 and T-3 100 g-l body mass). Serum and tissue samples (cardiac, pectoralis and semimembranosus leg muscl e, liver, pituitary and cerebral cortex) were collected from these four gro ups, and from a group of untreated controls, at 8 weeks of age. Development of duckling morphology was retarded in methimazole-treated birds compared with that in euthyroid controls, as evidenced by differences in skeletal di mensions, primary feather length, and body and muscle masses. Body mass was lower by 18%, and relative masses of cardiac and pectoralis muscles were l ower by 28% and 32% respectively. Heterologous oligonucleotides for TR alph a, TR beta 0, TR beta 2 and the housekeeping gene beta-actin were derived f rom chicken sequences. RT-PCR showed that TR alpha rnRNA was expressed in a ll tissues but was not significantly affected by any of the experimental tr eatments. TR beta 0 mRNA expression was significantly lower in the leg musc les of ducklings treated with 12 mg methimazole 100 g(-1) body mass (0.109 +/- 0.047 TR:beta-actin ratio, P < 0.05) compared with that in euthyroid co ntrols (0.380 +/- 0.202), but was unaltered in the pectoralis and cardiac m uscles. Expression of TR beta 0 mRNA was significantly higher in pectoralis (by 3.5-fold, P < 0.05), cardiac (by 4.2-fold, P = 0.003) and leg (by 4.0- fold, P < 0.001) muscles of ducklings treated with thyroid hormones compare d with those in euthyroid controls (0.098 +/- 0.019, 0.822 +/- 0.297 and 0. 38 +/- 0.202 TR:beta-actin respectively). Only the pituitary gland expresse d significant levels of TR beta 2 mRNA.