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. The 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 microg thyroxine (T(4)) and tri-iodothyronine (T(3)) 100 g(-1) body mass or 10 microg T(4) and T(3) 100 g(-1) body mass). Serum and tissue samples (cardiac, pectoralis and semimembranosus leg muscle, liver, pituitary and cerebral cortex) were collected from these four groups, 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 dimensions, primary feather length, and body and muscle masses. Body mass was lower by 18%, and relative masses of cardiac and pectoralis muscles were lower by 28% and 32% respectively. Heterologous oligonucleotides for TR alpha, TR beta 0, TR beta2 and the housekeeping gene beta-actin were derived from chicken sequences. RT-PCR showed that TR alpha mRNA was expressed in all tissues but was not significantly affected by any of the experimental treatments. TR beta 0 mRNA expression was significantly lower in the leg muscles 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 controls (0.380+/-0.202), but was unaltered in the pectoralis and cardiac muscles. 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 compared 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 expressed significant levels of TR beta 2 mRNA.