Kinetic Parameters of Plasma Thyroid Hormone and Thyroid Hormone Receptors in a Dwarf and Control Line of Chicken

Abstract

Sex-linked dwarf chickens have in plasma low triiodothyronine (T3) levels and slightly raised thyroxine (T4) concentrations and are functional hypothyroid. The kinetic parameters of T4 and T3 were investigated using 125I-labeled hormones. In addition the nuclear T3-receptors in the liver were examined using a radioreceptorassay and Scatchard analysis. Four-week-old dwarf (dw) and normal (Dw) chickens were injected with 125I-labeled T3 or T4 and blood samples taken 60, 120, 180, and 300 min after 125I-T3 injection and 120, 240, 360, and 480 minutes after 125I-T4 treatment. Labeled T4 and T3 and the degradation products were separated by paper chromatography. After the paper strips were dried, the iodinated compounds were visualized and counted in the gamma counter. The kinetic parameters, the half-life time (T1/2), the apparent distribution volume (Vd) and the metabolic clearance rate (MCR) were calculated using the natural base logarithm values of the measured radioactivity plotted against time and used for linear regression. T4 was cleared from circulation more slowly in dwarf than in control chicks and reflected a longer T1/2 (21.8%) and a reduced MCR (45%). The Vd tended to be lower (34.7%) in dwarfs. While the T1/2 of T3 was longer (28.1%) in dwarf chickens than in control animals, the MCR for T3 was considerably increased (31.8%). This results from an increased Vd (63.1%) in the dwarf chicks. The T3-receptor study in the liver of dwarf and non-dwarf chickens from Week 1 to Week 4 posthatching revealed that the total capacity and the affinity constant of the binding sites were comparable in dwarf and normal chickens. However, the occupancy of the receptors was higher in the dwarf animals.