KINETIC-PARAMETERS OF PLASMA THYROID-HORMONE AND THYROID-HORMONE RECEPTORS IN A DWARF AND CONTROL LINE OF CHICKEN

Sex-linked dwarf chickens have in plasma low triiodothyronine (T-3) le vels and slightly raised thyroxine (T-4) concentrations and are functi onal hypothyroid. The kinetic parameters of T-4 and T-3 were investiga ted using I-125-labeled hormones. In addition the nuclear T-3-receptor s in the liver were examined using a radioreceptorassay and Scatchard analysis. Four-week-old dwarf (dw) and normal (Dw) chickens were injec ted with I-125-labeled T-3 or T-4 and blood samples taken 60, 120, 180 , and 300 min after I-125-T-3 injection and 120, 240, 360, and 480 min utes after I-125-T-4 treatment. Labeled T-4 and T-3 and the degradatio n products were separated by paper chromatography. After the paper str ips were dried, the iodinated compounds were visualized and counted in the gamma counter. The kinetic parameters, the half-life time (T-1/2) , the apparent distribution volume (V-d) and the metabolic clearance r ate (MCR) were calculated using the natural base logarithm values of t he measured radioactivity plotted against time and used for linear reg ression. T-4 was cleared from circulation more slowly in dwarf than in control chicks and reflected a longer T-1/2 (2 1.8%) and a reduced MC R (45%). The V-d tended to be lower (34.7%) in dwarfs. While the T-1/2 of T-3 was longer (28.1%) in dwarf chickens than in control animals, the MCR for T-3 was considerably increased (31.8%). This results from an increased V-d (63.1%) in the dwarf chicks. The T-3-receptor study i n the liver of dwarf and non-dwarf chickens from Week 1 to Week 4 post hatching revealed that the total capacity and the affinity constant of the binding sites were comparable in dwarf and normal chickens. Howev er, the occupancy of the receptors was higher in the dwarf animals. (C ) 1994 Academic Press, Inc.