High-affinity binding of T-3 to epididymis nuclei

Thyroid hormones play an important role in epididymal function. Hypothyroid animals experience a significant decrease in the number and forward motili ty of sperm and a remarkable impairment of epididymal morphology. However, it is yet unknown if such activity is due to direct actions of iodothyronin es on the target epididymis. The eventual identification of T-3 receptors i n the nucleous of epididymal cells becomes relevant. For this reason, the a uthors searched for specific high-affinity binding of T-3 to these nuclei. Twenty prepuberal male Wistar rats were used. The testes and epididymis wer e approached as one unit through a scrotal incision. The fat-free epididymi des were subjected to standard techniques to prepare the nuclei for incubat ions with I-125-T-3 concentrations, ranging from 0.5 x 10(-9) to 2.0 x 10(- 11) M. Calculations of association constants and binding capacities were pe rformed according to Scatchard. A single binding site with a K-a of 3.06 +/ - 0.6 x 10(9) M-1 or K-d of 3.26 +/- 0.6 x 10(-10) M and a maximal binding capacity of 0.11 +/- 0.02 pmol T-3/mu g DNA were observed. It is concluded that these nuclei contain a specific T-3 receptor. This finding strongly su ggests that thyroid hormones have direct effects on the epididymis.