A cytoplasmic estradiol receptor in rat testicular tissue

Testicular cytosol from Sprague-Dawley rats was shown to possess an estradiol binding component which exhibited the properties of a receptor. Specific binding of estradiol by the receptor was shown, at 4C in vitro, to reach equilibrium by 6 h. That level of binding was maintained at a plateau for 18 h. Estradiol binding was demonstrated to increase linearly as a function of testicular cytosol concentration ultilized. The Ka at equilibrium was determined as 4.07 .05 x 1010M-1, while the number of binding sites for whole testicular tissue from mature rats was 1.16 x 10-14 +/- 0.061 moles/mg cytosol protein. Sucrose gradient sedimentation analysis of the cytosol receptor revealed that the major peak of radioactivity migrated in the 8S region. Enzymatic treatment of the cytosol demonstrated that the binding component was protein in nature. Heat treatment, 70 C for 10 min, resulted in 83% loss of the receptor activity. Storage in liquid N2 (for as long as 60 days) was shown to maintain receptor activity. Steroid specificity studies revealed that other estrogens were competitive with 17beta-estradiol for binding sites, but progestins, testosterone, and dihydrotestosterone did not affect estradiol binding. Tissue specificity studies showed that the capacity of testicular cytosol to bind estradiol was much greater than that of several non-target organs. Ontogenically, the cytoplasmic estradiol-binding capacity/testis was low from 7 to 21 days of age, but rose dramatically between 3 and 7 weeks of age, and an apparent plateau was reached at 15 weeks. The injection of unlabeled 17beta-estradiolin vvio was shown to deplete the cytosol of estradiol binding sites as determined in vitro. Depletion was rapid, occurring within 30 min, and the cytoplasmic estradiol receptors remained low for a period of 4 h. At 6 h, replenishment has occurred and control levels of cytoplasmic receptors were restored by 12 h. These results suggest that an estradiol receptor is present in testicular tissue of the rat, and that binding capacity is maximum in the mature testis.