Progestin receptors in testes from various animal species

Using a synthetic progestin, R5020 (17 alpha, 21-dimethyl-19-nor-4, 9-pregnadiene-3, 20-dione), we investigated the presence of progestin receptor(s) in testicular cytosol from various animal species including humans. After incubation of cytosol from adult chickens with [3H]R5020 in the presence or absence of various unlabeled steroids, specific progestin binding was found in the 8 S region with an equilibrium dissociation constant of 2 nM and a binding capacity of 75 fmoles/mg protein. Specific progestin binding was also found in cytosol from immature chickens sedimenting in the 4 S region with a dissociation constant of 61 nM and a binding capacity of 1 pmole/mg protein. Although 5 S binding was found in the cytosols from humans, a monkey and rats, the binding specificity was low. These results clearly indicate that the testicular cytosols from chickens contain progestin receptor(s), whereas those from the other species do not.