Differences between the Estradiol and Tstosterone Receptors in Rat Uterus

Immature rát uteri contain specific binding proteins for both estradiol and testosterone. In an attempt to compare these two hormonal sites and to determine their localisation, the following was successively established. 1 Testosterone did not compete with [3H]estradiol (3 nM) for the estrogen receptor sites, whereas high concentrations of estradiol did compete with [3H]testosterone on the androgen receptor. 2 After incubation of uteri at low physiological concentration of testosterone, the majority of the testosterone receptor sites was localised in the nucleus, whereas the estradiol receptor sites were found in the cytosol fraction. For higher concentration (≥ 100 nM) of testosterone the two steroid receptor sites were mainly present in the nuclear fraction. 3 The complexes of [3H]testosterone and [14C]estradiol with cytosol receptors, when simultaneously prepared and analysed, were shown to behave differently after sucrose-gradient ultracentrifugation and Sepharose chromatography. These results strongly suggest that the receptor sites for testosterone and estradiol in rat uterus are localised on two different multi-unit proteins in which the steroid binding units are different.