This paper describes the effect of oestradiol-17β implants on unbound cytosolic and nuclear oestrogen receptors in the uterus and anterior pituitary gland of ovariectomized adult rats. Rats were ovariectomized and implanted 1 week later with oil or oestradiol-17β oil solution in silicone elastomer capsules. In the latter animals the capsules either remained in situ until decapitation (subgroup 1) or were removed 48 h after implantation (subgroup 2). Implants of oestradiol-17β caused a significant depletion in both forms of oestrogen receptor in the uterus and anterior pituitary gland of rats in subgroup 1. In subgroup 2, however, there was an almost complete return to control concentrations of uterine cytosolic receptors and anterior pituitary cytosolic and nuclear receptors. Concentrations of uterine nuclear oestrogen receptors showed only a partial recovery.
These data suggest that both forms of oestrogen receptor constitute an integrated population of oestrogen receptors, without dependence on intracellular location, and that in the presence of oestradiol these receptors are bound more quickly than they are synthesized. The results also indicate the existence of a dynamic equilibrium of unbound receptors between the cytosolic and nuclear compartments for both target organs, but show that in the absence of oestradiol this equilibrium is restored in the anterior pituitary sooner than in the uterus.