Abstract
Cytosol binding sites for the synthetic progestin [
3H]R5020 have been identified in the brain and oviduct from untreated and estrogen primed ovariectomized female lizards (
Anolis carolinensis). Competition of various unlabeled steroids at either 10 nM (brain) or 10 and 100 nM (oviduct) revealed that progestins were effective competitors whereas two glucocorticoids as well as testosterone and estradiol were ineffective. The apparent dissociation constant (
K
d
) of the receptor for [
3H]R5020 in the hypothalamus and telencephalon of the brain was determined to be 0.7–0.8 nM. The concentration of binding sites in the hypothalamus was approximately twice as great as in the telencephalon. The dissociation constant of the binding site for [
3H]R5020 in the oviduct was determined to be 1.4–1.7 nM.
Although sucrose density gradient centrifugation of brain cytosols labeled with [
3H]R5020 failed to reveal a discretely sedimenting peak of radioactivity, oviduct cytosol gradients contained two broad peaks of [
3H]R5020 binding at 3–6S and 8–9S. The concentration of [
3H]R5020 binding sites in both the oviduct and hypothalamus was found to increase after estrogen treatment. Scatchard analysis of oviduct cytosol [
3H]R5020 binding showed that estrogen priming increased binding levels 3-fold. Single point assays with 0.4 nM [
3H]R5020 demonstrated that estrogen priming increased binding by 55% in hypothalamus but did not alter binding in cytosol from the telencephalon sample. These results suggest that the [
3H]R5020 binding sites identified in the brain and oviduct of the lizard
A. carolinensis may correspond to cytoplasmic progestin receptors. Furthermore, the finding of an estrogen-induced increase in the concentration of these receptors in the hypothalamus and oviduct indicate that the capability of estrogen to modulate the concentration of progestin receptor is present in a representative of a vertebrate class whose progenitors gave rise to birds and mammals.
