Cytoplasmic progesterone receptors in uterine tissue of the snapping turtle (Chelydra serpentina)

Abstract

ABSTRACT A high affinity progesterone-binding component was detected in the cytosol of the uterus of the snapping turtle, Chelydra serpentina. Density gradient centrifugation indicated that binding of [3H]progesterone and [3H]promegestone (R5020) was to a fraction with a heavier sedimentation coefficient than bovine serum albumin (BSA) appearing as a broader peak in the 6–7 S region; it was not affected by excess cortisol. Another binding peak, lighter than BSA and appearing with [3H]R5020 and [3H]progesterone near the 4 S region, was affected by excess cortisol. Excess progesterone decreased both the heavier and lighter peaks. Analysis of steroid specificity revealed that, of the natural steroids, progesterone had the highest affinity for the uterine cytosol. This was followed by deoxycorticosterone, 5α-pregnanedione, testosterone, oestradiol-17β, corticosterone, 5α-dihydrotestosterone and cortisol. Non-linear regression analysis of saturation data indicated the presence of two classes of high affinity binding sites: progesterone-binding sites (R-sites) with equilibrium association constants (Ka) of 2·9 ± 0·28 litres/nmol (mean ± 95% confidence limit) for [3H]R5020 and 0·34 ± 0·20 litres/nmol for [3H]progesterone, and corticosteroid-binding globulin-like sites (G-sites) with Ka of 4·5 ± 1·6 litres/nmol for progesterone. The concentration of R-sites was between 0·66 ± 0·10 and 2·6 ± 0·55 pmol/mg protein while that of G-sites was between 0·73 ± 0·05 and 5·0 ± 0·27 pmol/mg protein. DEAE-cellulose filtration assay also confirmed the presence of R-sites and G-sites in the cytosol. R-sites were detectable without oestrogen priming during the preovulatory and vitellogenic phases (low progesterone, high oestrogen concentrations) when the ovarian follicles are mature (18–22 mm diameter). During the early postovulatory phase (high progesterone, low oestrogen concentrations), when the ovarian follicles are immature (5–7 mm diameter) and active corpora lutea are present, the R-sites were undetectable even after oestrogen priming. This indicates that progesterone might have an inhibitory and oestrogen a stimulatory role in the regulation of R-sites. J. Endocr. (1986) 109, 385–392