Effects of Progesterone on Nuclear and Cytosol Steroid Receptor Levels in the Oestrogen-stimulated Uterus: Comparison of the Sheep and Mouse

Abstract

Methods for the measurement of nuclear receptors for oestradiol (Ez) and progesterone (P) in mouse uterus and sheep endometrium have been established. Scatchard analysis of nuclear receptors gave the following dissociation constants (nM) and binding site concentrations (pmol steroid/mg DNA): Ez receptor, mouse 3· 76 and 2· 68, sheep 2· 03 and 5· 37; P receptor, mouse 5· 77 and 2· 52, sheep 5 34 and 3· 58. The effects of a single injection of P on nuclear and cytosol levels of Ez and P receptors have been contrasted in these tissues from Ez-treated mice and sheep. In both species, P treatment resulted, in 30-120 min, in depletion of its own cytosol receptor and accumulation of its nuclear receptor. A significant reduction of cytosol E2 receptor was seen only in the mouse at 24 h. P decreased the nuclear Ez-receptor level in the mouse at 2-8 h, but had no such effect in the sheep. The results indicate that the anti-uterotrophic action of P in mouse uterus is caused by an early direct effect of P on nuclear Ez-receptor retention, and appear also to explain why P is not anti-uterotrophic in sheep uterus.