A rapid assay for progesterone receptors in rat uterine cytosol: Technique, changes at induction of decidualization

A rapid assay for progesterone receptors was developed using rat uteri as the tissue source. The method was based on Scatchard analysis of the binding to proteins of the cytosol fraction, “stripped” with dextran-coated charcoal. The stripped cytosol was incubated with tritiated promegestone (R-5020) over a Sephadex G-25 microcolumn; maximal binding of the steroid to the high-affinity binders was achieved in 60 min. The bound steroid was separated from the unbound fraction in the Sephadex column. The progesterone receptor had an association constant, K a of approximately 1.5 nM −1 for promegestone. The only steroid tested which competed significantly with promegestone was progesterone. The receptor activity in the stripped cytosol was lost at the rate of 4.5–5% per hour. The level of progesterone receptors in the uterus on day 4 of pseudopregnancy was 7 ± 1 pmol/g tissue. Induction of decidualization caused a linear decrease in the concentration of these receptors, at the rate of about 66 ± 18 fmol/g min, during the first 80 min.