Receptor progesterone complex in the nuclear fraction of the rat uterus: demonstration by 3H-progesterone exchange

We have previously shown that ³H-estradiol exchange can be used to measure the quantity of estrogen receptor complex in the nuclear fraction of target tissue cells. This method has been modified for the measurement of the progesterone receptor complex (R_n·P) in the nuclear fraction of uterine cells. Nuclear fractions were incubated for 5 hrs. at 15°C in the presence of varying concentrations of ³H-progesterone (³H-P) with or without a 250-fold excess of non-labeled progesterone (P). R_n·P was determined by subtracting the ³H-P bound in the presence of excess P (non-specific binding) from ³H-P bound in the absence of excess P (total binding). All R_n·P studies were done in adult castrate female rats that had received estradiol benzoate (0.4 mg) one week before use. The quantity of R_n·P increased in the uterine nuclear fractions by 280% 30 min. after injection of 5 mg of P. R_n·P was not increased in muscle or fat pad by this treatment. Injections of corticosterone (B), cortisol (F), dexamethasone (Dx) or testosterone (T) failed to increase R_n·P. The exchange reaction was specific for P; B, F, Dx or T did not compete. These results demonstrate the existence of a low capacity, high affinity, stereospecific progesterone binding site in the nuclear fraction of the uterus.