Progesterone receptors in the chick oviduct. Determination of the total concentration of binding sites in the cytosol and nuclear fraction and effect of progesterone on their distribution.

1. Exchange techniques were developed for measurement of total progesterone receptor binding sites concentration in the cytosol and nuclei of chicken oviduct. 2. The level of progesterone receptor in the cytosol was under both oestrogen and progesterone control. Primary stimulation by oestradiol benzoate increased the receptor concentration from approximately 10 000 sites/cell to approximately 40 000 sites/cell in the magnum cytosol; subsequent withdrawal from oestrogen treatment led to a decrease to approximately 14 000 sites/cell after 6 weeks. After progesterone administration (3 mg/kg) to oestrogen-stimulated, withdrawn chicken, the receptor concentration decreased to approximately 40% of the initial level within the first 4 h; afterwards the receptor level rose again and by 40 h exceeded slightly the initial one. 3. The nuclear levels of the receptor reached a maximum at 1 h after the progesterone injection; however, the gain of the binding sites by the nuclei (approximately 900/nucleus) did not account for their loss from the cytosol. The maximum nuclear receptor level was not influenced by the dose of progesterone within range of 1-10 mg/kg. 4. The extractibility of the nuclear progesterone receptor by 0.5 M NaCl was strongly influenced by preincubation of the nuclei at 30 degrees C; irrespective of this preincubation, approximately 12% of the nuclear receptor resisted extraction with 2 M NaCl/5 M urea.