Progesterone Receptors in the Chick Oviduct.

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Abstract
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 10000 sites/cell to approximately 40000 sites/cell in the magnum cytosol; subsequent withdrawal from oestrogen treatment led to a decrease to approximately 14000 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 After progesterone administration (3 mg/kg) to oestrogen-stimulated, withdrawn chicken, the receptor concentration decreased to approximately 40% of the initial level within the first 4 h; afterwards the receptor level rose again and by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 (∼ 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 C; irrespective of this preincubation, approximately 12% of the nuclear receptor resisted extraction with 2 M NaCl/5 M urea.