A Cytoplasmic Progesterone Receptor in Hen Pituitary and Hypothalamic Tissues

The cytosol fraction of the anterior lobe of the pituitary in laying White Leghorn hens was shown to possess a progesterone-binding component which perferentially bound progesterone. A similar component was also found in the cytosol of the hypothalamus and of the oviduct magnum. The dissociation constant (Kd) at equilibrium was determined as 1.68 +/- 0.10 (SE) X 10(-10) M in the pituitary, 3.29 +/- 0.10 x 10(-10) M in the hypothalamus, and 3.02 +/- 0.40 x 10(-10) M in the oviduct magnum, whereas the number of binding sites/mg cytosol protein was 5.99 +/- 0.084 (SE) X 10(-13) mol in the pituitary, 0.43 +/- 0.003 x 10(-13) mol in the hypothalamus, and 2.88 +/- 0.114 x 10(-13) mol in the oviduct magnum. The results suggest that a cytoplasmic progesterone receptor is present in the pituitary and hypothalamic tissue, as well as in the oviduct magnum, of the hen.