In VivoUptake of Progesterone by the Hypothalamus and Pituitary of the Female Ovariectomized Rat and its Relationship to Cytoplasmic Progesterone-Binding Protein

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The in vivo uptake of 3H-progesterone by the hypothalamus and pituitary of the female ovariectomized estrogen-primed rat was studied following an iv. injection of the radiolabeled steroid. 3H-progesterone was preferentially taken up by the pituitary and median eminence of the hypothalamus, but neither by the cerebral cortex nor by the remainder of hypothalamus. In addition, 3H-progesterone taken up by the tissues was mainly distributed in the cytoplasm, but not in the nucleus. Utilizing Sephadex G-100 column chromatographic analysis, characterization of progesterone-binding protein in the cytosol of the median eminence and pituitary was attempted. The binding protein in both tissues was of macromolecular component with 4.3-7.1S and preferentially bound progesterone, but neither corticosterone nor estrogen. The binding protein was easily saturated by a large amount of unlabeled progesterone, having a limited capacity to bind progesterone. It has been concluded that the cytosol progesterone-binding protein in the median eminence and pituitary is specific for progesterone with molecular size of 4.3-7.1S and has a limited capacity to bind progesterone.
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