Secretion of newly synthesized proteins by rabbit endometrial stromal cells in culture was studied. Progesterone (P) stimulated the synthesis and secretion of a protein with a mol. wt of approx 62 K and a pI of 6.5-7.0. Induction of 62 K protein synthesis was dose dependent; addition of 10 nM P to primary cultures caused a 2-fold or greater increase in the amount of this protein in the medium while addition of 1 μM P resulted in a 4.3-fold increase. Synthesis of this 62 K protein was not induced by estradiol, dexamethasone, or testosterone, nor was progesterone stimulation of the protein modified by estradiol. Thus, induction appears to be specific for a progesterone receptor mediated effect. Synthesis and secretion of this protein in the culture system described is potentially useful as a progestin bioassay.
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