ROLE OF OVARIAN-STEROIDS IN DEVELOPMENT OF UTERINE BINDING-SITES FOR PROLACTIN IN THE FERRET

The objectives of this study were to investigate 1) the presence of sp ecific prolactin (PRL) binding sites in the ferret uterus, 2) the uter ine location of I-125-labeled ovine PRL (oPRL) binding sites, 3) chang es in uterine PRL binding sites during pseudopregnancy, and 4) regulat ion of PRL binding sites by ovarian steroids. Binding was determined t hrough use of I-125-oPRL and 300-800 mug of protein from the 50 000 x g particulate fraction. Optimal binding occurred within 6 h at 25-degr ees-C. Scatchard analysis of saturation data revealed a single set of high-affinity (K(d) = 4.99 x 10(-11) +/- 0.88 M), low-capacity (22.76 +/- 1.62 fmol/mg) binding sites. Analysis of hormonal specificity reve aled that ovine growth hormone (oGH) cross-reacted with oPRL for the u terine binding sites, displacing 38% of the bound ligand. However, no inhibition of I-125-oPRL binding occurred in the presence of a 500-fol d excess of bovine thyroid-stimulating hormone (bTSH), ovine LH (oLH), or ovine FSH (oFSH), suggesting hormonal specificity of the binding s ites that are located in the luminal and glandular epithelium. Prolact in binding to ferret uterine membranes increased during the first half of pseudopregnancy, plateaued between Days 21 and 28, and then declin ed. The concentration of PRL binding sites in uteri of ferrets on Day 1 of pseudopregnancy was 4.91 +/- 0.42 fmol/mg of protein. Ovariectomy increased PRL binding (7.51 +/- 0. 1 2 fmol/mg), whereas ovariectomy and treatment with either estradiol (4.42 +/- 0.12 fmol/mg) or progest erone (4.57 +/- 0.03 fmol/mg) reduced the concentration of PRL binding sites to levels not significantly different from that on Day 1 of pse udopregnancy. Ovariectomized ferrets treated with estradiol and proges terone exhibited the highest concentration of PRL binding sites (8.09 +/- 0.02 fmol/mg). These data suggest that ovarian steroids have a rol e in the development of uterine PRL binding sites, but the mechanism b y which these steroids interact to enhance PRL binding is unknown.