Steroid regulation of retinol-binding protein in the ovine oviduct

Two studies were conducted to identify retinol-binding protein (RBP) expression in the ovine oviduct and to determine the role of ovarian steroids in its regulation. Ewes were salpingectomized on Days 1, 5, or 10 of their respective estrous cycles, and oviducts were homogenized for RNA analysis, fixed for immunocytochemistry (ICC), or cultured for 24 h for protein analysis. ICC localized REP to the epithelium of all oviducts. REP synthesis was demonstrated by immunoprecipitation of radio-labeled REP from the medium of oviductal explant cultures. Explant culture medium from oviducts harvested on Day 1 contained significantly more REP than medium from oviducts collected on Days 5 or 10. Slot-blot analysis demonstrated that steady-state REP mRNA levels were significantly higher on Day 1 than Day 5 or 10. In the second experiment, ovariectomized ewes were treated with estradiol-17 beta (E-2), progesterone (P-4), E-2+P-4 (E-2+P-4), or vehicle control, and oviducts were analyzed as above. P-4 alone or in combination with E-2 significantly reduced steady-state REP mRNA levels compared to those in E-2-treated animals. Oviductal explants from E-2- and E2+P4-treated animals released 3- to 5-fold more REP into the medium than control and P-4 treatments as determined by ELISA. REP synthesis of metabolically labeled REP was increased by E-2 and E2+P4 treatments. This study demonstrates that P-4 applied on an estradiol background negatively regulates REP gene expression in the oviduct whereas estradiol appears to stimulate REP synthesis and secretion.