Steroid regulation of the synthesis and secretion of retinol-binding protein by the uterus of the pig.

The endometrium of the pig secretes retinol-binding protein (RBP) under the influence of progesterone (P4). The objective of this study was to determine how conceptus-derived estrogen might modulate this production of RBP around days 11-13 of pregnancy when conceptuses elongate from spheres to long thread-like forms. Concentrations of retinol and RBP were low (35 +/- 7 ng/ml) in uterine flushings obtained on days 10-12 of the estrous cycle or from pregnant gilts in which conceptuses had not elongated. Concentrations of retinol and RBP increased 7- to 8-fold (P less than 0.01) in flushings where filamentous conceptuses were present. Size exclusion and ion exchange chromatography demonstrated that virtually all retinol assayed in uterine flushings was associated with RBP. Northern blot analysis with a cDNA representing uterine RBP revealed a single endometrial mRNA 1.1 kilobases in length. Expression of RBP mRNA in uterine endometrium was measured in ovariectomized prepubertal gilts after the administration of steroids according to the following regimens: I, corn oil (days 0-16; n = 10); II, estradiol benzoate (EB; days 13-14; n = 11); III, EB (days 1-2; n = 12); IV, EB (days 1-2) plus P4 (days 3-16; n = 12); and V, EB (days 1-2) plus P4 (days 3-16) plus EB (days 13-14; n = 12). EB (200 micrograms) and P4 (100 mg) were administered twice daily. Treatment IV was designed to stimulate the estrous cycle, and treatment V simulated early pregnancy. All gilts were hysterectomized on day 16, and total uterine mRNA (3 micrograms) was analyzed by Northern blotting. No RBP mRNA was detected in groups I, II, or III. In group IV, 5 of 12 gilts had detectable RBP mRNA, as measured by densitometric scanning (OD = 0.35 +/- 0.14). RNA isolated from all gilts in group V (12 of 12) gave a strong hybridization signal (OD = 1.58 +/- 0.22) for RBP. Finally, RBP mRNA was examined in the uterine endometrium of mature gilts on day 13 of the estrous cycle (n = 4), day 13 of pseudopregnancy (2.5 mg EB given on days 11-12; n = 4), or day 13 of pregnancy after conceptuses had elongated (n = 4). RBP mRNA was present in all groups, but was enhanced approximately 12-fold (P less than 0.01) in pregnant and pseudopregnant gilts compared to that in control gilts.(ABSTRACT TRUNCATED AT 400 WORDS)