Progesterone binding was studied in the uterus of rabbits at two different hormonal stages; either after oestrogen priming or a short time before implantation of the blastocyst (162 h post coitum). Uterine cytosols were incubated with [3H]progesterone, or the labelled hormone was injected into the uterine lumen 1 h before killing the animals. Gel filtration, ion exchange chromatography, sucrose gradient centrifugation, isoelectric focusing and saturation analysis indicate that during the period prior to implantation, the uterine progesterone receptor disappears and progesterone binding is performed by uteroglobin. These findings support the hypothesis that the physiological role of uteroglobin in the reproductive process is connected with its hormone-binding ability.

No abstract

This study was undertaken to determine whether the influx of progesterone into the uterine lumen of the rabbit, in the preimplantation period, is dependent onuteroglobin (UGL). Rabbits were ovariectomized and, three months later, treated with two defferent doses of progesterone. Purified UGL was injected into one uterine horn and, as a control,immunoglobulin G (IgG) was injected into the other. After four days, the animals were sacrificed their uteri flushed, and the progesterone content of the washes was determined by radioimmunoassay.Animals with the lower serum progesterone level (2.8 ng/ml) had a significantly different uterine horn progesterone content. The hormone accumulation in the horn containing UGL was 2.3 to 7.5 times higher than in the horn containing IgG. Animals with a higher serum progesterone level (7.2 ng/ml) showed no differences. The hormone content was equally high in both horns, presumably due to the synthesis of endogenous UGL being reactivated by the hormone treatment.The validity of these experiments as models for the events during early pregnancy and the physiological role of progesterone available inside the uterus are discussed.

[3H] progesterone was injected into the uterine lumen of rabbits toward the end of preimplantation period (162 h post coitum). Light-microscopic autoradiography showed accumulation of label in single cell groups of the uterine epithelium. Fluorographs of thin layer chromatograms of steroid extracts indicated the metabolization of progesterone in the uterine tissue. Incubation of uterine sections with fluorescein isothiocyanate-conjugated progesterone-rabbit serum albumin revealed binding sites for this reagent: 162 h post coitum, staining was also localized in single cell groups of the uterine epithelium. Pretreatment with a monospecific antiserum showed uteroglobin to be the binding protein.

Unlabeled or 3H-labeled UGL, isolated from rabbit lungs, was injected intraluminally into the uteri of ovariectomized low dose progesterone substituted and unsubstituted animals as well as into mated animals on the fifth day of pregnancy. It was offered to the endometrium, in this last case, in competition with endogenously synthesized UGL. 3H-labeled IgG served as a control. The animals were killed 24 h after the experiment. Immunohistograms and autoradiograms showed that UGL was taken up by the whole endometrium of the ovariectomized animals but only when progesterone was present. In the preimplantation uterus, uptake was limited to clusters of smaller cells accumulated in the lumen adjacent to the epithelium, whereas IgG remained within the uterine lumen, forming a dense layer on the endometrial surface.