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.

[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.

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