Chorionic gonadotropin receptors and immunoreactive chorionic gonadotropin in implantation of the rabbit blastocyst

To determine the temporal relationship between immunoreactive chorionic gonadotropin, chorionic gonadotropin receptors, and implantation of the rabbit blastocyst, (1) immunoreactive chorionic gonadotropin in plasma, uterine flushings and blastocysts; (2) chorionic gonadotropin receptors on blastocysts (day 5 and 6) and embryonic and interembryonic segments of the uterus (day 7); and (3) chorionic gonadotropin receptors on the endometrium and myometrium (day 1 through 6) were measured. Binding of I125-labeled beta-subunit of human chorionic gonadotropin (hCG) by cell membranes from blastocysts increased significantly from 6.0 +/- 1.1 fmol/mg of protein (mean +/- SE) on day 5 (N = 6) to 16.1 +/- 1.3 fmol/mg of protein on day 6 (n = 6) (P less than 0.001). Immunoreactive chorionic gonadotropin levels in blastocyst fluid increased from 0.3 ng/ml of day 5 to 0.65 ng/ml on day 6. Specific binding of I125-labeled hCG was absent in endometrial and myometrial cell membranes before implantation (days 1 to 6) but was found in decidual cells from embryonic segments on day 7. Plasma immunoreactive chorionic gonadotropin or chorionic gonadotropin--like material increased from 6 ng/ml of day 1 to 52 ng/ml on day 7. Uterine flushings had chorionic gonadotropin levels of 0.4 ng/ml of day 2, which increased slowly to 1.0 ng/ml on day 7. Intrauterine instillation of hCG into nonpregnant uterine horns demonstrated transuterine absorption of hCG with plasma hCG levels showing a dose-related response. Our findings demonstrate that (1) immunoreactive chorionic gonadotropin or chorionic gonadotropin--like material is detectable in plasma, uterine flushings, and blastocyst fluid before implantation, (2) chorionic gonadotropin receptors are present on the blastocyst cells before implantation, and (3) the uterus can absorb chorionic gonadotropin from its lumen.