Gonadotropin-like substance in the preimplanted rabbit blastocyst

The presence of a gonadotropin-like substance in preimplanted rabbit blastocyst fluid was determined bu radioreceptorassay of human chorionic gonadotropin (hCG)/luteinizing hormone (LH), using receptors prepared from bovine corpus luteum, rat testis, and rat ovaries. An average of 16.6 microliter of fluid containing 0.83 ng of luteotropic material was recovered from each blastocyst. An intense fluorescence was exhibited by the trophoblastic cells of the blastocysts treated with fluorescein-conjugated gamma-globulin isolated from antiserum raised against human chorionic gonadotropin. Serum concentrations of LH as well as estradiol, progesterone, and 17 alpha-hydroxyprogesterone up to day 6 after mating were determined in pregnant rabbits and compared with those in pseudopregnant and normal rabbits. In pregnant rabbits after mating, a surge of 62 +/- 15 ng of luteinizing hormone and 73 +/- 22 ng of progesterone/ml of serum occurred which returned to basal levels at 6 hours and on day 1, respectively. Secondary increases in serum luteinizing hormone of 26 +/- 12 to 36 +/- 16 ng/ml on days 3 and 5 and in serum progesterone of 16 +/- 4 and 14 +/- 5 ng/ml on days 5 and 6, respectively, were observed in pregnant rabbits.