Superovulatory Doses of Pregnant Mare Serum Gonadotropin Cause Delayed Implantation and Infertility in Immature Rats12

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Abstract
Immature Sprague-Dawley rats were maintained with lights off between 1900 and 0500 h and treated with pregnant mare serum gonadotropin (PMSG) at 30 days of age (0800-0900 h, Day 0). One hundred and ninety-eight animals were caged individually with mature fertile males at 1500 h on Day 2, scored for the occurrence of mating on the morning of Day 3, and killed on Day 22. Rats received 4, 8, 16, or 40 IU PMSG; a high proportion of those receiving each dose mated. Of those which mated, almost all receiving 4 or 8 IU, 76% of those receiving 16 IU, and none receiving 40 IU were pregnant on Day 22. The mean numbers of live fetuses in pregnant rats that received 4, 8, and 16 IU PMSG were 8.8, 12.9, and 14.2, respectively. The corresponding means for numbers of implanted embryos were 9.7, 15.4, and 18.8; for numbers of corpora lutea of pregnancy (CLP), 10.8, 20.8, and 48.1; for fetal weight, 2.25, 1.65, and 1.38 g; for placental weight, 0.47, 0.38, and 0.36 g; for peripheral serum estradiol concentration, 104, 125, and 112 pg/ml; and for peripheral serum progesterone concentration, 91, 112, and 164 ng/ml. Serum estradiol levels were more correlated to numbers of live fetuses than to numbers of CLP (r = 0.32, 0.01<P<0.05; r = 0.26, 0.01<P<0.05, n = 57, respectively), whereas serum progesterone levels were more correlated to numbers of CLP than to numbers of live fetuses (r = 0.71, P<0.001; r = 0.36, 0.001<P<0.01, n = 57, respectively).