OVULATION IN ADULT RATS AFTER TREATMENT WITH PREGNANT MARE SERUM GONADOTROPHIN DURING OESTRUS

ABSTRACT Treatment of the adult rat with pregnant mare serum gonadotrophin (PMS) followed by human chorionic gonadotrophin (HCG) is the standard procedure for inducing superovulation. Experiments were performed on rats with a 5 day cycle to determine why treatment with PMS only does not produce superovulation. In untreated animals all follicles in a range of [2267] 55 × 106 μm3 take part in ovulation. Similarly, in precocious ovulation induced by HCG in otherwise untreated animals, all follicles in this size range produce ovulations. After the injection of 5 IU of PMS into rats during oestrus the number of follicles in the size range of [2267] 55 × 106 μm3 is doubled, but only half of them take part in spontaneous ovulation, which occurs one day earlier than in untreated animals. An additional ovulating stimulus by means of treatment with HCG causes no increase in the number of ovulations. Data from hypophysectomized animals receiving HCG indicate that the ovulatory release of luteinizing hormone (LH) is not subnormal following treatment with 5 IU of PMS. After the administration of 10–35 IU PMS in oestrus, spontaneous ovulation does not occur. Data on hypophysectomized animals receiving HCG indicate that at this dose level of PMS, the ovulatory release of LH is subnormal. Indirect evidence suggests that this is due to high oestrogen levels in the blood, blocking the ovulatory release of LH. After 50–80 IU of PMS spontaneous ovulation of a small number of ova occurs on day 3. The ovulatory release of LH, estimated as in the previous experiments, is not distinctly subnormal. Therefore at this dose level of PMS a diminished responsiveness of the ovaries is responsible for the subnormal number of ovulations.