Multiple causes of pregnancy failure in hamsters precociously ovulated by human chorionic gonadotropin.

Cycling adult female hamsters can be induced to mate and ovulate 24 h early by the injection of 20 IU human chorionic gonadotropin (hCG) at 1500 h on Day 3 (day before proestrus), but pregnancy is not established. Although there is evidence of decreased sperm transport in precociously ovulated females, this does not appear to be the primary cause of infertility. Reduced size and vascularity of corpora lutea (CL) in treated females suggests incomplete or failed CL activation. Control and hCG-treated females were killed by exsanguination under ether anesthesia at intervals for the first 5 days after mating. Serum luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, estradiol, and progesterone were measured by radioimmunoassay. Luteinizing hormone in treated animals was very high at 2200 h on Day 1 after mating (31 h after the hCG injection), due to endogenous release, and dropped below control levels thereafter. Follicle-stimulating hormone, by contrast, was significantly lower than controls at 2200 h on Day 1 and remained low until 2200 h on Day 3 after mating. Prolactin in treated animals was not different from that in controls, except for 1000 h on Day 4, when it showed a significant dip. Estradiol in treated animals was significantly higher than in controls at 2200 h on Day 1 (when LH was also high and FSH was low), and remained high at 1000 h and 2200 h on Day 2, dropping thereafter to control levels. Progesterone was initially at control levels but had dropped significantly by 1000 h on Day 2 and remained low for the next 24 h. These results suggest that pregnancy failure is due to inadequate activation of corpora lutea. This may be due to: 1) immaturity of follicles at the time of ovulation; 2) inappropriate timing of preovulatory events; 3) the luteolytic effects of high levels of LH or estradiol or both; 4) the low level of FSH in the early stages of corpus luteum development; or 5) a combination of the above. Abnormalities of prolactin secretion were not investigated in detail but cannot be ruled out at this time.