The Effects of Luteinizing Hormone Releasing Hormone (LHRH) in Pregnant Rats 2. Prenidatory Effects and Delayed Parturition

During the prenidatory period, pregnancy is terminated in rats by doses of 150 g twice daily (b.i.d.) of LHRH from Days 1-7; a partial effect is seen at 50 Mg. Treatment with doses as large as 500 Mg b.i.d. of LHRH over Days 1-3 did not result in termination of pregnancy, but the fetuses were smaller in size than controls at autopsy on Day 15. When injected twice on either Day 3 or 4, 500 g of LHRH markedly depressed fetal size; in contrast, the same treatment on Days 1, 2, 5, 6 or 7 was without effect. The depressions in fetal size were explainable on the basis of a 2-3 day delay of implantation following LHRH (500 Mg b.i.d.) on Day 3 or 4, which was secondary to retardation of tubal transport (Days 3 and 4) and slowed cleavage (Day 3). Estradiol given on Day 4, but not on Day 3, normalized the effects of LHRH treatment on Day 3 or 4, suggesting that the delay of nidation resulted from estrogen (ovarian) failure. Progesterone had no effect on the delayed implantation. Delayed nidation was followed by postponement of parturi-tion, albeit litters appeared normal. INTRO DUCT IO