Suppression of ovarian function by LHRH and its analogues in pregnant rats.

1. LHRH and its analogues, when infused continuously at the doses that inhibit ovum implantation, reduced serum concentration of progesterone in pregnant rats. The progesterone concentration fell between Days 3 and 4 of pregnancy in rats treated with LHRH and Analogue I (Des-Gly10-LHRH-ethylamide), but fell between Days 4 and 5 in rats treated with Analogue II [Des-Gly10-(D-Ala6)-LHRH-ethylamide]. 2. Ovarian LH/hCG receptor levels were markedly reduced in rats treated with LHRH and its analogues in the first week of pregnancy. 3. Continuous infusion of LH to pregnant rats did not reduce the serum concentration of progesterone even with the highest dose that inhibited implantation. Interruption of pregnancy by LH appears to be due to excessive secretion of estrogen as indicated by uterine swellings, presence of large follicles in the ovaries, and absence of free blastocysts in the uterine lumen. Thus, the antifertility effects of LHRH and its analogues were not reproduced by LH administration.