Biology of Reproduction, Vol 12, 584-589, Copyright © 1975 by Society for the Study of Reproduction

Progestin Levels after Inhibition of Postpartum Ovulation in Rats

¹ Laboratory of Human Reproduction and Reproductive Biology and Department of Anatomy, Harvard Medical School, Boston, Massachusetts 02115

Postpartum ovulation was blocked in rats by treatment with antisera against LH releasing hormone (A-LHRH) beginning on day 20 of pregnancy. The decline in peripheral progesterone levels at the end of pregnancy was similar in control and A-LHRH animals. After parturition, progesterone levels increased in controls but remained low in postpartum ovulation blocked rats and ovarian weights in these animals were less. On day 8 postpartum, ovarian progesterone secretion was 270 times greater in controls than A-LHRH treated rats; however, 20-OH-P levels were similar in both groups. Blood samples were obtained from 12 postpartum ovulation blocked and 11 control rats at 0, 24, 48, 60 and 72 h after pups were removed on day 7 postpartum. The return to ovulatory cycles was synchronized as all 12 postpartum ovulation blocked rats showed a preovulatory progesterone rise at 60 h and tubal ova at 72 h postweaning. In controls 5 of 11 animals ovulated or were expected to ovulate after pups were removed while the remaining 6 were classified as "postpartum pseudopregnant". From the results of these studies, we concluded that during lactation the corpora lutea of gestation do not secrete progesterone but are the major contributor to 20-OH-P secretion during midlactation, and the return to ovulatory cycles is highly synchronized after pups are removed from rats without active corpora lutea.