On the role of prostaglandins in parturition in the rat.

Treatment of both intact and hypophysectomized pregnant rats on days 19 and 20 of pregnancy with PGF2alpha results in a rapid fall in plasma progesterone concentrations by day 20 followed by premature delivery on day 21. Administration of depoprovera did not prevent the PGF2alpha-induced fall in progesterone levels but premature littering was adverted. Indomethacin, an inhibitor of prostaglandin biosynthesis, postponed the onset of parturition by 20 h when administered in late pregnancy but rats treated with both indomethacin and PGF2alpha delivered at the normal time. Treatment of both intact and hypophysectomized animals with indomethacin delayed the fall in plasma progesterone concentrations which normally occurs between days 21 and 22 of pregnancy. Concomitant PGF2alpha treatment of rats receiving indomethacin caused plasma progesterone levels to fall in a pattern similar to controls. These findings indicate that endogenous PGF2alpha induces a fall in blood progesterone concentrations which is followed by increased myometrial activity.