Inhibitory effect of progesterone on the lactogenic and abortive action of prostaglandin F2alpha.

The effect of ovariectomy, progesterone and prolactin treatment on the action of prostaglandin F2alpha (PGF2alpha) was determined in pregnant rats. PGF2alpha (150 mug times 2) injected i.p. on day 1. or 18 of pregnancy induced lactogenesis about 25 h later and abortion on days 20 and 21 of pregnancy. Treatment with PGF2alpha (100 mug times 2 or 50 mug times 2) on day 19 induced lactogenesis around 22 or 38 h later, respectively, and abortion on day 21. PHF2alpha treatment on day 17 was less effective. Unilateral ovariectomy on day 17 of pregnancy induced lactogenesis 32 h later but not abortion. PGF2alpha (150 mug times 2) given on the day of surgery advanced lactogenesis 12 h and rats aborted on day 19. Bilateral overiectomy on day 17 induced abortion between days 20 to 21, but if a single dose of PGF2alpha (300 mug) was injected on day 18. all the ovariectomized rats aborted on day 19. Progesterone (10 mg) injected into rats treated with PGF2alpha (150 mug times 2) on day 18, prevented abortion and delayed lactogenesis. Prolactin (1 mg times 4) treatment delayed only abortion. Serum prolactin levels were significantly higher 12 h after the last dose of PGF2alpha (150 mug times 2) in rats treated on days 17, 18 or 19 of pregnancy. Pretreatment with progesterone prevented the rise in prolactin concentration. These result suggest that the lactogenic and abortive action of PGF2alpha may be dependent on the uterine and plasma concentration of progesterone.