Inability of actinomycin D to act upon the uterine refractory state resulting from nidatory oestrogen action in rats.

After priming with oestradiol, ovariectomized rats were given 6 days of progesterone treatment in which two doses of 50 ng oestradiol were superimposed on days 3 and 6. This basic regimen allows the oestradiol-induced (1st injection) inhibition of uterine sensitivity to decidual stimuli to appear. Actinomycin D given instead of, or together with, the first dose of oestradiol could not mimic, or prevent oestrogen action in the occurrence of the uterine refractory state. This drug was unable to interfere with the minimal 48 h progesterone treatment which is requisite to uterine sensitization. Our results together with other data from the literature, suggest that luteal hormones do not need to act at the transcriptional level to sensitize and desensitize the uterus to implantation.