Progesterone in vitro and the output of prostaglandin F from the uterus of ovariectomized rats. Priming or withdrawal?

The effects of progesterone in the solution suspending uterine preparations isolated from ovariectomized rats on the output of PGF2 infinity and PGE2-like material, were explored. Uterine incubation for one hour with progesterone (50 or 100 micrograms/ml, final concentration) did not alter PGF or PGE release into the bath medium, when compared with untreated controls. One hour after hormone removal the output of either prostaglandin, from uteri previously primed with progesterone at 50 micrograms/ml, was also similar to controls. However, preparations exposed to progesterone at 100 micrograms/ml for one hour and subjected afterwards to a second incubation interval of 60 minutes without the hormone, released into the suspending medium significantly more PGF, but not PGE, than controls. On the other hand, incubation with progesterone (at 100 micrograms/ml) for 120 minutes (not followed by a period of steroid removal) failed to change PGF and PGE uterine output. These findings show that there is a direct but complex effect of progesterone on rat uterine release of PGF, i.e., the positive correlation between the steroid and PGF required not only a priming period with a high concentration of progesterone, but also its subsequent withdrawal. Such peculiarity is discussed in terms of a possible influence of progesterone on enhancing intramural uterine levels of PGF and inhibiting prostaglandin release, and also in relation to the role of hormone withdrawal after corpora lutea regression on PGF concentration in blood from uterine veins during the sex cycle.