Prostaglandins in the uterus: modulation by steroid hormones.

Phospholipase A2 (PLA2), one of the enzymes considered to be rate-limiting in generating free arachidonic acid for prostaglandin (PG) synthesis, endogenous concentrations and in vitro production of PGs in the rat uterus were studied under various experimental conditions. Uterine PLA2 activity showed a 167-fold increase in ovariectomized rats bearing estradiol-17 beta (E2)-implants as compared to those treated with vehicle only. On the other hand, dexamethasone treatment reduced the E2-stimulable PLA2 activity by about 24-fold. The uterine PLA2 activity in the ovariectomized rat uterus was low and not altered by instillation of progesterone (P4) implants or by administration of dexamethasone. On the contrary, simultaneous placement of E2- and P4-implants prevented significantly the rise in PLA2 activity as observed under unopposed E2 exposure. Dexamethasone treatment further reduced the activity. The endogenous concentration of uterine PGF was several fold higher in the E2-implanted ovariectomized rats as compared to those without the E2-implants or carrying only P4-implants. The simultaneous treatment of the E2-implanted rats with P4 and/or dexamethasone reduced the uterine PGF concentrations considerably. The uterine PGF concentration was always lower in the ovariectomized rats under any condition if they were not treated with E2. Uterine PGE-A concentration did not change significantly between the ovariectomized rats and the ovariectomized rats carrying E2-implants. The treatment with P4 and/or dexamethasone, however, tended to decrease the PGE-A concentration. The production of PGF by the uterine homogenate increased by several fold in ovariectomized rats implanted with E2-silastic capsules as compared to those without the E2 implants. The treatments of the E2-implanted rats with P4 or dexamethasone did not alter this production. However, simultaneous exposure of E2-implanted rats to P4 and dexamethasone lowered the production rate of PGF in the uterus. The treatment of the ovariectomized rats with dexamethasone of P4 tended to elevate the uterine PGF production. The uterine PGE-A production followed more or less the same pattern. The analysis of our present data suggests that although a relationship exists between uterine PLA2 activity and PGF concentration, the role of PG synthetase could also be important in regulating PGF synthesis. Our study with dexamethasone, which showed inhibition of uterine PLA2 activity and decline in endogenous but not in vitro production of PGs, indicate that cellular integrity is essential for PLA2 to function as a rate-limiting step in PG synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)