Interaction between uterine PGE and PGF2α production and the nitridergic system during embryonic implantation in the rat

Embryonic implantation is a complex process in which both maternal andembryonic signals are involved. In the present study, we evaluated changes in uterine prostaglandins production and nitric oxide synthase (NOS) activity during the course of early pregnancy and their interaction during implantation in rats. Uterine phospholipase A₂ (PLA₂) activity is increased on days 5 (day of ovoimplantation) and 6, compared to preimplantation days (3 and 4). This enhanced activity might be responsible for the observed increase in uterine PGE and PGF_2α production observed on day 5 of pregnancy, which induces endometrial vascular permeability and decidualization. When embryo access to the uterus is impaired, the increase of PG production is suppressed. During postimplantation, PGE levels return to preimplantation values, while PGF_2α decreased with respect to preimplantation values. Uterine NOS activity is also increased on day 4 and reaches a maximum on day 5, with a profile similar to PGE and PGF_2α Dexamethasone administered in vivo decreased uterine NOS activity on day 4 of pregnancy but not on day 5, suggesting the presence of at least two types of NOS enzymes in the early days of pregnancy. A competitive inhibitor of NOS, L-NAME (600 and 1000 μM) induced a decrease in PGE and PGF_2α production in uterine tissue on day 5 of pregnancy. These results suggest the existence of a physiologically relevant nitridergic system which modulates prostaglandin production in the rat uterus during embryonic implantation.