Phospholipase A2 activity in the rat uterus during early pregnancy.

The activity of phospholipase A2 (PLA2), which provides free arachidonic acid and is widely accepted as the rate-limiting step in the biosynthesis of prostaglandins (PGs), was determined by radioisotopic method in the rat uterus on days 1 through 6 of pregnancy. Serum levels of progesterone (P4) and estradiol-17 beta (E2) were also determined by radioimmunoassay. Uterine PLA2 activity peaked in the morning of day 4 of pregnancy (15.85 +/- 2.44 nmol/h/mg protein). While the activity was low (5.78 +/- 0.80) in the morning of day 5 of pregnancy, a small peak (9.12 +/- 1.62) was noticed in the afternoon of day 5. Serum P4 levels, as expected, showed a gradual increase with the advancement of pregnancy, but the serum E2 levels in the same samples were undetectable under our assay conditions. The results are discussed in relation to the hormonal control of PLA2 activity and hence PG production in the uterus during early pregnancy.