In the present study we examined the effects of estrogens on plasma platelet-activating factor acetylhydrolase (PAF-AH) activity in pregnant rats in an attempt to further define the role of platelet-activating factor (PAF) in parturition. The plasma PAF-AH activity decreased starting on Day 13 of pregnancy and reached its lowest level by Day 19 (36.5 ± 5.7 nmol.min⁻¹.ml⁻¹ plasma, mean ± SD). The maternal plasma PAF-AH activity rapidly increased to 67.3 ± 3.5 nmol.min⁻¹.ml⁻¹ plasma by Day 2 postpartum, to 88.8 ± 14.4 by Day 7, and to 109.1 ± 4.4 by Day 21, returning to nonpregnant levels by Day 35 in the dams that were allowed to nurse through Day 21. The PAF-AH activity did not increase in dams from which the pups were removed 2 days after delivery.

When 17α-ethynylestradiol (0.25 mg/kg) was administered s.c. to pregnant rats for 3 days starting at Day 17 of pregnancy, the plasma PAF-AH activity in the animals decreased by 80% on Day 19, and the rats delivered prematurely on Day 21. When a similar dose of 17α-ethynylestradiol was injected into the pregnant rats for 3 days starting on Day 15, the enzyme activity again decreased by approximately 80% on Day 17 and Day 19 of pregnancy; however, it returned to a value similar to that seen in the controls by Day 21, and the time at which the treated rats delivered was similar to that for the control animals.

These observations provide further support for a role of PAF and PAF-AH and their possible regulation by estrogens in the initiation and maintenance of parturition.