Platelet-activating factor (PAF) and PAF acetylhydrolase activity in rat uterus and placenta during the late stages of pregnancy.

We evaluated the roles of platelet-activating factor (PAF) and PAF-acetylhydrolase (PAF-AH) activity in late pregnancy. Uterine and placental concentrations of PAF were determined by the washed rabbit platelet aggregation bioassay. Uterine, placental, and plasma PAF-AH activities were also assayed. PAF concentration in the uterus increased 4-fold between Days 15 and 21 of pregnancy. PAF was also determined in the placenta on Days 15 and 21. In contrast to findings in the uterus, the concentration in the placenta was decreased by 75%. Platelet aggregation caused by uterine and placental PAF was inhibited by the PAF receptor antagonists CV-3988 and TCV-309. Plasma and uterine PAF-AH activities decreased significantly between Days 15 and 21. In contrast, the placental PAF-AH activity significantly increased during this same time period. On the basis of these findings, it is suggested that the PAF concentration in the uterus and placenta may be regulated by intracellular PAF-AH and/or plasma PAF-AH activities. Increased PAF activity in the pregnant rat uterus may be related to the initiation of labor due to its known effect on myometrial contraction. Decreased PAF concentration in the placenta may contribute to the fetoplacental circulation due to its known hypotensive activity and the increase in vascular permeability.