Diminished responsiveness of human platelets to platelet-activating factor during pregnancy

Platelet-activating factor (PAF; PAF-acether) is a phospholipid that is synthesized and released by a number of inflammatory cells and is a potent inducer of platelet aggregation and secretion. In the present study we examined the responsiveness of human platelets to various concentrations of PAF during pregnancy and postpregnancy. We found that in pregnant individuals the responses of washed gel-filtered platelets to PAF were either absent or greatly reduced during the 22nd-30th wk of pregnancy and returned to normal in the postpartum period. In contrast, the responses of platelets to various concentrations of ADP were not altered during pregnancy. We propose that during pregnancy circulating levels of PAF in the maternal circulation, possibly originating from the fetus, may desensitize maternal platelet receptors to PAF, resulting in a diminished or a complete lack of response of platelets, specifically to PAF. These findings may have important implications for our understanding of the role of PAF in hemostasis during pregnancy and in embryonic and fetal development.