Effects of estradiol on platelet aggregation in mouse mesenteric arterioles and ex vivo.

Mice were implanted subcutaneously with a pellet containing 0.5 mg estradiol or with a placebo. Eight to 12 days later platelet aggregation was produced in mesenteric arterioles by injuring their endothelium in vivo with a noxious light/dye stimulus. The time between the onset of the noxious stimulus and the appearance of platelet aggregates was significantly shortened (p less than .01) in the estradiol treated mice. In contrast to this enhancement of aggregation, when platelets were tested ex vivo in platelet rich plasma (PRP), aggregation to sodium arachidonate was inhibited in estradiol treated mice. Aggregation of PRP by ADP was not affected by estradiol treatment of the mice. Thus the enhanced aggregation observed in injured mesenteric arterioles of estradiol treated mice may not reflect direct effects of estradiol on the platelet itself. Rather enhanced aggregation may reflect an effect of estradiol on endothelium or adjacent tissue. The data are discussed in light of other literature concerning estradiol effects in rodents, and in light of literature suggesting that an increased number of thromboembolic events occurs in humans receiving high doses of estrogens.