Effects of Estrogen Replacement Therapies on Mouse Platelet Funcion.

Clinical trials of hormone replacement therapy (HRT) have demonstrated an increase in stroke and myocardial infarction despite the beneficial changes in LDL and HDL cholesterols, and it has been suggested that HRT may adversely affect thrombosis and/or inflammation. Platelets participate in both of these processes and play a central role in thrombosis formation in high shear arterial vasculature. The effects of HRT on platelet function have been controversially reported, probably due to different hormone regimes used (e.g. different estrogenic compounds, different dosages and routes of administration). We used a murine model of menopause to test the hypothesis that HRT alters platelet reactivity. Bilaterally ovariectomized female littermates were treated with various estrogen regimes or matching placebo for 21 days. Three estrogen replacement therapy (ERT) regimes were studied: 1) oral conjugated equine estrogen (CEE) vs. placebo (n=5 pairs), 2) oral 17-beta estradiol (E2) vs. placebo (n=4 pairs), and 3) subcutaneously implanted E2 (SQ E2) vs. placebo (n=8 pairs). Washed platelets were used to focus on intrinsic platelet reactivity in the absence of plasma coagulation factors. Platelet reactivity was assessed by flow cytometric quantification of agonist-induced fibrinogen binding. Oral CEE, but not oral E2 enhanced the platelet response to collagen-related-peptide (CRP) (P<0.001 and P=0.87 for oral CEE and oral E2, respectively). This enhancement of platelet function was agonist specific since neither oral CEE (P=0.862) nor oral E2 (P=0.917) affected the platelet response to thrombin. We also tested whether the route of hormone administration affected platelet function. SQ E2 induced a condition different from that of oral E2: the platelet response to thrombin was enhanced (P=0.028), but only to low (<10 mU/ml), not high concentrations of thrombin. In contrast to oral E2, SQ E2 caused a reduction in CRP-induced platelet fibrinogen binding (P=0.030). Platelet surface expression of GPVI was reduced in response to SQ E2 (P<0.001), and the level of GPVI correlated with CRP-induced platelet fibrinogen binding (P<0.0001). In conclusion: 1) the effects of different estrogen replacement therapies on platelet function differ significantly, perhaps explaining some of the conflicts in the literature, 2) the effects of ERT on platelet function are dependent upon the ERT preparations and routes, 3) the effects of ERT on platelet function are agonist and agonist dose-dependent, and 4) SQ E2 induces lower platelet GPVI expression, most likely explaining the reduced response to CRP. A potential practical consequence of our findings is that oral E2 may be less prothrombotic than either oral CEE or transdermal E2.