Effects of oral combined hormone replacement therapy on platelet aggregation in postmenopausal women

The effects of combined oestrogen/progestin hormone replacement therapy (HR T) on platelet aggregation were studied using women on HRT or placebo. The study involved 32 postmenopausal women (aged 50-75 years) who were enrolled in a double-blind randomized controlled trial, and who received either ora l continuous combined HRT (Kliogest(R); 2 mg of oestradiol + I mg of noreth isterone)or placebo for a minimum of b months. Platelet aggregation was mea sured by whole-blood impedance aggregometry in response to the agonists col lagen, arachidonic acid and ADP. To determine whether the effects of oestro gen on platelets were influenced by platelet-derived nitric oxide, exposure to collagen was repeated in the presence of the nitric oxide synthase inhi bitor N-G-monomethyl-L-arginine (L-NMMA). Mean platelet volume was similar in the two groups. Compared with the placebo group. the women on HRT had si milar rates and maximum values of platelet aggregation in response to colla gen, arachidonic acid and ADP. Addition of L-NMMA did not alter the aggrega tion response to collagen in either the HRT or the placebo group. In conclu sion, postmenopausal women on oral combined continuous HRT comprising oestr adiol and norethisterone had similar whole-blood platelet aggregation rates and maximum platelet aggregation responses to higher doses of platelet ago nists when compared with those on placebo. The endogenous platelet nitric o xide system did not appear to affect aggregation in either group.