Platelet alpha-2 adrenoceptors and the menstrual cycle.

To clarify further the suggested influence of menstrual cycle phase on platelet alpha 2-adrenoceptors, we carried out cross-sectional studies in 77 subjects in a clinical trial of weight control strategies. Blood samples were drawn at baseline and after 6 weeks of diet, behavior modification, and exercise, a program that resulted in a mean weight loss of 4.5 kg. For the analyses, 42 premenopausal women were divided into four groups according to the week of menstrual cycle at the time of blood sampling. At baseline, there was no significant difference in mean platelet alpha 2-adrenoceptor numbers among the four groups. At week 6 accompanying the weight loss, there was a significant increase in the platelet alpha 2-adrenoceptor number for all groups. Despite the fact that the women were at a different phase of the menstrual cycle than at baseline, there was again no significant difference in mean platelet alpha 2-adrenoceptor number. Mean baseline platelet alpha 2-adrenoceptor number in the premenopausal women (113.7 +/- 5.5 fmol/mg protein) did not differ from values in 12 postmenopausal women (113.7 +/- 12.0 fmol/mg protein), four women with hysterectomies (105.9 +/- 8.9 fmol/mg protein), or 19 men (101.8 +/- 6.2 fmol/mg protein). Numbers at 6 weeks also did not differ. We conclude that the menstrual cycle has minimal effects on platelet alpha 2-adrenoceptor number and should not confound clinical studies of platelet alpha 2-adrenoceptors.