[Progestogens, progesterone, coagulation and vascular tone]

Epidemiological studies suggest that estrogens may have opposite effects on vessels: estrogens used for contraception are known to increase both arterial and venous risk, while hormone replacement therapy could reduce the risk of cardiovascular disease. In both situations, estrogens are associated with progestogens. Progestogens are rarely used alone, thus the effect of progestogens on haemostasis or vessel wall is unclear. Data can be obtained from studies using progestogens alone or from studies comparing unopposed estrogens to combined estrogen-progestogen therapy. Progestogens alone have few effect on the haemostatic system. In combined therapy used for contraception, progestogens modify the effects of estrogens on haemostasis and endothelium: the overall effect, including modifications of coagulation factors and inhibitors could a prothrombogen trend, mainly for third generation progestogens. Unopposed estrogens are also rarely used for post menopausal hormone replacement therapy (HRT). Experimental studies have shown that progestogens are able to inhibit the beneficial effect of estrogens. Two mechanisms have been suggested: first, progestogens may reduce the endothelium-dependent vasodilatator action of estrogens. Another explanation concerns the neointimal proliferation leading to atherosclerosis: Estradiol are known to reduce this proliferation. Progestogens could reduce the protective effect of estrogens. These pharmacological effect of progestogens must be taken in account to interpret the negative results of HERS study that failed to demonstrate a cardiovascular benefit of estrogens plus progestin therapy in postmenopausal women.