Reduced plasminogen activator content of the endometrium in oral contraceptive users

Human endometrium was found to contain two different plasminogen activators, urokinase and tissue activator. Urokinase was released in higher amounts from endometrial tissue explants obtained in the midcycle phase than from those obtained in the luteal phase. Plasminogen activator activity of the culture medium followed the same pattern. Treatment of postmenopausal patients with ethinylestradiol resulted in liberation of urokinase and tissue activator from endometrial explants in concentrations similar to those found in the normal midcycle phase. In contrast, treatment with oral contraceptives (OCs), containing ethinylestradiol and a progestagen, resulted in lowered release of both activators, even lower than was found during the normal luteal phase. Also the amounts of extractable urokinase from endometrial tissue samples were significantly lower in OC-users than in non-users. Estradiol seems to have a stimulatory effect on the release of plasminogen activators from the endometrium; whereas, gestagens depress the content and release of activators. The low content of plasminogen activators in the endometrium explains the reduced menstrual bleedings found in OC-users.