Increased euglobulin fibrinolytic potential in women on oral contraceptives low in oestrogen--levels of extrinsic and intrinsic plasminogen activators, prekallikrein, factor XII, and C1-inactivator.

Components of the fibrinolytic system were studied in samples of plasma from 15 normal, young women and from 11 women taking oral contraceptives containing 30 micrograms ethinyl oestradiol and 150 micrograms levo-norgestrel. Fibrinolytic activity of euglobulins precipitated at pH 5.9 was higher than normal in the hormone group, with significant fluctuations related to the cycle. Normal women showed only minor fluctuations. The concentration of C1-inactivator was lower in euglobulins of the hormone group. However, the difference in fibrinolytic activity was retained, when C1-inactivator was inactivated with sodium flufenamate. Fluctuations of the extrinsic (tissue-type) plasminogen activator (t-PA) activity parallelled those of the euglobulin activity. The intrinsic plasminogen activator activity (dextran sulphate precipitated euglobulin) was significantly increased in the hormone group and the cyclic pattern differed from that of the normal group. The increased activity was factor XII-dependent. Plasma prekallikrein did not differ. The factor XII level was increased in the hormone group but this could not explain the increased intrinsic fibrinolytic activity, suggesting an increase in the quantity of an additional factor XII-dependent proactivator.