Suppression of ovulation rate by antibodies to tissue-type plasminogen activator and alpha 2-antiplasmin

Indirect evidence has suggested a role for plasminogen activator (PA) in ovulation. Our recent studies demonstrated that 1) tissue-type PA (tPA) is the predominant PA produced by preovulatory rat follicles in response to gonadotropins or GnRH; and 2) several inhibitors of the serine proteases, to which PA and plasmin belong, block ovulation. Here, the role of tPA and plasmin in ovulation was examined directly by the use of specific antibodies to tPA and alpha 2-antiplasmin (alpha 2AP). Immature female rats at 25-26 days of age were treated (sc) with 15 IU PMSG to induce multiple preovulatory follicles. Fifty-four hours later, tPA antibodies and alpha 2AP were injected into one of the ovarian bursae to check their ability to block ovulation, which was initiated with an ovulatory dose (4 IU) of hCG. The data are expressed as percent inhibition of ovulation in the treated vs. the untreated ovaries. A significant decrease in the ovulation rate was obtained by administration of 500 micrograms antibodies to tPA (39.6%) or 1-50 micrograms alpha 2AP (36-44%), whereas minimal inhibition (12%) was found at lower doses of anti-tPA (10 micrograms) or alpha 2AP (0.1 micrograms). Furthermore, nonimmune immunoglobulin G (500 micrograms) and heat-inactivated alpha 2AP were not effective. Anti-tPA and alpha 2AP suppressed ovulation only when injected at the time of hCG administration; later injections (4-h delay) were ineffective, suggesting that PA and plasmin are involved in the early follicular responses to the ovulatory stimulus. Histological observation of the ovaries did not reveal any pathological changes associated with the anti-tPA and alpha 2AP treatment. Suppression of ovulation, as evidenced by decreased number of tubal ova, was frequently accompanied with intraovarian release of the eggs into the follicular thecal compartment. Thus, these results provide direct evidence for an essential role of tPA and plasmin in ovulation.