Secretion of a progesterone-induced inhibitor of plasminogen activator by the porcine uterus

The indirect ¹²⁵I-fibrin plate assay has been used to measure the levels of plasminogen activator (PA) in uterine flushings from pigs through the estrous cycle and during early pregnancy, and to measure the production of PA by pig conceptuses cultured in vitro. Activity in the flushings was high at the beginning and end of the estrous cycle, but only low levels were detected in mid cycle (the luteal phase). In pregnant animals, uterine PA levels became low around day 12 and did not show any further increase. Cultured day 12 blastocysts, however, released large amounts of PA into the medium in a time-dependent fashion over a 48 hr period, suggesting that this activity was inhibited in vivo. The presence of a protease inhibitor in uterine flushings has been demonstrated in cycling gilts, and follows a hormone-directed trend, with flushings taken during the luteal phase showing inhibitory activity against PA secreted early or late in the cycle. By assaying flushings from ovariectomized gilts given daily injections of progesterone, estrogen, both hormones together, or corn coil, it has been verified that the inhibitor is progesterone-induced and is also active against both PA produced by day 12 conceptuses and urokinase. It also inhibits PA, as determined using a direct fluorometric assay with glutaryl-glycyl-L-arginine-4-methyl-coumarinyl-7-amide as substrate. The PA inhibitor is acid-stable, and of low molecular weight (15,000 ± 5000), as determined by Sephacryl S-200 gel filtration. Unlike most animals, the trophoblast of the pig is not invasive in the uterus, but is invasive if transplanted to some ectopic site. The progesterone-induced inhibitor may possibly play a role in preventing invasive implantation.