Changes in Ovarian Expression of Tissue-Type Plasminogen Activator and Plasminogen Activator Inhibitor Type-1 Messenger Ribonucleic Acids during Ovulation in Rat

We investigated the time course and localization of ovarian tissue-type plasminogen activator (tPA) and plasminogen activator inhibitor type-1(PAI-1) expression during the ovulatory period in rat by RNase protection assay and in situ hybridization. Immature female Wistar rats were injected with 25 IU pregnant mare serum gonadotropin (PMSG), followed 50h later by 25 IU human chorionic gonadotropin (hCG). Levels of tPA mRNA were low before hormone treatment and after PMSG treatment. After hCG treatment, tPA mRNA levels increased rapidly, the first peak at 4h after hCG treatment and reached a maximum just prior to ovulation, 12h later, before declining again. PAI-1 mRNA was barely detectable before hormone treatment but was transiently induced by hCG treatment, reaching peak levels after 4h. Subsequently, PAI-1 mRNA levels decreased until early luteinization. The expression of tPA mRNA 4h after hCG treatment occurred mainly in the follicular thecal-interstitial cells, but was barely detectable in the granulosa cells, whereas 12h after hCG treatment it was maximal in the granulosa cells of the large follicles destined to ovulate. PAI-1 mRNA was expressed mainly in ovarian stromal tissue and in the thecal external interstitial cells encapsulating the follicles at 4h after hCG treatment. These results suggest that the temporal regulation of tPA biosynthesis after hCG induction depends on the cell types and size classes in the various ovarian compartments. PAI-1 may be produced by the stormal tissue and the thecal external interstitial cells and is perhaps implicated in structural changes during follicular growth, ovulation and luteinization.