Plasminogen activators (PAs) are serine proteases, known to be secreted by a large number of cell type. PAs are reported to play a role in variety of physiologic processes, including fibrinolysis, ovulation, mammary involution, implantation, and fertilization. The present study investigated the effects of cumulus cells and porcine follicular fluid (pFF) on PA activity and oocyte maturation in vitro in the pig. Porcine oocytes were harvested from slaughterhouse ovaries, selected, and matured in modified North Carolina State University-23 (NCSU-23) media. After culture, cumulus-oocyte complexes (COCs) and denuded oocytes (DOs) were separately put into microtubes containing 20 �L of sample buffer [5.0% (w:v) sodium dodecyl sulfate, 20% (v:v) glycerol, and 0.0025% (w:v) bromophenol blue in 0.125 M Tris-HCl buffer] and frozen at -80�C until used for zymographic analysis. Differences in data were evaluated by Duncan's multiple-range test using the General Linear Models procedure in the Statistical Analysis System (SAS Institue, Inc., Cary, NC, USA). To determine the effect of porcine follicular fluid (pFF) on PA activity in porcine oocytes during maturation, the COCs and DOs were incubated in NCSU-23 medium with or without 10% (v/v) pFF for 0, 24, or 48 h. In the presence of cumulus cells, the proportions of oocytes matured to metaphase-II stage were significantly (P < 0.05) higher in medium with pFF than without pFF (69.8% vs. 37.7%, respectively). When COCs and DOs were cultured in the presence of pFF, tissue-type PA (tPA), urokinase-type PA (uPA), and tPA-PA inhibitor (tPA-PAI) were observed in COCs, and PA activities were higher at 48 h than 24 h. However, no PA activity was detected in DOs. Under the same conditions, when COCs and DOs were cultured in the absence of pFF, tPA and tPA-PAI were observed in COCs, and PA activities were increased as duration of culture increased. However, no PA activity was detected in DOs. When porcine oocytes were cultured in the presence of pFF, the activities of tPA-PAI, tPA, and uPA were observed in conditioned medium with COCs and DOs cultured for 24 h and 48 h. In the absence of pFF, PA activities were observed only in conditioned medium with COCs, and no PA activities were detected in conditioned medium with DOs. On the other hand, three plasminogen-dependent lytic bands (tPA-PAI, tPA, and uPA) were observed in pFF cultures. Particularly uPA activity was higher than the other kinds of PA activity. When oocytes and cumulus cells were separated from porcine COCs at 0 h of cultrue, tPA-PAI, tPA, and uPA were detected in cumulus cells at 48-h culture, but no PA activities were in DOs. The presence of pFF and cumulus cells in maturation medium stimulated not only nuclear and cytoplasmic maturation in porcine COCs, but also PA production by cumulus cells and COCs. It is possible that PAs produced by cumulus cells migrated through the gap junction between oocyte and cumulus cells. These results suggest that porcine oocytes have no ability to produce PA themselves.
Sequential exposure of porcine cumulus cells to FSH and/or LH is critical for appropriate expression of steroidogenic and ovulation-related genes that impact oocyte maturation in vivo and in vitro
