295 RELATIONSHIP BETWEEN PLASMINOGEN ACTIVATOR/PLASMIN SYSTEM AND IN VITRO FERTILIZATION ABILITY IN THE PIG

Abstract

Plasminogen activators (PAs) are serine proteases, known to be secreted by a large number of cell types, that convert plasminogen into plasmin. The present study was undertaken to identify PAs in porcine gametes and to investigate a possible role of plasminogen in fertilization in vitro in the pig. The oocyte maturation medium used was North Carolina State University-23 (NCSU-23) medium supplemented with 10% (v/v) porcine follicular fluid (pFF), 0.6 mM cysteine, 10 IU/mL human chorionic gonadotropin (hCG), and 10 IU/mL pregnant mare's serum gonadotropin (PMSG). To determine the changes of PA activities in porcine oocytes during maturation, the cumulus-oocyte complexes (COCs) were incubated in NCSU-33 in an atmosphere of 5% CO₂ in air at 39°C for 0, 24, or 48 h. On the other hand, to investigate the release of PAs by boar spermatozoa, fresh spermatozoa were pre-incubated in fertilization medium (mTBM) for 0, 2, 4, or 6 h. After culture, 40 COCs, 40 cumulus-free oocytes, and 40 × 10⁶ spermatozoa were separately put into microtubes containing 20 ¼L of sample buffer [5.0% (w:v) sodium dodecyl sulfate (SDS), 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 analysis. PA activities in porcine oocytes and spermatozoa were quantified using SDS-PAGE, casein-agar zymography, and densitometry. Data were analyzed by ANOVA and Duncan's multiple-range test using the Statistical Analysis System (SAS Institute, Inc., Cary, NC, USA). In the COCs cultured for 24-18 h, tissue-type plasminogen activator (tPA), urokinase-type PA (uPA), and tPA-PA inhibitor (tPA-PAI) were observed. Also, PA activities increased as duration of culture increased. However, no uPA activity was detected in cumulus-free oocytes. In procine fresh spermatozoa, tPA, uPA, and tPA-PAI were observed. When spermatozoa were incubated for 2, 4, or 6 h in fertilization medium, the rate of acrosome reaction (AR) in spermatozoa increased as the duration of culture increased, but PA activities decreased gradually. However, PA activities in sperm-conditioned medium increased as duration of culture increased. On the other hand, to determine the effect of plasminogen on fertilization ability of porcine oocyte and spermatozoa, plasminogen (50 ¼g/mL) was added in fertilization medium. Addition of plasminogen to fertilization medium increased (P < 0.05) AR in spermatozoa and sperm binding to the zona pellucida (ZP), compared with control group. The ZP solubility (zona digestion time) was higher in medium with than that without plasminogen. When porcine oocytes and spermatozoa were co-incubated in fertilization medium with plasminogen, the polyspermic rate was lower in medium with than that without plasminogen. Also, plasminogen significantly (P < 0.05) increased formation rate of the male pronucleus in oocytes penetrated by spermatozoa. These results suggest that supplementing of plasminogen in fertilization medium may play a positive role in improving of fertilization ability in vitro in the pig.