Effects of plasmin on sperm–oocyte interactions during in vitro fertilization in the pig

The present study was undertaken to examine the effect of plasmin on sperm viability and sperm–oocyte interaction during in vitro fertilization in the pig. Porcine sperm, which were washed in Dulbecco's PBS were re-suspended and incubated in fertilization medium (mTBM; modified Tris-buffered medium) containing 0, 0.1, 1.0, 10.0 or 100.0 ng/mL of plasmin. Sperm viability was not affected by plasmin treatment. Addition of plasmin in doses ranging from 0.1 to 100.0 ng/mL for 2, 4 or 6 h to washed boar sperm resulted in enhancement of acrosome reaction (AR) compared with untreated cells. The concentration of 0.1 ng/mL plasmin (95 ± 18 sperm/oocyte) had no effect on sperm binding, whereas 1.0 ng/mL (123 ± 21 sperm/oocyte), 10.0 ng/mL (124 ± 16 sperm/oocyte) and 100.0 ng/mL (124 ± 15 sperm/oocyte) of plasmin increased sperm binding compared with the control (83 ± 15 sperm/oocyte). The zona pellucida solubility (zona dissolution time) was less in medium with 1.0 ng/mL (123 ± 24 s), 10.0 ng/mL (99 ± 15 s) or 100.0 ng/mL (95 ± 19 s) plasmin compared with control (176 ± 27 s). When pig oocytes and sperm were co-incubated in various concentrations of plasmin for 6 h, the penetration rate was greater in medium with 1.0 ng/mL plasmin (77.5 ± 3.1%) compared with the control. However, there were no differences in the polyspermic rates and mean number of sperm (MNS)/oocyte between the groups treated with plasmin and control. These results suggest that plasmin might play a role in events related to fertilization.