The objective of this study was to elucidate the role of a [Ca2 ]i rise and protein kinase C (PKC) activation on decreases of p34cdc2 kinase and mitogen-activated protein (MAP) kinase activity during parthenogenetic activation of porcine oocytes. In oocytes treated with 50 µM Ca2 ionophore, degradations of both p34cdc2 kinase and MAP kinase activity were observed and half of these oocytes formed pronuclei. However, a supplement of PKC inhibitor, calphostin C, after 50 µM Ca2 ionophore treatment, was sufficient to inhibit the inactivation of MAP kinase and pronuclear formation in the oocytes. These results showed that PKC played an important role in Ca2 -induced oocyte activation. On the other hand, 10 µM Ca2 ionophore treatment could not affect the MAP kinase activity but induced a transient decrease of p34cdc2 kinase activity, which resulted in recovery of p34cdc2 kinase activity and progression to meiotic metaphase III stage. To investigate the effects of PKC activator on oocytes treated with 10 µM Ca2 ionophore, matured oocytes were cultured with phorbol 12-myriatate 13-acetate (PMA), after 10 µM Ca2 ionophore treatment. The additional treatment suppressed the recovery of p34cdc2 kinase activity and rapidly induced a decrease of MAP kinase activity, and these low activities were maintained until 12-h cultivation. As a result, a significantly higher percentage of these oocytes (67%) had pronuclei at 12-h cultivation. Moreover, PMA treatment without Ca2 ionophore treatment effectively led to a decrease of MAP kinase activity in a dose-dependent manner but not p34cdc2 kinase activity in matured porcine oocytes. In conclusion, the parthenogenetic activation of porcine oocytes was mediated by the inactivation of p34cdc2 kinase via a calcium-dependent pathway and thereafter by the inactivation of MAP kinase via a PKC-dependent pathway.
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