Activation of protein kinase C induces cortical granule exocytosis in a Ca(2+)-independent manner, but not the resumption of cell cycle in porcine eggs.

The effects of protein kinase C (PKC) activation on meiotic resumption and cortical granule (CG) exocytosis as well as its dependence on Ca2+ in porcine eggs matured in vitro were studied. Cortical granule release was judged by both confocal laser microscopy after the eggs were labeled with fluorescein isothiocyanate-peanut agglutinin (FITC-PNA) and electron microscopy. Meiotic resumption and pronuclear formation were observed after eggs were stained with acetic orcein. When eggs were treated with PKC activators, 1-oleyl-2-acetyl-glycerol (OAG) or phorbol 12-myristate 13-acetate (PMA), the pronuclear formation percentage was significantly lower than that of Ca2+ ionophore A23187-treated group, but not statistically different from that in negative control group (P > 0.05), and most of the eggs were still arrested at metaphase II stage, suggesting that PKC activation does not induce the resumption of meiosis and pronuclear formation. In contrast, PKC activation induced 89.1% to 100% of the eggs completely or partially released their CG in different groups, not statistically different from A23187-treated group, and this effect could be overcome by PKC inhibition. When the intracellular free Ca2+ was chelated with acetoxymethal ester form of 1,2-bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM), and then treated with PMA or OAG in Ca(2+)-free medium, the proportions of eggs with CG release were 90.9% and 78.1%, respectively, not statistically different from the above-treated groups, suggesting that CG exocytosis induced by PKC activation is independent of Ca2+ rise. The results indicate that different events of porcine egg activation may be uncoupled from one another.