THE PROTEIN PHOSPHATASE INHIBITOR OKADAIC ACID INHIBITS EXIT FROM METAPHASE-II IN PARTHENOGENETICALLY ACTIVATED PIG OOCYTES

The exposure of in vitro matured pig oocytes to the calcium ionophore A 23187 (50 mu M, 7 min) resulted in parthenogenetic activation in 67% of the oocytes. When the activated oocytes were cultured, they formed pronuclei. In these oocytes, tubulin labelling revealed a rearrangeme nt of the microtubules into an interphase meshwork. The activated oocy tes also lost their ability to form cytoplasmic asters after short-ter m taxol treatment. The activation rate of the oocytes was further incr eased when they were cultured with a protein synthesis inhibitor, cycl oheximide, after ionophore treatment. A culture of ionophore-treated o ocytes with okadaic acid, the inhibitor of protein phosphatases 1 and 2A, prevents the events characterizing oocyte activation. In oocytes c ultured with okadaic acid, chromatin remained condensed, and cytoplasm retained its ability to respond to taxol treatment by the formation o f cytoplasmic asters. This effect of okadaic acid was observed even in oocytes in which the activating stimulus was followed by a culture wi th cycloheximide. This data allows us to conclude that protein phospha tases 1 and 2A play an important role during the transition from metap hase II to interphase after activation of the pig oocyte. (C) 1997 Wil ey-Liss, Inc.