The effects of α‐amanitin and cycloheximide on nuclear progression, protein synthesis, and phosphorylation during bovine oocyte maturation in vitro

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Abstract
Bovine cumulus oocyte complexes were cultured for various periods and either denuded and orcein stained or radiolabeled with 35S‐methionine or 32P‐orthophosphate. Specific inhibitors were added to the culture medium to investigate mRNA and protein synthesis requirements for both nuclear and cytoplasmic changes during maturation in vitro. Inhibition of mRNA synthesis by α‐amanitin during the first 2 h of culture prevented the phosphorylation of some specific proteins preceding GVBD and decreased the occurrence of GVBD from 97% to 27%. In addition, in oocytes that had undergone GVBD, only part of the changes in protein synthesis after GVBD were observed. Addition of α‐amanitin after 3 h of culture had no effect on meiotic maturation. When cumulus oocyte complexes were cultured in the presence of cycloheximide, the phosphorylation of specific proteins was also blocked and only 5% of the oocytes underwent GVBD. Addition of cycloheximide after 4, 6, or 8 h of culture resulted in an increasing percentage of GVBD, but the oocytes became arrested in metaphase I. When cycloheximide was added from 12 h of culture onwards, nuclear progression to metaphase II was increasingly restored. It is concluded that after the onset of culture, both mRNA and protein synthesis are necessary for the phosphorylation of specific proteins and for GVBD. Further‐more, transcription during the first hours of culture is needed for the synthesis of new proteins after GVBD.