Effect of 6‐dimethylaminopurine on germinal vesicle breakdown of bovine oocytes

The effect of 6‐dimethylaminopurine (6‐DMAP) on germinal vesicle breakdown (GVBD) and maturation in bovine oocytes was investigated in this study. This puromycin analog has been shown to be an inhibitor of phosphorylation. Whereas GVBD occurred in nearly all oocytes (96.8%; 120/124) in control medium, presence of 6‐DMAP (2 mM) blocked this process almost completely, irrespective of the presence (98.3% GV, 349/355) or absence (97.1% GV, 165/170) of cumulus cells. When lower concentrations of 6‐DMAP were used (100–500 μM), GVBD was observed in 87.9% of oocytes, but their maturation was arrested at late diakinesis‐metaphase I stage. The inhibition of GVBD was fully reversible, but most of the metaphase II plates were abnormal (80%). To assess whether the action of 6‐DMAP is different from the inhibitors of protein synthesis, metaphase II oocytes were exposed to either cycloheximide or 6‐DMAP, respectively. Whereas in cycloheximide‐supplemented medium approximately 80% of the oocytes were activated, parthenogenetic activation was much less frequent after incubation in 6‐DMAP (14.5%). Fusion studies showed that, even if GVBD occurs in 6‐DMAP supplemented medium, the level of the maturation‐promoting factor (MPF) is decreased. These experiments may indicate the importance of phosphorylation for GVBD in cattle oocytes.