Involvement of cyclic AMP-dependent protein kinase in chromatin condensation before germinal vesicle breakdown in bovine oocytes

The effects of follicle-stimulating hormone (FSH), cyclic AMP-dependent protein kinase (PK-A) inhibitor and dibutyryl cAMP (dbcAMP) on chromatin condensation of bovine oocytes at the germinal vesicle (GV) stage were investigated in vitro. Although germinal vesicle breakdowns (GVBD) of cumulus-enclosed and denuded oocytes were completely blocked by the protein synthesis inhibitor cycloheximide (CX), chromatin condensations in GVs occurred in 58% and 67% of the oocytes, respectively. However, when cumulus-enclosed oocytes were treated with CX plus FSH, this proportion was significantly reduced to 19%. In order to elucidate a direct correlation between PK-A activity and chromatin condensation before GVBD, GV oocytes were treated with a specific PK-A inhibitor (H-8). Under conditions inhibiting protein synthesis, treatment with H-8 dramatically increased the proportion of cumulus-enclosed oocytes possessing GVs with highly condensed bivalents, even in the presence of FSH (93%). By contrast, the proportion of denuded oocytes possessing highly condensed bivalents within GVs was significantly reduced to 26% by treatment with CX plus dbcAMP. These results indicate that PK-A activated by treatment with FSH or dbcAMP is responsible for preventing chromatin condensation before GVBD. Thus, we conclude that PK-A activity is involved in the regulation of chromatin condensation at the GV stage in bovine oocytes, regardless of the presence or absence of protein synthesis.