Developmental competence of bovine oocytes is stimulated by low doses of dibutyryl-cAMP during cycloheximide-induced block of meiosis

The time that precedes the resumption of meiosis is crucial for the acquisition of oocyte developmental competence. However, at present, the mechanisms regulating this process are largely unknown though their understanding would enhance our capacity to improve the quality of in vitro produced embryos. This study was designed to assess the role of cAMP in the acquisition of oocyte developmental competence before meiotic resumption. To achieve this, bovine cumulusoocyte complexes (COCs) were reversibly maintained in meiotic arrest for 6 h by a protein synthesis inhibitor, cycloheximide (CHX), and during this period the intracellular cAMP concentration were modulated with different concentrations of a cAMP analog, dibutyryl-cAMP (dbcAMP) before being matured under standard conditions with serum and gonadotropins. COCs were isolated from ovaries recovered at slaughter by aspirating 2-5 mm follicles in TALP Hepes alone or supplemented with 2 gg/ml of CHX. COCs with compact cumulus layers and homogeneous cytoplasm were selected and incubated for 6 h with or without CHX and various concentrations of dbcAMP. At the end of culture COCs were washed and placed in M199 supplemented with 10% FCS, 1 gg/ml FSH and 5 pg/ml LH for 24 h'then fertilized for 16 h and cultured for 5 d in the presence of bovine oviductal epithelial cells. Frequency of cleavage, morula and blastocyst formation were determined 2 and 6 d post IVF (p.i.) respectively.