Effect of gonadotropins on the ability of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) to inhibit germinal vesicle breakdown in bovine oocytes.

The transcriptional inhibitor, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), prevents germinal vesicle breakdown (GVBD) in bovine oocytes only in the presence of gonadotropins. The objectives of this study were to examine the ability of gonadotropins to facilitate transcriptional inhibition of GVBD in bovine oocytes and to examine the effect of gonadotropins on transcriptional inhibition of cumulus expansion. Cumulus-oocyte complexes (COC) from 2 to 7 mm follicles were cultured in TCM-199 with 1 microg/ml estradiol, 50 microg/ml gentamicin and hormonal treatments for 20 to 24 h at 39 degrees C in 5% CO2 in air. After culture, COC were assessed for degree of cumulus expansion and oocytes were then denuded, fixed and stained to determine stage of meiosis. In the presence of LH and FSH the proportion of oocytes arrested at germinal vesicle (GV) stage was significantly increased with DRB treatment (58 vs 3% GV for LH/FSH + DRB vs LH/FSH-DRB; P < 0.001). However, maximal inhibition of GVBD by treatment with DRB could also be achieved in the presence of FSH alone (60% GV for FSH + DRB). The ability of DRB to block GVBD was significantly reduced in the presence of LH alone (20 to 28% GV for LH + DRB; P < 0.05 vs FSH + DRB), and treatment with DRB did not block GVBD in the presence of hCG (6.8 to 13.3% GV for hCG + DRB; P < 0.001 vs FSH + DRB). Inhibition of cumulus expansion by treatment with DRB occurred in the presence of either FSH or LH. Based on these results, it is suggested that DRB prevents GVBD in cultured bovine COC by interfering with a transcriptional event mediated primarily by FSH.