Demonstration of a suppressive effect of inhibin alpha-subunit on the developmental competence of in vitro matured bovine oocytes

Changes in intrafollicular concentrations of different forms of inhibin (fr ee alpha-subunits and ap dimers) occur during follicle development and may influence the oocyte maturation process. The aim of this study was to inves tigate the effects of inhibin A and free alpha-subunit (pro-alpha C) isolat ed from bovine follicular fluid on maturation of bovine cumulus-oocyte comp lexes, as reflected by their competence for embryo development after in vit ro fertilization. Bovine cumulus-oocyte complexes were isolated from ovarie s obtained from an abattoir and were cultured for 22-24 h at 38.5 degrees C in TCM-199 medium supplemented with 10% oestrous cow serum, pregnant mares ' serum gonadotrophin (2.5 iu ml(-1)) and either inhibin A (0, 0.2 and 1.0 mu g ml(-1)) or pro-alpha C (0, 2 and 10 mu g ml(-1)). Neither inhibin A no r free a-subunit affected the cleavage rate of cumulus-oocyte complexes aft er fertilization (approximately 60%). Inhibin A reduced the proportion of c leaved oocytes reaching the eight-cell stage by 19% (P < 0.05), but did not affect the yield of blastocysts. However, pro-alpha C decreased the propor tion of cleaved oocytes that reached the eight-cell (25%; P < 0.05) and bla stocyst (28%; P < 0.05) stages. In addition, a negative correlation (r = -0 .55, P < 0.001) was found between concentrations of total immunoreactive (i r) a-inhibin (measured by radioimmunoassay) produced by untreated control c umulus-oocyte complexes and their post-cleavage development to the blastocy st stage. In a second experiment, mouse monoclonal antibodies (20 mu g ml(- 1)) against two different regions of the inhibin alpha-subunit precursor (p ro-region and alpha C fragment) were tested for their ability to neutralize Endogenous inhibin alpha-subunit-related molecules produced by cumulus cel ls; control cumulus-oocyte complexes were treated with normal mouse IgG (20 mu g ml(-1)). Although the cleavage rate was not affected, the yield of bl astocysts was significantly higher in the presence of mouse monoclonal anti bodies to both pro-alpha (77% increase; P < 0.05) and alpha C (48% increase ; P < 0.05). None of the treatments tested affected endogenous production o f activin-A or follistatin by cumulus-oocyte complexes. Overall, these resu lts indicate that the inhibin alpha-subunit (pro-alpha C) has an inhibitory role in oocyte maturation which is independent of the modulatory effects o f activin and follistatin.