The influence of the preovulatory LH peak on protein synthesis and phosphorylation in bovine oocytes from follicles of different size and quality

Abstract

Whether the luteinizing hormone (LH) peak induces oocyte maturation in other than preovulatory follicles was examined by analysing protein synthesis and phosphorylation patterns of bovine oocytes from non-atretic and atretic follicles of various sizes, present in the ovaries of oestrous animals treated for superovulation. The oocytes were collected at consecutive times after the preovulatory LH peak, radiolabelled for 3 h and individually processed for SDS-PAGE. Furthermore, the nuclear status of the oocytes, the expansion of their cumulus investment, the luteinization of their granulosa cells and steroid levels in the corresponding follicular fluids, were scored and related to the protein synthesis and phosphorylation patterns obtained. Oocytes from non-atretic follicles of 3–8 mm, collected between 3 and 20 h after the LH peak, exhibited consistent protein synthesis and phosphorylation patterns similar to those of oocytes from preovulatory follicles collected 3–8 h after the LH peak in the germinal vesicle (GV) stage. The majority of oocytes from small follicles were in the GV stage and surrounded by a compact cumulus investment throughout the collection period. In these follicles, no luteinization of the membrana granulosa was observed, while the oestradiol-17β level declined during the collection period. The results indicate that the LH peak affects the theca interna of small follicles, but does not induce further follicular differentiation or oocyte maturation. The protein synthesis and phosphorylation patterns of oocytes from atretic follicles larger than 10 mm, as well as their nuclear status and cumulus expansion, were similar to those of preovulatory follicles during the collection period. The timing of changes in the steroid levels during this period was similar in non-atretic and atretic follicles. The results show that oocyte maturation is induced in all follicles larger than 10 mm.