Possible Involvement of Insulin-like Growth Factor-I in Mediating the Stimulatory Effect of Recombinant Bovine Growth Hormone on Maturation of Bovine Oocytes In Vitro.

Abstract

The present study was undertaken to determine whether insulin-like growth factor-I (IGF-I) is involved in mediating the stimulatory effects of growth hormone (GH) on nuclear maturation of bovine oocytes in vitro and subsequent embryonic development after in vitro fertilization. When cumulus-oocyte complexes (COCs) or cumulus-free oocytes were cultured in modified Medium 199 supplemented with 0-100 ng/ml recombinant human IGF-I (rhIGF-I), the proportion of COCs with slight cumulus expansion was increased with increasing concentration of rhIGF-I after 12 and 24 h of culture. Nuclear maturation of oocytes to the metaphase II (M-II) stage was promoted by rhIGF-I only in COCs, with higher proportions of M-II oocytes at 10-100 ng/ml (74 ± 3-76 ± 3%) than at 0-1 ng/ml (53 ± 4-61 ± 1%) after 24 h of culture. When COCs were cultured in the presence of 100 ng/ml rhIGF-I or 10 ng/ml recombinant bovine GH (rbGH) and/or anti-IGF-I antibody (Ab) for 24 h, the proportion of oocytes reaching the M-II stage was higher with rhIGF-I (73 ± 2%) or rbGH (74 ± 1%) than in the absence of these additives (54 ± 3%), in Ab alone (54 ± 3%), or with IGF-I or GH in combination with Ab (54 ± 3-58 ± 1%). The inhibition of the stimulatory effects of rbGH on nuclear maturation by Ab was dose-dependent. After in vitro maturation and fertilization, no differences were observed in the penetration rate (75 ± 3-77 ± 6%), pronuclear formation (56 ± 3-64 ± 10%) and polyspermy (28 ± 4-36 ± 5%) at 8 h post-insemination among cumulus enclosed oocytes cultured in the presence of different additives. When the oocytes were cultured in a chemically defined medium starting from 8 h after insemination, 20 ± 1-26 ± 2% of oocytes developed to the ≥ morula stage 144 h post-insemination without difference among different additives during maturation. However, at 192 h post-insemination, higher proportions of oocytes developed to the blastocyst stage when COCs were matured in the presence of rhIGF-I (16 ± 1%) or rbGH (15 ± 2%) than in the absence of the additives (4 ± 2%) or when IGF-I or GH was combined with Ab (4 ± 2-5 ± 3%). These results suggest that the stimulatory effects of GH on bovine oocyte maturation may not be direct, but mediated via IGF-I probably produced by the cumulus cells.