Effects of insulin-like growth factor-I on in vitro final oocyte maturation and ovarian steroidogenesis in striped bass, Morone saxatilis

1Recombinant human (rh) insulin-like growth factor-I (IGF-I) was more poten t than rhIGF-II at inducing in vitro germinal vesicle breakdown (GVBD), a m arker for resumption of meiosis, in oocytes of striped bass. Treatment of o varian fragments containing oocytes in intact follicles with rhIGF-I increa sed concentrations of estradiol-17 beta and maturation-inducing steroid (MI S) 17,20 beta,21-trihydoxy-4-pregnen-3-one (20 beta-S) in the culture mediu m and decreased testosterone levels. The follicles were too immature for oo cytes to complete GVBD in response to 20 beta-S (MIS incompetent) or hCG. A ddition of 20 beta-S to cultures did not increase the percentage of oocytes completing GVBD in response to rhIGF-I or rhIGF-II. Bovine insulin was wit hout effect on GVBD or steroid production. Incubation of MIS-competent foll icles with actinomycin D, cyanoketone, trilostane, 1-heptanol, or 1-octanol had no effect on rhIGF-I-induced GVBD, but attenuated hCG-induced GVBD and 20 beta-S production. Cycloheximide inhibited rhIGF-I-induced GVBD. Collec tively, these observations indicate that IGF-I can induce GVBD via MIS- and transcription-independent pathways without coupled gap junctions between o ocytes and granulosa cells or among granulosa cells, but requires protein s ynthesis to do so. An rhIGF-I analogue that does not bind IGF-binding prote ins, des(1,3)IGF-I, was more potent than rhIGF-I in inducing GVBD, suggesti ng ovarian IGF-binding proteins may inhibit IGF-I action.