In vitro actions of insulin-like growth factor-I on ovarian follicle maturation in white perch ( Morone americana)

Abstract

Previous studies of follicle maturation in temperate basses showed that insulin-like growth factor (IGF)-I and -II can induce meiotic resumption, indicated by germinal vesicle breakdown (GVBD), and oocyte maturational competence (OMC), the ability to respond to the maturation-inducing hormone (MIH, 17,20β-21-trihydroxy-4-pregnen-3-one, 20β-S). The IGFs-induced GVBD but not OMC in striped bass follicles in vitro, but OMC and not GVBD in white bass follicles. Striped bass are group-synchronous single-clutch spawners whereas white bass and white perch are group-synchronous multiple-clutch spawners. In the present study, we found that IGFs-induced OMC in white perch. Although IGF-I weakly stimulated GVBD in follicles from some late stage fish, it is likely that IGF-I did not directly induce GVBD but instead induced OMC, enabling endogenous MIH to act. Bovine insulin was less potent than IGFs at inducing OMC, suggesting that the IGFs were acting through an IGF-I receptor. IGF-I increased testosterone and estradiol-17β production by ovarian fragments but decreased production of 17,20β-dihydroxy-4-pregnen-3-one, a precursor to the MIH, which was below detection levels. As with the other Morone species, phosphatidylinositiol 3-kinase inhibitors, wortmannin and LY 294002, and the translation inhibitor cyclohexamide, attenuated GVBD induced by human chorionic gonadotropin (hCG), 20β-S, and a combination of IGF-I and 20β-S. Only hCG-induced GVBD was attenuated by the transcription inhibitor actinomycin D. The IGFs have shared and disparate actions on ovarian follicle maturation among Morone species that appear to be linked to reproductive strategy and exhibit similarities in mechanisms of action.