Insulin and insulin-like growth factors I and II induce final maturation of oocytes of red seabream, Pagrus major, in vitro.

The effects of various growth factors and peptides on in vitro germinal vesicle breakdown (GVBD) were examined in oocytes of the red seabream, Pagrus major, a daily spawning marine teleost. The oocytes used underwent GVBD in response to HCG but not to 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (DHP). Insulin (1 microM) and insulin-like growth factor-I (IGF-I, 10 nM) and IGF-II (13 nM) induced GVBD. Salmon gonadotropin-releasing hormone (GnRH), chicken GnRH (100 nM), angiotensin II and III (10 micrograms/ml), inhibin A, activin A (200 ng/ml), and epidermal growth factor (1 nM) were ineffective. IGF-I was the most potent inducer of GVBD. In the presence of DHP, oocytes underwent GVBD in response to low doses of IGF-I or -II or insulin which alone were ineffective. GVBD occurred in response to DHP after oocytes were preincubated with IGF-I or with HCG. Actinomycin D inhibit GVBD induced by HCG alone or in combination with DHP, but actinomycin D did not block GVBD induced by IGF-I alone or in combination with DHP. Cycloheximide totally inhibited induction of GVBD by HCG or IGF-I, alone or in combination with DHP. These results suggest, for the first time in teleost, that growth factors, such as IGF-I, are involved in the induction of GVBD of oocytes. IGF-I appears to act directly on oocytes, and not via maturation-inducing hormone production, to induce GVBD.