Spontaneous maturation in Bufo arenarum oocytes: participation of protein kinase C

Although progesterone is the maturation inducer in amphibians, it has been demonstrated that in Bufo arenarum oocytes resumed meiosis with no need of exogenous hormonal stimulus if derived of their enveloping follicle cells. This phenomenon, called spontaneous maturation, is quite rare in amphibians. In B. arenarum, spontaneous maturation took place only in oocytes obtained during the reproductive period (spring-summer). During this period the oocytes also demonstrated a respiratory activity characteristic of mature oocytes. Interestingly, full-grown B. arenarum oocytes always responded to progesterone regardless of the season in which they were obtained and of their respiratory activity. The disposition of oocytes competent or not competent to mature spontaneously provides a useful system for the study of molecular mechanisms involved in the maturation process. The data presented here indicate that the activation of protein kinase C (PKC) induces germinal vesicle breakdown (GVBD) in denuded oocytes unable to mature spontaneously (winter oocytes) and is involved in the in vitro spontaneous maturation of B. arenarum full-grown oocytes. The inhibition of PKC by 1-(5-isoquinolynyl-sulphonyl-2-methyl-piperazine (H-7) impeded spontaneous maturation in a dose-dependent manner, thus supporting the participation of the PKC pathway during this process. Interestingly phorbol 12-myristate-13-acetate (PMA)-induced GVBD is inhibited by the incubation of the oocytes in dibutyryl cAMP (dbcAMP), indicating that both pathways, PKC and protein kinase A (PKA), are related at a certain point. However, spontaneous GVBD is less sensitive than PMA-induced GVBD to dbcAMP. This fact would support the suggestion that in spontaneous GVBD mechanisms different from activation of PKC are at work.