The in vitro effects of forskolin, IBMX and cyanoketone on meiotic maturation in follicle-enclosed catfish (Clarias batrachus) oocytes.

The effects of an adenylate cyclase activator (forskolin, FK), phosphodiesterase inhibitor (3-isobutyl-l-methyl-xanthine, IBMX) and an inhibitor of steroidogenesis (cyanoketone, CK) on germinal vesicle breakdown (GVBD) in the catfish (Clarias batrachus) were investigated in vitro. In most of the experiments GVBD was induced by using 1 microgram/ml 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-DP), which is the maturation-inducing steroid (MIS) for this species. Adenosine 3':5'-cyclic monophosphate (cAMP) levels were also measured in the control, MIS-induced and/or FK- and IBMX-treated follicle-enclosed oocytes. MIS-induced GVBD was inhibited by FK (> or = 0.5 microM) or IBMX (> or = 1.0 mM), but oocyte exposed to 0.1 microM FK or 0.5 mM IBMX, after MIS stimulation, underwent GVBD. However, an inhibition of GVBD was recorded when the MIS-induced folliculated oocytes were preincubated with CK (1 microgram/ml) and subsequently treated with 0.1 microM FK. In the time course study, when the oocytes were stimulated by MIS for various time intervals and then treated with 1.0 microM FK or 1.0 mM IBMX, both the substances blocked maturation if they were added up to 12 hr after MIS. The extent of inhibition was gradually decreased and was completely removed after 30 hr of post-MIS stimulation. The stimulatory dose of 17 alpha,20 beta-DP (1 microgram/ml) not only induced GVBD (83.2 +/- 1.50%) in vitro but also reduced oocyte cAMP level (65.3 +/- 2.85 pmol/100 micrograms protein) significantly after 6 hr of incubation. However, FK (10.0 microM) or IBMX (1.0 mM) countered these effects and promoted the accumulation of cAMP in the oocytes; FK being more potent. On the other hand, when unstimulated full-grown but immature oocytes were cultured in vitro in the presence of different concentrations of FK, an induction of oocyte maturation was recorded in dose- and time-dependent manner. These results strongly suggest the involvement of cAMP in the regulation of catfish oocyte maturation.