2-Hydroxyestradiol-17β-induced oocyte maturation: Involvement of cAMP-protein kinase A and okadaic acid-sensitive protein phosphatases, and their interplay in oocyte maturation in the catfish Heteropneustes fossilis

Abstract

In Heteropneustes fossilis, in vitro incubation of postvitellogenic follicles with 2-hydroxyestradiol-17β (2-OHE2, 5 μmol l -1) decreased significantly the total cAMP level, concomitant with germinal vesicle breakdown (GVBD). The incubation of the follicles with cAMP or cAMP-elevating drugs [phosphodiesterase (PDE) inhibitors], such as IBMX (3-isobutyl-1-methyl-xanthine), theophylline and caffeine, inhibited the 2-OHE2-induced GVBD in a concentration-dependent manner. The magnitude of the response varied: both cAMP and IBMX were effective at all concentrations (0.1-2.0 mmol l-1), followed by theophylline (0.5-2.0 mmol l-1) and caffeine (1-2.0 mmol l-1). The protein kinase A (PKA) inhibitor H89 stimulated oocyte maturation in a concentration-dependent manner. However, when co-incubated with 2-OHE 2 for 24 h it produced a biphasic effect: low concentrations (0.1 and 1.0 μmol l-1) did not alter the 2-OHE2-induced GVBD, but high concentrations (5 and 10 μmol l-1) inhibited it. The incubation of the follicles with H89 lowered the inhibitory effect of IBMX on the 2-OHE2-induced GVBD. The incubation of the follicles with okadaic acid (OA), a protein phosphatase 1 and 2A inhibitor did not affect GVBD but when co-incubated with 2-OHE2, it enhanced the GVBD response. OA reversed the inhibitory effect of IBMX. The results suggest that OA may overcome the inhibition of 2-OHE2-induced GVBD by IBMX at a step distal to the cAMP-PKA pathway.