Inhibitory Action of Cyclic Guanosine 5′-Phosphoric Acid (GMP) on Oocyte Maturation: Dependence on an Intact Cumulus

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Abstract
Oocyte cumulus complexes (OCC) were isolated from antral follicles of proestrous hamsters prior to the luteinizing hormone (LH)/follicle-stimulating hormone (FSH) surge and then incubated from 4 to 24 h at 37°C. Under these conditions approximately 89% of the oocytes exhibited metaphase plates (11% were in the dictyate stage) and addition of LH (200 ng/ml ovine NIH-LH-S20) did not significantly alter the percentage of oocyte maturation. Approximately 50% of the OCC incubated for 24 h with 6 mM 8-bromo-cyclic quanine 3′:5′ monophosphate (8-Br-cGMP) were prevented from maturing beyond the dictyate stage. OCC incubated with 6 mM 8-bromo-cyclic adenosine 3′:5′ monophosphate (8-Br-cAMP) were also prevented from maturing (59.5% 8-Br-cAMP) in vitro. LH (200 ng/ml) was able to overcome the 8-Br-cAMP-induced inhibition of oocyte maturation in OCC. It also produced a decrease in 8-Br-cGMP mediated inhibition which was more pronounced when the dosage of LH was increased to 10 µg/ml. 8-Br-cGMP prevented oocyte maturation in a dose and time dependent manner. Only 8-Br-cAMP prevented maturation (∼60% inhibition) of denuded oocytes in vitro. Denuded oocytes incubated with and without 8-Br-cGMP exhibited no inhibited of maturation. These results indicate that 8-Br-cGMP may exert its inhibitory effects through the cumulus cells. On the other hand, cAMP appears to directly inhibit oocyte maturation.