IN VITRO MEIOTIC MATURATION OF MOUSE OOCYTES: ROLE OF NITRIC OXIDE

In this experiment we used cultured mouse cumulus cell-enclosed oocytes (CEOs) and denuded oocytes (DOs) to study the function of nitric oxide (NO) in mouse oocyte meiotic maturation. CEOs and DOs were cultured in a medium containing 4 mM hypoxanthine (HX) to maintain meiotic arrest, in maturation medium (without HX) supplemented with different doses of sodium nitroprusside (SNP, a NO donor), and in N-omega-nitro-L-arginine methyl ester (L-NAME) (inhibitor of NO synthase). NOS inhibitor suppressed the formation of first polar body (PB1) of the oocytes in CEOs in a dose dependent manner, but no effect on germinal vesicle break down (GVBD) was observed. Treatments of low concentrations of SNP stimulated significantly the oocyte meiotic maturation of CEOs which were inhibited with HX, but had no effect on DOs in the same culture medium. The treatment with high concentrations of SNP (0.1-2 mM) during the CEOs cultured in maturation medium resulted in a lower percentage of oocytes at PB1 stage and a higher percentage of atypical oocytes. A dose of SNP at 1 mM exhibited significant inhibitory effect on the formation of PB1, and the number of atypical oocytes compared with control. The results showed that the treatment with 1 mM concentration of SNP could significantly delay GVBD during the first 5 h culture period. The concomitant addition of L-NAME with SNP did not reverse the inhibitory effect of SNP on CEOs. Pre-incubation use of SNP did not have any effect on oocyte maturation. These data support the idea that NO could act in mouse meiotic maturation depending on its concentration.