Nitricoxide (NO) is a biological signalling molecule that is generated by NO synthase (NOS) from L-arginine. It has been demonstrated that NO has a crucial role in maturation of mammalian oocytes. In this study, the importance of NO/NOS system in in-vitro maturation of ovine oocytes was investigated. Different concentrations (0.1, 1, 10 mM) of L-NAME, a NOS inhibitor, were used to evaluate the effect of the inhibition of NOS on cumulus expansion and meiotic resumption of sheep oocytes. the results were evaluated by chi-square test and p<0.05 was considered significant. L-NAME in the highest concentration (10 mM) inhibited total cumulus expansion as compared to control (p<0.05). It also suppressed the meiotic maturation and the extrusion of first polar body in a dose-dependent manner. The percentage of oocytes at MΙΙ stage was 26.47%, 40.72%, 63.78% and 75.16% for 10, 1, 0.1 mM and control group, respectively. To evaluate if the effect is reversible, 0.1 mM sodiumnitroprusside (SNP, a NO donor) was added in the treatment containing 10 mM L-NAME. The concomitant addition of L-NAME with SNP reversed the inhibitory effect of L-NMAE on cumulus expansion and meiotic maturation. These results indicate that NO/NOS system is involved in maturation of sheep oocytes.

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