Effects of inhibiting nitric oxide synthase on cumulus expansion and nuclear maturation of sheep oocytes

Nitric oxide is a biological signaling molecule that plays a crucial role in oocyte maturation of mammalians. It is generated by the nitric oxide synthase (NOS) enzyme from L-arginine. This study aimed to investigate the importance of NO/NOS system in the in vitro maturation of ovine oocytes. Different concentrations of L-NAME, a NOS inhibitor, (0.1, 1 and 10mM) were added to maturation medium to evaluate the effect of inhibiting NOS on cumulus expansion and meiotic resumption of sheep oocytes. After 26 h culture, low and medium concentrations of L-NAME (0.1 and 1mM) had no significant effect on cumulus expansion, however, its higher concentration (10mM) decreased the percentage of oocytes with total cumulus expansion (P less than 0.05). The extrusion of the first polar body was also suppressed in a dose-dependent manner, so that the addition of 10mM L-NAME to maturation medium significantly stopped oocytes in GV stage (P less than 0.05). Moreover, to confirm the results and to evaluate if this effect is reversible, 0.1mM sodium nitroprusside (SNP, a NO donor) was added only to the maturation medium which had the highest concentration of L-NAME (10mM). The concomitant addition of NOS inhibitor with NO donor reversed the inhibitory effect of L-NAME on cumulus expansion and meiotic maturation. These results indicated that NO/NOS system is involved in the maturation of sheep oocytes.