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Inhibitors of nitric oxide synthase influence oocyte maturation in rats
A Jablonka-Shariff1, R Basuray, L M Olson
1Department of Obstetrics and Gynecology, Washington University, St. Louis, Missouri, USA.
Objective:
To examine the effect of inhibiting nitric oxide synthase (NOS) on the number of ovulated oocytes and on oocyte meiotic maturation.
Methods:
Female Sprague-Dawley rats (25 days old) were superovulated with a subcutaneous injection of 10 U pregnant mare's serum gonadotropin, followed 52 hours later by a subcutaneous injection of 10 U human chorionic gonadotropin (hCG). Three hours before and 3 hours after hCG injection, the rats were treated orally with the vehicle (0.5% methylcellulose) as the control or with either of two NOS inhibitors, N omega-nitro-L-arginine methyl ester (L-NAME) and L-N6-(1-iminoethyl)-lysine (L-NIL). The rats were killed 20 hours after hCG injection, and oocytes present in the oviduct were flushed, counted, and classified for stages of meiosis. In addition, ovarian oocytes (12 hours post-hCG) and ovulated oocytes were treated with an immunofluorescent stain for the presence of endothelial NOS (eNOS).
Results:
Strong positive staining for eNOS was observed in the cytoplasm of ovarian and ovulated oocytes. Control rats ovulated an average 43.0 +/- 4.1 oocytes each, which was lowered with either L-NAME or L-NIL (23.8 +/- 4.3 and 23.5 +/- 4.0 oocytes per rat, respectively; P < .002). We observed that significantly fewer ovulated oocytes obtained from rats treated with NOS inhibitors were at metaphase II (P < .006), the normal stage of meiosis for unfertilized oocytes, and a significantly greater percentage of oocytes displayed atypical morphology as compared with control oocytes (P < .0001).
Conclusion:
Ovarian nitric oxide synthesis is required for maximal ovulation, and a lack of nitric oxide during the periovulatory period results in severe defects in oocyte maturation.
Insights
Inhibiting nitric oxide synthase (NOS) significantly reduces oocyte ovulation and impairs meiotic maturation in rats. This suggests ovarian nitric oxide is crucial for successful ovulation and oocyte quality.
Area of Science:
- Reproductive biology
- Endocrinology
- Cell biology
Background:
- Nitric oxide (NO) plays a role in various physiological processes, including reproduction.
- The specific role of nitric oxide synthase (NOS) in ovulation and oocyte maturation requires further elucidation.
Purpose of the Study:
- To investigate the impact of inhibiting nitric oxide synthase (NOS) on oocyte ovulation and meiotic maturation.
- To determine the necessity of ovarian nitric oxide synthesis for ovulation and oocyte quality.
Main Methods:
- Female Sprague-Dawley rats were induced to superovulate using pregnant mare's serum gonadotropin and human chorionic gonadotropin (hCG).
- Rats were treated with NOS inhibitors (L-NAME, L-NIL) or vehicle control before and after hCG administration.
- Oocytes were collected, counted, and assessed for meiotic stage and morphology; eNOS presence was confirmed via immunofluorescence.
Main Results:
- Strong eNOS staining was observed in ovarian and ovulated oocytes.
- NOS inhibition significantly reduced the number of ovulated oocytes compared to controls.
- Inhibitor-treated groups showed fewer oocytes at metaphase II and a higher incidence of atypical morphology.
Conclusions:
- Ovarian nitric oxide synthesis is essential for achieving maximal ovulation rates.
- Disruption of nitric oxide during the periovulatory period leads to significant defects in oocyte maturation and quality.