Abnormal estrous cyclicity after disruption of endothelial and inducible nitric oxide synthase in mice

The roles of nitric oxide (NO) and nitric oxide synthase (NOS) in reproduction were studied by examining the estrous cycle of wild-type (WT) mice, inducible NOS (iNOS)-, and endothelial NOS (eNOS)-knockout mice. We observed an average estrous cycle of 4.8 ± 0.2 days in WT mice. While we observed no significant influence of iNOS deficiency on cycle length, eNOS- knockout females showed a significantly longer estrous cycle (6.6 ± 0.6 days; p < 0.03) than WT females, due to an extension of diestrus (p < 0.03). There was no influence of iNOS deficiency on ovulation rate compared with that in WT females; however, eNOS-knockout mice showed a significant reduction (p < 0.05) in ovulatory efficiency relative to WT or iNOS-knockout females. In contrast to WT females, in which the highest level of estradiol (E₂) was observed at 1500 h of proestrus, iNOS-knockout females reached a peak of E₂ at 1830 h of proestrus. In eNOS-knockout females, the peak of E₂ occurred at 1830 h, as in iNOS-knockout mice; however, E₂ levels were 5- fold and 3-fold higher (p < 0.05) than levels observed in WT and iNOS- knockout females, respectively. There was no effect of genotype on the plasma LH concentrations at proestrus. On the first day of diestrus, eNOS-knockout females showed significantly higher plasma E₂ and progesterone levels (p < 0.05) relative to WT and iNOS-knockout females. The dysfunction in cyclicity, ovulation rate, ovarian morphology, and steroidogenesis in eNOS-knockout female mice strongly supports the concept that eNOS/NO plays critical roles in ovulation and follicular development.