Pharmacological manipulation of nitric oxide levels in mouse follicle cultures demonstrates key role of extrafollicular control of ovulation.

Background: Nitric oxide (NO) is involved in ovulation, but it is uncertain whether the mechanisms responsible are systemic or local. In vivo and in perfused ovaries, inhibition of nitric oxide synthase (NOS) reduces ovulation. This study used a well-characterized system of isolated follicle culture to assess which isoforms of NOS might be involved at the follicular level.

Methods: NOS inhibitors, stimulators and NO donors were used to manipulate the environment of mouse cultured follicles, selectively targeting different isoforms of NOS. Follicle survival and ovulation in vitro were monitored using established markers.

Results: Inhibition of endothelial NOS did not affect survival or ovulation of cultured follicles. Inhibition of inducible nitric oxide synthase (iNOS) had mild effects in this system, generally inhibitory of ovulation. NO donors had variable inhibitory effects, including toxic effects. Stimulators of iNOS appeared to show mixed, mostly inhibitory, effects that possibly involved different mechanisms.

Conclusions: The results suggest that relatively minor effects on survival and ovulation in vitro are achieved by modulating the NO pathway, and the marked effects of NOS inhibitors in vivo do not occur in vitro. It is therefore probable that the effects of NOS inhibitors in vivo are mediated via effects outside the follicle.