Ovary and ovulation: Cell-specific localization of nitric oxide synthases (NOS) in the rat ovary during follicular development, ovulation and luteal formation

Nitric oxide (NO) has emerged as one of several important intraovarianregulatory factors. In particular, NO has been implicated inthe processes of ovulation and atresia-related apoptosis. Theaim of the present study was to investigate the presence anddistribution of the NO-generating nitric oxide synthase (NOS)enzymes in the ovary during follicular development, ovulationand luteal formation of the equine chorionic gonadotrophin (ECG)/humanchorionic gonado-trophin (HCG)-primed rat NADPH diaphorase activitywas used as a histochemical marker for NOS within the ovary.Diaphorase reactivity was most abundant in the stroma (S) ofthe ovary and in the theca (T) layer of the follicle. In luteinizedovaries, weaker diaphorase reactivity was present within thecorpora lutea (CL). Two different isoforms of NOS, the constitutivelyexpressed endothelial NOS (eNOS) and the inducible isoform ofNOS (iNOS), were immunolocalized in ovaries of immature ratsand in ECG/HCG-primed rats during the periovulatory period fromHCG injection until 2 days after ovulation. In addition, ovarianconcentrations of eNOS and iNOS were quantified by immunoblotting.Immunoblotting with a monoclonal anti-eNOS antibody demonstratedthe presence of eNOS mainly in the residual ovary (ROV) duringthe periovulatory period. In luteinized ovaries, higher concentrationsof eNOS were seen in CL, while those in the ROV at this stagewere lower than in the periovulatory ovary. Immature ovariescontained diminutive amounts of eNOS, detectable mostly in theROV compartment. In contrast, iNOS was barely detectable duringfollicular development to the preovulatory stage. A slight elevationof iNOS was observed in the granulosa cells at 6 h after theHCG injection. The levels of iNOS during the luteal phase werealso low. Immunohistochemical analysis using polyclonal eNOSand iNOS antibodies revealed the localization of these two isoformsprimarily in the S and the T of the periovulatory ovary. Inluteinized ovaries, positive immunoreactivity was also seenwithin the CL. With a monoclonal antibody against eNOS, intenseimmunoreactivity was observed in the S, T and within CL. Therewas a particularly strong staining in blood vessels. These datademonstrate the presence of an intraovarian NO-generating system.The localization of this system to the S, T and CL suggestsa role for NO in the ovulatory process and in the regulationof CL function.