Nitric oxide decreases estradiol synthesis of rat luteinized ovarian cells: Possible role for nitric oxide in functional luteal Regression

This study was designed to examine the synthesis and function of nitric oxide during follicular development and luteinization in the rat ovary. Cells were obtained from hypophysectomized diethylstilbestrol-implanted immature female rats subjected to several hormonal regimens that resulted in preantral, Graafian, ovulatory, atretic, or luteinized ovaries. Cells obtained from ovaries at all stages of follicular development synthesized nitric oxide in a linear manner over time. The basal production of nitric oxide was 6- to 14-fold higher (P < 0.009) in cells obtained from luteinized ovaries than that in cells obtained from ovaries at all other developmental stages. Inhibiting endogenous nitric oxide synthesis in cells obtained from luteinized ovaries resulted in a 3-fold increase (P < 0.007) in the estradiol level without affecting progesterone synthesis. We used isoform-specific antisera to determine the cellular location and isoform(s) of nitric oxide synthase expressed in our cell culture system and in the luteinized ovary in vivo. Positive immunofluorescent staining for both the endothelial and inducible isoforms was observed in separate cell types. Immunoblotting experiments also showed that luteinized ovaries express the endothelial and inducible isoforms of nitric oxide synthase. Nitric oxide synthesis inhibits estradiol synthesis in vitro. We suggest that nitric oxide participates in functional luteal regression by inhibiting steroidogenesis.