Estrogen regulation of nitric oxide synthesis in the procine oocyte

The endothelial type (NOS-3) of three isoforms of nitric oxide (NO) synthase occurs in porcine oocytes and granulosa cells, but the regulation of NO synthesis in oocytes remains unknown. The present study was designed to evaluate steroid control in the process of oocyte NO synthesis. Cumulus-oocyte complexes (COCs), obtained from small-sized antral follicles of immature porcine ovaries, were cultured in estrogen-deprived medium, and the effect of steroids or steroid-free porcine follicular fluids on the NO release from oocytes was investigated. Oocytes that were isolated from cultured COCs were incubated with 1 M ionomycin. The NO metabolites were identified using a NO detector-high-pressure liquid chromatography system. Oocytes from COCs cultured with 10 nM 17-estradiol (E₂) released NO in response to ionomycin, whereas progesterone and testosterone had little effect on the synthesis of NO. An inhibitor of NOS suppressed the synthesis of NO. The maximal synthesis was observed after a 15 h-culture with E₂. However, oocytes freshly obtained from antral follicles did not response to ionomycin, and the E₂ action was suppressed by the addition of steroid-free follicular fluids. Analyses of RT-PCR and Western blotting showed that E₂ did not increase NOS-3 expression. In addition, estrogen receptor was detected in oocytes and cumulus cells, and estrogen receptor was detected only in cumulus cells. These findings suggest that oocyte NOS-3 is promoted for the synthesis of NO by E₂ without increases in NOS-3 expression, but the synthesis of NO is suppressed, at least in the oocytes of early antral follicles.