Changes in nitric oxide production levels and expression of nitric oxide synthase isoforms in the rat uterus during pregnancy.

We clarified nitric oxide (NO) production in the rat uterus by electron paramagnetic resonance spectroscopy and with Fe-N-(dithiocarboxy) sarcosine complex (an NO-trapping reagent). We examined changes in NO production in the whole uterus, decidua, and myometrium (gestational days 13.5–21.5). The expression of nitric oxide synthase (NOS) isoforms was also examined by quantitative reverse transcription-polymerase chain reaction. The uterine NO levels were low on day 13.5, peaked on day 17.5, and thereafter decreased significantly. The NO production levels in the decidua and myometrium were the same on day 13.5, but the levels in the decidua were 2- to 4-fold higher than those in the myometrium from day 15.5 onwards. The NOS-2 mRNA expression pattern correlated well with changes in the NO levels in the decidua, whereas the NOS-3 mRNA was expressed constantly during gestation. Thus NOS-2-generated NO in the decidua contributed significantly to uterine NO levels.