Gestational changes in production of NO and expression of NOS mRNA isoforms in the rat placenta.

To monitor changes in NO production over time in the fetal placenta of rats, we used electron paramagnetic resonance spectroscopy with the Fe-N-(dithiocarboxy) sarcosine (Fe-DTCS) complex as an NO-trapping reagent. The expression of nitric oxide synthase (NOS) isoforms was examined in parallel using quantitative RT-PCR. NO production was first detected on day 13.5 of gestation. NO levels reached a peak on day 15.5, then decreased significantly during the last few days of gestation. The pattern of expression of NOS II mRNA was in good agreement with changes in NO levels, whereas NOS III mRNA expression did not change markedly during gestation. Thus, it appears that NO levels in the placenta are NOS II-dependent and differ at different gestational stages.