Fetal growth retardation in rats may result from apoptosis: role of peroxynitrite

Administration of the nitric oxide synthase (NOS) inhibitor NG-nitro-L-arginine methyl ester (L-NAME) results in fetal growth retardation. This study was designed to further examine the influence of NO on fetal growth, specifically, the potential role of inducible NOS and to evaluate the possibility that apoptosis contributed to uteroplacental dysfunction. L-NAME administration caused a paradoxical increase in NO synthesis determined by direct detection of NO by electrochemistry, nitrite accumulation, and cGMP levels, indicating that a lack of NO was not the cause of the fetal growth retardation. Additionally, supplemental L-arginine or NO donors failed to reverse the effects of L-NAME on fetal and placental size. Administration of low dose endotoxin (30 micrograms/kg IP daily for 6 d) also caused significant reductions in fetal and placental size and increased NO synthesis comparable to that seen with L-NAME. Inducible NOS was constitutively expressed in the pregnant uterus (smooth muscle and epithelia) and placenta (sinusoids and macrophages) but was absent in the nonpregnant state as determined by RT-PCR and immunohistochemistry. Neither L-NAME nor endotoxin modified the expression of iNOS. In situ evidence for apoptosis (DNA fragmentation) was minimal to absent in control pregnant rats, but markedly evident in the placenta (decidua) and uterus of rats treated with L-NAME or endotoxin. Immunohistochemical evidence for nitrotyrosine, a marker for peroxynitrite formation, was absent in control rats but colocalized with apoptosis in the L-NAME and LPS groups. We conclude that L-NAME-induced fetal growth retardation is not due to a lack of NO, but as for endotoxin, results from a net reduction in cellular proliferation due to the induction of apoptosis, possibly in response to peroxynitrite formation.