Limb reduction defects after prenatal inhibition of nitric oxide synthase in rats

To determine the influence of nitric oxide (NO) on vascular tone during fetal development, timed pregnant rats received the NO synthase inhibitor NG-nitro-L-arginine methyl ester for consecutive 4, 7, or 14 d before parturition (postorganogenesis). Offspring demonstrated limb reduction defects (incidence, 53%) involving either or both hindlimbs, whereas forelimbs were uniformly spared. Defects were dose-dependent but independent of the duration of administration occurring with equal frequency in 4-, 7-, and 14-d treatment groups. Histologic analysis revealed features characteristic of vascular disruption with hemorrhagic necrosis and loss of structure. The defects were prevented by concurrent maternal administration of L-arginine or the NO donors S-nitroso-N-acetyl-penicillamine and sodium nitroprusside. Defects were not seen after prenatal treatment with aminoguanidine. To study basal and agonist-mediated NO release, newborn femoral and brachial arteries were cannulated with a glass micropipette under constant pressure, and changes in intraluminal diameter (micrometers) were measured in response to acetylcholine and the NO synthase inhibitor N omega-nitro-L-arginine. Newborn femoral and brachial vessels demonstrated a dramatic (59%) decrease in resting diameter compared with adult vessels (16%). These findings suggest that basal NO release is upregulated during fetal development concurrent with the processes that increase maternal NO release. The data also suggest that up-regulation of NO release occurs throughout the fetal systemic circulation and is not restricted to hindlimbs. This is the first study to demonstrate inhibition of NO release in the pathogenesis of limb reduction defects.