Background Nitric oxide plays a role in the regulation of fetal great vessels and ductus arteriosus caliber near the end of gestation but the enzyme source is unknown. Aims: To determine if inducible nitric oxide synthase (iNOS) is present in the cardiovascular system of fetal rats, and to compare the effects ofL- NG-(1-lminoethyl)lysine (L-NIL) a selective inhibitor of iNOS, with lipopolysacharide (LPS), sodium nitroprusside (SNP) and untreated controls near the end of gestation.Methods and Results iNOS inthe rat fetus was demonstrated in heart/great vessels and placenta using RT-PCR. Rats in the last week of pregnancy were treated for 5 d with L-NIL (1, 10, and 100 μg/ml in the drinking water). LPS 30 μg/kg od ip 5 d. SNP in mini-osmotic pumps to deliver 10 μg/kg/min. Control group was undisturbed. On day 21 of gestation, dams were anesthetized, fetuses delivered by cesarean section and rapidly frozen in isopentane chilled in liquid nitrogen. Cryostat sections(10μm) were used to reconstruct a computer generated 3D image of the great vessels and ductus arteriosus, calibers were measured with electronic calipers in mm. Table shows Mean±SEM Significant dose-dependent constriction of the great vessels and ductus arteriosus was observed with L-NIL while both LPS and SNP significantly dilated the vessels.Conclusions Nitric oxide, generated by inducible nitric oxide synthase plays a significant role in the control of major vessels and ductus arteriosus caliber in the fetal rat. In this regard nitrergic regulation of vascular tone in the fetus differs from the adult.

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