Type I nitric oxide synthase is decreased in the fetal pulmonary circulation of hypertensive lambs

The nitric oxide (NO)/guanosine 3′,5′-cyclic monophosphate (cGMP) pathway plays an essential role in mediating pulmonary vasodilatation during transition of the pulmonary circulation at birth. We used immunoblot analysis (Western) and semiquantitative immunohistochemistry to study the presence, distribution, and relative amounts of type I nitric oxide synthase (NOS-I). Immunoblots were performed on normal fetal sheep lungs, whereas immunohistochemistry for NOS-I was compared between lungs from normal fetal lambs vs. fetal lambs with persistent pulmonary hypertension of the newborn (PPHN) induced by ligation of the ductus arteriosus. Western blot analysis using a polyclonal antibody detected NOS-I protein in homogenates of normal fetal sheep lungs. Abundant NOS-I immunoreactivity was observed exclusively in the precapillary resistance vessels, i.e., terminal bronchiole-associated arteries (TA) and respiratory bronchiole-associated arteries (RA) in normal fetal lung. In marked contrast, immunoreactivity for NOS-I was significantly reduced in the TA and RA of hypertensive lungs. We conclude that there is a heterogeneous distribution of NOS-I in the normal fetal sheep lung, but that NOS-I staining is significantly reduced in lambs with PPHN.