Birth upregulates nitric oxide synthase activity in the porcine lung.

Developmental changes in the lung occur at birth, allowing for the transition from placental to air breathing. Here we have measured nitric oxide synthase (NOS) activity in the porcine lung pre and post partum. NOS activity, which was predominantly calcium dependent, was low in full term fetal tissue compared to that present in lungs from the newborn (5 minutes post partum), 1, 3, 6 and 14 day old animals. No increase in activity was seen when fetal pigs were allowed to breathe for 5 minutes. Specific activity remained low in fetal tissue following partial purification. By contrast, levels of NOS III protein in tissue extracts and in pulmonary arterial endothelial cells, demonstrated by immunohistochemistry, were similar in tissue from the fetal and newborn animals. Thus NOS activity is significantly lower in fetal when compared to postnatal lung tissue despite comparable amounts of NOS III protein being expressed, and birth is followed by an abrupt increase in enzyme activity in animals born at term which correlates with an increase in protein expression.

Newborn intrapulmonary veins are more reactive than arteries in normal and hypertensive piglets.

The reactivity of pulmonary veins during adaptation from pre- to postnatal life is not well characterized. With an in vitro organ bath technique, the responses to the contractile and relaxant agonists U-46619 (10(-10) to 3 x 10(-6) M) and acetylcholine (10(-9) to 10(-4) M) were compared in adjacent conduit pulmonary vein and artery rings from 66 piglets aged 1 wk preterm to 14 days of postnatal life and from adult tissue. Five additional piglets were made hypertensive by exposure to chronic hypoxia for 3 days after birth. Both arteries and veins showed smaller contractile and relaxant responses before birth than after. By 5 min after birth, the contraction by arteries and relaxation by veins had increased (P < 0.05). By 3 days of age, arterial relaxation increased, but in all animals, venous relaxation exceeded that in arteries (P < 0.05). Veins contracted more than arteries in animals aged 3-14 days. Neonatal hypoxia diminished the responses to both agonists in the veins (P < 0.05), whereas the response in the arteries remained similar to that in the normal newborn. We speculate that veins may be more important in postnatal adaptation than previously suggested.