Changes in ANP responsiveness of normal and hypertensive porcine intrapulmonary arteries during maturation

Advances in the diagnosis and management of persistent pulmonary hypertension of the newborn

Rapid evaluation of a neonate who is cyanotic and in respiratory distress is essential for achieving a good outcome. Persistent pulmonary hypertension of the newborn (PPHN) can be a primary cause or a contributing factor to respiratory failure, particularly in neonates born at 34 weeks or more of gestation. PPHN represents a failure of normal postnatal adaptation that occurs at birth in the pulmonary circulation. Rapid advances in therapy in recent years have led to a remarkable decrease in mortality for the affected infants. Infants who survive PPHN are at significant risk for long-term hearing and neurodevelopmental impairments, however. This review focuses on the diagnosis, recent advances in management, and recommendations for the long-term follow-up of infants who have PPHN.

Rho kinase activation maintains high pulmonary vascular resistance in the ovine fetal lung

Mechanisms that maintain high pulmonary vascular resistance (PVR) in the fetal lung are poorly understood. Activation of the Rho kinase signal transduction pathway, which promotes actin-myosin interaction in vascular smooth muscle cells, is increased in the pulmonary circulation of adult animals with experimental pulmonary hypertension. However, the role of Rho kinase has not been studied in the fetal lung. We hypothesized that activation of Rho kinase contributes to elevated PVR in the fetus. To address this hypothesis, we studied the pulmonary hemodynamic effects of brief (10 min) intrapulmonary infusions of two specific Rho kinase inhibitors, Y-27632 (15-500 microg) and HA-1077 (500 microg), in chronically prepared late-gestation fetal lambs (n = 9). Y-27632 caused potent, dose-dependent pulmonary vasodilation, lowering PVR from 0.67 +/- 0.18 to 0.16 +/- 0.02 mmHg x ml(-1) x min(-1) (P < 0.01) at the highest dose tested without lowering systemic arterial pressure. Despite brief infusions, Y-27632-induced pulmonary vasodilation was sustained for 50 min. HA-1077 caused a similar fall in PVR, from 0.39 +/- 0.03 to 0.19 +/- 0.03 (P < 0.05). To study nitric oxide (NO)-Rho kinase interactions in the fetal lung, we tested the effect of Rho kinase inhibition on pulmonary vasoconstriction caused by inhibition of endogenous NO production with nitro-L-arginine (L-NA; 15-30 mg), a selective NO synthase antagonist. L-NA increased PVR by 127 +/- 73% above baseline under control conditions, but this vasoconstrictor response was completely prevented by treatment with Y-27632 (P < 0.05). We conclude that the Rho kinase signal transduction pathway maintains high PVR in the normal fetal lung and that activation of the Rho kinase pathway mediates pulmonary vasoconstriction after NO synthase inhibition. We speculate that Rho kinase plays an essential role in the normal fetal pulmonary circulation and that Rho kinase inhibitors may provide novel therapy for neonatal pulmonary hypertension.

Role of veins in regulation of pulmonary circulation

Pulmonary veins have been seen primarily as conduit vessels; however, over the past two decades, a large amount of evidence has accumulated to indicate that pulmonary veins can exhibit substantial vasoactivity. In this review, the role of veins in regulation of the pulmonary circulation, particularly during the perinatal period and under certain pathophysiological conditions, is discussed. In the fetus, pulmonary veins contribute a significant fraction to total pulmonary vascular resistance. At birth, the veins as well as the arteries relax in response to endothelium-derived nitric oxide and dilator prostaglandins, thereby assisting in the fall in pulmonary vascular resistance. These effects are oxygen dependent and modulated by cGMP-dependent protein kinase. Under chronic hypoxic conditions, pulmonary veins undergo remodeling and demonstrate substantial constriction and hypertrophy. In a number of species, including the human, pulmonary veins are also the primary sites of action of certain vasoconstrictors such as endothelin and thromboxane. In various pathological conditions, there is an increased synthesis of these vasoactive agents that may lead to pulmonary venous constriction, increased microvascular pressures for fluid filtration, and formation of pulmonary edema. In conclusion, the significant role of veins in regulation of the pulmonary circulation needs to be appreciated to better prevent, diagnose, and treat lung disease.

Postnatal changes in beta-adrenoceptors in the lung and the effect of hypoxia induced pulmonary hypertension of the newborn

1. beta-adrenoceptor activation leads to pulmonary vasodilatation. The increase in circulating catecholamines at birth may assist the postnatal fall in vascular resistance by their activation. To study beta(1)- and beta(2)-adrenoceptors during postnatal adaptation, we used [(125)I]-iodocyanopindolol (ICYP) binding to lung membranes and sections to quantify and locate the binding sites in piglets from birth to 14 days of age and compared them with those in adult pigs. In addition, pulmonary hypertension was induced in newborn piglets by hypobaric hypoxia. 2. In lung membranes the equilibrium dissociation constant (K(d)) did not change with age for total beta-adrenoceptors or for beta(2)-adrenoceptors, but there was a significant increase in maximum binding sites (B(max)) between birth and 3 days of age. On tissue sections, B(max) increased between 3 days and adulthood with no change in K(d). 3. Binding sites of beta(1)- and beta(2)-adrenoceptors were localized to the bronchial epithelium, to endothelium of extra- and intra-pulmonary arteries and to lung parenchyma. Total beta-adrenoceptor density increased with age at all locations (P<0.05 - 0.01). At birth intrapulmonary arteries showed no binding, beta(2)-adrenoceptors appeared on day 1 and increased up to 14 days of age. beta(1)-adrenoceptors appeared by 3 days of age and increased with age. 4. Hypobaric hypoxia from birth led to attenuation in the normal postnatal increase in receptor number, but hypoxia from 3 - 6 days did not decrease receptor density. 5. The normal postnatal increase in beta-adrenoceptors suggests a potential for catecholamine induced dilatation in the lung during adaptation which is attenuated in pulmonary hypertension.

Differential responses of newborn pulmonary arteries and veins to atrial and C-type natriuretic peptides

Atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are important dilators of the pulmonary circulation during the perinatal period. We compared the responses of pulmonary arteries (PA) and veins (PV) of newborn lambs to these peptides. ANP caused a greater relaxation of PA than of PV, and CNP caused a greater relaxation of PV than of PA. RIA showed that ANP induced a greater increase in cGMP content of PA than CNP. In PV, ANP and CNP caused a similar moderate increase in cGMP content. Receptor binding study showed more specific binding sites for ANP than for CNP in PA and more for CNP than for ANP in PV. Relative quantitative RT-PCR for natriuretic peptide receptor A (NPR-A) and B (NPR-B) mRNAs show that, in PA, NPR-A mRNA is more prevalent than NPR-B mRNA, whereas, in PV, NPR-B mRNA is more prevalent than NPR-A mRNA. In conclusion, in the pulmonary circulation, arteries are the major site of action for ANP, and veins are the major site for CNP. Furthermore, the differences in receptor abundance and the involvement of a cGMP-independent mechanism may contribute to the heterogeneous effects of the natriuretic peptides in PA and PV of newborn lambs.