Estradiol improves pulmonary hemodynamics and vascular remodeling in perinatal pulmonary hypertension

Partial ligation of the ductus arteriosus (DA) in the fetal lamb causes sustained elevation of pulmonary vascular resistance (PVR) and hypertensive structural changes in small pulmonary arteries, providing an animal model for persistent pulmonary hypertension of the newborn. Based on its vasodilator and antimitogenic properties in other experimental studies, we hypothesized that estradiol (E(2)) would attenuate the pulmonary vascular structural and hemodynamic changes caused by pulmonary hypertension in utero. To test our hypothesis, we treated chronically instrumented fetal lambs (128 days, term = 147 days) with daily infusions of E(2) (10 microg; E(2) group, n = 6) or saline (control group, n = 5) after partial ligation of the DA. We measured intrauterine pulmonary and systemic artery pressures in both groups throughout the study period. After 8 days, we delivered the study animals by cesarean section to measure their hemodynamic responses to birth-related stimuli. Although pulmonary and systemic arterial pressures were not different in utero, fetal PVR immediately before ventilation was reduced in the E(2)-treated group (2.43 +/- 0.79 vs. 1.48 +/- 0.26 mmHg. ml(-1). min, control vs. E(2), P < 0.05). During the subsequent delivery study, PVR was lower in the E(2)-treated group in response to ventilation with hypoxic gas but was not different between groups with ventilation with 100% O(2). During mechanical ventilation after delivery, arterial partial O(2) pressure was higher in E(2) animals than controls (41 +/- 11 vs. 80 +/- 35 Torr, control vs. E(2), P < 0. 05). Morphometric studies of hypertensive vascular changes revealed that E(2) treatment decreased wall thickness of small pulmonary arteries (59 +/- 1 vs. 48 +/- 1%, control vs. E(2), P < 0.01). We conclude that chronic E(2) treatment in utero attenuates the pulmonary hemodynamic and histological changes caused by DA ligation in fetal lambs.     

Inhibition of 20-HETE abolishes the myogenic response during NOS antagonism in the ovine fetal pulmonary circulation

Mechanisms that maintain high pulmonary vascular resistance (PVR) and oppose vasodilation in the fetal lung are poorly understood. In fetal lambs, increased pulmonary artery pressure evokes a potent vasoconstriction, suggesting that a myogenic response contributes to high PVR in the fetus. In adult systemic circulations, the arachidonic acid metabolite 20-hydroxyeicosatetraenoic acid (20-HETE) has been shown to modulate the myogenic response, but its role in the fetal lung is unknown. We hypothesized that acute increases in pulmonary artery pressure release 20-HETE, which causes vasoconstriction, or a myogenic response, in the fetal lung. To address this hypothesis, we studied the hemodynamic effects of N-methylsufonyl-12,12-dibromododec-11-enamide (DDMS), a specific inhibitor of 20-HETE production, on the pulmonary vasoconstriction caused by acute compression of the ductus arteriosus (DA) in chronically prepared fetal sheep. An inflatable vascular occluder around the DA was used to increase pulmonary artery pressure under three study conditions: control, after pretreatment with nitro-L-arginine (L-NA; to inhibit shear-stress vasodilation), and after combined treatment with both L-NA and a specific 20-HETE inhibitor, DDMS. We found that DA compression after L-NA treatment increased PVR by 44 +/- 12%. Although intrapulmonary DDMS infusion did not affect basal PVR, DDMS completely abolished the vasoconstrictor response to DA compression in the presence of L-NA (44 +/- 12% vs. 2 +/- 4% change in PVR, L-NA vs. L-NA + DDMS, P < 0.05). We conclude that 20-HETE mediates the myogenic response in the fetal pulmonary circulation and speculate that pharmacological inhibition of 20-HETE might have a therapeutic role in neonatal conditions characterized by pulmonary hypertension.     

Pulmonary hypertension impairs alveolarization and reduces lung growth in the ovine fetus

Persistent pulmonary hypertension of the newborn (PPHN) is a clinical disorder characterized by abnormal vascular structure, growth, and reactivity. Disruption of vascular growth during early postnatal lung development impairs alveolarization, and newborns with lung hypoplasia often have severe pulmonary hypertension. To determine whether pulmonary hypertension can directly impair vascular growth and alveolarization in the fetus, we studied the effects of chronic intrauterine pulmonary hypertension on lung growth in fetal lambs. We performed surgery, which included partial constriction of the ductus arteriosus (DA) to induce pulmonary hypertension (PH, n = 14) or sham surgery (controls, n = 13) in fetal lambs at 112-125 days (term = 147 days). Tissues were harvested near term for measurement of right ventricular hypertrophy (RVH), radial alveolar counts (RAC), mean linear intercepts (MLI), wall thickness, and vessel density of small pulmonary arteries. Chronic DA constriction caused RVH (P < 0.0001), increased wall thickness of small pulmonary arteries (P < 0.002), and reduced small pulmonary artery density (P < 0.005). PH also reduced alveolarization, causing a 27% reduction in RAC and 20% increase in MLI. Furthermore, prolonged DA constriction (21 days) not only decreased RAC and increased MLI by 30% but also caused a 25% reduction of lung-body weight ratio. We conclude that chronic PH reduces pulmonary arterial growth, decreases alveolar complexity, and impairs lung growth. We speculate that chronic hypertension impairs vascular growth, which disrupts critical signaling pathways regulating lung vascular and alveolar development, thereby interfering with alveolarization and ultimately resulting in lung hypoplasia.     

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.     

Effects of sildenafil on hypoxic pulmonary vascular function in dogs.

Sildenafil has been shown to be an effective treatment of pulmonary arterial hypertension and is believed to present with pulmonary selectivity. This study was designed to determine the site of action of sildenafil compared with inhaled nitric oxide (NO) and intravenous sodium nitroprusside (SNP), known as selective and nonselective pulmonary vasodilators, respectively. Inhaled NO (40 ppm), and maximum tolerated doses of intravenous SNP and sildenafil, (5 microg x kg(-1) x min(-1) and 0.1 mg x kg(-1) x h(-1)), respectively, were administered to eight dogs ventilated in hypoxia. Pulmonary vascular resistance (PVR) was evaluated by pulmonary arterial pressure (Ppa) minus left atrial pressure (Pla) vs. flow curves, and partitioned into arterial and venous segments by the occlusion method. Right ventricular hydraulic load was defined by pulmonary arterial characteristic impedance (Zc) and elastance (Ea) calculations. Right ventricular arterial coupling was estimated by the ratio of end-systolic elastance (Ees) to Ea. Decreasing the inspired oxygen fraction from 0.4 to 0.1 increased Ppa - Pla at a standardized flow of 3 l x min(-1) x m(-2) from 6 +/- 1 to 18 +/- 1 mmHg (mean +/- SE). Ppa - Pla was decreased to 9 +/- 1 by inhaled NO, 14 +/- 1 by SNP, and 14 +/- 1 mmHg by sildenafil. The partition of PVR, Zc, Ea, and Ees/Ea was not affected by the three interventions. Inhaled NO did not affect systemic arterial pressure, which was similarly decreased by sildenafil and SNP, from 115 +/- 4 to 101 +/- 4 and 98 +/- 5 mmHg, respectively. We conclude that inhaled NO inhibits hypoxic pulmonary vasoconstriction more effectively than sildenafil or SNP, and sildenafil shows no more selectivity for the pulmonary circulation than SNP.     

Umbilical cord compression produces pulmonary hypertension in newborn lambs: a model to study the pathophysiology of persistent pulmonary hypertension in the newborn

We investigated the effects of chronic intrauterine hypoxaemia produced by prolonged partial umbilical cord compression on the circulation shortly after birth in lambs. Vascular catheters were inserted in 10 fetal sheep at 120 to 130 days gestation to measure descending aortic blood gases, arterial pH, and arterial O2 saturation. An inflatable silicone rubber balloon cuff was also placed around the umbilical cord. After recovery and the return of descending aortic blood gases to the normal range, the balloon was gradually inflated, decreasing the PaO2 from 21.2 +/- 3.6 to 17.5 +/- 1.3 mm Hg and the arterial O2 saturation from 57.1 +/- 9.2% to 37.2% +/- 5.2. After 14.3 +/- 3.7 days of partial umbilical cord compression, the lambs were delivered by Caesarean section, instrumented to measure systemic and pulmonary arterial, right atrial and pulmonary arterial wedge pressures, pulmonary and systemic blood flows, and mechanically ventilated. Five normal lambs were also studied. From 60 to 120 min after delivery, when compared to normal lambs, the umbilical compression lambs had an increased pulmonary arterial pressure (P less than 0.05) pulmonary vascular resistance (P less than 0.05), and right atrial pressure (P less than 0.05) with similar arterial blood gases. In both groups, hypoxic ventilation produced an increase in pulmonary arterial pressure (P less than 0.05) which on return to room air ventilation decreased to baseline in the normal lambs but not in the umbilical cord compression lambs (P less than 0.05). Prolonged partial umbilical cord compression produces chronic fetal hypoxaemia and pulmonary arterial hypertension after birth. This may represent a model to study the pathophysiology of persistent pulmonary hypertension syndrome.     

Fasudil inhibits the myogenic response in the fetal pulmonary circulation

In addition to high pulmonary vascular resistance (PVR) and low pulmonary blood flow, the fetal pulmonary circulation is characterized by mechanisms that oppose vasodilation. Past work suggests that high myogenic tone contributes to high PVR and may contribute to autoregulation of blood flow in the fetal lung. Rho-kinase (ROCK) can mediate the myogenic response in the adult systemic circulation, but whether high ROCK activity contributes to the myogenic response and modulates time-dependent vasodilation in the developing lung circulation are unknown. We studied the effects of fasudil, a ROCK inhibitor, on the hemodynamic response during acute compression of the ductus arteriosus (DA) in chronically prepared, late-gestation fetal sheep. Acute DA compression simultaneously induces two opposing responses: 1) blood flow-induced vasodilation through increased shear stress that is mediated by NO release and 2) stretch-induced vasoconstriction (i.e., the myogenic response). The myogenic response was assessed during acute DA compression after treatment with N(omega)-nitro-L-arginine, an inhibitor of nitric oxide synthase, to block flow-induced vasodilation and unmask the myogenic response. Intrapulmonary fasudil infusion (100 microg over 10 min) did not enhance flow-induced vasodilation during brief DA compression but reduced the myogenic response by 90% (P<0.05). During prolonged DA compression, fasudil prevented the time-dependent decline in left pulmonary artery blood flow at 2 h (183+/-29 vs. 110+/-11 ml/min with and without fasudil, respectively; P<0.001). We conclude that high ROCK activity opposes pulmonary vasodilation in utero and that the myogenic response maintains high PVR in the normal fetal lung through ROCK activation.     

Effects of platelet-activating factor antagonist SRI 63-441 on endotoxemia in sheep

We investigated whether platelet-activating factor (PAF) mediates endotoxin-induced systemic and pulmonary vascular derangements by studying the effects of a selective PAF receptor antagonist, SRI 63-441, during endotoxemia in sheep. Endotoxin infusion (1.3 micrograms/kg over 0.5 h) caused a rapid, transient rise in pulmonary arterial pressure (Ppa) from 16 +/- 3 to 36 +/- 10 mmHg (P less than 0.001) and pulmonary vascular resistance (PVR) from 187 +/- 84 to 682 +/- 340 dyn.s.cm-5 (P less than 0.05) at 0.5 h, followed by a persistent elevation in Ppa to 22 +/- 3 mmHg and in PVR to 522 +/- 285 dyn.s.cm-5 at 5 h in anesthetized sheep. Arterial PO2 (PaO2) decreased from 341 +/- 79 to 198 +/- 97 (P less than 0.01) and 202 +/- 161 Torr at 0.5 and 5 h, respectively (inspired O2 fraction = 1.0). SRI 63-441, 20 mg.kg-1.h-1 infused for 5 h, blocked the early rise in Ppa and PVR and fall in PaO2, but had no effect on the late phase pulmonary hypertension or hypoxemia. Endotoxin caused a gradual decrease in mean aortic pressure, which was unaffected by SRI 63-441. Infusion of SRI 63-441 alone caused no hemodynamic alterations. In follow-up studies, endotoxin caused an increase in lung lymph flow (QL) from 3.8 +/- 1.1 to 14.1 +/- 8.0 (P less than 0.05) and 12.7 +/- 8.6 ml/h at 1 and 4 h, respectively. SRI 63-441 abolished the early and attenuated the late increase in QL.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of neuronal nitric oxide synthase in regulation of vascular and ductus arteriosus tone in the ovine fetus

Nitric oxide (NO) is produced by NO synthase (NOS) and contributes to the regulation of vascular tone in the perinatal lung. Although the neuronal or type I NOS (NOS I) isoform has been identified in the fetal lung, it is not known whether NO produced by the NOS I isoform plays a role in fetal pulmonary vasoregulation. To study the potential contribution of NOS I in the regulation of basal fetal pulmonary vascular resistance (PVR), we studied the hemodynamic effects of a selective NOS I antagonist, 7-nitroindazole (7-NINA), and a nonselective NOS antagonist, N-nitro-L-arginine (L-NNA), in chronically prepared fetal lambs (mean age 128 +/- 3 days, term 147 days). Brief intrapulmonary infusions of 7-NINA (1 mg) increased basal PVR by 37% (P < 0.05). The maximum increase in PVR occurred within 20 min after infusion, and PVR remained elevated for up to 60 min. Treatment with 7-NINA also increased the pressure gradient between the pulmonary artery and aorta, suggesting constriction of the ductus arteriosus (DA). To test whether 7-NINA treatment selectively inhibits the NOS I isoform, we studied the effects of 7-NINA and L-NNA on acetylcholine-induced pulmonary vasodilation. The vasodilator response to acetylcholine remained intact after treatment with 7-NINA but was completely inhibited after L-NNA, suggesting minimal effects on endothelial or type III NOS after 7-NINA infusion. Western blot analysis detected NOS I protein in the fetal lung and great vessels including the DA. NOS I protein was detected in intact and endothelium-denuded vessels, suggesting that NOS I is present in the medial or adventitial layer. We conclude that 7-NINA, a selective NOS I antagonist, increases basal PVR, systemic arterial pressure, and DA tone in the late-gestation fetus and that NOS I protein is present in the fetal lung and great vessels. We speculate that NOS I may contribute to NO production in the regulation of basal vascular tone in the pulmonary and systemic circulations and the DA.     

NO and prostaglandin interactions during hemodynamic stress in the fetal ovine pulmonary circulation

Nitric oxide (NO) and prostacyclin (PGI(2)) are potent fetal pulmonary vasodilators, but their relative roles and interactions in the regulation of the perinatal pulmonary circulation are poorly understood. We compared the separate and combined effects of nitric oxide synthase (NOS) and cyclooxygenase (COX) inhibition during acute hemodynamic stress caused by brief mechanical compression of the ductus arteriosus (DA) in chronically prepared fetal lambs. Nitro-L-arginine (L-NNA; NOS antagonist), meclofenamate (Mec; COX inhibitor), combined drugs (L-NNA-Mec), or saline (control) was infused into the left pulmonary artery (LPA) before DA compression. In controls, DA compression decreased pulmonary vascular resistance (PVR) by 43% (P < 0.01). L-NNA, but not Mec, treatment completely blocked vasodilation and caused a paradoxical increase in PVR (+31%; P < 0.05). The effects of L-NNA-Mec and L-NNA on PVR were similar. To determine if the vasodilator effect of PGI(2) is partly mediated by NO release, we studied PGI(2)-induced vasodilation before and after NOS inhibition. L-NNA treatment blocked the PGI(2)-induced rise in LPA blood flow by 73% (P < 0.001). We conclude that NO has a greater role than PGs in fetal pulmonary vasoregulation during acute hemodynamic stress and that PGI(2)-induced pulmonary vasodilation is largely mediated by NO release in the fetal lung.     

Pulmonary vascular effects of nitric oxide-cGMP augmentation in a model of chronic pulmonary hypertension in fetal and neonatal sheep

Persistent pulmonary hypertension of the newborn (PPHN) is partly due to impaired nitric oxide (NO)-cGMP signaling. BAY 41-2272 is a novel direct activator of soluble guanylate cyclase, but whether this drug may be an effective therapy for PPHN is unknown. We hypothesized that BAY 41-2272 would cause pulmonary vasodilation in a model of severe PPHN. To test this hypothesis, we compared the hemodynamic response of BAY 41-2272 to acetylcholine, an endothelium-dependent vasodilator, and sildenafil, a selective inhibitor of PDE5 in chronically instrumented fetal lambs at 1 and 5 days after partial ligation of the ductus arteriosus. After 9 days, we delivered the animals by cesarean section to measure their hemodynamic responses to inhaled NO (iNO), sildenafil, and BAY 41-2272 alone or combined with iNO. BAY 41-2272 caused marked pulmonary vasodilation, as characterized by a twofold increase in blood flow and a nearly 60% fall in PVR at day 1. Effectiveness of BAY 41-2272-induced pulmonary vasodilation increased during the development of pulmonary hypertension. Despite a similar effect at day 1, the pulmonary vasodilator response to BAY 41-2272 was greater than sildenafil at day 5. At birth, BAY 41-2272 dramatically reduced PVR and augmented the pulmonary vasodilation induced by iNO. We concluded that BAY 41-2272 causes potent pulmonary vasodilation in fetal and neonatal sheep with severe pulmonary hypertension. We speculate that BAY 41-2272 may provide a novel treatment for severe PPHN, especially in newborns with partial response to iNO therapy.     

High-altitude chronic hypoxia during gestation and after birth modifies cardiovascular responses in newborn sheep

Perinatal exposure to chronic hypoxia induces sustained pulmonary hypertension and structural and functional changes in both pulmonary and systemic vascular beds. The aim of this study was to analyze consequences of high-altitude chronic hypoxia during gestation and early after birth in pulmonary and femoral vascular responses in newborn sheep. Lowland (LLNB; 580 m) and highland (HLNB; 3,600 m) newborn lambs were cathetherized under general anesthesia and submitted to acute sustained or stepwise hypoxic episodes. Contractile and dilator responses of isolated pulmonary and femoral small arteries were analyzed in a wire myograph. Under basal conditions, HLNB had a higher pulmonary arterial pressure (PAP; 20.2 +/- 2.4 vs. 13.6 +/- 0.5 mmHg, P < 0.05) and cardiac output (342 +/- 23 vs. 279 +/- 13 ml x min(-1) x kg(-1), P < 0.05) compared with LLNB. In small pulmonary arteries, HLNB showed greater contractile capacity and higher sensitivity to nitric oxide. In small femoral arteries, HLNB had lower maximal contraction than LLNB with higher maximal response and sensitivity to noradrenaline and phenylephrine. In acute superimposed hypoxia, HLNB reached higher PAP and femoral vascular resistance than LLNB. Graded hypoxia showed that average PAP was always higher in HLNB compared with LLNB at any Po2. Newborn lambs from pregnancies at high altitude have stronger pulmonary vascular responses to acute hypoxia associated with higher arterial contractile status. In addition, systemic vascular response to acute hypoxia is increased in high-altitude newborns, associated with higher arterial adrenergic responses. These responses determined in intrauterine life and early after birth could be adaptive to chronic hypoxia in the Andean altiplano.     

Effects of chronic estrogen-receptor blockade on ovine perinatal pulmonary circulation

Prolonged infusions of 17beta-estradiol reduce fetal pulmonary vascular resistance (PVR), but the effects of endogenous estrogens in the fetal pulmonary circulation are unknown. To test the hypothesis that endogenous estrogen promotes pulmonary vasodilation at birth, we studied the hemodynamic effects of prolonged estrogen-receptor blockade during late gestation and at birth in fetal lambs. We treated chronically prepared fetal lambs with ICI-182,780 (ICI, a specific estrogen-receptor blocker, n = 5) or 1% DMSO (CTRL, n = 5) for 7 days and then measured pulmonary hemodynamic responses to ventilation with low- and high-fraction inspired oxygen (FI(O(2))). Treatment with ICI did not change basal fetal PVR or arterial blood gas tensions. However, treatment with ICI abolished the vasodilator response to ventilation with low FI(O(2)) [change in PVR -30 +/- 6% (CTRL) vs. +10 +/- 13%, (ICI), P < 0.05] without reducing the vasodilator response to ventilation with high FI(O(2)) [change in PVR, -73 +/- 3% (CTRL) vs. -77 +/- 4%, (ICI); P = not significant]. ICI treatment reduced prostacyclin synthase (PGIS) expression by 33% (P < 0.05) without altering expression of endothelial nitric oxide synthase or cyclooxygenase-1 and -2. In situ hybridization and immunohistochemistry revealed that PGIS is predominantly expressed in the airway epithelium of late gestation fetal lambs. We conclude that prolonged estrogen-receptor blockade inhibits the pulmonary vasodilator response at birth and that this effect may be mediated by downregulation of PGIS. We speculate that estrogen exposure during late gestation prepares the pulmonary circulation for postnatal adaptation.     

Role of platelet-derived growth factor in vascular remodeling during pulmonary hypertension in the ovine fetus

Platelet-derived growth factor (PDGF) is a potent smooth muscle cell mitogen that may contribute to smooth muscle hyperplasia during the development of chronic pulmonary hypertension (PH). We studied changes in PDGFalpha- and beta-receptor and ligand expression in lambs with chronic intrauterine PH induced by partial ligation of the ductus arteriosus (DA) at gestational age 124-128 days (term = 147 days). Western blot analysis performed on whole lung homogenates from PH animals after 8 days of DA ligation showed a twofold increase in PDGFalpha- and beta-receptor proteins compared with age-matched controls (P < 0.05). Lung PDGF-A and -B mRNA expression did not differ between PH and control animals. We treated PH animals with NX1975, an aptamer that selectively inhibits PDGF-B, by infusion into the left pulmonary artery for 7 days after DA ligation. NX1975 reduced the development of muscular thickening of small pulmonary arteries by 47% (P < 0.05) and right ventricular hypertrophy (RVH) by 66% (P < 0.02). Lung PDGFalpha- and beta-receptor expression is increased in perinatal PH, and NX1975 reduces the increase in wall thickness of small pulmonary arteries and RVH in this model. We speculate that PDGF signaling contributes to structural vascular remodeling in perinatal PH and that selective PDGF inhibition may provide a novel therapeutic strategy for the treatment of chronic PH.     

Pulmonary vascular dysfunction in preterm lambs with chronic lung disease

Chronic lung injury from prolonged mechanical ventilation after premature birth inhibits the normal postnatal decrease in pulmonary vascular resistance (PVR) and leads to structural abnormalities of the lung circulation in newborn sheep. Compared with normal lambs born at term, chronically ventilated preterm lambs have increased pulmonary arterial smooth muscle and elastin, fewer lung microvessels, and reduced abundance of endothelial nitric oxide synthase. These abnormalities may contribute to impaired respiratory gas exchange that often exists in infants with chronic lung disease (CLD). Nitric oxide inhalation (iNO) reduces PVR in human infants and lambs with persistent pulmonary hypertension. We wondered whether iNO might have a similar effect in lambs with CLD. We therefore studied the effect of iNO on PVR in lambs that were delivered prematurely at approximately 125 days of gestation (term = 147 days) and mechanically ventilated for 3 wk. All of the lambs had chronically implanted catheters for measurement of pulmonary vascular pressures and blood flow. During week 2 of mechanical ventilation, iNO at 15 parts/million for 1 h decreased PVR by approximately 20% in 12 lambs with evolving CLD. When the same study was repeated in eight lambs at the end of week 3, iNO had no significant effect on PVR. To see whether this loss of iNO effect on PVR might reflect dysfunction of lung vascular smooth muscle, we infused 8-bromo-guanosine 3',5'-cyclic monophosphate (cGMP; 150 microg. kg(-1). min(-1) iv) for 15-30 min in four of these lambs at the end of week 3. PVR consistently decreased by 30-35%. Lung immunohistochemistry and immunoblot analysis of excised pulmonary arteries from lambs with CLD, compared with control term lambs, showed decreased soluble guanylate cyclase (sGC). These results suggest that loss of pulmonary vascular responsiveness to iNO in preterm lambs with CLD results from impaired signaling, possibly related to deficient or defective activation of sGC, the intermediary enzyme through which iNO induces increased vascular smooth muscle cell cGMP and resultant vasodilation.     

The role of leukotrienes in the late hemodynamic manifestations of group B streptococcal sepsis in piglets

In order to evaluate the role of leukotrienes in group B streptococcal (GBS) sepsis we studied the effect of a leukotriene receptor antagonist, FPL 57231, on the late hemodynamic changes occurring secondary to an infusion of live GBS. Paralyzed, mechanically ventilated piglets received a continuous intravenous infusion of bacteria (5 x 10(7) org/kg/min) while systemic arterial (Psa) and pulmonary artery pressures (Ppa) were measured. To separate the effects of the lipoxygenase products of arachidonic acid from those of the cyclooxygenase by-products, animals in control and treatment groups received indomethacin, a cyclooxygenase blocking agent, 15 min after the infusion of GBS was begun. In addition to GBS and indomethacin, treatment animals received a 30 min infusion of FPL 57231 starting 120 min after the bacterial infusion was begun. All study animals responded to bacteria within 15 min with marked elevation in pulmonary artery pressure (X +/- SD) (12 +/- 3 to 49 +/- 5 mmHg; p less than .01), and a decline in PaO2 (84 +/- 9 to 49 +/- 5 mmHg; p less than .01) and cardiac output (0.29 +/- 0.04 to 0.18 +/- .07 liter/min/kg; p less than .01). These changes were reversed by indomethacin. Subsequent values remained relatively stable until approximately 90 min when a gradual decrease in cardiac output (CO) and PaO2, and an increase in Ppa, and calculated systemic (SVR) and pulmonary (PVR) vascular resistances occurred. After the initial increase in TxB2 and 6-keto-PGF1 alpha, indomethacin treatment resulted in return of these values to baseline with no further increase throughout the study period.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhaled nitric oxide-induced rebound pulmonary hypertension: role for endothelin-1

Clinically significant increases in pulmonary vascular resistance have been noted on acute withdrawal of inhaled nitric oxide (NO). Endothelin (ET)-1 is a vasoactive peptide produced by the vascular endothelium that may participate in the pathophysiology of pulmonary hypertension. The objectives of this study were to determine the effects of inhaled NO on endogenous ET-1 production in vivo in the intact lamb and to determine the potential role of ET-1 in the rebound pulmonary hypertension associated with the withdrawal of inhaled NO. Seven 1-mo-old vehicle-treated control lambs and six PD-156707 (an ET(A) receptor antagonist)-treated lambs were mechanically ventilated. Inhaled NO (40 parts per million) was administered for 24 h and then acutely withdrawn. After 24 h of inhaled NO, plasma ET-1 levels increased by 119.5 +/- 42.2% (P < 0.05). Western blot analysis revealed that protein levels of preproET-1, endothelin-converting enzyme-1alpha, and ET(A) and ET(B) receptors were unchanged. On acute withdrawal of NO, pulmonary vascular resistance (PVR) increased by 77.8% (P < 0.05) in control lambs but was unchanged (-5.5%) in PD-156707-treated lambs. Inhaled NO increased plasma ET-1 concentrations but not gene expression in the intact lamb, and ET(A) receptor blockade prevented the increase in PVR after NO withdrawal. These data suggest a role for ET-1 in the rebound pulmonary hypertension noted on acute withdrawal of inhaled NO.     

Sildenafil is more selective pulmonary vasodilator than prostaglandin E1 in patients with pulmonary hypertension due to heart failure

In some patients, heart failure (HF) is associated with increased pulmonary vascular resistance (PVR). The magnitude and the reversibility of PVR elevation affect the HF management. Sildenafil has been recently recognized as potent PVR-lowering drug in HF. The aim of the study was to compare hemodynamic effects and pulmonary selectivity of sildenafil to prostaglandin E(1) (PGE(1)). Right-heart catheterization was performed in 13 euvolemic advanced HF patients with elevated PVR (6.3+/-2 Wood's units). Hemodynamic parameters were measured at the baseline, during i.v. infusion of PGE1 (alprostadil 200 ng · kg(-1) · min(-1)) and after 40 mg oral dose of sildenafil. Both drugs similarly reduced systemic vascular resistance (SVR), but sildenafil had higher effect on PVR (-28 % vs. -49 %, p = 0.05) and transpulmonary pressure gradient than PGE(1). The PVR/SVR ratio--an index of pulmonary selectivity, did not change after PGE(1) (p = 0.7) but it decreased by -32 % (p = 0.004) after sildenafil. Both drugs similarly reduced pulmonary artery mean and wedge pressures and increased cardiac index (+27 % and +28 %). Sildenafil led more often to transplant-acceptable PVR while causing smaller drop of mean systemic pressure than PGE(1). In conclusion, vasodilatatory effects of sildenafil in patients with heart failure are more pronounced in pulmonary than in systemic circulation.     

The role of peripheral chemoreceptors in the rapid response of the pulmonary vasculature of the late gestation sheep fetus to changes in PaO2.

The effect of removing the input from the peripheral arterial chemoreceptors on pulmonary vascular responses to changes in PaO2 was examined in late gestation fetal sheep. Blood flow in the left pulmonary artery and driving pressure across the pulmonary vascular bed were monitored in chronically prepared fetal sheep at 126-129 days gestation. Five fetuses had carotid sinus and vagus nerves sectioned bilaterally and four were left intact. In normoxia (PaO2 ca. 23 mmHg) pulmonary vascular resistance was slightly greater and pulmonary blood flow reduced in the denervated group relative to the intact group but these differences were not significant. When made hypoxic (PaO2 ca. 14 mmHg), pulmonary blood flow fell and pulmonary vascular resistance increased in all fetuses. However, in the intact fetuses these changes were significantly more rapid. In all fetuses the vasoconstriction was prolonged after their return to normoxia. When made hyperoxic (PaO2 ca. 27 mmHg), pulmonary blood flow increased by a similar amount in all fetuses. We conclude that in the term fetus the peripheral chemoreceptors play no appreciable role in the maintenance of the high pulmonary vascular resistance in normoxia, or the fall in resistance produced by a rise in PaO2. The chemoreceptors do however initiate the rapid phase of pulmonary vasoconstriction in hypoxia.     

Increases in lung expansion alter pulmonary hemodynamics in fetal sheep.

Prolonged increases in fetal lung expansion stimulate fetal lung growth and development, but the effects on pulmonary hemodynamics are unknown. Our aim was to determine the effect of increased fetal lung expansion, induced by tracheal obstruction (TO), on pulmonary blood flow (PBF) and vascular resistance (PVR). Chronically catheterized fetal sheep (n = 6) underwent TO from 120 to 127 days of gestational age (term approximately 147 days); tracheas were not obstructed in control fetuses (n = 6). PBF, PVR, and changes to the PBF waveform were determined. TO significantly increased lung wet weight compared with control (166.3 +/- 20.2 vs. 102.0 +/- 18.8 g; P < 0.05). Despite the increase in intraluminal pressure caused by TO (5.0 +/- 0.9 vs. 2.4 +/- 1.0 mmHg; P < 0.001), PBF and PVR were similar between groups after 7 days (TO 28.1 +/- 3.2 vs. control 34.1 +/- 10.0 ml.min(-1).100 g lung wt(-1)). However, TO markedly altered pulmonary hemodynamics associated with accentuated fetal breathing movements, causing a reduction rather than an increase in PBF at 7 days of TO. To account for the increase in intraluminal pressure, the pressure was equalized by draining the lungs of liquid on day 7 of TO. Pressure equalization increased PBF from 36.8 +/- 5.2 to 112.4 +/- 22.8 ml/min (P = 0.01) and markedly altered the PBF waveform. These studies provide further evidence to indicate that intraluminal pressure is an important determinant of PBF and PVR in the fetus. We suggest that the increase in PBF associated with pressure equalization following TO reflects an increase in growth of the pulmonary vascular bed, leading to an increase in its cross-sectional area.