Endothelin-1 decreases endothelial NOS expression and activity through ETA receptor-mediated generation of hydrogen peroxide

Similar to infants born with persistent pulmonary hypertension of the newborn (PPHN), there is an increase in circulating endothelin-1 (ET-1) and decreased endothelial nitric oxide synthase (eNOS) gene expression in an ovine model of PPHN. These abnormalities lead to vasoconstriction and vascular remodeling. Our previous studies have demonstrated that reactive oxygen species (ROS) levels are elevated in the pulmonary arteries from PPHN lambs and that ET-1 increases ROS production in pulmonary arterial smooth muscle cells (PASMC) in culture. Thus the objective of this study was to determine whether there was a feedback mechanism between the ET-1-mediated increase in ROS in fetal PASMC (FPASMC) and a decrease in eNOS gene expression in fetal pulmonary arterial endothelial cells (FPAEC). Our results indicate that ET-1 increased H2O2 levels in FPASMC in an endothelin A receptor-dependent fashion. This was observed in both FPASMC monoculture and in cocultures of FPASMC and FPAEC. Conversely, ET-1 decreased H2O2 levels in FPAEC monoculture in an endothelin B receptor-dependent fashion. Furthermore, ET-1 decreased eNOS promoter activity by 40% in FPAEC in coculture with FPASMC. Promoter activity was restored in the presence of catalase. In FPAEC in monoculture treated with 0-100 microM H2O2, 12 microM had no effect on eNOS promoter activity, but it increased eNOS protein levels by 50%. However, at 100 microM, H2O2 decreased eNOS promoter activity and protein levels in FPAEC by 79 and 40%, respectively. These data suggest a role for smooth muscle cell-derived H2O2 in ET-1-mediated downregulation of eNOS expression in children born with PPHN.     

Cinaciguat, a soluble guanylate cyclase activator, augments cGMP after oxidative stress and causes pulmonary vasodilation in neonatal pulmonary hypertension

Although inhaled NO (iNO) therapy is often effective in treating infants with persistent pulmonary hypertension of the newborn (PPHN), up to 40% of patients fail to respond, which may be partly due to abnormal expression and function of soluble guanylate cyclase (sGC). To determine whether altered sGC expression or activity due to oxidized sGC contributes to high pulmonary vascular resistance (PVR) and poor NO responsiveness, we studied the effects of cinaciguat (BAY 58-2667), an sGC activator, on pulmonary artery smooth muscle cells (PASMC) from normal fetal sheep and sheep exposed to chronic intrauterine pulmonary hypertension (i.e., PPHN). We found increased sGC α(1)- and β(1)-subunit protein expression but lower basal cGMP levels in PPHN PASMC compared with normal PASMC. To determine the effects of cinaciguat and NO after sGC oxidation in vitro, we measured cGMP production by normal and PPHN PASMC treated with cinaciguat and the NO donor, sodium nitroprusside (SNP), before and after exposure to 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, an sGC oxidizer), hyperoxia (fraction of inspired oxygen 0.50), or hydrogen peroxide (H(2)O(2)). After treatment with ODQ, SNP-induced cGMP generation was markedly reduced but the effects of cinaciguat were increased by 14- and 64-fold in PPHN fetal PASMC, respectively (P < 0.01 vs. controls). Hyperoxia or H(2)O(2) enhanced cGMP production by cinaciguat but not SNP in PASMC. To determine the hemodynamic effects of cinaciguat in vivo, we compared serial responses to cinaciguat and ACh in fetal lambs after ductus arteriosus ligation. In contrast with the impaired vasodilator response to ACh, cinaciguat-induced pulmonary vasodilation was significantly increased. After birth, cinaciguat caused a significantly greater fall in PVR than either 100% oxygen, iNO, or ACh. We conclude that cinaciguat causes more potent pulmonary vasodilation than iNO in experimental PPHN. We speculate that increased NO-insensitive sGC may contribute to the pathogenesis of PPHN, and cinaciguat may provide a novel treatment of severe pulmonary hypertension.     

Contribution of the K(Ca) channel to membrane potential and O2 sensitivity is decreased in an ovine PPHN model

Ca2+-sensitive K+ (K(Ca)) channels play an important role in mediating perinatal pulmonary vasodilation. We hypothesized that lung K(Ca) channel function may be decreased in persistent pulmonary hypertension of the newborn (PPHN). To test this hypothesis, pulmonary artery smooth muscle cells (PASMC) were isolated from fetal lambs with severe pulmonary hypertension induced by ligation of the ductus arteriosus in fetal lambs at 125-128 days gestation. Fetal lambs were killed after pulmonary hypertension had been maintained for at least 7 days. Age-matched, sham-operated animals were used as controls. PASMC K+ currents and membrane potentials were recorded using amphotericin B-perforated patch-clamp techniques. The increase in whole cell current normally seen in response to normoxia was decreased (333.9 +/- 63.6% in control vs. 133.1 +/- 16.0% in hypertensive fetuses). The contribution of the K(Ca) channel to the whole cell current was diminished in hypertensive, compared with control, fetal PASMC. In PASMC from hypertensive fetuses, a change from hypoxia to normoxia caused no change in membrane potential compared with a -14.6 +/- 2.8 mV decrease in membrane potential in PASMC from control animals. In PASMC from animals with pulmonary hypertension, 4-aminopyridine (4-AP) caused a larger depolarization than iberiotoxin, whereas in PASMC from control animals, iberiotoxin caused a larger depolarization than 4-AP. These data confirm the hypothesis that the contribution of the K(Ca) channel to membrane potential and O2 sensitivity is decreased in an ovine model of PPHN, and this may contribute to the abnormal perinatal pulmonary vasoreactivity associated with PPHN.     

Oxidant stress from uncoupled nitric oxide synthase impairs vasodilation in fetal lambs with persistent pulmonary hypertension

Persistent pulmonary hypertension of newborn (PPHN) is associated with decreased NO release and impaired pulmonary vasodilation. We investigated the hypothesis that increased superoxide (O(2)(*-)) release by an uncoupled endothelial nitric oxide synthase (eNOS) contributes to impaired pulmonary vasodilation in PPHN. We investigated the response of isolated pulmonary arteries to the NOS agonist ATP and the NO donor S-nitroso-N-acetylpenicillamine (SNAP) in fetal lambs with PPHN induced by prenatal ligation of ductus arteriosus and in sham-ligated controls in the presence or absence of the NOS antagonist nitro-L-arginine methyl ester (L-NAME) or the O(2)(*-) scavenger 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron). ATP caused dose-dependent relaxation of pulmonary artery rings in control lambs but induced constriction of the rings in PPHN lambs. L-NAME, the NO precursor L-arginine, and Tiron restored the relaxation response of pulmonary artery rings to ATP in PPHN. Relaxation to NO was attenuated in arteries from PPHN lambs, and the response was improved by L-NAME and by Tiron. We also investigated the alteration in heat shock protein (HSP)90-eNOS interactions and release of NO and O(2)(*-) in response to ATP in the pulmonary artery endothelial cells (PAEC) from these lambs. Cultured PAEC and endothelium of freshly isolated pulmonary arteries from PPHN lambs released O(2)(*-) in response to ATP, and this was attenuated by the NOS antagonist L-NAME and superoxide dismutase (SOD). ATP stimulated HSP90-eNOS interactions in PAEC from control but not PPHN lambs. HSP90 immunoprecipitated from PPHN pulmonary arteries had increased nitrotyrosine signal. Oxidant stress from uncoupled eNOS contributes to impaired pulmonary vasodilation in PPHN induced by ductal ligation in fetal lambs.     

Hydrocortisone normalizes oxygenation and cGMP regulation in lambs with persistent pulmonary hypertension of the newborn

In the pulmonary vasculature, cGMP levels are regulated by soluble guanylate cyclase (sGC) and phosphodiesterase 5 (PDE5). We previously reported that lambs with persistent pulmonary hypertension of the newborn (PPHN) demonstrate increased reactive oxygen species (ROS) and altered sGC and PDE5 activity, with resultant decreased cGMP. The objective of this study was to evaluate the effects of hydrocortisone on pulmonary vascular function, ROS, and cGMP in the ovine ductal ligation model of PPHN. PPHN lambs were ventilated with 100% O(2) for 24 h. Six lambs received 5 mg/kg hydrocortisone every 8 h times three doses (PPHN-hiHC), five lambs received 3 mg/kg hydrocortisone followed by 1 mg·kg(-1)·dose(-1) times two doses (PPHN-loHC), and six lambs were ventilated with O(2) alone (PPHN). All groups were compared with healthy 1-day spontaneously breathing lambs (1DSB). O(2) ventilation of PPHN lambs decreased sGC activity, increased PDE5 activity, and increased ROS vs. 1DSB lambs. Both hydrocortisone doses significantly improved arterial-to-alveolar ratios relative to PPHN lambs, decreased PDE5 activity, and increased cGMP relative to PPHN lambs. High-dose hydrocortisone also increased sGC activity, decreased PDE5 expression, decreased ROS, and increased total vascular SOD activity vs. PPHN lambs. These data suggest that hydrocortisone treatment in clinically relevant doses improves oxygenation and decreases hyperoxia-induced changes in sGC and PDE5 activity, increasing cGMP levels. Hydrocortisone reduces ROS levels in part by increasing SOD activity in PPHN lambs ventilated with 100% O(2.) We speculate that hydrocortisone increases cGMP by direct effects on sGC and PDE5 expression and by attenuating abnormalities induced by oxidant stress.     

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.     

Superoxide dismutase restores eNOS expression and function in resistance pulmonary arteries from neonatal lambs with persistent pulmonary hypertension

Endothelial nitric oxide (NO) synthase (eNOS) expression and activity are decreased in fetal lambs with persistent pulmonary hypertension (PPHN). We sought to determine the impact of mechanical ventilation with O(2) with or without inhaled NO (iNO) or recombinant human SOD (rhSOD) on eNOS in the ductal ligation model of PPHN. PPHN lambs and age-matched controls were ventilated with 100% O(2) for 24 h alone or combined with 20 ppm iNO continuously or a single dose of rhSOD (5 mg/kg) given intratracheally at delivery. In 1-day spontaneously breathing lambs, eNOS expression in resistance pulmonary arteries increased relative to fetal levels. eNOS expression increased in control lambs ventilated with 100% O(2), but not in PPHN lambs. Addition of iNO or rhSOD increased eNOS expression and decreased generation of reactive oxygen species (ROS) in PPHN lambs relative to those ventilated with 100% O(2) alone. However, only rhSOD restored eNOS function, increased tetrahydrobiopterin (BH(4)), a critical cofactor for eNOS function, and restored GTP cyclohydrolase I expression in isolated vessels and lungs from PPHN lambs. These data suggest that ventilation of PPHN lambs with 100% O(2) increases ROS production, blunts postnatal increases in eNOS expression, and decreases available BH(4) in PPHN lambs. Although the addition of iNO or rhSOD diminished ROS production and increased eNOS expression, only rhSOD improved eNOS function and levels of available BH(4). Thus therapies designed to decrease oxidative stress and restore eNOS coupling, such as rhSOD, may prove useful in the treatment of PPHN in newborn infants.     

H2O2 and cGMP may function as an O2 sensor in the pulmonary artery

The effects of O2 tension on force in precontracted isolated pulmonary arterial smooth muscle from calf lungs was characterized to investigate the mechanism of O2 tension sensing. These arteries display a decrease in force with increasing O2 tension that is antagonized via inhibition of soluble guanylate cyclase activation by 10 microM methylene blue or inactivation of catalase by pretreatment with 50 mM 3-amino-1,2,4-triazole for 30 min. O2 tension-dependent relaxation is associated with an increase in intracellular H2O2 metabolism through catalase (detected as the peroxide-dependent inactivation of tissue catalase activity by aminotriazole) and cyclic guanosine 5'-monophosphate (cGMP), known mediators of relaxation in calf pulmonary arteries. Thus a recently reconstructed mechanism of activation of soluble guanylate cyclase involving the metabolism of H2O2 by catalase appears to function as an O2 tension sensor in pulmonary arteries.     

Chronic intrauterine pulmonary hypertension decreases calcium-sensitive potassium channel mRNA expression

Calcium-sensitive potassium (K(Ca)) channels play a critical role in mediating perinatal pulmonary vasodilation. Because infants with persistent pulmonary hypertension of the newborn (PPHN) have blunted vasodilator responses to birth-related stimuli, we hypothesized that lung K(Ca) channel gene expression is decreased in PPHN. To test this hypothesis, we measured K(Ca) channel gene expression in distal lung homogenates from both fetal lambs with severe pulmonary hypertension caused by prolonged compression of the ductus arteriosus and age-matched, sham-operated animals (controls). After at least 9 days of compression of the ductus arteriosus, fetal lambs were killed. To determine lung K(Ca) channel mRNA levels, primers were designed against the known sequence of the K(Ca) channel and used in semiquantitative RT-PCR, with lung 18S rRNA content as an internal control. Compared to that in control lambs, lung K(Ca) channel mRNA content in the PPHN group was reduced by 26 +/- 6% (P < 0.02), whereas lung voltage-gated K(+) 2.1 mRNA content was unchanged. We conclude that lung K(Ca) channel mRNA expression is decreased in an ovine model of PPHN. Decreased K(Ca) channel gene expression may contribute to the abnormal pulmonary vascular reactivity associated with PPHN.     

Decreased association of HSP90 impairs endothelial nitric oxide synthase in fetal lambs with persistent pulmonary hypertension

Persistent pulmonary hypertension of newborn (PPHN) is associated with decreased nitric oxide (NO) release and impaired pulmonary vasodilation. We investigated the hypothesis that decreased association of heat shock protein 90 (HSP90) with endothelial NO synthase (eNOS) impairs NO release and vasodilation in PPHN. The responses to the NOS agonist ATP were investigated in fetal lambs with PPHN induced by prenatal ligation of ductus arteriosus, and in sham ligation controls. ATP caused dose-dependent vasodilation in control pulmonary resistance arteries, and this response was attenuated in PPHN vessels. The response of control pulmonary arteries to ATP was attenuated by NG-nitro-l-arginine methyl ester (l-NAME), a NOS antagonist, and geldanamycin, an inhibitor of HSP90-eNOS interaction. The attenuated response to ATP observed in PPHN was improved by pretreatment of vessels with l-NAME or 4,5-dihydroxy-1,3-benzene-disulfonate, a superoxide scavenger. Pulmonary arteries from PPHN lambs had decreased basal levels of HSP90 in association with eNOS. Association of HSP90 with eNOS and NO release increased in response to ATP in control pulmonary artery endothelial cells, but not in cells from PPHN lambs. Decreased HSP90-eNOS interactions may contribute to the impaired NO release and vasodilation observed in the ductal ligation model of PPHN.     

Inhaled carbon monoxide does not cause pulmonary vasodilation in the late-gestation fetal lamb

As observed with nitric oxide (NO), carbon monoxide (CO) binds and may activate soluble guanylate cyclase and increase cGMP levels in smooth muscle cells in vitro. Because inhaled NO (I(NO)) causes potent and sustained pulmonary vasodilation, we hypothesized that inhaled CO (I(CO)) may have similar effects on the perinatal lung. To determine whether I(CO) can lower pulmonary vascular resistance (PVR) during the perinatal period, we studied the effects of I(CO) on late-gestation fetal lambs. Catheters were placed in the main pulmonary artery, left pulmonary artery (LPA), aorta, and left atrium to measure pressure. An ultrasonic flow transducer was placed on the LPA to measure blood flow to the left lung. After baseline measurements, fetal lambs were mechanically ventilated with a hypoxic gas mixture (inspired O(2) fraction < 0.10) to maintain a constant fetal arterial PO(2). After 60 min (baseline), the lambs were treated with I(CO) [5-2,500 parts/million (ppm)]. Comparisons were made with I(NO) (5 and 20 ppm) and combined I(NO) (5 ppm) and I(CO) (100 and 2,500 ppm). We found that I(CO) did not alter left lung blood flow or PVR at any of the study doses. In contrast, low-dose I(NO) decreased PVR by 47% (P < 0.005). The combination of I(NO) and I(CO) did not enhance the vasodilator response to I(NO). To determine whether endogenous CO contributes to vascular tone in the fetal lung, zinc protoporphyrin IX, an inhibitor of heme oxygenase, was infused into the LPA in three lambs. Zinc protoporphyrin IX had no effect on baseline PVR, aortic pressure, or the pressure gradient across the ductus arteriosus. We conclude that I(CO) does not cause vasodilation in the near-term ovine transitional circulation, and endogenous CO does not contribute significantly to baseline pulmonary vascular tone or ductus arteriosus tone in the late-gestation ovine fetus.     

The role of nitric oxide synthase-derived reactive oxygen species in the altered relaxation of pulmonary arteries from lambs with increased pulmonary blood flow

Congenital cardiac defects associated with increased pulmonary blood flow (Q(p)) produce pulmonary hypertension. We have previously reported attenuated endothelium-dependent relaxations in pulmonary arteries (PA) isolated from lambs with increased Q(p) and pulmonary hypertension. To better characterize the vascular alterations in the nitric oxide-superoxide system, 12 fetal lambs underwent in utero placement of an aortopulmonary vascular graft (shunt). Twin lambs served as controls. PA were isolated from these lambs at 4-6 wk of age. Electron paramagnetic resonance spectroscopy on fourth-generation PA showed significantly increased superoxide anion generation in shunt PA that were decreased to control levels following inhibition of nitric oxide synthase (NOS) with 2-ethyl-2-thiopseudourea. Preconstricted fifth-generation PA rings were relaxed with a NOS agonist (A-23187), a nitric oxide donor [S-nitrosyl amino penicillamine (SNAP)], polyethylene glycol-conjugated superoxide dismutase (PEG-SOD), or H(2)O(2). A-23187-, PEG-SOD-, and H(2)O(2)-mediated relaxations were impaired in shunt PA compared with controls. Pretreatment with PEG-SOD significantly enhanced the relaxation response to A-23187 and SNAP in shunt but not control PA. Inhibition of NOS with nitro-L-arginine or scavenging superoxide anions with tiron enhanced relaxation to SNAP and inhibited relaxation to PEG-SOD in shunt PA. Pretreatment with catalase inhibited relaxation of shunt PA to A-23187, SOD, and H(2)O(2). We conclude that NOS catalyzes the production of superoxide anions in shunt PA. PEG-SOD relaxes shunt PA by converting these anions to H(2)O(2), a pulmonary vasodilator. The redox environment, influenced by the balance between production and scavenging of ROS, may have important consequences on pulmonary vascular reactivity in the setting of increased Q(p).     

Contribution of oxygen radicals to altered NO-dependent pulmonary vasodilation in acute and chronic hypoxia

Chronic hypoxia (CH) increases pulmonary arterial endothelial nitric oxide (NO) synthase (NOS) expression and augments endothelium-derived nitric oxide (EDNO)-dependent vasodilation, whereas vasodilatory responses to exogenous NO are attenuated in CH rat lungs. We hypothesized that reactive oxygen species (ROS) inhibit NO-dependent pulmonary vasodilation following CH. To test this hypothesis, we examined responses to the EDNO-dependent vasodilator endothelin-1 (ET-1) and the NO donor S-nitroso-N-acetyl penicillamine (SNAP) in isolated lungs from control and CH rats in the presence or absence of ROS scavengers under normoxic or hypoxic ventilation. NOS was inhibited in lungs used for SNAP experiments to eliminate influences of endogenously produced NO. Additionally, dichlorofluorescein (DCF) fluorescence was measured as an index of ROS levels in isolated pressurized small pulmonary arteries from each group. We found that acute hypoxia increased DCF fluorescence and attenuated vasodilatory responses to ET-1 in lungs from control rats. The addition of ROS scavengers augmented ET-1-induced vasodilation in lungs from both groups during hypoxic ventilation. In contrast, upon NOS inhibition, DCF fluorescence was elevated and SNAP-induced vasodilation diminished in arteries from CH rats during normoxia, whereas acute hypoxia decreased DCF fluorescence, which correlated with augmented reactivity to SNAP in both groups. ROS scavengers enhanced SNAP-induced vasodilation in normoxia-ventilated lungs from CH rats similar to effects of hypoxic ventilation. We conclude that inhibition of NOS during normoxia leads to greater ROS generation in lungs from both control and CH rats. Furthermore, NOS inhibition reveals an effect of acute hypoxia to diminish ROS levels and augment NO-mediated pulmonary vasodilation.     

rhVEGF treatment preserves pulmonary vascular reactivity and structure in an experimental model of pulmonary hypertension in fetal sheep

We have previously shown that lung VEGF expression is decreased in a fetal lamb model of PPHN and that VEGF165 inhibition causes severe pulmonary hypertension in fetal lambs. Therefore, we hypothesized that treatment with rhVEGF165 would preserve endothelium-dependent vasodilation and reduce the severity of pulmonary vascular remodeling in an experimental model of PPHN. We studied the effects of daily intrapulmonary infusions of rhVEGF after partial ligation of the ductus arteriosus (DA). We performed surgery in 24 late-gestation fetal lambs and placed catheters in the main pulmonary artery, left atrium, and aorta for pressure measurements and in the left pulmonary artery for drug infusions. A pressure transducer was placed around the LPA to measure blood flow to the left lung (Qp), and the DA was surgically constricted to induce pulmonary hypertension. rhVEGF165 or vehicle was infused for 7 or 14 days. ACh or 8-BrcGMP was infused on days 2 and 13 to assess endothelium-dependent and -independent vasodilation, respectively. ACh-induced vasodilation was reduced in PPHN lambs after 14 days (change in Qp from baseline, 106% vs. 11%). In contrast, the response to ACh was preserved in lambs treated with rhVEGF (change in Qp, 94% vs. 90%). Pulmonary vasodilation to 8-BrcGMP was not altered in PPHN lambs or enhanced by VEGF treatment. rhVEGF treatment increased expression of lung eNOS protein and decreased pulmonary artery wall thickness by 34% vs. PPHN lambs. We conclude that VEGF165 preserves endothelium-dependent vasodilation, upregulates eNOS expression, and reduces the severity of pulmonary vascular remodeling in experimental PPHN.     

Endothelin-1-induced pulmonary arterial dilation is reduced by N omega-nitro-L-arginine in fetal lambs.

Endothelin-1 (ET-1) is a pulmonary vasodilator in the unventilated fetal lamb. The site and mechanism of this vasodilator response were investigated in isolated blood-perfused lungs from nine fetal lambs delivered at 127-140 days gestation. The vascular occlusion technique was used to partition the total pulmonary pressure gradient into pressure gradients across large and small arteries (delta PLA and delta PSA, respectively) and veins (delta PV). Injection of ET-1 (74 ng/kg) into the pulmonary artery significantly decreased delta PLA from 12.4 +/- 2.1 to 5.2 +/- 1.1 mmHg and delta PSA from 49.2 +/- 2.7 to 31.3 +/- 4.9 mmHg. The pressure measured by double occlusion, an estimate of pulmonary capillary pressure, was not altered by ET-1 (15.5 +/- 1.0 vs. 14.8 +/- 1.0 mmHg), indicating that ET-1 had no effect on pulmonary veins. Addition of N omega-nitro-L-arginine (estimated perfusate concentration 2-6 mM), an analogue of L-arginine that inhibits the production of endothelium-derived relaxing factor (EDRF), significantly attenuated the dilator responses to acetylcholine (10 micrograms) and ET-1 (74 ng/kg) by 35 and 56%, respectively. These results in unventilated fetal lungs indicate that 1) ET-1 dilates both large and small pulmonary arteries with no effect on pulmonary veins, and 2) this effect is mediated in part through the action of the EDRF pathway.     

Inhibition of phosphodiesterase 1 augments the pulmonary vasodilator response to inhaled nitric oxide in awake lambs with acute pulmonary hypertension

Phosphodiesterase 1 (PDE1) modulates vascular tone and the development of tolerance to nitric oxide (NO)-releasing drugs in the systemic circulation. Any role of PDE1 in the pulmonary circulation remains largely uncertain. We measured the expression of genes encoding PDE1 isozymes in the pulmonary vasculature and examined whether or not selective inhibition of PDE1 by vinpocetine attenuates pulmonary hypertension and augments the pulmonary vasodilator response to inhaled NO in lambs. Using RT-PCR, we detected PDE1A, PDE1B, and PDE1C mRNAs in pulmonary arteries and veins isolated from healthy lambs. In 13 lambs, the thromboxane A(2) analog U-46619 was infused intravenously to increase mean pulmonary arterial pressure to 35 mmHg. Four animals received an intravenous infusion of vinpocetine at incremental doses of 0.3, 1, and 3 mg.kg(-1).h(-1). In nine lambs, inhaled NO was administered in a random order at 2, 5, 10, and 20 ppm before and after an intravenous infusion of 1 mg.kg(-1).h(-1) vinpocetine. Administration of vinpocetine did not alter pulmonary and systemic hemodynamics or transpulmonary cGMP or cAMP release. Inhaled NO selectively reduced mean pulmonary arterial pressure, pulmonary capillary pressure, and pulmonary vascular resistance index, while increasing transpulmonary cGMP release. The addition of vinpocetine enhanced pulmonary vasodilation and transpulmonary cGMP release induced by NO breathing without causing systemic vasodilation but did not prolong the duration of pulmonary vasodilation after NO inhalation was discontinued. Our findings demonstrate that selective inhibition of PDE1 augments the therapeutic efficacy of inhaled NO in an ovine model of acute chemically induced pulmonary hypertension.     

Altered endothelium-dependent relaxations in lambs with high pulmonary blood flow and pulmonary hypertension

Congenital heart disease associated with increased pulmonary blood flow produces pulmonary hypertension. To characterize vascular alterations in the nitric oxide (NO)-cGMP cascade induced by increased pulmonary blood flow and pulmonary hypertension, 10 fetal lambs underwent in utero placement of an aortopulmonary vascular graft (shunt). When the lambs were 4-6 wk of age, we assessed responses of pulmonary arteries (PAs) and pulmonary veins (PVs) isolated from lungs of control and shunted lambs. PVs from control and shunted lambs relaxed similarly to exogenous NO (S-nitrosyl-acetyl-penicillamine), to NO produced endogenously (zaprinast and A-23187), and to cGMP (atrial natriuretic peptide). In contrast, relaxations to A-23187 and zaprinast were blunted in PAs isolated from shunted lambs relative to controls. Inhibitors of NO synthase (NOS) and soluble guanylate cyclase constricted control but not shunt PAs, indicating reduced basal NOS activity in shunt PAs. Pretreatment of shunt PAs with the substrates L-arginine and sepiapterin, a precursor for tetrahydrobiopterin synthesis, did not augment A-23187 relaxations. However, pretreatment with superoxide dismutase and catalase significantly enhanced A-23187 relaxations in shunt PAs. We conclude that increased pulmonary blood flow induces an impairment of endothelium-dependent relaxation that is selective to PAs. The impaired relaxation may be mediated in part by excess superoxide production.     

Apocynin improves oxygenation and increases eNOS in persistent pulmonary hypertension of the newborn

NADPH oxidase is a major source of superoxide anions in the pulmonary arteries (PA). We previously reported that intratracheal SOD improves oxygenation and restores endothelial nitric oxide (NO) synthase (eNOS) function in lambs with persistent pulmonary hypertension of the newborn (PPHN). In this study, we determined the effects of the NADPH oxidase inhibitor apocynin on oxygenation, reactive oxygen species (ROS) levels, and NO signaling in PPHN lambs. PPHN was induced in lambs by antenatal ligation of the ductus arteriosus 9 days prior to delivery. Lambs were treated with vehicle or apocynin (3 mg/kg intratracheally) at birth and then ventilated with 100% O(2) for 24 h. A significant improvement in oxygenation was observed in apocynin-treated lambs after 24 h of ventilation. Contractility of isolated fifth-generation PA to norepinephrine was attenuated in apocynin-treated lambs. PA constrictions to NO synthase (NOS) inhibition with N-nitro-l-arginine were blunted in PPHN lambs; apocynin restored contractility to N-nitro-l-arginine, suggesting increased NOS activity. Intratracheal apocynin also enhanced PA relaxations to the eNOS activator A-23187 and to the NO donor S-nitrosyl-N-acetyl-penicillamine. Apocynin decreased the interaction between NADPH oxidase subunits p22(phox) and p47(phox) and decreased the expression of Nox2 and p22(phox) in ventilated PPHN lungs. These findings were associated with decreased superoxide and 3-nitrotyrosine levels in the PA of apocynin-treated PPHN lambs. eNOS protein expression, endothelial NO levels, and tetrahydrobiopterin-to-dihydrobiopterin ratios were significantly increased in PA from apocynin-treated lambs, although cGMP levels did not significantly increase and phosphodiesterase-5 activity did not significantly decrease. NADPH oxidase inhibition with apocynin may improve oxygenation, in part, by attenuating ROS-mediated vasoconstriction and by increasing NOS activity.     

Effects of BAY 41-2272, a soluble guanylate cyclase activator, on pulmonary vascular reactivity in the ovine fetus

Nitric oxide (NO)-cGMP signaling plays a critical role during the transition of the pulmonary circulation at birth. BAY 41-2272 is a novel NO-independent direct stimulator of soluble guanylate cyclase that causes vasodilation in systemic and local circulations. However, the hemodynamic effects of BAY 41-2272 have not been studied in the perinatal pulmonary circulation. We hypothesized that BAY 41-2272 causes potent and sustained fetal pulmonary vasodilation. We performed surgery on 14 fetal lambs (125-130 days gestation; term = 147 days) and placed catheters in the main pulmonary artery, aorta, and left atrium to measure pressures. An ultrasonic flow transducer was placed on the left pulmonary artery (LPA) to measure blood flow, and a catheter was placed in the LPA for drug infusion. Pulmonary vascular resistance (PVR) was calculated as pulmonary artery pressure minus left atrial pressure divided by LPA blood flow. BAY 41-2272 caused dose-related increases in pulmonary blood flow up to threefold above baseline and reduced PVR by 75% (P < 0.01). Prolonged infusion of BAY 41-2272 caused sustained pulmonary vasodilation throughout the 120-min infusion period. The pulmonary vasodilator effect of BAY 41-2272 was not attenuated by N(omega)-nitro-l-arginine, a NO synthase inhibitor. In addition, compared with sildenafil, a phosphodiesterase 5 inhibitor, the pulmonary vasodilator response to BAY 41-2272 was more prolonged. We conclude that BAY 41-2272 causes potent and sustained fetal pulmonary vasodilation independent of NO release. We speculate that BAY 41-2272 may have therapeutic potential for pulmonary hypertension associated with failure to circulatory adaptation at birth, especially in the setting of impaired NO production.     

Endothelin blockade augments pulmonary vasodilation in the ovine fetus

Ivy, D. Dunbar, John P. Kinsella, and Steven H. Abman.Endothelin blockade augments pulmonary vasodilation in the ovine fetus. J. Appl. Physiol. 81(6):2481-2487, 1996.The physiological role of endothelin-1 (ET-1) inregulation of vascular tone in the perinatal lung is controversial.Recent studies suggest that ET-1 contributes to high basal pulmonaryvascular resistance in the normal fetus, but its role in the modulationof pulmonary vascular tone remains uncertain. We hypothesized that highET-1 activity opposes the vasodilator response to some physiological stimuli such as increased pressure. To test the hypothesis that ET-1modulates fetal pulmonary vascular responses to acute and prolongedphysiological stimuli, we performed a series of experiments in thelate-gestation ovine fetus. We studied the hemodynamic effects of twoET-1 antagonists, BQ-123 (a selectiveET_A-receptor antagonist) andphosphoramidon (a nonselective ET-1-converting enzyme inhibitor) duringmechanical increases in pressure due to partial ductus arteriosuscompression in chronically prepared late-gestation fetal lambs. Incontrol studies, partial ductus arteriosus compression decreased theratio of pulmonary arterial pressure to pulmonary artery flow in theleft lung 34 ± 6% from baseline. Intrapulmonary infusions ofBQ-123 (0.5 µg/min for 10 min; 0.025 µg/min for 2 h) orphosphoramidon (1.0 mg/min for 10 min) augmented the peak vasodilatorresponse during ductus arteriosus compression (52 ± 3 and 49 ± 6% from baseline, respectively, P < 0.05 vs. control). In addition, unlike the transient vasodilator response to ductus arteriosus compression in control studies, ET-1blockade with BQ-123 or phosphoramidon prolonged the increase in flowcaused by ductus arteriosus compression. In summary,ET_A-receptor blockade andET-1-converting enzyme inhibition augment and prolong fetal pulmonaryvasodilation during partial compression of the ductus arteriosus. Weconclude that ET-1 activity modulates acute and prolonged responses ofthe fetal pulmonary circulation to changes in vascular pressure. Wespeculate that ET-1 contributes to regulation and maintenance of highpulmonary vascular resistance in the normal ovine fetal lung.