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.

High-density lipoprotein binding to scavenger receptor-BI activates endothelial nitric oxide synthase

Atherosclerosis is the primary cause of cardiovascular disease, and the risk for atherosclerosis is inversely proportional to circulating levels of high-density lipoprotein (HDL) cholesterol. However, the mechanisms by which HDL is atheroprotective are complex and not well understood. Here we show that HDL stimulates endothelial nitric oxide synthase (eNOS) in cultured endothelial cells. In contrast, eNOS is not activated by purified forms of the major HDL apolipoproteins ApoA-I and ApoA-II or by low-density lipoprotein. Heterologous expression experiments in Chinese hamster ovary cells reveal that scavenger receptor-BI (SR-BI) mediates the effects of HDL on the enzyme. HDL activation of eNOS is demonstrable in isolated endothelial-cell caveolae where SR-BI and eNOS are colocalized, and the response in isolated plasma membranes is blocked by antibodies to ApoA-I and SR-BI, but not by antibody to ApoA-II. HDL also enhances endothelium- and nitric-oxide-dependent relaxation in aortae from wild-type mice, but not in aortae from homozygous null SR-BI knockout mice. Thus, HDL activates eNOS via SR-BI through a process that requires ApoA-I binding. The resulting increase in nitric-oxide production might be critical to the atheroprotective properties of HDL and ApoA-I.

Plasma Membrane Estrogen Receptors Are Coupled to Endothelial Nitric-oxide Synthase through Gαi

Estrogen causes rapid endothelial nitric oxide (NO) production because of the activation of plasma membrane-associated estrogen receptors (ER) coupled to endothelial NO synthase (eNOS). In the present study, we determined the role of G proteins in eNOS activation by estrogen. Estradiol-17beta (E(2), 10(-8) m) and acetylcholine (10(-5) m) caused comparable increases in NOS activity (15 min) in intact endothelial cells that were fully blocked by pertussis toxin (Ptox). In addition, exogenous guanosine 5'-O-(2- thiodiphosphate) inhibited E(2)-mediated eNOS stimulation in isolated endothelial plasma membranes, and Ptox prevented enzyme activation by E(2) in COS-7 cells expressing ERalpha and eNOS. Coimmunoprecipitation studies of plasma membranes from COS-7 cells transfected with ERalpha and specific Galpha proteins demonstrated E(2)-stimulated interaction between ERalpha and Galpha(i) but not between ERalpha and either Galpha(q) or Galpha(s); the observed ERalpha-Galpha(i) interaction was blocked by the ER antagonist ICI 182,780 and by Ptox. E(2)-stimulated ERalpha-Galpha(i) interaction was also demonstrable in endothelial cell plasma membranes. Cotransfection of Galpha(i) into COS-7 cells expressing ERalpha and eNOS yielded a 3-fold increase in E(2)-mediated eNOS stimulation, whereas cotransfection with a protein regulator of G protein signaling, RGS4, inhibited the E(2) response. These findings indicate that eNOS stimulation by E(2) requires plasma membrane ERalpha coupling to Galpha(i) and that activated Galpha(i) mediates the requisite downstream signaling events. Thus, novel G protein coupling enables a subpopulation of ERalpha to initiate signal transduction at the cell surface. Similar mechanisms may underly the nongenomic actions of other steroid hormones.

Estrogen receptor alpha and endothelial nitric oxide synthase are organized into a functional signaling module in caveolae

Estrogen causes nitric oxide (NO)-dependent vasodilation due to estrogen receptor (ER) alpha-mediated, nongenomic activation of endothelial NO synthase (eNOS). The subcellular site of interaction between ERalpha and eNOS was determined in studies of isolated endothelial cell plasma membranes. Estradiol (E(2), 10(-8) mol/L) caused an increase in eNOS activity in plasma membranes in the absence of added calcium, calmodulin, or eNOS cofactors, which was blocked by ICI 182,780 and ERalpha antibody. Immunoidentification studies detected the same 67-kDa protein in endothelial cell nucleus, cytosol, and plasma membrane. Plasma membranes from COS-7 cells expressing eNOS and ERalpha displayed ER-mediated eNOS stimulation, whereas membranes from cells expressing eNOS alone or ERalpha plus a myristoylation-deficient mutant eNOS were insensitive. Fractionation of endothelial cell plasma membranes revealed ERalpha protein in caveolae, and E(2) caused stimulation of eNOS in isolated caveolae that was ER-dependent; noncaveolae membranes were insensitive. Acetylcholine and bradykinin also activated eNOS in isolated caveolae. Furthermore, the effect of E(2) on eNOS in caveolae was prevented by calcium chelation. Thus, a subpopulation of ERalpha is localized to endothelial cell caveolae where they are coupled to eNOS in a functional signaling module that may regulate the local calcium environment. The full text of this article is available at http://www.circresaha.org.

Sustained endothelial nitric-oxide synthase activation requires capacitative Ca2+ entry

Endothelial nitric-oxide synthase (eNOS), a Ca(2+)/calmodulin-dependent enzyme, is critical for vascular homeostasis. While eNOS is membrane-associated through its N-myristoylation, the significance of membrane association in locating eNOS near sources of Ca(2+) entry is uncertain. To assess the Ca(2+) source required for eNOS activation, chimera containing the full-length eNOS cDNA and HA-tagged aequorin sequence (EHA), and MHA (myristoylation-deficient EHA) were generated and transfected into COS-7 cells. The EHA chimera was primarily targeted to the plasma membrane while MHA was located intracellularly. Both constructs retained enzymatic eNOS activity and aequorin-mediated Ca(2+) sensitivity. The plasma membrane-associated EHA and intracellular MHA were compared in their ability to sense changes in local Ca(2+) concentration, demonstrating preferential sensitivity to Ca(2+) originating from intracellular pools (MHA) or from capacitative Ca(2+) entry (EHA). Measurements of eNOS activation in intact cells revealed that the eNOS enzymatic activity of EHA was more sensitive to Ca(2+) influx via capacitative Ca(2+) entry than intracellular release, whereas MHA eNOS activity was more responsive to intracellular Ca(2+) release. When eNOS activation by CCE was compared with that generated by an equal rise in [Ca(2+)](i) due to the Ca(2+) ionophore ionomycin, a 10-fold greater increase in NO production was found in the former condition. These results demonstrate that EHA and MHA chimera are properly targeted and retain full functions of eNOS and aequorin, and that capacitative Ca(2+) influx is the principle stimulus for sustained activation of eNOS on the plasma membrane in intact cells.

Regulation of types I and III NOS in ovine uterine arteries by daily and acute estrogen exposure

Nitric oxide contributes to estrogen-mediated uterine vasodilation; however, the nitric oxide synthases (NOS) involved and their location within uterine arteries are incompletely documented. We investigated the effects of repetitive daily and acute estradiol-17beta (E(2)beta) exposure on uterine hemodynamics and NOS abundance and localization in uterine arteries from nonpregnant ovariectomized ewes receiving daily intravenous E(2)beta (1 microg/kg, n = 5) or no E(2)beta (n = 7) for 5 days to determine NOS abundance, cGMP contents, and NOS immunohistochemistry. Daily E(2)beta increased basal and E(2)beta-mediated rises in uterine blood flow (UBF) 36 and 43% (<0.01), respectively, calcium-dependent NOS activity 150% (P < 0.02) in endothelium-intact and -denuded ( approximately 40% of total NOS) arteries, and cGMP contents 39% (P < 0.05). Endothelial (eNOS) was detected in luminal endothelium, whereas neuronal NOS (nNOS) protein was only in the media. A second group of ewes received E(2)beta (1 microg/kg iv) for 4 days and acute intravenous E(2)beta (n = 8) or vehicle (n = 4) on day 5. UBF rose 5.5-fold (P < 0.001) 115 min after E(2)beta, at which time only endothelium-derived calcium-dependent NOS activity increased 30 +/- 13% (P < 0.05). Daily E(2)beta enhances basal and E(2)beta-mediated increases in UBF, which parallel increases in endothelium-derived eNOS and smooth muscle-derived nNOS. Acute E(2)beta, however, selectively increases endothelium-derived eNOS.

High density lipoprotein prevents oxidized low density lipoprotein-induced inhibition of endothelial nitric-oxide synthase localization and activation in caveolae

Oxidized LDL (oxLDL) depletes caveolae of cholesterol, resulting in the displacement of endothelial nitric-oxide synthase (eNOS) from caveolae and impaired eNOS activation. In the present study, we determined if the class B scavenger receptors, CD36 and SR-BI, are involved in regulating nitric-oxide synthase localization and function. We demonstrate that CD36 and SR-BI are expressed in endothelial cells, co-fractionate with caveolae, and co-immunoprecipitate with caveolin-1. Co-incubation of cells with 10 microgram/ml high density lipoprotein (HDL) prevented oxLDL-induced translocation of eNOS from caveolae and restored acetylcholine-induced nitric-oxide synthase stimulation. Acetylcholine caused eNOS activation in cells incubated with 10 microgram/ml oxLDL (10-15 thiobarbituric acid-reactive substances) and blocking antibodies to CD36, whereas cells treated with only oxLDL were unresponsive. Furthermore, CD36-blocking antibodies prevented oxLDL-induced redistribution of eNOS. SR-BI-blocking antibodies were used to demonstrate that the effects of HDL are mediate by SR-BI. HDL binding to SR-BI maintained the concentration of caveola-associated cholesterol by promoting the uptake of cholesterol esters, thereby preventing oxLDL-induced depletion of caveola cholesterol. We conclude that CD36 mediates the effects of oxLDL on caveola composition and eNOS activation. Furthermore, HDL prevents oxLDL from decreasing the capacity for eNOS activation by preserving the cholesterol concentration in caveolae and, thereby maintaining the subcellular location of eNOS.

Molecular basis of cell-specific endothelial nitric-oxide synthase expression in airway epithelium

Nitric oxide (NO) plays an important role in airway function, and endothelial NO synthase (eNOS) is expressed in airway epithelium. To determine the basis of cell-specific eNOS expression in airway epithelium, studies were performed in NCI-H441 human bronchiolar epithelial cells transfected with the human eNOS promoter fused to luciferase. Transfection with 1624 base pairs of sequence 5' to the initiation ATG (position -1624) yielded a 19-fold increase in promoter activity versus vector alone. No activity was found in lung fibroblasts, which do not express eNOS. 5' deletions from -1624 to -279 had modest effects on promoter activity in H441 cells. Further deletion to -248 reduced activity by 65%, and activity was lost with deletion to -79. Point mutations revealed that the GATA binding motif at -254 is mandatory for promoter activity and that the positive regulatory element between -248 and -79 is the Sp1 binding motif at -125. Electrophoretic mobility shift assays yielded two complexes with the GATA site and three with the Sp1 site. Immunodepletion with antiserum to GATA-2 prevented formation of the slowest migrating GATA complex, and antiserum to Sp1 supershifted the slowest migrating Sp1 complex. An electrophoretic mobility shift assay with H441 versus fibroblast nuclei revealed that the slowest migrating GATA complex is unique to airway epithelium. Thus, cell-specific eNOS expression in airway epithelium is dependent on the interaction of GATA-2 with the core eNOS promoter, and the proximal Sp1 binding site is also an important positive regulatory element.

Specific association of estrogen receptor beta with the cell cycle spindle assembly checkpoint protein, MAD2

Estrogen receptors (ERs) are ligand-activated transcription factors that regulate gene expression and cell growth. Two ERs now have been identified: ERalpha and the more recently discovered ERbeta. The physiological function of ERbeta remains unclear, but evidence from vascular injury studies and from ERbeta knockout mice suggests that ERbeta may be involved in the regulation of cellular proliferation. Here we show a direct and specific interaction between ERbeta and the cell cycle mitotic spindle assembly checkpoint protein, MAD2 (mitosis arrest-deficient 2). The ERbeta-MAD2 interaction was identified by screening of a yeast two-hybrid system vascular endothelial cell library with ERbeta and confirmed with glutathione S-transferase-fusion protein interaction studies. In contrast, ERalpha did not interact with MAD2 in either the two-hybrid system or in the protein-protein interaction experiments. Amino acids 173-208 in the hinge region of ERbeta were sufficient to mediate the interaction with MAD2 in the two-hybrid system and in glutathione S-transferase-fusion protein studies. These data identify a link between ERbeta and MAD2 of potential importance to regulation of the cell cycle and support a function of ERbeta distinct from the established role of ERs as transcription factors.

Novel role of estrogen receptors in vascular endothelium

Estrogen has a variety of effects on the vascular wall including rapid vasodilation due to the stimulation of endothelial nitric oxide synthase (eNOS). Studies in cultured endothelium indicate that the hormone cause acute, direct activation of eNOS that is unaffected by actinomycin D but fully inhibited by estrogen receptor (ER) antagonism. Overexpression of ERalpha leads to marked enhancement of the acute response to estrogen, and this process is blocked by ER antagonism and requires the ERalpha hormone binding domain. The acute response of eNOS to estrogen can also be reconstituted in COS-7 cells cotransfected with ERalpha and eNOS, but not by transfection with eNOS alone. The inhibition of calcium influx, or tyrosine kinases or mitogen-activated protein (MAP) kinase prevents eNOS stimulation by estrogen, and estrogen causes rapid ER-dependent activation of MAP kinase. Thus, the acute effect of estrogen on eNOS is mediated by ERalpha functioning in a novel, nongenomic manner to activate the enzyme via calcium-dependent, MAP kinase-dependent mechanisms.

Cell growth modulates nitric oxide synthase expression in fetal pulmonary artery endothelial cells

Nitric oxide (NO), produced by endothelial (e) nitric oxide synthase (NOS), is a critical mediator of vascular function and growth in the developing lung. Pulmonary eNOS expression is diminished in conditions associated with altered pulmonary vascular development, suggesting that eNOS may be modulated by changes in pulmonary artery endothelial cell (PAEC) growth. We determined the effects of cell growth on eNOS expression in cultured ovine fetal PAEC studied at varying levels of confluence. NOS enzymatic activity was sixfold greater in quiescent PAEC at 100% confluence compared with more rapidly replicating cells at 50% confluence. To determine if there is a reciprocal effect of NO on PAEC growth, studies of NOS inhibition or the provision of exogenous NO from spermine NONOate were performed. Neither intervention had a discernable effect on PAEC growth. The influence of cell growth on NOS activity was unique to pulmonary endothelium, because varying confluence did not alter NOS activity in fetal systemic endothelial cells. The effects of cell growth induced by serum stimulation were also evaluated, and NOS enzymatic activity was threefold greater in quiescent, serum-deprived cells compared with that in serum-stimulated cells. The increase in NOS activity observed at full confluence was accompanied by parallel increases in eNOS protein and mRNA expression. These findings indicate that eNOS gene expression in fetal PAEC is upregulated during cell quiescence and downregulated during rapid cell growth. Furthermore, the interaction between cell growth and NO in the PAEC is unidirectional.

Oxidized low density lipoprotein displaces endothelial nitric-oxide synthase (eNOS) from plasmalemmal caveolae and impairs eNOS activation

Hypercholesterolemia-induced vascular disease and atherosclerosis are characterized by a decrease in the bioavailability of endothelium-derived nitric oxide. Endothelial nitric-oxide synthase (eNOS) associates with caveolae and is directly regulated by the caveola protein, caveolin. In the present study, we examined the effects of oxidized low density lipoprotein (oxLDL) on the subcellular location of eNOS, on eNOS activation, and on caveola cholesterol in endothelial cells. We found that treatment with 10 microgram/ml oxLDL for 60 min caused greater than 90% of eNOS and caveolin to leave caveolae. Treatment with oxLDL also inhibited acetylcholine-induced activation of eNOS but not prostacyclin production. oxLDL did not affect total cellular eNOS abundance. Oxidized LDL also did not affect the palmitoylation, myristoylation or phosphorylation of eNOS. Oxidized LDL, but not native LDL, or HDL depleted caveolae of cholesterol by serving as an acceptor for cholesterol. Cyclodextrin also depleted caveolae of cholesterol and caused eNOS and caveolin to translocate from caveolae. Furthermore, removal of oxLDL allowed eNOS and caveolin to return to caveolae. We conclude that oxLDL-induced depletion of caveola cholesterol causes eNOS to leave caveolae and inhibits acetylcholine-induced activation of the enzyme. This process may be an important mechanism in the early pathogenesis of atherosclerosis.

Nitric oxide (NO) upregulates NO synthase expression in fetal intrapulmonary artery endothelial cells

Endothelium-derived nitric oxide (NO) generated by endothelial NO synthase (eNOS) is critically involved in pulmonary vasodilation during cardiopulmonary transition at birth. Inhaled NO therapy has recently been considered for patients with persistent pulmonary hypertension of the newborn (PPHN). To better understand the mechanisms regulating NO synthesis in the developing pulmonary circulation and the possible ramifications of NO therapy, studies were performed with early passage ovine fetal intrapulmonary artery endothelial cells (PAEC) to determine whether NO directly modulates eNOS expression. To examine the effects of exogenous NO, PAEC were treated with the NO donor spermine NONOate or the parent compound spermine. Exogenous NO caused increases in eNOS protein expression and NOS enzymatic activity that were detectable within 16 h of exposure. In contrast, the inhibition of endogenous NO production with nitro-L-arginine-methyl ester (L-NAME) caused a reduction in eNOS protein expression that was evident within 8 h. Paralleling the changes in eNOS protein, eNOS messenger RNA (mRNA) abundance was upregulated by exogenous NO and downregulated by L-NAME, suggesting that NO modulation of eNOS expression involves processes at the level of gene transcription or mRNA stability. Thus, in fetal PAEC there is positive-feedback regulation of eNOS expression by both exogenous and endogenous NO. These findings suggest that difficulties with transient effectiveness or prolonged requirements for NO therapy in certain PPHN patients are not due to declines in eNOS expression. Further, conditions such as fetal hypoxemia that impair PAEC NO production may attenuate eNOS expression through this mechanism, thereby contributing to the pathogenesis of PPHN.

Establishment of an immortalized fetal intrapulmonary artery endothelial cell line

The investigation of fetal pulmonary endothelial cell gene expression and function has been limited by the requirement for primary cells. In an effort to establish an immortalized cell line, ovine fetal pulmonary artery endothelial cells (PAECs; passage 5) were permanently transfected with the E6 and E7 open reading frames of human papillomavirus type 16, and phenotypes related to nitric oxide (NO) production were evaluated up to passage 28. Acetylated low-density lipoprotein uptake, endothelial NO synthase (eNOS) expression, and proliferation rates were unaltered by immortalization. Acetylcholine-stimulated eNOS activity was 218-255% above basal levels in immortalized cells, and this was comparable to the 250% increase seen in primary PAECs (passage 6). eNOS was also acutely activated by estradiol to levels 197-309% above basal, paralleling the stimulation obtained in primary cells. In addition, the expression of estrogen receptor-alpha, which has recently been shown to mediate the acute response in primary PAECs, was conserved. Thus fetal PAECs transfected with E6 and E7 show no signs of senescence with passage, and mechanisms of NO production, including those mediated by estradiol, are conserved. Immortalized PAECs will provide an excellent model for further studies of eNOS gene expression and function in fetal pulmonary endothelium.

Regulation of vasodilator synthesis during lung development

The endothelium-derived vasodilator molecules prostaglandin I2 (PGI2) and nitric oxide (NO) are critically involved in the dramatic increase in pulmonary blood flow that occurs during cardiopulmonary transition at birth. Studies in animal and cell culture models have revealed that there is increased PGI2 and NO production in the pulmonary circulation of the late fetus in direct response to increased oxygenation, and that this response is unique to the pulmonary endothelium. Additional work has demonstrated that there is normally marked upregulation in the expression of the key synthetic enzymes cyclooxygenase type I and endothelial NO synthase in the lung during late gestation, thereby maximizing the capacity for vasodilator production at the time of birth. Furthermore, studies in animal models of neonatal pulmonary hypertension indicate that attenuated expression of these genes may frequently contribute to the pathogenesis of the disorder. A greater understanding of the mechanisms regulating PGI2 and NO synthesis in the developing lung will potentially lead to novel therapies for neonatal pulmonary hypertension aimed at optimizing endogenous vasodilator production.

Estrogen acutely stimulates endothelial nitric oxide synthase in H441 human airway epithelial cells

Nitric oxide (NO) is an important mediator of physiologic processes in the airway. Levels of exhaled NO are greatest and asthma symptoms are least in menstruating women during midcycle, when estrogen levels are highest. To better understand the role of estrogen in airway function, we tested the hypothesis that estrogen stimulates endothelial NO synthase (eNOS) in NCI-H441 human bronchiolar epithelial cells. eNOS activation was assessed by measuring conversion of [3H]L-arginine to [3H]L-citrulline in intact cells. eNOS activity rose in the presence of estradiol-17beta (E2beta), with a maximum stimulation of 243% at 10(-8) M E2beta. This response was comparable to the 201% increase elicited by the calcium (Ca2+) ionophore A23187 (10(-5) M), and was evident as early as 5 min after such treatment. Actinomycin D had no effect on the response to E2beta, and eNOS abundance was similar in control and E2beta-treated cells. E2beta-stimulated eNOS activity was dependent on the influx of extracellular Ca2+, and was completely inhibited by the estrogen receptor (ER) antagonist ICI182,780. Messenger RNA and protein for the alpha isoform of ER (ERalpha) were evident in the H441 cells, and freshly isolated ovine airway epithelial cells also coexpressed eNOS and ERalpha. These findings indicate that estrogen acutely activates existing eNOS in H441 airway epithelial cells, through a process that involves the stimulation of epithelial ER and Ca2+ influx. This process may play a role in the hormonal modulation of airway function.

Glucocorticoids downregulate cyclooxygenase-1 gene expression and prostacyclin synthesis in fetal pulmonary artery endothelium

Prostacyclin (prostaglandin I2 [PGI2]) is a key mediator of pulmonary vascular function during early postnatal life, and its production in the pulmonary vasculature rises markedly during that period because of increasing expression of cyclooxygenase type 1 (COX-1). The postnatal rise in COX-1 may be due to the release of inhibition by glucocorticoids, since plasma glucocorticoid levels fall after birth and glucocorticoids decrease PGI2 synthesis in certain nonpulmonary cell types. We therefore studied the direct effects of dexamethasone (DEX) on COX-1 expression in early-passage ovine fetal pulmonary-artery endothelial cells (PAECs). DEX (10(-10) to 10(-6) mol/L) caused a dose-related decrease in COX-1 mRNA expression that was evident by 24 hours, was maximal at 10(-6) mol/L (50% inhibition), and was not due to changes in mRNA stability. There was a parallel decline in COX-1 protein expression. COX-1 protein rose following DEX withdrawal, and DEX blunted the stimulatory effect of 17beta-estradiol on COX-1 expression. DEX alone (10(-8) mol/L for 48 hours) caused a 93% fall in basal PGI2 production, and bradykinin- and A23187-stimulated PGI2 were diminished 96% and 94%, respectively. Similarly, PGI2 synthesis from arachidonic acid fell 86% with DEX; all of the above effects are consistent with COX-1 downregulation. The glucocorticoid receptor (GR) antagonist mifepristone (RU-486; 10(-6) mol/L) blocked the inhibitory effect of DEX, and GR expression was evident by immunoblot analysis. These findings indicate that glucocorticoids downregulate COX-1 expression and PGI2 synthesis in fetal PAECs through the activation of PAEC GR and effects on COX-1 gene transcription. This mechanism may modulate pulmonary PGI2 production in the perinatal period, and it may also play a role in the effects of glucocorticoids on the systemic circulation at a variety of ages.

Estrogen receptor alpha mediates the nongenomic activation of endothelial nitric oxide synthase by estrogen

Estrogen is an important vasoprotective molecule that causes the rapid dilation of blood vessels by activating endothelial nitric oxide synthase (eNOS) through an unknown mechanism. In studies of intact ovine endothelial cells, 17beta-estradiol (E2) caused acute (five-minute) activation of eNOS that was unaffected by actinomycin D but was fully inhibited by concomitant acute treatment with specific estrogen receptor (ER) antagonists. Overexpression of the known transcription factor ERalpha led to marked enhancement of the acute response to E2, and this was blocked by ER antagonists, was specific to E2, and required the ERalpha hormone-binding domain. In addition, the acute response of eNOS to E2 was reconstituted in COS-7 cells cotransfected with wild-type ERalpha and eNOS, but not by transfection with eNOS alone. Furthermore, the inhibition of tyrosine kinases or mitogen-activated protein (MAP) kinase kinase prevented the activation of eNOS by E2, and E2 caused rapid ER-dependent activation of MAP kinase. These findings demonstrate that the short-term effects of estrogen central to cardiovascular physiology are mediated by ERalpha functioning in a novel, nongenomic manner to activate eNOS via MAP kinase-dependent mechanisms.

Nitric oxide synthase isoform expression in the developing lung epithelium

Nitric oxide (NO), generated by NO synthase (NOS), is an important mediator of physiological processes in the airway and lung parenchyma, and there is evidence that the pulmonary expression of the endothelial isoform of NOS (eNOS) is developmentally regulated. The purpose of the present study was to delineate the cellular distribution of expression of eNOS in the developing respiratory epithelium and to compare it with inducible (iNOS) and neuronal (nNOS) NOS. Immunohistochemistry was performed on fetal (125-135 days gestation, term 144 days), newborn (2-4 wk), and maternal sheep lungs. In fetal lung, eNOS expression was evident in bronchial and proximal bronchiolar epithelia but was absent in terminal and respiratory bronchioles and alveolar epithelium. Similar to eNOS, iNOS was detected in bronchial and proximal bronchiolar epithelia but not in alveolar epithelium. However, iNOS was also detected in terminal and respiratory bronchioles. nNOS was found in epithelium at all levels including the alveolar wall. iNOS and nNOS were also detected in airway and vascular smooth muscle. The cellular distribution of all three isoforms was similar in fetal, newborn, and adult lungs. Findings in the epithelium were confirmed by isoform-specific reverse transcription-polymerase chain reaction assays and NADPH diaphorase histochemistry. Thus the three NOS isoforms are commonly expressed in proximal lung epithelium and are differentially expressed in distal lung epithelium. All three isoforms may be important sources of epithelium-derived NO throughout lung development.

Developmental changes in prostacyclin synthesis are conserved in cultured pulmonary endothelium and vascular smooth muscle

Prostacyclin (PGI2) is a key mediator of pulmonary vascular and parenchymal function during late fetal and early postnatal life, and its synthesis in intrapulmonary arteries increases markedly during that period. The rate-limiting enzyme in PGI2 synthesis in the developing lung is cyclooxygenase (COX). To understand better the mechanisms underlying the developmental increase in PGI2 synthesis, we evaluated PGI2 production in early-passage, cultured pulmonary artery endothelial cells (PAEC) and pulmonary vascular smooth-muscle cells (VSM) from fetal and newborn lambs. In arterial segments, PGI2 synthesis was sevenfold greater in intact arteries from newborn than from fetal lambs, and it was 12-fold greater in endothelium-denuded newborn than in fetal arteries, indicating that the developmental increase occurs in both the endothelium and medial layer. Similarly, basal PGI2 production was three-fold greater in newborn than in fetal PAEC, and 2.5-fold greater in newborn than in fetal pulmonary VSM cells. Calcium ionophore (A23187)-stimulated and arachidonic acid-stimulated PGI2 synthesis were also greater in newborn than in fetal PAEC and VSM, revealing a developmental upregulation in COX enzymatic activity in both cell types. Immunoblot analysis showed that this is due to greater COX-1 protein expression in newborn than in fetal vascular cells; COX-2 protein expression was not detected. In addition, COX-1 messenger RNA (mRNA) abundance was greater in newborn than in fetal PAEC, and this was not due to a difference in COX-1 mRNA stability. Thus, the developmental upregulation of PGI2 synthesis is conserved in early-passage PAEC and pulmonary VSM, and is related to a maturational increase in COX-1 gene expression. Further studies with the cultured cell model will enable determination of the factors that directly regulate COX-1 expression in the developing pulmonary vasculature.

Rapid activation of endothelial nitric oxide synthase by estrogen

Estrogen is an important atheroprotective molecule that causes the rapid dilation of blood vessels by stimulating endothelial nitric oxide synthase (eNOS). There is also evidence that estrogen modulates airway epithelial NO production, thereby potentially affecting bronchial hyperresponsiveness. Studies in cultured endothelial and airway epithelial cells indicate that physiologic concentrations of estrogen cause rapid direct activation of eNOS that is unaffected by actinomycin D, but fully inhibited by estrogen receptor (ER) antagonism. Overexpression of ERalpha leads to marked enhancement of the acute response to estrogen, and this process is blocked by ER antagonism, it is specific to estrogen, and it requires the ERalpha hormone binding domain. In addition, the acute response of eNOS to estrogen can be reconstituted in COS-7 cells cotransfected with wild-type ERalpha and eNOS, but not by transfection with eNOS alone. Furthermore, the inhibition of calcium influx, or tyrosine kinases or MAP kinase prevents the stimulation of eNOS by estrogen, and estrogen causes rapid ER-dependent activation of MAP kinase. These findings indicate that the acute effects of estrogen on both endothelial and airway epithelial eNOS are mediated by ERalpha functioning in a novel, nongenomic manner to activate the enzyme via calcium-dependent, MAP kinase-dependent mechanisms.

Role of plasmalemmal caveolae in signal transduction

Caveolae are specialized plasmalemmal microdomains originally studied in numerous cell types for their involvement in the transcytosis of macromolecules. They are enriched in glycosphingolipids, cholesterol, sphingomyelin, and lipid-anchored membrane proteins, and they are characterized by a light buoyant density and resistance to solubilization by Triton X-100 at 4 degreesC. Once the identification of the marker protein caveolin made it possible to purify this specialized membrane domain, it was discovered that caveolae also contain a variety of signal transduction molecules. This includes G protein-coupled receptors, G proteins and adenylyl cyclase, molecules involved in the regulation of intracellular calcium homeostasis, and their effectors including the endothelial isoform of nitric oxide synthase, multiple components of the tyrosine kinase-mitogen-activated protein kinase pathway, and numerous lipid signaling molecules. More recent work has indicated that caveolae further serve to compartmentalize, modulate, and integrate signaling events at the cell surface. This specialized plasmalemmal domain warrants direct consideration in future investigations of both normal and pathological signal transduction in pulmonary cell types.

Estrogen upregulates cyclooxygenase-1 gene expression in ovine fetal pulmonary artery endothelium

Prostacyclin (PGI2) is a key mediator of pulmonary vasodilation in the perinatal period and its synthesis in the pulmonary vasculature increases markedly during late gestation due to enhanced expression of the rate-limiting enzyme cyclooxygenase-1 (COX-1). The hormone estrogen may play a role in COX-1 upregulation since fetal estrogen levels rise dramatically during late gestation and estrogen enhances PGI2 synthesis in nonpulmonary vascular cells. We therefore studied the direct effects of estrogen on COX-1 expression in ovine fetal pulmonary artery endothelial cells (PAEC). Exposure to estradiol-17beta (E2beta, 10(-)10 to 10(-)6 M) caused a dose-related increase in COX-1 mRNA expression that was evident after 48 h and maximal at 10(-)8 M (fourfold increase). COX-1 mRNA stability was unchanged, suggesting that the upregulation is mediated at the level of transcription. E2beta treatment (10(-)8 M for 48 h) also caused a threefold increase in COX-1 protein expression and a threefold increase in PGI2 synthesis stimulated by bradykinin, the calcium ionophore A23187, or arachidonic acid. The estrogen receptor (ER) antagonist ICI 182,780 fully reversed the effects of the hormone on COX-1 protein expression and on arachidonic acid-stimulated PGI2 synthesis, and ER expression was evident in the PAEC by immunoblot analysis. These findings indicate that physiologic levels of estrogen cause upregulation of COX-1 expression and PGI2 synthesis in fetal PAEC via activation of PAEC ER. This process may play a critical role in optimizing the capacity for PGI2-mediated pulmonary vasodilation at birth, and it may also be involved in estrogen responsiveness in other vascular beds.

Ontogeny of cyclooxygenase-1 and cyclooxygenase-2 gene expression in ovine lung

Prostacyclin is a key mediator of pulmonary vascular and parenchymal function during late fetal and early postnatal life, and its synthesis in whole lung increases during that period. The rate-limiting enzyme in prostacyclin synthesis in the developing lung is cyclooxygenase (COX). We investigated the ontogeny and cellular localization of COX-1 (constitutive) and COX-2 (inducible) gene expression in lungs from late-gestation fetal lambs, 1-wk-old newborn lambs (NB1), and 1- to 4-mo-old newborn lambs (NB2). COX-1 mRNA abundance rose progressively from fetal to NB1 to NB2, increasing 12-fold overall. In parallel, immunoblot analysis revealed a progressive increase in COX-1 protein, rising fourfold from fetal lambs to NB2. COX-2 mRNA levels increased fivefold from fetal to NB1 but were similar in NB1 and NB2. However, COX-2 protein was not detectable by immunoblot analysis in any age group. Immunohistochemistry for COX-1 showed intense immunostaining in endothelial cells at all ages. COX-1 was also expressed in airway epithelium at all ages, with a greater number of epithelial cells staining positively in NB2 compared with fetal and NB1 groups. In addition, COX-1 was expressed in airway smooth muscle from NB1. COX-2 immunostaining was absent in all age groups. These findings indicate that there is differential expression of COX-1 and COX-2 in the developing lung and that the enzymes are expressed in a cell-specific manner. The developmental upregulation in COX-1 may optimize the capacity for prostaglandin-mediated vasodilation, bronchodilation, and surfactant synthesis in the newborn lung.

Ontogeny of nitric oxide in the pulmonary vasculature

The vasodilator molecule nitric oxide is critically involved in the successful cardiopulmonary transition from fetal to postnatal life. It is produced in the pulmonary endothelium by the endothelial isoform of the enzyme nitric oxide synthase. The expression of endothelial nitric oxide synthase in the lung increases dramatically during late gestation, optimizing the capacity for nitric oxide production at the time of birth. Studies in cultured cell models indicate that the developmental upregulation may be mediated by estrogen, and that the expression of the enzyme is also upregulated by oxygen. Pulmonary endothelial nitric oxide synthase expression is diminished in models of congenital diaphragmatic hernia and neonatal pulmonary hypertension induced by fetal ductal ligation. Thus, there is normally a marked developmental upregulation in endothelial nitric oxide synthase expression in the lung during late fetal life, and attenuated expression of the enzyme may contribute to the pathophysiology of a variety of forms of neonatal pulmonary vascular disease.

Estrogen upregulates endothelial nitric oxide synthase gene expression in fetal pulmonary artery endothelium

NO, produced by endothelial NO synthase (eNOS), is a key mediator of pulmonary vasodilation during cardiopulmonary transition at birth. The capacity for NO production is maximal at term because pulmonary eNOS expression increases during late gestation. Since fetal estrogen levels rise markedly during late gestation and there is indirect evidence that the hormone enhances nonpulmonary NO production in adults, estrogen may upregulate eNOS in fetal pulmonary artery endothelium. Therefore, we studied the direct effects of estrogen on eNOS expression in ovine fetal pulmonary artery endothelial cells (PAECs). Estradiol-17beta caused a 2.5-fold increase in NOS enzymatic activity in PAEC lysates. This effect was evident after 48 hours, and it occurred in response to physiological concentrations of the hormone (10(-10) to 10(-6) mol/L). The increase in NOS activity was related to an upregulation in eNOS protein expression, and eNOS mRNA abundance was also enhanced. Estrogen receptor antagonism with ICI 182,780 completely inhibited estrogen-mediated eNOS upregulation, indicating that estrogen receptor activation is necessary for this response. In addition, immunocytochemistry revealed that fetal PAECs express estrogen receptor protein. Furthermore, transient transfection assays with a specific estrogen-responsive reporter system have demonstrated that the endothelial estrogen receptor is capable of estrogen-induced transcriptional transactivation. Thus, estrogen upregulates eNOS gene expression in fetal PAECs through the activation of PAEC estrogen receptors. This mechanism may be responsible for pulmonary eNOS upregulation during late gestation, thereby optimizing the capacity for NO-mediated pulmonary vasodilation at birth.

Estrogen acutely stimulates nitric oxide synthase activity in fetal pulmonary artery endothelium

Estrogen (E) has nitric oxide (NO)-mediated effects in certain vascular beds, and fetal E levels rise acutely with parturition, suggesting that E may be involved in NO-mediated pulmonary vasodilation at birth. We tested the hypothesis that E acutely stimulates NO synthase (NOS) activity in ovine fetal pulmonary artery endothelial cells (PAEC) by measuring L-[3H]arginine conversion to L-[3H]citrulline in intact cells. NOS activity in the presence of 17 beta-estradiol (E2 beta) rose in a dose-dependent manner, increasing 70-100%, with a threshold concentration of 10(-10) M. This effect was detectable within 5 min of E2 beta exposure, and the maximal response was comparable to that obtained with acetylcholine, which had a threshold concentration of 10(-8) M. Ca2+ removal completely inhibited E2 beta-stimulated NOS activity, and activity with E2 beta and the Ca2+ ionophore A-23187 was not additive. In addition, the expression of the endothelial isoform of NOS (eNOS) was not altered, and the inducible and neuronal NOS isoforms were not detected by immunoblot analysis. These findings indicate that E2 beta acutely stimulates eNOS by Ca2+ influx. Furthermore, E2 beta-stimulated NOS activity was fully inhibited by the E receptor (ER) antagonists tamoxifen and ICI-182,780, and ER mRNA expression was evident in reverse transcription-polymerase chain reaction assays. Thus E acutely stimulates eNOS activity in fetal PAEC via the activation of endothelial ER and increases in intracellular Ca2+.

Pulmonary endothelial NO synthase gene expression is decreased in fetal lambs with pulmonary hypertension

Nitric oxide (NO), produced by endothelial (e) NO synthase (NOS), is critically involved in the cardiopulmonary transition from fetal to neonatal life. We have previously shown that NO-dependent relaxation is attenuated in intrapulmonary arteries from fetal lambs with pulmonary hypertension (PHT) created by prenatal ligation of the ductus arteriosus. In the present study, we determined whether this is due to altered pulmonary eNOS expression. eNOS and neuronal NOS (nNOS) protein expression were assessed in lungs from near-term control lambs and PHT lambs that underwent ductal ligation 10 days earlier. eNOS protein expression was decreased 49% in PHT lung. In contrast, nNOS protein abundance was unchanged. NOS enzymatic activity was also diminished in PHT vs. control lung (60 +/- 3 vs. 110 +/- 7 fmol.mg protein-1.min-1, respectively). Paralleling the declines in eNOS protein and NOS enzymatic activity, eNOS mRNA abundance was decreased 64% in PHT lung. Thus pulmonary eNOS gene expression is attenuated in the lamb model of fetal PHT. Because NO modulates both vasodilation and vascular smooth muscle growth, diminished eNOS expression may contribute to both the abnormal vasoreactivity and the excessive muscularization of the pulmonary circulation in fetal PHT.

Inhaled nitric oxide and persistent pulmonary hypertension of the newborn. The Inhaled Nitric Oxide Study Group

BACKGROUND

Persistent pulmonary hypertension of the newborn causes systemic arterial hypoxemia because of increased pulmonary vascular resistance and right-to-left shunting of deoxygenated blood. Inhaled nitric oxide decreases pulmonary vascular resistance in newborns. We studied whether inhaled nitric oxide decreases severe hypoxemia in infants with persistent pulmonary hypertension.

METHODS

In a prospective, multicenter study, 58 full-term infants with severe hypoxemia and persistent pulmonary hypertension were randomly assigned to breathe either a control gas (nitrogen) or nitric oxide (80 parts per million), mixed with oxygen from a ventilator. If oxygenation increased after 20 minutes and systemic blood pressure did not decrease, the treatment was considered successful and was continued at lower concentrations. Otherwise, it was discontinued and alternative therapies, including extracorporeal membrane oxygenation, were used.

RESULTS

Inhaled nitric oxide successfully doubled systemic oxygenation in 16 of 30 infants (53 percent), whereas conventional therapy without inhaled nitric oxide increased oxygenation in only 2 of 28 infants (7 percent). Long-term therapy with inhaled nitric oxide sustained systemic oxygenation in 75 percent of the infants who had initial improvement. Extracorporeal membrane oxygenation was required in 71 percent of the control group and 40 percent of the nitric oxide group (P=0.02). The number of deaths was similar in the two groups. Inhaled nitric oxide did not cause systemic hypotension or increase methemoglobin levels.

CONCLUSIONS

Inhaled nitric oxide improves systemic oxygenation in infants with persistent pulmonary hypertension and may reduce the need for more invasive treatments.

Tissue specific expression of vascular smooth muscle angiotensin II receptor subtypes during ovine pregnancy

Uteroplacentral responses to infused angiotensin II (ANG II) are less than those elicited by systemic vasculature. This does not reflect ANG II receptor (AT) downregulation but may reflect differences in AT-receptor subtypes expressed. We examined AT-receptor subtypes in smooth muscle (SM) from uterine (UA), mesenteric, renal, and mammary arteries and aorta from nulliparous (n = 12), pregnant (n = 18; 105-140 days, term = 145 days), postpartum (n = 5; 6-9 days after delivery), and nonpregnant parous (n = 14) ewes by assessing displacement of 125I-labeled ANG II binding by [Sar1, Ile8]ANG II (AT1 and AT2), losartan (AT1) PD-123319 (AT2), and CGP-42112A (AT2). AT2 receptors accounted for 75-90% of total binding in UA. Except for mammary arteries, other arteries expressed only AT1 receptors. Receptor subtype expression was not altered by reproductive state in any artery studied. With the use of autoradiography, AT2 receptors appear to predominate in media of small intramyometrial arteries, whereas AT1 receptors predominate in the luminal portion. We therefore determined which subtype mediates endothelium-derived ANG II-induced increases in UA PGI2 synthesis during pregnancy. ANG II (0.05 microM) increased PGI2 synthesis 62%, from 214 +/- 13 to 346 +/- 23 pg.mg-1.h-1 (P < 0.05). Losartan (1.0 microM) inhibited the rise in PGI2 (257 +/- 24 vs. 238 +/- 25 pg.mg-1.h-1), whereas 1.0 microM PD-123319 had no effect (231 +/- 23 vs. 337 +/- 31 pg.mg-1.h-1; P < 0.05). AT2 receptors do not mediate ANG II-induced vasoconstriction, thus differences in uteroplacental and systemic sensitivity to ANG II may reflect predominance of AT2 receptors in UASM and ANG II-induced increases in UA prostacyclin synthesis by endothelial AT1 receptors.

Endothelial vasodilator production by uterine and systemic arteries. I. Effects of ANG II on PGI2 and NO in pregnancy

Uterine vasculature is less responsive than systemic vasculature to angiotensin II (ANG II)-induced vasoconstriction. We hypothesized that pregnancy augments basal and ANG II-stimulated endothelial prostacyclin (PGI2) and/or nitric oxide (NO) production, which locally increase vascular smooth muscle (VSM) adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP), respectively. Uterine (UA) and systemic arteries (SA) from pregnant (P) and nonpregnant (NP) sheep were incubated with isobutylmethylxanthine. Basal PGI2, cAMP, and cGMP production was 2.4-, 1.6-, and 5.9-fold greater (P < 0.01) in UA from P vs. NP sheep; endothelium removal lowered (P < 0.05) values 69, 44, and 88%. Basal SA PGI2 and cAMP, but not cGMP, also were elevated by pregnancy. Indomethacin (Indo; 100 microM) decreased PGI2 and cAMP, but not cGMP production; N omega-nitro-L-arginine methyl ester (L-NAME; 10 microM) and methylene blue (MB, 10 microM) only decreased cGMP. Basal UA, but not SA, NO synthase activity (conversion of [3H]arginine to [3H]citrulline), was 1.8-fold higher in pregnancy and decreased (P < 0.01) after endothelium removal and with L-NAME. ANG II (50 nM) increased PGI2 (86%) and cAMP (56%) production only in UA from P sheep (P < 0.05); this was abolished by endothelium removal or Indo. ANG II also increased (P < 0.01) cGMP production by UA from both groups but only by SA from P ewes; this was absent in denuded, L-NAME-, or MB-treated vessels. Stimulation of VSM cGMP production with sodium nitroprusside (50 microM) was inhibited by MB, but not L-NAME or endothelium removal. In pregnancy, endothelial PGI2 and NO production are enhanced and may contribute to attenuated ANG II vasoconstriction via VSM cAMP and cGMP.

Oxygen upregulates nitric oxide synthase gene expression in ovine fetal pulmonary artery endothelial cells

Nitric oxide (NO) is critically involved in oxygen-mediated pulmonary vasodilatation in the fetus and newborn. We determined the effects of prolonged alterations in oxygenation on endothelial NO synthase (eNOS) gene expression in early passage ovine fetal intrapulmonary artery endothelial cells (PAEC). PAEC were exposed to PO2 = 50 or 150 mmHg for 48 h, and eNOS protein expression was evaluated by immunoblot analysis. eNOS protein expression was 2.7-fold greater at higher oxygen tension; eNOS upregulation was also evident after 24 h. Inducible NOS protein was not detectable by immunoblot at either level of oxygenation. In the lung, the effect of oxygen on eNOS expression may be specific to the endothelium, as eNOS expression in bronchiolar epithelial cells of Clara cell lineage was not altered by varying oxygen tension. The oxygen-related increase in eNOS protein in the fetal PAEC was associated with 2.5-fold greater NOS enzymatic activity. In parallel, there was a 2.8-fold rise in eNOS mRNA abundance. Thus eNOS gene expression in ovine fetal PAEC is upregulated by oxygen, and this is mediated at the level of gene transcription or mRNA stability. This process may play an important role in oxygen modulation of pulmonary vasomotor tone in the fetus and newborn.

Acylation targets emdothelial nitric-oxide synthase to plasmalemmal caveolae

Endothelial nitric-oxide synthase (eNOS) generates the key signaling molecule nitric oxide in response to intralumenal hormonal and mechanical stimuli. We designed studies to determine whether eNOS is localized to plasmalemmal microdomains implicated in signal transduction called caveolae. Using immunoblot analysis, eNOS protein was detected in caveolar membrane fractions isolated from endothelial cell plasma membranes by a newly developed detergent-free method; eNOS protein was not found in noneaveolar plasma membrane. Similarly, NOS enzymatic activity was 9.4-fold enriched in caveolar membrane versus whole plasma membrane, whereas it was undetectable in non-caveolar plasma membrane. 51-86% of total NOS activity in postnuclear supernatant was recovered in plasma membrane, and 57-100% of activity in plasma membrane was recovered in caveolae. Immunoelectron microscopy showed that eNOS heavily decorated endothelial caveolae, whereas coated pits and smooth plasma membrane were devoid of gold particles. Furthermore, eNOS was targeted to caveolae in COS-7 cells transfected with wild-type eNOS cDNA. Studies with eNOS mutants revealed that both myristoylation and palmitoylation are required to target the enzyme to caveolae and that each acylation process enhances targeting by 10-fold. Thus, acylation targets eNOS to plasmalemmal caveolae. Localization to this microdomain is likely to optimize eNOS activation and the extracellular release of nitric oxide.

Pulmonary endothelial nitric oxide synthase gene expression is decreased in a rat model of congenital diaphragmatic hernia

Nitric oxide (NO) produced by the enzyme nitric oxide synthase (NOS) is critically involved in the cardiopulmonary transition from fetal to neonatal life. In congenital diaphragmatic hernia (CDH) this transition often does not occur normally, resulting in persistent pulmonary hypertension of the newborn (PPHN). We sought to determine if pulmonary NOS expression is altered in a rat model of CDH induced by maternal ingestion of the herbicide 2,4-dichlorophenyl-p-nitrophenyl ether (Nitrofen) on day 9 of gestation (term = 22 days). Sixty-three percent of Nitrofen-exposed fetuses developed CDH. Endothelial NOS (eNOS) and neuronal NOS (nNOS) protein expression were assessed in ipsilateral CDH lungs and in control lungs (Nitrofen-treated, no hernia) at 20 d gestation using immunoblot analyses. eNOS and nNOS have been immunohistochemically localized to rat pulmonary endothelium and bronchiolar epithelium, respectively, and we have previously demonstrated that their expression normally increases during late gestation to be maximal near term. eNOS protein expression was decreased in CDH versus control lung (58 +/- 6 versus 100 +/- 6% of control, n = 5). In contrast, nNOS protein abundance was similar. Factor VIII-associated antigen expression was comparable in CDH and control lung, indicating that the change in eNOS is not related to differences in endothelial cell density. eNOS mRNA abundance was evaluated in semiquantitative reverse transcription-polymerase chain reaction assays. Paralleling the decline in eNOS protein expression, eNOS mRNA was decreased in CDH versus control lung (22 +/- 8 versus 100 +/- 31% of control, n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged in vivo hypoxia enhances nitric oxide synthase type I and type III gene expression in adult rat lung

Prolonged hypoxia in the adult rat causes a decline in endothelium-derived nitric oxide (NO) production in the pulmonary circulation. To evaluate whether this is related to a decrease in endothelial NO synthase (NOS-III) expression, we determined the effects of hypobaric hypoxia (7 or 21 days) on NOS-III gene expression in adult rat lung. Neuronal NOS (NOS-I) expression was also examined; NOS-I has been immunohistochemically localized to rat bronchiolar epithelium. NOS-III and NOS-I mRNA abundance were assessed in reverse transcription-polymerase chain reaction assays and the proteins were evaluated by immunoblot analysis. After 7 and 21 days of hypoxia, there were increases in the steady-state levels of both NOS-III and NOS-I mRNA, rising 2.7- to 3.0-fold and 2.5- to 2.8-fold, respectively. These findings were confirmed by Northern analyses. In parallel, NOS-III and NOS-I protein abundance were also increased with hypoxia by 3.0- to 3.5-fold and 2.4- to 3.0-fold, respectively. NOS activity detected by [3H]arginine to [3H]citrulline conversion rose 109%. Thus, prolonged in vivo hypoxia causes enhancement of NOS-III and NOS-I gene expression in adult rat lung, indicating that the pulmonary expression of these genes is modulated in vivo. The increase in NOS-III expression does not explain the declines in pulmonary endothelial NO production previously observed following prolonged hypoxia in this model. Alternatively, the fall in NO production may be related to diminished NOS co-factor availability.

Endothelial nitric oxide synthase is expressed in cultured human bronchiolar epithelium

Nitric oxide (NO) is an important mediator of physiologic and inflammatory processes in the lung. To better understand the role of NO in the airway, we examined constitutive NO synthase (NOS) gene expression and function in NCI-H441 human bronchiolar epithelial cells, which are believed to be of Clara cell lineage. NOS activity was detected by [3H]arginine to [3H]citrulline conversion (1,070 +/- 260 fmol/mg protein per minute); enzyme activity was inhibited 91% by EGTA, consistent with the expression of a calcium-dependent NOS isoform. Immunoblot analyses with antisera directed against neuronal, inducible, or endothelial NOS revealed expression solely of endothelial NOS protein. Immunocytochemistry for endothelial NOS revealed staining predominantly in the cell periphery, consistent with the association of this isoform with the cellular membrane. To definitively identify the NOS isoform expressed in H441 cells, NOS cDNA was obtained by degenerate PCR. Sequencing of the H441 NOS cDNA revealed 100% identity with human endothelial NOS at the amino acid level. Furthermore, the H441 NOS cDNA hybridized to a single 4.7-kb mRNA species in poly(A)+ RNA isolated from H441 cells, from rat, sheep, and pig lung, and from ovine endothelial cells, coinciding with the predicted size of 4.7 kb for endothelial NOS mRNA. Guanylyl cyclase activity in H441 cells, assessed by measuring cGMP accumulation, rose 6.6- and 5.4-fold with calcium-mediated activation of NOS by thapsigargin and A23187, respectively. These findings indicate that endothelial NOS is expressed in select bronchiolar epithelial cells, where it may have autocrine effects through activation of guanylyl cyclase. Based on these observations and the previous identification of endothelial NOS in a kidney epithelial cell line, it is postulated that endothelial NOS may be expressed in unique subsets of epithelial cells in a variety of organs, serving to modulate ion flux and/or secretory function.

Oxygen modulates nitric oxide production selectively in fetal pulmonary endothelial cells

Acute hypoxia causes pulmonary hypertension in the fetus and newborn that is contrasted by systemic hypotension or normotension. To better understand the role of nitric oxide (NO) in this specific pulmonary vascular response, we determined the acute effects of decreased oxygenation on NO production in ovine fetal pulmonary and systemic (mesenteric) endothelial cells. NO was assessed by measuring cGMP accumulation in fetal vascular smooth muscle (VSM) cells during co-culture incubations of endothelium and VSM (40 s) in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine. Changes in cGMP were dependent on the endothelium and on NO synthase and guanylate cyclase activity. At high O2 (680 mm Hg), basal NO was detectable and NO increased 6- to 10-fold with bradykinin or A23187. In pulmonary endothelium, basal NO fell 58% at pO2 = 150 mm Hg and 51% at 40 mm Hg versus 680 mm Hg, while NO with bradykinin fell 56% and 63%, respectively. NO with A23187, however, was unchanged at 150 mm Hg, but it fell 56% at 40 mm Hg. In contrast, in systemic endothelium basal and stimulated NO production were not altered at lower O2. Findings were similar using pulmonary or systemic detector VSM cells, and exogenous L-arginine had no effect. Thus, decreased O2 acutely attenuates NO production specifically in fetal pulmonary endothelial cells. This process is not related to changes in O2 or L-arginine availability as substrates for NO synthase; alternatively, it may be partially mediated by specific effects of O2 on pulmonary endothelial cell calcium homeostasis.

Hypoxia stimulates prostacyclin synthesis in newborn pulmonary artery endothelium by increasing cyclooxygenase-1 protein

In newborn lambs, pulmonary prostacyclin (PGI2) production increases acutely in response to low oxygen. We tested the hypothesis that decreased oxygenation directly stimulates PGI2 synthesis in arterial segments and cultured endothelial cells from newborn lamb intrapulmonary arteries. In segments studied at PO2 of 680 mm Hg, the synthesis of PGI2 exceeded prostaglandin E2 (PGE2) by 73%. Endothelium removal lowered PGI2 by 77% and PGE2 by 66%. At low oxygen tension (PO2, 40 mm Hg), PGI2 and PGE2 synthesis rose by 96% and 102%, respectively. Similarly, in endothelial cells studied at PO2 of 680 mm Hg, the synthesis of PGI2 exceeded PGE2 by 50%, and at low oxygen tension both PGI2 and PGE2 increased (89% and 64%, respectively). Endothelial cell PGI2 synthesis maximally stimulated by bradykinin, A23187, or arachidonic acid was also increased at low PO2 by 50%, 66%, and 48%, respectively. PGE2 synthesis was similarly altered, increasing by 33%, 37%, and 41%, respectively. In contrast, lowering oxygen had minimal effect on PGI2 and PGE2 synthesis with exogenous PGH2, which is the product of cyclooxygenase. Immunoblot analyses revealed that there was a 2.6-fold greater abundance of cyclooxygenase-1 protein at PO2 of 40 versus 680 mm Hg, and the increase at lower oxygen tension was inhibited by cycloheximide. The cyclooxygenase-2 isoform was not detected. Thus, attenuated oxygenation directly stimulates PGI2 and PGE2 synthesis in intrapulmonary arterial segments and endothelial cells from newborn lambs. This process is due to enhanced cyclooxygenase activity related to increased abundance of the cyclooxygenase-1 protein, and this effect may be due to increased synthesis of the enzyme protein.

Glomerular losartan (DuP 753)-sensitive angiotensin II receptor density is increased in young spontaneously hypertensive rats

There is increasing evidence that an activated intrarenal renin-angiotensin system (RAS) alters renal hemodynamics and fluid balance and that such events may lead to the development of hypertension. To examine the role of the glomerular RAS in the development of hypertension in the spontaneously hypertensive (SHR) rat, we studied angiotensin (ANG) II receptors in isolated glomeruli from young (4- to 5-wk-old) and adult (10- to 12-wk-old) SHR and from age-matched, normotensive Wistar-Kyoto (WKY) rats. Glomerular ANG II receptor density in young SHR is 3-fold higher than in age-matched WKY rats (2033 +/- 154 versus 742 +/- 151 receptors/microns2; p < 0.05) and 1.5-fold higher than in adult SHR and WKY rats (1128 +/- 85 and 1198 +/- 181 receptors/microns2, respectively; p < 0.05). Additional studies demonstrated that the differences in receptor density are not related to disparity in receptor occupancy and that they are also independent of systemic ANG levels. Suppression of RAS by ANG converting enzyme inhibitors resulted in a 3-fold increase in receptor density in young SHR rats and a 4.5-fold increase in young WKY rats; receptor density remained greater in young SHR rats (5915 +/- 318 versus 3358 +/- 234 receptors/microns2, p < 0.05). Furthermore, competitive binding experiments using the nonpeptide ANG II antagonists losartan (AT1) and PD 123319 (AT2) indicate that the greater ANG II receptor density in the young SHR rats represents an increase in the number of a single population of AT1 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide synthase type I and type III gene expression are developmentally regulated in rat lung

The successful transition from fetal to neonatal life involves a marked decline in pulmonary vascular resistance which is modulated in part by endothelium-derived nitric oxide. To define the molecular processes which prepare the pulmonary circulation for nitric oxide mediation of vasodilatation at the time of birth, we determined the ontogeny of endothelial nitric oxide synthase (NOS-III) gene expression in lungs from fetal and newborn rats. Maturational changes in lung neuronal NOS (NOS-I) expression were also investigated; the latter isoform has been localized to rat bronchiolar epithelium. NOS proteins were examined by immunoblot analysis, and mRNA abundance was assessed in reverse transcription-polymerase chain reaction assays. Both NOS-III and NOS-I protein were detectable in 16-day fetal lung, they increased 3.8- and 3.1-fold, respectively, to maximal levels at 20 days of gestation (term = 22 day), and they fell postnatally (1-5 days). In parallel with the findings for NOS-III protein, NOS-III mRNA increased from 16 to 20 days gestation and fell after birth. In contrast, NOS-I mRNA abundance declined during late fetal life and rose postnatally. These findings were confirmed by Northern analyses. Thus NOS-III and NOS-I gene expression are developmentally regulated in rat lung, with maximal NOS-III and NOS-I protein present near term. The regulation of pulmonary NOS-III may primarily involve alterations in transcription or mRNA stability, whereas NOS-I expression in the maturing lung may also be mediated by additional posttranscriptional processes.

Prostacyclin synthesis in ovine pulmonary artery is developmentally regulated by changes in cyclooxygenase-1 gene expression

Prostacyclin (PGI2) is a key mediator of pulmonary vasomotor tone during late gestation and in the newborn, and its production in whole lung increases during that period. We investigated the developmental regulation of PGI2 synthesis in ovine intrapulmonary artery (PA) segments from 110 to 115 d (F1) and 125 to 135 d gestation fetal lambs (F2, term = 144 d) and 1- and 4-wk-old newborn lambs (NB1 and NB2). Basal PGI2 rose fourfold from F1 to F2, fourfold from F2 to NB1, and twofold from NB1 to NB2. In all age groups 66-72% of PGI2 was derived from the endothelium. Similar fold increases in PGI2 were observed with maturation in intact and endothelium-denuded segments. In intact PA from F2, NB1, and NB2, basal PGI2 synthesis and synthesis maximally stimulated by bradykinin, A23187, or arachidonic acid rose with development in a comparable manner. In contrast, PGI2 synthesis stimulated by exogenous PGH2, the product of cyclooxygenase, was similar at all ages. Immunoblot analyses of PA from F2, NB1, and NB2 revealed that there is a sixfold maturational increase in cyclooxygenase-1 protein; the cyclooxygenase-2 isoform was not detectable. Cyclooxygenase-1 mRNA abundance in whole lung also rose with development. Thus, PGI2 synthesis in ovine PA endothelium and vascular smooth muscle increases markedly during late fetal and early newborn life; the increase is due to a rise in cyclooxygenase activity related to enhanced expression of cyclooxygenase-1. We conclude that there is developmental regulation of PA cyclooxygenase-1 gene expression, and that this may be critical to successful cardiopulmonary transition and function in the newborn.

Acute and prolonged hypoxia attenuate endothelial nitric oxide production in rat pulmonary arteries by different mechanisms

Hypoxic pulmonary hypertension complicates many primary respiratory and cardiac conditions. To define the potential role of endothelial nitric oxide (NO) further in both the acute and chronic forms of this disorder, we determined the effects of acute changes in O2 in vitro and prolonged variations in O2 in vivo on endothelial NO production in rat main pulmonary arteries. NO production was assessed by measuring segment cyclic GMP synthesis, which was dependent on the presence of the endothelium and on NO synthase and soluble guanylate cyclase activity. With an acute decrease in pO2 in vitro from 150 to 40 mm Hg, basal endothelial NO production was attenuated by 52%. NO production stimulated by acetylcholine (ACh) or A23187, however, was not altered, suggesting that the underlying mechanism involves acute changes in endothelial intracellular calcium homeostasis or in the production or action of a local activator of endothelial NO synthase. Although prolonged hypoxia in vivo (7 days) also caused a 52% decrease in basal endothelial NO production, ACh- and A23187-stimulated production were diminished as well, by 69 and 73%, respectively; the attenuation in NO production was evident when tested at high pO2 in vitro, was not altered by exogenous L-arginine, and was reversed by 3 days of normoxic recovery, indicating that the chronic process may involve diminished availability of cofactor(s) required for NO synthase activity. Parallel studies of aortic segments showed that these effects are specific to the pulmonary endothelium. Thus, both acute and prolonged hypoxia selectively attenuate pulmonary endothelial NO production by different mechanisms.

Myometrial angiotensin II receptor subtypes change during ovine pregnancy

Although regulation of angiotensin II receptor (AT) binding in vascular and uterine smooth muscle is similar in nonpregnant animals, studies suggest it may differ during pregnancy. We, therefore, examined binding characteristics of myometrial AT receptors in nulliparous (n = 7), pregnant (n = 24, 110-139 d of gestation), and postpartum (n = 21, 5 to > or = 130 d) sheep and compared this to vascular receptor binding. We also determined if changes in myometrial binding reflect alterations in receptor subtype. By using plasma membrane preparations from myometrium and medial layer of abdominal aorta, we determined receptor density and affinity employing radioligand binding; myometrial AT receptor subtypes were assessed by inhibiting [125I]-ANG II binding with subtype-specific antagonists. Compared to nulliparous ewes, myometrial AT receptor density fell approximately 90% during pregnancy (1,486 +/- 167 vs. 130 +/- 16 fmol/mg protein) and returned to nulliparous values > or = 4 wk postpartum; vascular binding was unchanged. Nulliparous myometrium expressed predominantly AT2 receptors (AT1/AT2 congruent to 15%/85%), whereas AT1 receptors predominated during pregnancy (AT1/AT2 congruent to 80%/20%). By 5 d postpartum AT1/AT2 congruent to 40%/60%, and > 4 wk postpartum AT2 receptors again predominated (AT1/AT2 congruent to 15%/85%). In studies of ANG II-induced force generation, myometrium from pregnant ewes (n = 10) demonstrated dose-dependent increases in force (P < 0.001), which were inhibited with an AT1 receptor antagonist. Postpartum myometrial responses were less at doses > or = 10(-9) M (P < 0.05) and unaffected by AT2 receptor antagonists. Vascular and myometrial AT receptor binding are differentially regulated during ovine pregnancy, the latter primarily reflecting decreases in AT2 receptor expression. This is the first description of reversible changes in AT receptor subtype in adult mammals.

Pulmonary endothelial nitric oxide production is developmentally regulated in the fetus and newborn

To define the role of endothelial nitric oxide (NO) in developmental changes in pulmonary vascular resistance and oxygen responsiveness, we determined the ontogeny of endothelial NO production and of oxygen modulation of that process in pulmonary arteries from fetal and newborn lambs. NO production was assessed by measuring endothelium-dependent arterial guanosine 3',5'-cyclic monophosphate synthesis. Basal NO rose two-fold from late gestation to 1 wk of age and another 1.6-fold from 1 to 4 wk. Acetylcholine-stimulated NO also increased 1.6-fold from 1 to 4 wk. The maturational rise in NO was evident at high Po2 in vitro, and it was not modified by L-arginine. This suggests that the developmental increase may alternatively involve enhanced calcium-calmodulin-mediated mechanisms, increased expression of NO synthase, or greater availability of required cofactor(s). With an acute decline in Po2 in vitro from 680 to 150 or 40 mmHg, there was 50-88% attenuation of basal and acetylcholine-stimulated NO late in the third trimester and in the newborn but not early in the third trimester. Parallel studies of mesenteric endothelium revealed postnatal increases in basal and stimulated NO but no decline in NO at lower Po2. Ontogenic changes in endothelial NO production and in oxygen modulation of that process may be involved in the maturational decrease in vascular resistance and the development of oxygen responsiveness in the pulmonary circulation.

Advances in the treatment of persistent pulmonary hypertension of the newborn

Despite an association with meconium and blood aspiration, pneumonia, sepsis, pneumothorax, prematurity, and congenital diaphragmatic hernia, no cause for persistent pulmonary hypertension of the newborn can be found in many cases. This article discusses the advances in current therapies including the promising new therapy of inhaled nitric oxide.

Oxygen modulation of pulmonary arterial prostacyclin synthesis is developmentally regulated

To better understand the role of prostacyclin [prostaglandin (PG) I2] in oxygen mediation of vasomotor tone in the developing lung, we determined the ontogeny of the direct effects of acute changes in oxygen on in vitro PGI2 synthesis and adenosine 3',5'-cyclic monophosphate (cAMP) production in intrapulmonary arteries from fetal and newborn lambs. In the absence of varying oxygen, PGI2 synthesis increased 6.9-fold from early to late in the third trimester, and it rose an additional 3.2-fold from late gestation to 1 wk of age, and another 2.1-fold from 1 to 4 wk. PGE2 synthesis similarly rose 4.9-fold during the third trimester, but it then fell 69% from late gestation to 1 wk of age and remained unchanged postnatally. Paralleling the developmental increase in PGI2 synthesis, basal cAMP production rose 6.2-fold from the early third trimester to 4 wk of age. In contrast, PGI2-stimulated cAMP production was similar in all age groups. With an acute decline in PO2 in vitro from 680 to 40 mmHg, PGI2 and PGE2 synthesis in fetal arteries fell 33-46 and 39-55%, respectively. In contrast, they were increased by 9-145% and 44-130%, respectively, at lower PO2 in arteries from newborn lambs. Basal cAMP production was altered by decreased oxygen in a similar manner, falling by 35-39% in fetal arteries yet increasing by 21-47% in the newborn. PGI2-stimulated cAMP production, however, was not affected by oxygen at all ages except in the early third trimester. Thus there is a dramatic developmental increase in pulmonary arterial PGI2 synthesis that causes a marked maturational rise in cAMP production.(ABSTRACT TRUNCATED AT 250 WORDS)