Oxygen increases ductus arteriosus smooth muscle cytosolic calcium via release of calcium from inositol triphosphate-sensitive stores

Using omics to breathe new life into our understanding of the ductus arteriosus oxygen response

The ductus arteriosus (DA) connects the aorta to the pulmonary artery (PA), directing placentally oxygenated blood away from the developing lungs. High pulmonary vascular resistance and low systemic vascular resistance facilitate shunting of blood in utero from the pulmonary to the systemic circulation through the widely patent DA, thereby optimizing fetal oxygen (O₂) delivery. With the transition from fetal (hypoxia) to neonatal (normoxia) oxygen conditions, the DA constricts while the PA dilates. This process often fails in prematurity, promoting congenital heart disease. Impaired O₂-responsivness in the DA promotes persistent ductus arteriosus (PDA), the most common form of congenital heart disease. Knowledge of DA oxygen sensing has greatly advanced in the past few decades, however we still lack a complete understanding of the sensing mechanism. The genomic revolution of the past two decades has facilitated unprecedented discovery in every biological system. This review will demonstrate how multiomic integration of data generated from the DA can breathe new life into our understanding of the DA's oxygen response.

The Association between Hemodynamically Significant Patent Ductus Arteriosus and 25-Hydroxyvitamin D Levels in Preterm Infants ≤32 Weeks Gestational Age

INTRODUCTION

We investigated the relationship between 25-hydroxyvitamin D (25-OHD) levels and the development of hemodynamically significant patent ductus arteriosus (hsPDA) in preterm infants.

METHODS

Newborns having a gestational age (GA) of ≤32 weeks with hsPDA consisted the study group (n = 25, 20%), while newborns ≤32 weeks of GA without PDA/hsPDA were the control group (n = 97, 80%).

RESULTS

The study group had lower GA, birth weight (BW) and 25-OHD levels (p < 0.0001, p = 0.002 and p = 0.003, respectively). After adjusting for the effects of GA, BW and the presence of respiratory distress syndrome, multivariable logistic regression analyses demonstrated that preterm infants with low 25-OHD levels were 6.407 (95% CI: 1.656-24.788, p = 0.007) times more likely to experience hsPDA than preterm infants with normal 25-OHD levels. Every 1 ng/mL increase in 25-OHD levels decreased the probability of hsPDA (OR: 0.894, 95% CI: 0.816-0.98, p = 0.016). Conclusion: Low 25-OHD levels may have a role in the development of hsPDA.

The role of interleukin-1 in perinatal inflammation and its impact on transitional circulation

Preterm birth is defined as delivery at <37 weeks of gestational age (GA) and exposes 15 million infants worldwide to serious early life diseases. Lowering the age of viability to 22 weeks GA entailed provision of intensive care to a greater number of extremely premature infants. Moreover, improved survival, especially at extremes of prematurity, comes with a rising incidence of early life diseases with short- and long-term sequelae. The transition from fetal to neonatal circulation is a substantial and complex physiologic adaptation, which normally happens rapidly and in an orderly sequence. Maternal chorioamnionitis or fetal growth restriction (FGR) are two common causes of preterm birth that are associated with impaired circulatory transition. Among many cytokines contributing to the pathogenesis of chorioamnionitis-related perinatal inflammatory diseases, the potent pro-inflammatory interleukin (IL)-1 has been shown to play a central role. The effects of utero-placental insufficiency-related FGR and in-utero hypoxia may also be mediated, in part, via the inflammatory cascade. In preclinical studies, blocking such inflammation, early and effectively, holds great promise for improving the transition of circulation. In this mini-review, we outline the mechanistic pathways leading to abnormalities in transitional circulation in chorioamnionitis and FGR. In addition, we explore the therapeutic potential of targeting IL-1 and its influence on perinatal transition in the context of chorioamnionitis and FGR.

Patent ductus arteriosus and oxidative stress in preterm infants: a narrative review

The role of oxygen, reactive oxygen species (ROS), and isoprostanes (IsoPs) in regulating patency and closure of patent ductus arteriosus (PDA) have been studied in preterm infants. Also the possible correlation between a hemodynamically significant PDA and its pharmacological treatment with oxidative stress has been investigated. The National Library of Medicine (MEDLINE) database was searched without time limits. Available data demonstrate that free radicals are not always harmful and that ROS and IsoPs play a relevant role in DA closure. On the other hand, a hemodynamically significant PDA can cause oxidative stress and this can partially explain its association with other complications of prematurity related to oxidative stress, such as bronchopulmonary dysplasia (BPD), intraventricular haemorrhage (IVH), and necrotizing enterocolitis (NEC). Some drugs used for pharmacological closure, such as ibuprofen, also have antioxidant effects, and the closure of PDA can restore a proper tissue oxygenation and the balance between pro-oxidant and antioxidant factors. These data support the importance of the relationship between PDA and oxidative stress whose understanding increase our awareness when we approach this prematurity complication in the clinical practice. Further studies might assess the reliability of ROS as possible biomarkers of the risk of developing a hsPDA.

Reactive oxygen species as mediators of oxygen signaling during fetal-to-neonatal circulatory transition

Reactive oxygen species (ROS) are frequently seen as pathological agents of oxidative stress. However, ROS are not always deleterious and can also act as cell signaling molecules. Vascular oxygen sensing and signaling during fetal-to-neonatal circulatory transition is a remarkable example of the physiological regulatory actions of ROS. The fetal relative hypoxic environment induces hypoxic pulmonary vasoconstriction (HPV) and ductus arteriosus (DA) relaxation favoring the presence of high pulmonary vascular resistance and right-to-left ductal shunt. At birth, the increase in oxygen tension causes relaxation of pulmonary arteries (PAs) and normoxic DA vasoconstriction (NDAV), thus diverting blood flow to the lungs. Although the response to changes in oxygen tension is diametrically opposite, the mechanisms responsible for HPV and NDAV appear to be the result of a similar interaction between triggering and modulating factors that lead to an increase in cytosolic Ca²⁺ concentration and Ca²⁺ sensitization of the contractile apparatus. Growing evidence points to an increase in ROS (mitochondria- and/or NADPH-derived superoxide and/or H₂O₂), leading to inhibition of voltage-gated K⁺ channels, membrane depolarization, and activation of voltage-gated L-type Ca²⁺ channels as critical events in the signaling pathway of both HPV and NDAV. Several groups of investigators have completed this pathway adding other elements such as neutral sphingomyelinase-derived ceramide, the sarcoplasmic/endoplasmic reticulum (through ryanodine and inositol 1,4,5-trisphosphate receptors), Rho kinase-mediated Ca²⁺ sensitization, or transient receptor potential channels. The present review focus on the role of ROS as mediators of the homeostatic oxygen sensing system during fetal and neonatal life not only in the PAs and DA but also in systemic arteries.

Unique aspects of the developing lung circulation: structural development and regulation of vasomotor tone

This review summarizes our current knowledge on lung vasculogenesis and angiogenesis during normal lung development and the regulation of fetal and postnatal pulmonary vascular tone. In comparison to that of the adult, the pulmonary circulation of the fetus and newborn displays many unique characteristics. Moreover, altered development of pulmonary vasculature plays a more prominent role in compromised pulmonary vasoreactivity than in the adult. Clinically, a better understanding of the developmental changes in pulmonary vasculature and vasomotor tone and the mechanisms that are disrupted in disease states can lead to the development of new therapies for lung diseases characterized by impaired alveolar structure and pulmonary hypertension.

The effects of preterm birth and its antecedents on the cardiovascular system

INTRODUCTION

Preterm birth occurs in approximately 10% of all births worldwide. It prematurely exposes the developing cardiovascular system to the hemodynamic transition that occurs at birth and to the subsequent functional demands of life ex utero. This review describes the current knowledge of the effects of preterm birth, and some of its common antecedents (chorioamnionitis, intra-uterine growth restriction, and maternal antenatal corticosteroid administration), on the structure of the myocardium.

MATERIAL AND METHODS

A thorough literature search was conducted for articles relating to how preterm birth, and its antecedents, affect development of the heart. Given that sheep are an excellent model for the studies of cardiac development, this review has focused on experimental studies in sheep as well as clinical findings.

RESULTS

Our review of the literature demonstrates that individuals born preterm are at an increased risk of cardiovascular disease later in life, including increased mean arterial pressure, abnormally shaped and sub-optimally performing hearts and changes in the vasculature. The review highlights how antenatal corticosteroids, intra-uterine growth restriction, and exposure to chorioamnionitis also have the potential to impact cardiac growth in the preterm newborn.

CONCLUSIONS

Preterm birth and its common antecedents (antenatal corticosteroids, intra-uterine growth restriction, and chorioamnionitis) have the potential to adversely impact cardiac structure immediately following birth and in later life.

Hypoxia-induced cytosolic calcium influx is mediated primarily by the reverse mode of Na+/Ca2+ exchanger in smooth muscle cells of fetal small pulmonary arteries

AIM

Constriction of small pulmonary arteries and high resistance of pulmonary circulation are important for maintaining fetal circulation before birth. In this study, we investigated how cytosolic free calcium concentration ([Ca(2+)]i) in fetal lamb small pulmonary artery smooth muscle cells (SPASMC) was affected by hypoxia and regulated by calcium pumps during this process.

METHODS

(Ca(2+))i in response to acute hypoxia was determined spectrofluorometrically with fluo-3AM in cultured fetal SPASMC. Chemicals or solutions, including ryanodine, 2-aminoethoxydiphenyl borate, Ca(2+)-free solution with 20 mmol ethyleneglycoltetraacetic (EGTA), nimodipine, Na(+)-free medium and KB-R7943, were administrated at the same time point when samples were exposed to acute hypoxia.

RESULTS

(Ca(2+))i in fetal lamb SPASMC increased under acute hypoxia. 2-Aminoethoxydiphenyl borate, an inhibitor of inositol triphosphate calcium store, partially attenuated the (Ca(2+))i increase after 6-min treatment. Ryanodine, an inhibitor of ryanodine-sensitive calcium stores, had no effect on the (Ca(2+))i increase. Ca(2+)-free solution with EGTA completely abolished this increase. Both nimodipine, that blocks the voltage-gated calcium channel, and KB-R7943, that inhibits the reverse mode of Na(+)/Ca(2+) exchanger, greatly diminished the hypoxia-induced (Ca(2+))i increase. The inhibitory effect of KB-R7943 was stronger than nimodipine, evidenced by the fact that (Ca(2+))i dropped near to the baseline level in the presence of KB-R7943 at a later time point. Low extracellular Na(+) concentration enhanced the hypoxia-induced increase of (Ca(2+))i.

CONCLUSION

These results suggest that hypoxia-induced Ca(2+) increase in fetal SPASMC results from cytosolic Ca(2+) influx mediated primarily by the reverse mode of Na(+)/Ca(2+) exchanger.

Transcriptional profiling reveals ductus arteriosus-specific genes that regulate vascular tone

Failure of the ductus arteriosus (DA) to close at birth can lead to serious complications. Conversely, certain profound congenital cardiac malformations require the DA to be patent until corrective surgery can be performed. In each instance, clinicians have a very limited repertoire of therapeutic options at their disposal - indomethacin or ibuprofen to close a patent DA (PDA) and prostaglandin E1 to maintain patency of the DA. Neither treatment is specific to the DA and both may have deleterious off-target effects. Therefore, more therapeutic options specifically targeted to the DA should be considered. We hypothesized the DA possesses a unique genetic signature that would set it apart from other vessels. A microarray was used to compare the genetic profiles of the murine DA and ascending aorta (AO). Over 4,000 genes were differentially expressed between these vessels including a subset of ion channel-related genes. Specifically, the alpha and beta subunits of large-conductance calcium-activated potassium (BKCa) channels are enriched in the DA. Gain- and loss-of-function studies showed inhibition of BKCa channels caused the DA to constrict, while activation caused DA relaxation even in the presence of O2. This study identifies subsets of genes that are enriched in the DA that may be used to develop DA-specific drugs. Ion channels that regulate DA tone, including BKCa channels, are promising targets. Specifically, BKCa channel agonists like NS1619 maintain DA patency even in the presence of O2 and may be clinically useful.

Ceramide mediates acute oxygen sensing in vascular tissues

AIMS

A variety of vessels, such as resistance pulmonary arteries (PA) and fetoplacental arteries and the ductus arteriosus (DA) are specialized in sensing and responding to changes in oxygen tension. Despite opposite stimuli, normoxic DA contraction and hypoxic fetoplacental and PA vasoconstriction share some mechanistic features. Activation of neutral sphingomyelinase (nSMase) and subsequent ceramide production has been involved in hypoxic pulmonary vasoconstriction (HPV). Herein we aimed to study the possible role of nSMase-derived ceramide as a common factor in the acute oxygen-sensing function of specialized vascular tissues.

RESULTS

The nSMase inhibitor GW4869 and an anticeramide antibody reduced the hypoxic vasoconstriction in chicken PA and chorioallantoic arteries (CA) and the normoxic contraction of chicken DA. Incubation with interference RNA targeted to SMPD3 also inhibited HPV. Moreover, ceramide and reactive oxygen species production were increased by hypoxia in PA and by normoxia in DA. Either bacterial sphingomyelinase or ceramide mimicked the contractile responses of hypoxia in PA and CA and those of normoxia in the DA. Furthermore, ceramide inhibited voltage-gated potassium currents present in smooth muscle cells from PA and DA. Finally, the role of nSMase in acute oxygen sensing was also observed in human PA and DA.

INNOVATION

These data provide evidence for the proposal that nSMase-derived ceramide is a critical player in acute oxygen-sensing in specialized vascular tissues.

CONCLUSION

Our results indicate that an increase in ceramide generation is involved in the vasoconstrictor responses induced by two opposite stimuli, such as hypoxia (in PA and CA) and normoxia (in DA).

Mechanisms for ductus arteriosus closure

Closure of the ductus arteriosus at birth is a complex phenomenon being conditioned by antenatal events and progressing in preprogrammed steps. Functional at first, narrowing of the vessel is determined by 2 overlapping processes--removal of the prostaglandin E(2)-based relaxation sustaining prenatal patency and activation of a constrictor mechanism by the natural rise in blood oxygen tension. Two schemes have been proposed for oxygen action--one involving a cytochrome P450 hemoprotein (sensor)/endothelin-1 (effector) complex and the other a set of voltage-gated K(+) channels. These proposals, however, are not mutually exclusive. Structural closure follows the constriction through a remodeling process initiated antenatally with the development of intimal cushions and completed postnatally by a host of humoral and mechanical stimuli. Research in this area has already provided clinical applications. Nevertheless, management of premature infants with persistent ductus remains troublesome and calls for an alternative approach to the prostaglandin E(2) inhibitors now in use. Studies in progress on the oxygen-sensing system may lead to a definitive solution for this problem.

Current Perspectives on Pathobiology of the Ductus Arteriosus

The ductus arteriosus (DA) shunts blood away from the lungs during fetal life, but at birth this shunt is no longer needed and the vessel rapidly constricts. Postnatal persistence of the DA, patent ductus arteriosus (PDA), is predominantly a detrimental condition for preterm infants but is simultaneously a condition required to maintain systemic blood flow for infants born with certain severe congenital heart defects. Although PDA in preterm infants is associated with significant morbidities, there is controversy regarding whether PDA is truly causative. Despite advances in our understanding of the pathobiology of PDA, the optimal treatment strategy for PDA in preterm infants is unclear. Here we review recent studies that have continued to elucidate the fundamental mechanisms of DA development and pathogenesis.

Cytochrome P-450 3A13 and endothelin jointly mediate ductus arteriosus constriction to oxygen in mice

The fetal ductus arteriosus (DA) contracts to oxygen, and this feature, maturing through gestation, is considered important for its closure at birth. We have previously obtained evidence of the involvement of cytochrome P-450, possibly of the 3A subfamily (CYP3A), in oxygen sensing and have also identified endothelin (ET)-1 as the attendant effector for the contraction. Here, we examined comparatively wild-type (WT) and CYP3A-null ( Cyp3a−/−) mice for direct validation of this concept. We found that the CYP3A subfamily is represented only by CYP3A13 in the WT DA. CYP3A13 was also detected in the DA by immunofluorescence microscopy, being primarily colocalized with the endoplasmic reticulum in both endothelial and muscle cells. However, a distinct signal was also evident in the plasma membrane. Isolated DAs from term WT animals developed a sustained contraction to oxygen with transient contractions superimposed. Conversely, no tonic response occurred in Cyp3a−/− DAs, whereas the phasic response persisted unabated. Oxygen did not contract the preterm WT DA but caused a full-fledged contraction after retinoic acid (RA) treatment. RA also promoted an oxygen contraction in the Cyp3a −/− DA. However, responses of RA-treated WT and Cyp3a−/− mice differed in that only the former abated with ET-1 suppression. This implies the existence of an alternative target for RA responsible for the oxygen-induced contraction in the absence of CYP3A13. In vivo, the DA was constricted in WT and Cyp3a−/− newborns, although with a tendency to be less narrowed in the mutant. We conclude that oxygen acts primarily through the complex CYP3A13 (sensor)/ET-1 (effector) and, in an accessory way, directly onto ET-1. However, even in the absence of CYP3A13, the DA may close postnatally thanks to the contribution of ET-1 and the likely involvement of compensating mechanism(s) identifiable with an alternative oxygen-sensing system and/or the withdrawal of relaxing influence(s) operating prenatally.

Rho kinase modulates postnatal adaptation of the pulmonary circulation through separate effects on pulmonary artery endothelial and smooth muscle cells

At birth, pulmonary vasodilation occurs concomitant with the onset of air-breathing life. Whether and how Rho kinase (ROCK) modulates the perinatal pulmonary vascular tone remains incompletely understood. To more fully characterize the separate and interactive effects of ROCK signaling, we hypothesized that ROCK has discrete effects on both pulmonary artery (PA): 1) endothelial cell (PAEC) nitric oxide (NO) production and contractile state; and 2) smooth muscle cell tone independent of endothelial NO synthase (eNOS) activity. To test these hypotheses, NO production and endothelial barrier function were determined in fetal PAEC under baseline hypoxia and following exposure to normoxia with and without treatment with Y-27632, a specific pharmacological inhibitor of ROCK. In acutely instrumented, late-gestation ovine fetuses, eNOS was inhibited by nitro-l-arginine infusion into the left PA (LPA). Subsequently, fetal lambs were mechanically ventilated (MV) with 100% oxygen in the absence (control period) and presence of Y-27632. In PAEC, treatment with Y-27632 had no effect on cytosolic calcium but did increase normoxia-induced NO production. Moreover, acute normoxia increased PAEC barrier function, an effect that was potentiated by Y-27632. In fetal lambs, MV during the control period had no effect on LPA flow. In contrast, MV after Y-27632 increased LPA flow and fetal arterial P(O)₂ (Pa(O₂)) and decreased PA pressure. In conclusion, ROCK activity modulates vascular tone in the perinatal pulmonary circulation via combined effects on PAEC NO production, barrier function, and smooth muscle tone. ROCK inhibition may represent a novel treatment strategy for neonatal pulmonary vascular disease.

Patent ductus arteriosus of the preterm infant

A persistently patent ductus arteriosus (PDA) in preterm infants can have significant clinical consequences, particularly during the recovery period from respiratory distress syndrome. With improvement of ventilation and oxygenation, the pulmonary vascular resistance decreases early and rapidly, especially in very immature infants with extremely low birth weight (<1000 g). Subsequently, the left-to-right shunt through the ductus arteriosus (DA) is augmented, thereby increasing pulmonary blood flow, which leads to pulmonary edema and overall worsening of cardiopulmonary status. Prolonged ventilation, with the potential risks of volutrauma, barotrauma, and hyperoxygenation, is strongly associated with the development and severity of bronchopulmonary dysplasia/chronic lung disease. Substantial left-to-right shunting through the ductus may also increase the risk of intraventricular hemorrhage, necrotizing enterocolitis, and death. Postnatal ductal closure is regulated by exposure to oxygen and vasodilators; the ensuing vascular responses, mediated by potassium channels, voltage-gated calcium channels, mitochondrial-derived reactive oxygen species, and endothelin 1, depend on gestational age. Platelets are recruited to the luminal aspect of the DA during closure and probably promote thrombotic sealing of the constricted DA. Currently, it is unclear whether and when a conservative, pharmacologic, or surgical approach for PDA closure may be advantageous. Furthermore, it is unknown if prophylactic and/or symptomatic PDA therapy will cause substantive improvements in outcome. In this article we review the mechanisms underlying DA closure, risk factors and comorbidities of significant DA shunting, and current clinical evidence and areas of uncertainty in the diagnosis and treatment of PDA of the preterm infant.

Hypoxia-induced cytosolic calcium decrease is mediated primarily by the forward mode of Na(+)/Ca(2+) exchanger in smooth muscle cells of fetal ductus arteriosus

Closure of the ductus arteriosus (DA) after birth, essential for postnatal adaptation, is initiated by the transition from hypoxia to normoxia. The current study investigated how hypoxia affects the level of cytosolic calcium ([Ca(2+)](i)) in fetal lamb DA smooth muscle cells (DASMCs) and the role of calcium pumps in this process. The [Ca(2+)](i) variation in response to acute hypoxia was determined spectrofluorometrically with fura-3-AM in cultured fetal DASMCs. Interventions using chemicals or solutions including thapsigargin, vanadate, KB-R7943, alkaline PH9.0 solution, or Na(+)-free medium were administered when samples were exposed to acute hypoxia. The results show that [Ca(2+)](i) decreased dramatically under acute hypoxia. This decrease was not attenuated completely by an inhibitor of sarcoplasmic/endoplasmic reticulum Ca(2+) adenosine triphosphatase (ATPase) (SERCA), a blocker of plasma membrane Ca(2+) ATPase (PMCA), or an inhibitor and activator of the reserve mode of the Na(+)/Ca(2+) exchanger (NCX). In contrast, KT-R9743, an inhibitor of the forward mode of NCX at a high concentration (30 microm), greatly diminished the hypoxia-induced [Ca(2+)](i) decrease in fetal DASMCs. These results suggest that a hypoxia-induced Ca(2+) decrease in fetal DASMCs results from cytosolic Ca(2+) efflux mediated primarily by the forward mode of NCX.

T-type Ca2+ channels promote oxygenation-induced closure of the rat ductus arteriosus not only by vasoconstriction but also by neointima formation

The ductus arteriosus (DA), an essential vascular shunt for fetal circulation, begins to close immediately after birth. Although Ca(2+) influx through several membrane Ca(2+) channels is known to regulate vasoconstriction of the DA, the role of the T-type voltage-dependent Ca(2+) channel (VDCC) in DA closure remains unclear. Here we found that the expression of alpha1G, a T-type isoform that is known to exhibit a tissue-restricted expression pattern in the rat neonatal DA, was significantly up-regulated in oxygenated rat DA tissues and smooth muscle cells (SMCs). Immunohistological analysis revealed that alpha1G was localized predominantly in the central core of neonatal DA at birth. DA SMC migration was significantly increased by alpha1G overexpression. Moreover, it was decreased by adding alpha1G-specific small interfering RNAs or using R(-)-efonidipine, a highly selective T-type VDCC blocker. Furthermore, an oxygenation-mediated increase in an intracellular Ca(2+) concentration of DA SMCs was significantly decreased by adding alpha1G-specific siRNAs or using R(-)-efonidipine. Although a prostaglandin E receptor EP4 agonist potently promoted intimal thickening of the DA explants, R(-)-efonidipine (10(-6) m) significantly inhibited EP4-promoted intimal thickening by 40% using DA tissues at preterm in organ culture. Moreover, R(-)-efonidipine (10(-6) m) significantly attenuated oxygenation-induced vasoconstriction by approximately 27% using a vascular ring of fetal DA at term. Finally, R(-)-efonidipine significantly delayed the closure of in vivo DA in neonatal rats. These results indicate that T-type VDCC, especially alpha1G, which is predominantly expressed in neonatal DA, plays a unique role in DA closure, implying that T-type VDCC is an alternative therapeutic target to regulate the patency of DA.

Maturation of O2 sensing and signaling in the chicken ductus arteriosus

The increase in O(2) tension after birth is a major factor stimulating ductus arteriosus (DA) constriction and closure. Here we studied the role of the mitochondrial electron transport chain (ETC) as sensor, H(2)O(2) as mediator, and voltage-gated potassium (K(V)) channels and Rho kinase as effectors of O(2)-induced contraction in the chicken DA during fetal development. Switching from 0% to 21% O(2) contracted the pulmonary side of the mature DA (mature pDA) but had no effect in immature pDA and relaxed the aortic side of the mature DA (mature aDA). This contraction of the pDA was attenuated by inhibitors of the mitochondrial ETC and by the H(2)O(2) scavenger polyethylene glycol (PEG)-catalase. Moreover, O(2) increased reactive oxygen species (ROS) production, measured with the fluorescent probes dihydroethidium and 2',7'-dichlorofluorescein, only in mature pDA. The H(2)O(2) analog t-butyl-hydroperoxide mimicked the responses to O(2) in the three vessels. In contrast to immature pDA cells, mature pDA cells exhibited high-amplitude O(2)-sensitive potassium currents. The K(V) channel blocker 4-aminopyridine prevented the current inhibition elicited by O(2). The L-type Ca(2+) (Ca(L)) channel blocker nifedipine and the Rho kinase inhibitors Y-27632 and hydroxyfasudil induced a similar relaxation when mature pDA were stimulated with O(2) or H(2)O(2). Moreover, the sensitivity to these drugs increased with maturation. Our results indicate the presence of a common mechanism for O(2) sensing/signaling in mammalian and nonmammalian DA and favor the idea that, rather than a single mechanism, a parallel maturation of the sensor and effectors is critical for O(2) sensitivity appearance during development.

Maturation of the contractile response of the Emu ductus arteriosus

The avian embryo has a pair of ductus arteriosi that allow the blood to bypass the pulmonary circulation prior to the initiation of lung ventilation. Our objective was to characterize the factors regulating DA tone during the later stages of development in the emu embryo. We examined in vitro the reactivity of the emu ductus from day 39 through 49 of a 50-day incubation. Steady state tension was not altered by the COX inhibitor indomethacin or the nitric oxide synthase inhibitor L-NAME. However, prostaglandin E(2) (PGE(2)) produced a significant relaxation. Norephinephrine and U-46619 produced strong significant contractions in the emu DA and the adrenergic response matured with development. The contractile response to oxygen matured as the embryo developed with significant oxygen-induced contraction on days 45 and 49, but not on day 39 of incubation. The Kv channel inhibitor 4-aminopyridine induced the contraction of the day 48-49 ductus of similar magnitude as the oxygen-induced contraction. The oxygen-induced contraction was reversed by the reducing agent DTT and the electron transport chain inhibitor rotenone. These results suggest that while the emu DA responds to PGE(2), locally produced PGE(2) are not the important regulators of vessel tone. Additionally, relaxation upon addition of the mitochondria electron transport chain inhibitor rotenone suggests that the mitochondria might be acting as vascular oxygen sensors in this system through the production of reactive oxygen species to stimulate the oxygen-induced contraction in a similar fashion to mammals.

The effects of caffeine on the preterm sheep ductus arteriosus

Caffeine and other methyl xanthines are widely used in the neonatal period. A recent, randomized, placebo-controlled, multicenter trial found that infants who were randomly assigned to caffeine treatment had less need for pharmacologic and/or surgical closure of a patent ductus arteriosus (PDA). We hypothesized that the decreased need for pharmacologic and surgical closure of the PDA after caffeine treatment might be due to a direct effect of caffeine on ductus contractility. We examined preterm fetal lamb ductus arteriosus (from 24 fetuses, 105 +/- 4 d of gestation, term = 147 d), in vitro to determine the direct effects of caffeine on the isometric tension of the ductus arteriosus. Caffeine (0.003-0.3 mM) had no direct effect on ductus arteriosus tension, nor did it affect the contractile response of the ductus arteriosus to increasing oxygen concentrations. Caffeine's lack of effect was observed in both the presence and absence of indomethacin and NG-nitro-L-arginine methyl ester (L-NAME) (inhibitors of prostaglandin and nitric oxide production). In conclusion, we found no evidence of a direct effect of therapeutic caffeine concentrations on ductus contractility.

Wnt5a is required for cardiac outflow tract septation in mice

Lack of septation of the cardiac outflow tract (OFT) results in persistent truncus arteriosus (PTA), a form of congenital heart disease. The outflow myocardium expands through addition of cells originating from the pharyngeal mesoderm referred to as secondary/anterior heart field, whereas cardiac neural crest (CNC) cell-derived mesenchyme condenses to form an aortopulmonary septum. We show for the first time that a mutation in Wnt5a in mice leads to PTA. We provide evidence that Wnt5a is expressed in the pharyngeal mesoderm adjacent to CNC cells in both mouse and chicken embryos and in the myocardial cell layer of the conotruncus at the time when CNC cells begin to form the aortopulmonary septum in mice. Although expression domains of secondary heart field markers are not altered in Wnt5a mutant embryos, the expression of CNC cell marker PlexinA2 is significantly reduced. Stimulation of CNC cells with Wnt5a protein elicits Ca2+ transients, suggesting that CNC cells are capable of responding to Wnt5a. We propose a novel model in which Wnt5a produced in the OFT by cells originating from the pharyngeal mesoderm signals to adjacent CNC cells during formation of the aortopulmonary septum through a noncanonical pathway via localized intracellular increases in Ca2+.

Oxygen activates the Rho/Rho-kinase pathway and induces RhoB and ROCK-1 expression in human and rabbit ductus arteriosus by increasing mitochondria-derived reactive oxygen species: a newly recognized mechanism for sustaining ductal constriction

BACKGROUND

Constriction of the ductus arteriosus (DA) is initiated at birth by inhibition of O2-sensitive K+ channels in DA smooth muscle cells. Subsequent membrane depolarization and calcium influx through L-type calcium channels initiates functional closure. We hypothesize that Rho-kinase activation is an additional mechanism that sustains DA constriction.

METHODS AND RESULTS

The effect of increased PO2 on the activity and expression of Rho-kinase was assessed in DAs from neonates with hypoplastic left-heart syndrome (n=15) and rabbits (339 term and 99 preterm rabbits). Rho-kinase inhibitors (Y-27632 and fasudil) prevent and reverse O2 constriction. Heterogeneity exists in the sensitivity of constrictors (PO2=endothelin=phenylephrine>KCl) and of fetal vessels (DA=pulmonary artery>aorta) to Rho-kinase inhibition. Inhibition of L-type calcium channels (nifedipine) or removal of extracellular calcium inhibits approximately two thirds of O2 constriction. Residual DA constriction reflects calcium sensitization, which persists after removal of extracellular calcium and blocking of sarcoplasmic reticulum Ca2+-ATPase. In term DA, an increase in PO2 activates Rho-kinase and thereby increases RhoB and ROCK-1 expression. Activation of Rho-kinase in DA smooth muscle cells is initiated by a PO2-dependent, rotenone-sensitive increase in mitochondrion-derived reactive O2 species. O2 effects on Rho-kinase are mimicked by exogenous H2O2. In preterm DAs, immaturity of mitochondrial reactive oxygen species generation is associated with reduced and delayed O2 constriction and lack of PO2-dependent upregulation of Rho-kinase expression.

CONCLUSIONS

O2 activates Rho-kinase and increases Rho-kinase expression in term DA smooth muscle cells by a redox-regulated, positive-feedback mechanism that promotes sustained vasoconstriction. Conversely, Rho-kinase inhibitors may be useful in maintaining DA patency, as a bridge to congenital heart surgery.

Persistent ductus arteriosus in the Brown-Norway inbred rat strain

Persistent ductus arteriosus (PDA) is a common cardiovascular anomaly in children caused by the pathologic persistence of the left sixth pharyngeal arch artery. The inbred Brown-Norway (BN) rat presents with increased vascular fragility due to an aortic elastin deficit resulting from decreased elastin synthesis. The strikingly high prevalence of PDA in BN rats in a pilot study led us to investigate this vascular anomaly in 12 adolescent BN rats. In all BN rats, a PDA was observed macroscopically, whereas a ligamentum arteriosum was found in adult controls. The macroscopic appearance of the PDA was tubular (n = 2), stenotic (n = 8), or diverticular (n = 2). The PDA had the structure of a muscular artery with intimal thickening. In the normal closing ductus of the neonatal controls, the media consisted of layers of smooth muscle cells (SMCs) intermingled with layers of elastin. The intima was thin and poor in elastin. By contrast, the media of PDA in BN rats elastin lamellae were absent and the intima contained many elastic fibers. The abnormal distribution of elastin in the PDA of BN rats suggests that impaired elastin metabolism is related to the persistence of the ductus and implicates a genetically determined factor that may link the PDA with aortic fragility.

Role of store-operated calcium channels and calcium sensitization in normoxic contraction of the ductus arteriosus

BACKGROUND

At birth, the increase in oxygen causes contraction of the ductus arteriosus, thus diverting blood flow to the lungs. Although this contraction is modulated by substances such as endothelin and dilator prostaglandins, normoxic contraction is an intrinsic property of ductus smooth muscle. Normoxic inhibition of potassium channels causes membrane depolarization and calcium entry through L-type calcium channels. However, the studies reported here show that after inhibition of this pathway there is still substantial normoxic contraction, indicating the involvement of additional mechanisms.

METHODS AND RESULTS

Using ductus ring experiments, calcium imaging, reverse-transcription polymerase chain reaction, Western blot, and cellular electrophysiology, we find that this depolarization-independent contraction is caused by release of calcium from the IP3-sensitive store in the sarcoplasmic reticulum, by subsequent calcium entry through store-operated channels, and by increased calcium sensitization of actin-myosin filaments, involving Rho-kinase.

CONCLUSIONS

Much of the normoxic contraction of the ductus arteriosus at birth is related to calcium entry through store-operated channels, encoded by the transient receptor potential superfamily of genes, and to increased calcium sensitization. A clearer understanding of the mechanisms involved in normoxic contraction of the ductus will permit the development of better therapy to close the patent ductus arteriosus, which constitutes approximately 10% of all congenital heart disease and is especially common in premature infants.

Gene expression in ductus arteriosus and aorta: comparison of birth and oxygen effects

Ductus arteriosus (DA) closure is initiated by oxygen rise postnatally and progresses in two, functional-to-permanent, stages. Here, using GeneChip Arrays in rats (normoxic and hyperoxic fetus, normoxic newborn), we examined whether oxygen alone duplicates the birth process in affecting DA genes. In addition, by comparing DA with aorta (Ao), we identified features in postnatal gene profile marking transitional adjustments in a closing (DA) vs. a persistent (Ao) vessel. We found changes in neonatal DA denoting enhanced formation and action of the constrictor endothelin-1 (ET-1). Likewise, ANG II type 1 receptor was upregulated, and the compound was a constrictor. Conversely, relaxant PGE2 became less effective. Among agents for functional closure, only ET-1 was affected similarly by oxygen and birth. Coincidentally, neonatal DA showed enhanced contractile drive with upregulation of Rho-Rho kinase and calcium signaling along with downregulation of contractile proteins. The latter effect was shared by oxygen. Changes denoting active remodeling were also seen in neonatal but not hyperoxic fetal DA. Ao, unlike DA, exhibited postnatal variations in noradrenergic, purinergic, and PGI2 systems with opposing effects on vasomotion. Contraction and remodeling processes were also less affected by birth, whereas lipid and glucose metabolism were upregulated. We conclude that several agents, including ANG II as novel effector, promote functional closure of DA, but only ET-1 is causally coupled with oxygen. Oxygen has no role in processes for permanent closure. Functional closure is associated with downregulation of contractile apparatus, and this may render neonatal DA less amenable to tone manipulation. Conceivably, activation of metabolism in neonatal Ao is a distinguishing feature for transitional adaptations in the permanent vasculature.