Interactions between NO, CO and an endothelium-derived hyperpolarizing factor (EDHF) in maintaining patency of the ductus arteriosus in the mouse

Oxygen sensing in the ductus arteriosus-A unifying vision for two concepts

Closure of the ductus arteriosus at birth is known to be related to the physiological rise in blood oxygen tension. Two main schemes have been proposed to explain the peculiar contraction of the ductus to oxygen, and their function is based on distinct sensor-effector complexes. Specifically, these schemes comprise a cytochrome P450 monooxygenase-endothelin complex or, alternatively, a mitochondrial redox mechanism-voltage gated potassium channel complex. However, it is not clear how these systems may relate to the closure process. Here, it is proposed that they operate jointly within a single functional entity, with their combined activation being required for a full-fledged contraction of the vessel to oxygen. This arrangement is thought to ensure a smooth transition from the powerful relaxant mechanism that is evident through the early postnatal period. Validation of this concept would not only settle a central question in the operation of the ductus but may also afford a broader perspective in any future translational research.

Role of Kinins in Hypertension and Heart Failure

The kallikrein-kinin system (KKS) is proposed to act as a counter regulatory system against the vasopressor hormonal systems such as the renin-angiotensin system (RAS), aldosterone, and catecholamines. Evidence exists that supports the idea that the KKS is not only critical to blood pressure but may also oppose target organ damage. Kinins are generated from kininogens by tissue and plasma kallikreins. The putative role of kinins in the pathogenesis of hypertension is discussed based on human mutation cases on the KKS or rats with spontaneous mutation in the kininogen gene sequence and mouse models in which the gene expressing only one of the components of the KKS has been deleted or over-expressed. Some of the effects of kinins are mediated via activation of the B2 and/or B1 receptor and downstream signaling such as eicosanoids, nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF) and/or tissue plasminogen activator (T-PA). The role of kinins in blood pressure regulation at normal or under hypertension conditions remains debatable due to contradictory reports from various laboratories. Nevertheless, published reports are consistent on the protective and mediating roles of kinins against ischemia and cardiac preconditioning; reports also demonstrate the roles of kinins in the cardiovascular protective effects of the angiotensin-converting enzyme (ACE) and angiotensin type 1 receptor blockers (ARBs).

Molecular and Mechanical Mechanisms Regulating Ductus Arteriosus Closure in Preterm Infants

Failure of ductus arteriosus closure after preterm birth is associated with significant morbidities. Ductal closure requires and is regulated by a complex interplay of molecular and mechanical mechanisms with underlying genetic factors. In utero patency of the ductus is maintained by low oxygen tension, high levels of prostaglandins, nitric oxide and carbon monoxide. After birth, ductal closure occurs first by functional closure, followed by anatomical remodeling. High oxygen tension and decreased prostaglandin levels mediated by numerous factors including potassium channels, endothelin-1, isoprostanes lead to the contraction of the ductus. Bradykinin and corticosteroids also induce ductal constriction by attenuating the sensitivity of the ductus to PGE2. Smooth muscle cells of the ductus can sense oxygen through a mitochondrial network by the role of Rho-kinase pathway which ends up with increased intracellular calcium levels and contraction of myosin light chains. Anatomical closure of the ductus is also complex with various mechanisms such as migration and proliferation of smooth muscle cells, extracellular matrix production, endothelial cell proliferation which mediate cushion formation with the interaction of blood cells. Regulation of vessel walls is affected by retinoic acid, TGF-β1, notch signaling, hyaluronan, fibronectin, chondroitin sulfate, elastin, and vascular endothelial cell growth factor (VEGF). Formation of the platelet plug facilitates luminal remodeling by the obstruction of the constricted ductal lumen. Vasa vasorum are more pronounced in the term ductus but are less active in the preterm ductus. More than 100 genes are effective in the prostaglandin pathway or in vascular smooth muscle development and structure may affect the patency of ductus. Hemodynamic changes after birth including fluid load and flow characteristics as well as shear forces within the ductus also stimulate closure. Current pharmacological treatment for the closure of a patent ductus is based on the blockage of the prostaglandin pathway mainly through COX or POX inhibition, albeit with some limitations and side effects. Further research for new agents aiming ductal closure should focus on a clear understanding of vascular biology of the ductus.

Understanding the pathobiology in patent ductus arteriosus in prematurity-beyond prostaglandins and oxygen

The ductus arteriosus (DA) is probably the most intriguing vessel in postnatal hemodynamic transition. DA patency in utero is an active state, in which prostaglandin E₂ (PGE₂) and nitric monoxide (NO), play an important role. Since the DA gets programmed for postnatal closure as gestation advances, in preterm infants the DA frequently remains patent (PDA). PGE₂ exposure programs functional postnatal closure by inducing gene expression of ion channels and phosphodiesterases and anatomical closure by inducing intimal thickening. Postnatally, oxygen inhibits potassium and activates calcium channels, which ultimately leads to a rise in intracellular calcium concentration consequently inducing phosphorylation of the myosin light chain and thereby vasoconstriction of the DA. Since ion channel expression is lower in preterm infants, oxygen induced functional vasoconstriction is attenuated in comparison with full term newborns. Furthermore, the preterm DA is more sensitive to both PGE₂ and NO compared to the term DA pushing the balance toward less constriction. In this review we explain the physiology of DA patency in utero and subsequent postnatal functional closure. We will focus on the pathobiology of PDA in preterm infants and the (un)intended effect of antenatal exposure to medication on both fetal and neonatal DA vascular tone.

Molecular and mechanical factors contributing to ductus arteriosus patency and closure

Regulation of the ductus arteriosus, an essential fetal vessel connecting the pulmonary artery and aorta, is complex. Failure of this vessel to close after birth may result in a persistent left-to-right shunt through the patent ductus arteriosus, a condition associated with significant morbidities. Numerous factors contribute to the shift from fetal ductus patency to postnatal closure, requiring precise coordination of molecular cues with biomechanical forces and underlying genetic influences. Despite significant advances, questions remain regarding signaling dynamics and the natural time course of ductus closure, particularly in preterm neonates. This review highlights the contributions of early investigators and more recent clinician scientists to our understanding of the molecular and mechanical factors that mediate ductus patency and closure.

Inducible nitric oxide synthase: Good or bad?

Nitric oxide synthases (NOS) are a family of isoforms responsible for the synthesis of the potent dilator nitric oxide (NO). Expression of inducible NOS (iNOS) occurs in conditions of inflammation, and produces large amounts of NO. In pathological conditions iNOS is regarded as a harmful enzyme and is proposed to be a major contributor to diseases of the cardiovascular system such as atherosclerosis. In this review, we address the notion that iNOS is a detrimental enzyme in disease and discuss its potentially beneficial roles. Additionally, we describe other molecules associated with iNOS in diseases such as atherosclerosis, and current research on therapeutic inhibitors tested to reduced pathology associated with cardiovascular diseases (CVD).

Functional differences between the arteries perfusing gas exchange and nutritional membranes in the late chicken embryo

The chicken extraembryonic arterial system comprises the allantoic arteries, which irrigate the gas exchange organ (the chorioallantoic membrane, CAM) and the yolk sac (YS) artery, which irrigates the nutritional organ (the YS membrane). We compared, using wire myography, the reactivity of allantoic and YS arteries from 19-day chicken embryos (total incubation 21 days). The contractions induced by KCl, the adrenergic agonists norepinephrine (NE, nonselective), phenylephrine (α1), and oxymetazoline (α2), electric field stimulation (EFS), serotonin, U46619 (TP receptor agonist), and endothelin (ET)-1 and the relaxations induced by acetylcholine (ACh), sodium nitroprusside (SNP, NO donor), forskolin (adenylate cyclase activator), and isoproterenol (β-adrenergic agonist) were investigated. Extraembryonic allantoic arteries did not show α-adrenergic-mediated contraction (either elicited by exogenous agonists or EFS) or ACh-induced (endothelium-dependent) relaxation, whereas these responses were present in YS arteries. Interestingly, the intraembryonic segment of the allantoic artery showed EFS- and α-adrenergic-induced contraction and ACh-mediated relaxation. Moreover, glyoxylic acid staining showed the presence of catecholamine-containing nerves in the YS and the intraembryonic allantoic artery, but not in the extraembryonic allantoic artery. Isoproterenol- and forskolin-induced relaxation and ET-1-induced contraction were higher in YS than in allantoic arteries, whereas serotonin- and U46619-induced contraction and SNP-induced relaxation did not significantly differ between the two arteries. In conclusion, our study demonstrates a different pattern of reactivity in the arteries perfusing the gas exchange and the nutritional membranes of the chicken embryo.

Aminoglycoside-mediated relaxation of the ductus arteriosus in sepsis-associated PDA

Sepsis is strongly associated with patency of the ductus arteriosus (PDA) in critically ill newborns. Inflammation and the aminoglycoside antibiotics used to treat neonatal sepsis cause smooth muscle relaxation, but their contribution to PDA is unknown. We examined whether: 1) lipopolysaccharide (LPS) or inflammatory cytokines cause relaxation of the ex vivo mouse DA; 2) the aminoglycosides gentamicin, tobramycin, or amikacin causes DA relaxation; and 3) newborn infants treated with aminoglycosides have an increased risk of symptomatic PDA (sPDA). Changes in fetal mouse DA tone were measured by pressure myography in response to LPS, TNF-α, IFN-γ, macrophage-inflammatory protein 2, IL-15, IL-13, CXC chemokine ligand 12, or three aminoglycosides. A clinical database of inborn patients of all gestations was analyzed for association between sPDA and aminoglycoside treatment. Contrary to expectation, neither LPS nor any of the inflammatory mediators caused DA relaxation. However, each of the aminoglycosides caused concentration-dependent vasodilation in term and preterm mouse DAs. Pretreatment with indomethacin and N-(G)-nitro-L-arginine methyl ester did not prevent gentamicin-induced DA relaxation. Gentamicin-exposed DAs developed less oxygen-induced constriction than unexposed DAs. Among 488,349 infants who met the study criteria, 40,472 (8.3%) had sPDA. Confounder-adjusted odds of sPDA were higher in gentamicin-exposed infants, <25 wk and >32 wk. Together, these findings suggest that factors other than inflammation contribute to PDA. Aminoglycoside-induced vasorelaxation and inhibition of oxygen-induced DA constriction support the paradox that antibiotic treatment of sepsis may contribute to DA relaxation. This association was also found in newborn infants, suggesting that antibiotic selection may be an important consideration in efforts to reduce sepsis-associated PDA.

Functional closure of the ductus arteriosus at birth: evidence against an intermediary role of angiotensin II

The fetal ductus arteriosus (DA) closes postnatally first functionally and then structurally. Normal rise in blood oxygenation is regarded as a prime trigger, but closure may occur more slowly without this stimulus. Here, our aim was to assess the role of angiotensin II (Ang II) in functional closure of DA since its action may not be conditioned by oxygen. Experiments were performed with wild-type fetal and neonatal mice, using whole-body freezing technique to assess DA caliber in vivo. Transcripts for Ang II type 1 (AT1R) and type 2 (AT2R) receptors were also examined. We found that the AT1R antagonist olmesartan had no effect in the fetus, but delayed ductus closure in the neonate. However, this response was short-lived and disappeared upon concomitant treatment with the AT2R antagonist PD123319. Coincidentally, olmesartan promoted the Agtr2 transcript. We conclude that AT1R-based Ang II has no role in the functional closure of DA. Conversely, the compound may modulate this process through AT2R-mediated vasodilatation.

Early end-tidal carbon monoxide levels, patency of the ductus arteriosus and regional cerebral oxygenation in preterm infants

BACKGROUND

Carbon monoxide (CO), a relaxant regulator of muscle tone and marker of oxidative stress and inflammation, can be measured in exhaled air by determination of end-tidal CO corrected for CO in ambient air (ETCOc).

OBJECTIVE

Increased endogenous production of CO may influence patency of the ductus arteriosus, cerebral perfusion and, subsequently, cerebral oxygenation. The aim was to study the relation between early ETCOc levels, hemodynamically significant patent ductus arteriosus (hsPDA) and cerebral oxygenation (rScO2) in preterm infants <32 weeks' gestational age and determine predictive values of ETCOc for hsPDA.

METHODS

ETCOc was measured in 91 infants within the first 24 h after birth. A hsPDA was diagnosed according to echocardiographic indices. In 78/91 infants, rScO2 was monitored with near-infrared spectroscopy to assess cerebral oxygenation.

RESULTS

ETCOc values were significantly higher in infants who subsequently developed hsPDA (2.3 ± 0.7 ppm) vs. no-hsPDA (1.7 ± 0.6 ppm), p < 0.001. With a cut-off value of 2.5 ppm, positive and negative predictive values of ETCOc for hsPDA were 55 and 88%, respectively. rScO2 values were not different between the two groups (64 ± 1 vs. 65 ± 3%, NS).

CONCLUSIONS

The higher ETCOc values in hsPDA infants early after birth reflect the early relaxant state of ductal muscular tone. ETCOc <2.5 ppm within 24 h after birth may predict the subsequent absence of hsPDA. ETCOc showed no correlation with cerebral oxygenation in both groups.

Transcription profiles of endothelial cells in the rat ductus arteriosus during a perinatal period

Endothelial cells (ECs) lining the blood vessels serve a variety of functions and play a central role in the homeostasis of the circulatory system. Since the ductus arteriosus (DA) has different arterial characteristics from its connecting vessels, we hypothesized that ECs of the DA exhibited a unique gene profile involved in the regulation of DA-specific morphology and function. Using a fluorescence-activated cell sorter, we isolated ECs from pooled tissues from the DA or the descending aorta of Wistar rat fetuses at full-term of gestation (F group) or neonates 30 minutes after birth (N group). Using anti-CD31 and anti-CD45 antibodies as cell surface markers for ECs and hematopoietic derived cells, respectively, cDNAs from the CD31-positive and CD45-negative cells were hybridized to the Affymetrix GeneChip® Rat Gene 1.0 ST Array. Among 26,469 gene-level probe sets, 82 genes in the F group and 81 genes in the N group were expressed at higher levels in DA ECs than in aortic ECs (p<0.05, fold change>2.0). In addition to well-known endothelium-enriched genes such as Tgfb2 and Vegfa, novel DA endothelium-dominant genes including Slc38a1, Capn6, and Lrat were discovered. Enrichment analysis using GeneGo MetaCore software showed that DA endothelium-related biological processes were involved in morphogenesis and development. We identified many overlapping genes in each process including neural crest-related genes (Hoxa1, Hoxa4, and Hand2, etc) and the second heart field-related genes (Tbx1, Isl1, and Fgf10, etc). Moreover, we found that regulation of epithelial-to-mesenchymal transition, cell adhesion, and retinol metabolism are the active pathways involved in the network via potential interactions with many of the identified genes to form DA-specific endothelia. In conclusion, the present study uncovered several significant differences of the transcriptional profile between the DA and aortic ECs. Newly identified DA endothelium-dominant genes may play an important role in DA-specific functional and morphologic characteristics.

Hydrogen sulfide in the mouse ductus arteriosus: a naturally occurring relaxant with potential EDHF function

We have previously reported that bradykinin relaxes the fetal ductus arteriosus via endothelium-derived hyperpolarizing factor (EDHF) when other naturally occurring relaxants (prostaglandin E2, nitric oxide, and carbon monoxide) are suppressed, but the identity of the agent could not be ascertained. Here, we have examined in the mouse whether hydrogen sulfide (H2S) is a relaxant of the ductus and, if so, whether it may also function as an EDHF. We found in the vessel transcripts for the H2S synthetic enzymes, cystathionine-γ-lyase (CSE) and cystathionine-β-synthase (CBS), and the presence of these enzymes was confirmed by immunofluorescence microscopy. CSE and CBS were distributed across the vessel wall with the former prevailing in the intimal layer. Both enzymes occurred within the endoplasmic reticulum of endothelial and muscle cells, whereas only CSE was located also in the plasma membrane. The isolated ductus contracted to inhibitors of CSE (d,l-propargylglycine, PPG) and CBS (amino-oxyacetic acid), and PPG contraction was attenuated by removal of the endothelium. EDHF-mediated bradykinin relaxation was curtailed by both PPG and amino-oxyacetic acid, whereas the relaxation to sodium nitroprusside was not affected by either treatment. The H2S donor sodium hydrogen sulfide (NaHS) was also a potent, concentration-dependent relaxant. We conclude that the ductus is endowed with a H2S system exerting a tonic relaxation. In addition, H2S, possibly via an overriding CSE source, qualifies as an EDHF. These findings introduce a novel vasoregulatory mechanism into the ductus, with implications for antenatal patency of the vessel and its transitional adjustments at birth.

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.

Vasoactivity of the gasotransmitters hydrogen sulfide and carbon monoxide in the chicken ductus arteriosus

Besides nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H(2)S) is a third gaseous messenger that may play a role in controlling vascular tone and has been proposed to serve as an O(2) sensor. However, whether H(2)S is vasoactive in the ductus arteriosus (DA) has not yet been studied. We investigated, using wire myography, the mechanical responses induced by Na(2)S (1 μM-1 mM), which forms H(2)S and HS(-) in solution, and by authentic CO (0.1 μM-0.1 mM) in DA rings from 19-day chicken embryos. Na(2)S elicited a 100% relaxation (pD(2) 4.02) of 21% O(2)-contracted and a 50.3% relaxation of 62.5 mM KCl-contracted DA rings. Na(2)S-induced relaxation was not affected by presence of the NO synthase inhibitor l-NAME, the soluble guanylate cyclase (sGC) inhibitor ODQ, or the K(+) channel inhibitors tetraethylammonium (TEA; nonselective), 4-aminopyridine (4-AP, K(V)), glibenclamide (K(ATP)), iberiotoxin (BK(Ca)), TRAM-34 (IK(Ca)), and apamin (SK(Ca)). CO also relaxed O(2)-contracted (60.8% relaxation) and KCl-contracted (18.6% relaxation) DA rings. CO-induced relaxation was impaired by ODQ, TEA, and 4-AP (but not by L-NAME, glibenclamide, iberiotoxin, TRAM-34 or apamin), suggesting the involvement of sGC and K(V) channel stimulation. The presence of inhibitors of H(2)S or CO synthesis as well as the H(2)S precursor L-cysteine or the CO precursor hemin did not significantly affect the response of the DA to changes in O(2) tension. Endothelium-dependent and -independent relaxations were also unaffected. In conclusion, our results indicate that the gasotransmitters H(2)S and CO are vasoactive in the chicken DA but they do not suggest an important role for endogenous H(2)S or CO in the control of chicken ductal reactivity.

Developmental changes in mesenteric artery reactivity in embryonic and newly hatched chicks

At birth, the intestine becomes the sole site for nutrient absorption requiring a dramatic increase in blood flow. The vascular changes accompanying this transition have been partly characterized in mammals. We investigated, using wire myography, the developmental changes in chick mesenteric artery (MA) reactivity. Rings of the MA from 15-day (E15) and 19-day (E19) chicken embryos (total incubation 21 days) as well as non-fed 0–3-h-old (NH3h) and first-fed 1-day-old (NH1d) newly hatched chicks contracted in response to KCl, norepinephrine (NE), U46619, and endothelin (ET)-1 and relaxed in response to acetylcholine (ACh), sodium nitroprusside (SNP), and forskolin indicating the presence of electro- and pharmaco-mechanical coupling as well as cGMP- and cAMP-mediated relaxation. In ovo development and transition to ex ovo life was accompanied by alterations in the response of the MAs, but a different developmental trajectory was observed for each reactivity pathway tested. Thus, the contractile efficacy of KCl underwent a linear increase (E15 < E19 < NH3h < NH1d). The efficacy of NE and U46619 increased in ovo, but not ex ovo (E15 < E19 = NH3h = NH1d) and the efficacy of ET-1 peaked at E19 (E15 < E19 > NH3h = NH1d). The relaxations elicited by ACh (endothelium-dependent), SNP, and forskolin did not undergo significant developmental changes. In conclusion, the ability of chick MAs to constrict in response to pharmacological stimuli increases during the embryonic period, but no dramatic changes are induced by hatching or the first feeding. Maturation of vasodilator mechanisms precedes that of vasoconstrictor mechanisms. Alterations of the delicate balance between vasoconstrictors and vasodilators may play an important role in perinatal intestinal diseases.

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.

EDHF function in the ductus arteriosus: evidence against involvement of epoxyeicosatrienoic acids and 12S-hydroxyeicosatetraenoic acid

We have previously shown (Ref. 2) that endothelium-derived hyperpolarizing factor (EDHF) becomes functional in the fetal ductus arteriosus on removal of nitric oxide and carbon monoxide. From this, it was proposed that EDHF originates from a cytochrome P-450 (CYP450)-catalyzed reaction being inhibited by the two agents. Here, we have examined in the mouse ductus whether EDHF can be identified as an arachidonic acid product of a CYP450 epoxygenase and allied pathways. We did not detect transcripts of the mouse CYP2C subfamily in vessel, while CYP2J subfamily transcripts were expressed with CYP2J6 and CYP2J9. These CYP2J hemoproteins were also detected in the ductus by immunofluorescence microscopy, being colocalized with the endoplasmic reticulum in both endothelial and muscle cells. Distinct CYP450 transcripts were also detected and were responsible for omega-hydroxylation (CYP4A31) and 12R-hydroxylation (CYP4B1). Mass spectrometric analysis showed formation of epoxyeicosatrienoic acids (EETs) in the intact ductus, with 11,12- and 14,15-EETs being more prominent than 5,6- and 8,9-EETs. However, their yield did not increase with nitric oxide/carbon monoxide suppression, nor did it abate with endothelium removal. No evidence was obtained for formation of 12R-hydroxyeicosatrienoic acid and omega-hydroxylation products. 2S-hydroxyeicosatetraenoic acid was instead detected, and, contrary to data implicating this compound as an alternative EDHF, its suppression with baicalein did not modify the EDHF-mediated relaxation to bradykinin. We conclude that none of the more common CYP450-linked arachidonic acid metabolites appears to qualify as EDHF in mouse ductus. We speculate that some novel eicosanoid or a totally unrelated compound requiring CYP450 for its synthesis accounts for EDHF in this vessel.

EDHF: an update

The endothelium controls vascular tone not only by releasing NO and prostacyclin, but also by other pathways causing hyperpolarization of the underlying smooth muscle cells. This characteristic was at the origin of the term 'endothelium-derived hyperpolarizing factor' (EDHF). However, this acronym includes different mechanisms. Arachidonic acid metabolites derived from the cyclo-oxygenases, lipoxygenases and cytochrome P450 pathways, H(2)O(2), CO, H(2)S and various peptides can be released by endothelial cells. These factors activate different families of K(+) channels and hyperpolarization of the vascular smooth muscle cells contribute to the mechanisms leading to their relaxation. Additionally, another pathway associated with the hyperpolarization of both endothelial and vascular smooth muscle cells contributes also to endothelium-dependent relaxations (EDHF-mediated responses). These responses involve an increase in the intracellular Ca(2+) concentration of the endothelial cells, followed by the opening of SK(Ca) and IK(Ca) channels (small and intermediate conductance Ca(2+)-activated K(+) channels respectively). These channels have a distinct subcellular distribution: SK(Ca) are widely distributed over the plasma membrane, whereas IK(Ca) are preferentially expressed in the endothelial projections toward the smooth muscle cells. Following SK(Ca) activation, smooth muscle hyperpolarization is preferentially evoked by electrical coupling through myoendothelial gap junctions, whereas, following IK(Ca) activation, K(+) efflux can activate smooth muscle Kir2.1 and/or Na(+)/K(+)-ATPase. EDHF-mediated responses are altered by aging and various pathologies. Therapeutic interventions can restore these responses, suggesting that the improvement in the EDHF pathway contributes to their beneficial effect. A better characterization of EDHF-mediated responses should allow the determination of whether or not new drugable targets can be identified for the treatment of cardiovascular diseases.

Carbon monoxide is a significant mediator of cardiovascular status following preterm birth

OBJECTIVE

With male gender as a strong predictor of cardiovascular instability, we hypothesized that gender-specific differences in circulating carbon monoxide levels contributed to dysregulated microvascular function in preterm male infants.

METHODS

Infants born at 24 to 34 weeks of gestation (N = 84) were studied in a regional tertiary neonatal unit. Carboxyhemoglobin levels were measured through spectrophotometry in umbilical arterial blood and at 24, 72, and 120 hours after birth. Microvascular blood flow was determined through laser Doppler flowmetry.

RESULTS

Carboxyhemoglobin levels demonstrated a strong inverse relationship with gestational age (r = -0.636; P < .001) and were higher in boys (P = .032). Repeated-measures analysis of variance showed a significant decrease in arterial carboxyhemoglobin levels over time (P < .001), with significant between-subjects effects for gestational age (P = .011) and gender (P = .025). Positive correlations with microvascular blood flow at 24 hours of age (r = 0.495; P < .001) and 120 hours of age (r = 0.548; P < .001) were observed. With controlling for gestational age, carboxyhemoglobin levels at 72 hours were greater for infants who died in the first week of life (P = .035).

CONCLUSIONS

The gestational age- and gender-specific differences in carboxyhemoglobin levels and the relationship with dysregulated microvascular blood flow, a state related to greater illness severity and hypotension, are novel findings not confined solely to sick preterm infants. Both inducible heme oxygenase-dependent and non-heme oxygenase-dependent pathways may initially play a central role in carbon monoxide production, inducing pathophysiologic processes in a gender-specific manner.

Regulation of the fetal mouse ductus arteriosus is dependent on interaction of nitric oxide and COX enzymes in the ductal wall

Nitric oxide (NO) and cyclooxygenase (COX)-derived prostaglandins are critical regulators of the fetal ductus arteriosus. To examine the interaction of these pathways within the ductus wall, the ductus arteriosus of term and preterm fetal mice was evaluated by pressurized myography. The isolated preterm ductus was more sensitive to NOS inhibition than at term. Sequential NOS and COX inhibition caused 36% constriction of the preterm ductus regardless of drug order. In contrast, constriction of the term ductus was dependent on the sequence of inhibition; NOS inhibition prior to COX inhibition produced greater constriction than when inhibitors were given in reverse order (36+/-6% versus 23+/-5%). Selective COX-1 or COX-2 inhibition prior to N(G)-nitro-l-arginine methyl ester (l-NAME) induced the expected degree of constriction. However, NOS inhibition followed by selective COX-2 inhibition caused unexpected ductal dilation. These findings are consistent with NO-induced activation of COX in the ductus arteriosus wall and the production of a COX-2-derived constrictor prostanoid that contributes to the balance of vasoactive forces that maintain fetal ductus arteriosus tone.

Role of microsomal prostaglandin E synthase-1 (mPGES1)-derived PGE2 in patency of the ductus arteriosus in the mouse

Prostaglandin E2 (PGE2) plays a key role in the ductus arteriosus, prenatally by maintaining patency and postnatally by promoting tissue remodeling for closure. Here, by using near-term mouse fetuses with (wild-type, WT) and without microsomal PGE synthase-1 (mPGES1-/-), we have examined the importance of this enzyme for PGE2 formation and function. mPGES1-/- ductus, unlike WT ductus, contracted little, or not all, to indomethacin in vitro. Coincidentally, as evident from responses to NG-nitro-L-arginine methyl ester and zinc photoporphyrin, the mutant showed no significant enhancement of nitric oxide (NO)- and carbon monoxide (CO)-based relaxation. mPGES1 suppression differs, therefore, from cyclooxygenase (COX) suppression, whether genetically or pharmacologically induced, where NO is markedly up-regulated. In vivo, the ductus was patent, albeit occasionally with a narrowed lumen, in all mPGES1-/- fetuses. Conversely, postnatal closure progressed regularly in mPGES1-/- animals thanks to residual PGE2 originating via mPGES2. We conclude that mPGES1 is critical for PGE2 formation in the ductus but its loss does not entail compensatory up-regulation of other relaxing mechanisms. Accordingly, an mPGES1 inhibitor stands out as a prospective better tool, compared with the currently used COX inhibitors, for the management of premature infants with persistent ductus.

Expression, activity, and function of phosphodiesterases in the mature and immature ductus arteriosus

A patent ductus arteriosus is due in large part to increased sensitivity of the premature ductus to PGE2. After PGE2 stimulation, cAMP concentrations are higher in the immature than in the mature ductus. cAMP concentrations depend on the rates of adenyl cyclase production and phosphodiesterase (PDE)-mediated degradation. We used ductus from immature (n = 25) and mature (n = 21) fetal sheep to investigate whether a developmental increase in PDE activity could explain the diminished cAMP accumulation that follows PGE2 stimulation in the mature ductus. With advancing gestation, mRNA expression of the smooth muscle PDE isoforms (PDE1A, 1B, 1C, 3A, 3B, 4D, and 5A) increased in the ductus as did their hydrolytic activities. Selective inhibitors of PDE1, PDE3, and PDE4 relaxed the mature and immature ductus in the presence of inhibitors of prostaglandin and nitric oxide production. The mature ductus required higher concentrations of each of the PDE inhibitors to inhibit its tension to the same extent as in the immature ductus. There were no developmental changes in PDE expression in the fetal aorta. In conclusion, we observed a developmental increase in cAMP and cGMP PDE activity that contributes to the decreased sensitivity of the late-gestation ductus arteriosus to vasodilators like PGE2.

Indomethacin promotes nitric oxide function in the ductus arteriosus in the mouse

BACKGROUND AND PURPOSE

Prenatal patency of ductus arteriosus is maintained by prostaglandin (PG) E(2) in concert with nitric oxide (NO) and carbon monoxide (CO). Accordingly, we have previously found that NO activity increases upon deletion of either COX. Here, we have examined whether COX inhibition by indomethacin mimics COX deletion in promoting NO.

EXPERIMENTAL APPROACH

Experiments were performed in vitro and in vivo with wild-type (WT) and eNOS-/-, near-term mouse foetuses. Indomethacin was given p.o. to the mother as single (acute treatment) or repeated (daily for 3 days; chronic treatment) doses within a therapeutic range (2 mg kg(-1)).

KEY RESULTS

Indomethacin promoted eNOS mRNA expression in the WT ductus. Coincidentally, the drug enhanced the contraction of the isolated ductus to the NOS inhibitor, N(G)-nitro-L-arginine methyl ester, and its effect augmented with the length of treatment. No such enhancement was seen with the eNOS-/- ductus. Chronic indomethacin also increased, albeit marginally, the contraction of the WT ductus to the CO synthesis inhibitor, zinc protoporphyrin. Whether given acutely or chronically, indomethacin induced a little narrowing of the ductus antenatally and had no effect on postnatal closure of the vessel.

CONCLUSIONS AND IMPLICATIONS

We conclude that activation of NO and, to a much lesser degree, CO mechanisms is an integral part of the indomethacin effect on the ductus. This relaxing influence may oppose the contraction from PGE(2) suppression and could explain the failures of indomethacin therapy in premature infants with persistent duct.

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.

Endothelium-dependent hyperpolarizations: past beliefs and present facts

Endothelium-dependent relaxations are attributed to the release of various factors, such as nitric oxide, carbon monoxide, reactive oxygen species, adenosine, peptides and arachidonic acid metabolites derived from the cyclooxygenases, lipoxygenases, and cytochrome P450 monooxygenases pathways. The hyperpolarization of the smooth muscle cell can contribute to or be an integral part of the mechanisms underlying the relaxations elicited by virtually all these endothelial mediators. These endothelium-derived factors can activate different families of K(+) channels of the vascular smooth muscle. Other events associated with the hyperpolarization of both the endothelial and the vascular smooth muscle cells (endothelium-derived hyperpolarizing factor (EDHF)-mediated responses) contribute also to endothelium-dependent relaxations. These responses involve an increase in the intracellular Ca(2+) concentration of the endothelial cells followed by the opening of Ca(2+)-activated K(+) channels of small and intermediate conductance and the subsequent hyperpolarization of these cells. Then, the endothelium-dependent hyperpolarization of the underlying smooth muscle cells can be evoked by direct electrical coupling through myoendothelial junctions and/or the accumulation of K(+) ions in the intercellular space between the two cell types. These various mechanisms are not necessarily mutually exclusive and, depending on the vascular bed and the experimental conditions, can occur simultaneously or sequentially, or also may act synergistically.