The role of nitric oxide signaling in pulmonary circulation of high- and low-altitude newborn sheep under basal and acute hypoxic conditions

The Oxygen Cascade from Atmosphere to Mitochondria as a Tool to Understand the (Mal)adaptation to Hypoxia

Hypoxia is a life-threatening challenge for about 1% of the world population, as well as a contributor to high morbidity and mortality scores in patients affected by various cardiopulmonary, hematological, and circulatory diseases. However, the adaptation to hypoxia represents a failure for a relevant portion of the cases as the pathways of potential adaptation often conflict with well-being and generate diseases that in certain areas of the world still afflict up to one-third of the populations living at altitude. To help understand the mechanisms of adaptation and maladaptation, this review examines the various steps of the oxygen cascade from the atmosphere to the mitochondria distinguishing the patterns related to physiological (i.e., due to altitude) and pathological (i.e., due to a pre-existing disease) hypoxia. The aim is to assess the ability of humans to adapt to hypoxia in a multidisciplinary approach that correlates the function of genes, molecules, and cells with the physiologic and pathological outcomes. We conclude that, in most cases, it is not hypoxia by itself that generates diseases, but rather the attempts to adapt to the hypoxia condition. This underlies the paradigm shift that when adaptation to hypoxia becomes excessive, it translates into maladaptation.

Chronic High-Altitude Hypoxia Alters Iron and Nitric Oxide Homeostasis in Fetal and Maternal Sheep Blood and Aorta

The mammalian fetus thrives at oxygen tensions much lower than those of adults. Gestation at high altitude superimposes hypoxic stresses on the fetus resulting in increased erythropoiesis. We hypothesized that chronic hypoxia at high altitude alters the homeostasis of iron and bioactive nitric oxide metabolites (NOx) in gestation. To test for this, electron paramagnetic resonance was used to provide unique measurements of iron, metalloproteins, and free radicals in the blood and aorta of fetal and maternal sheep from either high or low altitudes (3801 or 300 m). Using ozone-based chemiluminescence with selectivity for various NOx species, we determined the NOx levels in these samples immediately after collection. These experiments demonstrated a systemic redistribution of iron in high altitude fetuses as manifested by a decrease in both chelatable and total iron in the aorta and an increase in non-transferrin bound iron and total iron in plasma. Likewise, high altitude altered the redox status diversely in fetal blood and aorta. This study also found significant increases in blood and aortic tissue NOx in fetuses and mothers at high altitude. In addition, gradients in NOx concentrations observed between fetus and mother, umbilical artery and vein, and plasma and RBCs demonstrated complex dynamic homeostasis of NOx among these circulatory compartments, such as placental generation and efflux as well as fetal consumption of iron-nitrosyls in RBCs, probably HbNO. In conclusion, these results may suggest the utilization of iron from non-hematopoietic tissues iron for erythropoiesis in the fetus and increased NO bioavailability in response to chronic hypoxic stress at high altitude during gestation.

Cinaciguat (BAY-582667) Modifies Cardiopulmonary and Systemic Circulation in Chronically Hypoxic and Pulmonary Hypertensive Neonatal Lambs in the Alto Andino

Neonatal pulmonary hypertension (NPHT) is produced by sustained pulmonary vasoconstriction and increased vascular remodeling. Soluble guanylyl cyclase (sGC) participates in signaling pathways that induce vascular vasodilation and reduce vascular remodeling. However, when sGC is oxidized and/or loses its heme group, it does not respond to nitric oxide (NO), losing its vasodilating effects. sGC protein expression and function is reduced in hypertensive neonatal lambs. Currently, NPHT is treated with NO inhalation therapy; however, new treatments are needed for improved outcomes. We used Cinaciguat (BAY-582667), which activates oxidized and/or without heme group sGC in pulmonary hypertensive lambs studied at 3,600 m. Our study included 6 Cinaciguat-treated (35 ug kg−1 day−1x 7 days) and 6 Control neonates. We measured acute and chronic basal cardiovascular variables in pulmonary and systemic circulation, cardiovascular variables during a superimposed episode of acute hypoxia, remodeling of pulmonary arteries and changes in the right ventricle weight, vasoactive functions in small pulmonary arteries, and expression of NO-sGC-cGMP signaling pathway proteins involved in vasodilation. We observed a decrease in pulmonary arterial pressure and vascular resistance during the acute treatment. In contrast, the pulmonary pressure did not change in the chronic study due to increased cardiac output, resulting in lower pulmonary vascular resistance in the last 2 days of chronic study. The latter may have had a role in decreasing right ventricular hypertrophy, although the direct effect of Cinaciguat on the heart should also be considered. During acute hypoxia, the pulmonary vascular resistance remained low compared to the Control lambs. We observed a higher lung artery density, accompanied by reduced smooth muscle and adventitia layers in the pulmonary arteries. Additionally, vasodilator function was increased, and vasoconstrictor function was decreased, with modifications in the expression of proteins linked to pulmonary vasodilation, consistent with low pulmonary vascular resistance. In summary, Cinaciguat, an activator of sGC, induces cardiopulmonary modifications in chronically hypoxic and pulmonary hypertensive newborn lambs. Therefore, Cinaciguat is a potential therapeutic tool for reducing pulmonary vascular remodeling and/or right ventricular hypertrophy in pulmonary arterial hypertension syndrome.

The Action of 2-Aminoethyldiphenyl Borinate on the Pulmonary Arterial Hypertension and Remodeling of High-Altitude Hypoxemic Lambs

Calcium signaling is key for the contraction, differentiation, and proliferation of pulmonary arterial smooth muscle cells. Furthermore, calcium influx through store-operated channels (SOCs) is particularly important in the vasoconstrictor response to hypoxia. Previously, we found a decrease in pulmonary hypertension and remodeling in normoxic newborn lambs partially gestated under chronic hypoxia, when treated with 2-aminoethyldiphenyl borinate (2-APB), a non-specific SOC blocker. However, the effects of 2-APB are unknown in neonates completely gestated, born, and raised under environmental hypoxia. Accordingly, we studied the effects of 2-APB-treatment on the cardiopulmonary variables in lambs under chronic hypobaric hypoxia. Experiments were done in nine newborn lambs gestated, born, and raised in high altitude (3,600 m): five animals were treated with 2-APB [intravenous (i.v.) 10 mg kg^–1] for 10 days, while other four animals received vehicle. During the treatment, cardiopulmonary variables were measured daily, and these were also evaluated during an acute episode of superimposed hypoxia, 1 day after the end of the treatment. Furthermore, pulmonary vascular remodeling was assessed by histological analysis 2 days after the end of the treatment. Basal cardiac output and mean systemic arterial pressure (SAP) and resistance from 2-APB- and vehicle-treated lambs did not differ along with the treatment. Mean pulmonary arterial pressure (mPAP) decreased after the first day of 2-APB treatment and remained lower than the vehicle-treated group until the third day, and during the fifth, sixth, and ninth day of treatment. The net mPAP increase in response to acute hypoxia did not change, but the pressure area under the curve (AUC) during hypoxia was slightly lower in 2-APB-treated lambs than in vehicle-treated lambs. Moreover, the 2-APB treatment decreased the pulmonary arterial wall thickness and the α-actin immunoreactivity and increased the luminal area with no changes in the vascular density. Our findings show that 2-APB treatment partially reduced the contractile hypoxic response and reverted the pulmonary vascular remodeling, but this is not enough to normalize the pulmonary hemodynamics in chronically hypoxic newborn lambs.

Enhanced Vasoconstriction Mediated by α1-Adrenergic Mechanisms in Small Femoral Arteries in Newborn Llama and Sheep Gestated at Low and High Altitudes

The authors previously demonstrated that newborn llama (NBLL) express high levels of α₁ adrenergic receptors, which provide a potent vasoconstriction response when compared with newborn sheep (NBSH) gestated at sea level. However, data regarding the impact of chronic gestational hypobaric hypoxia on α-adrenergic vasoconstriction in the neonatal life has not been studied. We evaluated if gestation under chronic hypobaric hypoxia modifies α₁-adrenergic vasoconstrictor function in NBLL and NBSH. We compared the vasoconstrictor response induced by potassium and α-adrenergic stimuli in isolated small femoral arteries of NBLL and NBSH gestated at high altitude (HA; 3,600 m) or low altitude (LA; 580 m). The maximal contraction (R_MAX) and potency (EC₅₀) to potassium, noradrenaline (NA), and phenylephrine (PHE) were larger in HA-NBLL than LA-NBLL. R_MAX to potassium, NA, and PHE were lower in HA-NBSH when compared with LA-NBSH and potency results were similar. Competitive blockade with prazosin showed that RNLL LA/HA have a similar pA₂. In contrast, NBSH had increased pA₂ values in HA when compared with LA. Finally, small femoral arteries denudated or treated with LNAME in LA and HA lacked NO or endothelium participation in response to PHE stimulation. In contrast, NBSH displayed that denudation or blockade with LNAME support NO or endothelium participation in response to PHE activation. In conclusion, HA chronic hypoxia enhances α₁ adrenergic receptor activity in small femoral arteries in NBLL to a higher degree than NBSH, implying a higher vasoconstriction function.

The newborn sheep translational model for pulmonary arterial hypertension of the neonate at high altitude

Chronic hypoxia during gestation induces greater occurrence of perinatal complications such as intrauterine growth restriction, fetal hypoxia, newborn asphyxia, and respiratory distress, among others. This condition may also cause a failure in the transition of the fetal to neonatal circulation, inducing pulmonary arterial hypertension of the neonate (PAHN), a syndrome that involves pulmonary vascular dysfunction, increased vasoconstrictor tone and pathological remodeling. As this syndrome has a relatively low prevalence in lowlands (~7 per 1000 live births) and very little is known about its prevalence and clinical evolution in highlands (above 2500 meters), our understanding is very limited. Therefore, studies on appropriate animal models have been crucial to comprehend the mechanisms underlying this pathology. Considering the strengths and weaknesses of any animal model of human disease is fundamental to achieve an effective and meaningful translation to clinical practice. The sheep model has been used to study the normal and abnormal cardiovascular development of the fetus and the neonate for almost a century. The aim of this review is to highlight the advances in our knowledge on the programming of cardiopulmonary function with the use of high-altitude newborn sheep as a translational model of PAHN.

MiR-21-5p directly contributes to regulating eNOS expression in human artery endothelial cells under normoxia and hypoxia

Clinical conditions associated with hypoxia and oxidative stress, such as fetal growth restriction (FGR), results in endothelial dysfunction. Previous reports show that changes in eNOS expression under these conditions are tightly controlled by DNA methylation and histone posttranslational modifications. However, the contribution of an orchestrating epigenetic mechanism, such as miRNAs, on the NO-related genes expression has not been addressed. We aimed to determine the levels of miRNAs highly expressed in normal endothelial cells (EC), miR-21 and miR-126, in FGR human umbilical artery EC (HUAEC), and their effects on hypoxia-dependent regulation of both, NO-related and oxidative stress-related genes. Results were validated by transcriptome analysis of HUAEC cultured under chronic low oxygen conditions. Cultured FGR-HUAEC showed decreased hsa-miR-21, DDAH1, SOD1, and NRF2, but increased miR-126, NOX4, and eNOS levels, compared with controls. MiR-21-5p levels in FGR were associated with increased hg-miR-21 gene promoter methylation, with no changes in hg-miR-126 gene promoter methylation. HUAEC exposed to hypoxia showed a transient increase in eNOS and DDAH11, paralleled by decrease miR-21-5p levels, but no changes in miR-126-3p and the other genes under study. Transcriptome profiling showed an inverse relationship among miR-21 and several transcripts targeted by miR-21 in HUAEC exposed to hypoxia, meanwhile miR-21-5p-mimic decreased eNOS and DDAH1 transcripts stability, blocking their induction by hypoxia. Consequently, FGR programs a hypoxia-related miRNA that contributes to the regulation of the NO pathway, involving a direct effect of miR-21-5p on eNOS transcript stability, not previously reported. Moreover, hypoxia downregulates miR-21-5p, contributing to increasing the expression of NO-related genes in arterial endothelial cells.

Perinatal cardiopulmonary adaptation to the thin air of the Alto Andino by a native Altiplano dweller, the llama

Most mammals have a poor tolerance to hypoxia, and prolonged O₂ restriction can lead to organ injury, particularly during fetal and early postnatal life. Nevertheless, the llama (Lama Glama) has evolved efficient mechanisms to adapt to acute and chronic perinatal hypoxia. One striking adaptation is the marked peripheral vasoconstriction measured in the llama fetus in response to acute hypoxia, which allows efficient redistribution of cardiac output toward the fetal heart and adrenal glands. This strong peripheral vasoconstrictor tone is triggered by a carotid body reflex and critically depends on α-adrenergic signaling. A second adaptation is the ability of the llama fetus to protect its brain against hypoxic damage. During hypoxia, in the llama fetus there is no significant increase in brain blood flow. Instead, there is a fall in brain O₂ consumption and temperature, together with a decrease of Na⁺-K⁺-ATPase activity and Na⁺ channels expression, protecting against seizures and neuronal death. Finally, the newborn llama does not develop pulmonary hypertension in response to chronic hypoxia. In addition to maintaining basal pulmonary arterial pressure at normal levels the pulmonary arterial pressor response to acute hypoxia is lower in highland than in lowland llamas. The protection against hypoxic pulmonary arterial hypertension and pulmonary contractile hyperreactivity is partly due to increased hemoxygenase-carbon monoxide signaling and decreased Ca²⁺ sensitization in the newborn llama pulmonary vasculature. These three striking physiological adaptations of the llama allow this species to live and thrive under the chronic influence of the hypobaric hypoxia of life at high altitude.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.