Response of small pulmonary arteries to unilobar hypoxia and hypercapnia

Obstructive Sleep Apnea and Pulmonary Hypertension: A Chicken-and-Egg Relationship

Obstructive sleep apnea (OSA) is characterized by repeated episodes of upper airway obstruction during sleep, and it is closely linked to several cardiovascular issues due to intermittent hypoxia, nocturnal hypoxemia, and disrupted sleep patterns. Pulmonary hypertension (PH), identified by elevated pulmonary arterial pressure, shares a complex interplay with OSA, contributing to cardiovascular complications and morbidity. The prevalence of OSA is alarmingly high, with studies indicating rates of 20-30% in males and 10-15% in females, escalating significantly with age and obesity. OSA's impact on cardiovascular health is profound, particularly in exacerbating conditions like systemic hypertension and heart failure. The pivotal role of hypoxemia increases intrathoracic pressure, inflammation, and autonomic nervous system dysregulation in this interplay, which all contribute to PH's pathogenesis. The prevalence of PH among OSA patients varies widely, with studies reporting rates from 15% to 80%, highlighting the variability in diagnostic criteria and methodologies. Conversely, OSA prevalence among PH patients also remains high, often exceeding 25%, stressing the need for careful screening and diagnosis. Treatment strategies like continuous positive airway pressure (CPAP) therapy show promise in mitigating PH progression in OSA patients. However, this review underscores the need for further research into long-term outcomes and the efficacy of these treatments. This review provides comprehensive insights into the epidemiology, pathophysiology, and treatment of the intricate interplay between OSA and PH, calling for integrated, personalized approaches in diagnosis and management. The future landscape of OSA and PH management hinges on continued research, technological advancements, and a holistic approach to improving patient outcomes.

Hypoxia in the pulmonary vein increases pulmonary vascular resistance independently of oxygen in the pulmonary artery

INTRODUCTION

Hypoxic pulmonary vasoconstriction (HPV) can be a challenging clinical problem. It is not fully elucidated where in the circulation the regulation of resistance takes place. It is often referred to as if it is in the arteries, but we hypothesized that it is in the venous side of the pulmonary circulation.

METHODS

In an open thorax model, pigs were treated with a veno-venous extra corporeal membrane oxygenator to either oxygenate or deoxygenate blood passing through the pulmonary vessels. At the same time the lungs were ventilated with extreme variations of inspired air from 5% to 100% oxygen, making it possible to make combinations of high and low oxygen content through the pulmonary circulation. A flow probe was inserted around the main pulmonary artery and catheters in the pulmonary artery and in the left atrium were used for pressure monitoring and blood tests. Under different combinations of oxygenation, pulmonary vascular resistance (PVR) was calculated.

RESULTS

With unchanged level of oxygen in the pulmonary artery and reduced inspired oxygen fraction lowering oxygen tension from 29 to 6.7 kPa in the pulmonary vein, PVR was doubled. With more extreme hypoxia PVR suddenly decreased. Combinations with low oxygenation in the pulmonary artery did not systematic influence PVR if there was enough oxygen in the inspired air and in the pulmonary veins.

DISCUSSION

The impact of hypoxia occurs from the alveolar level and forward with the blood flow. The experiments indicated that the regulation of PVR is mediated from the venous side.

Atorvastatin rescues pulmonary artery hypertension by inhibiting the AKT/ERK-dependent PDGF-BB/HIF-1α axis

BACKGROUND

The aim of this study is to explore the role of atorvastatin in rescuing pulmonary artery hypertension (PAH) by inhibiting the AKT/ERK-dependent PDGF-BB/HIF-1α axis.

METHODS

PAH model in rats was established by MCT induction, followed by Atorvastatin intervention. Pulmonary hemodynamic measurement and pulmonary morphological evaluation in rats were conducted. Human pulmonary artery smooth muscle cells (hPASMCs) were subjected to hypoxic exposure or PDGF-BB treatment, followed by atorvastatin induction. Relative levels of HIF-1α, p-ERK and p-Akt were detected. Viability and apoptosis were respectively determined by cell counting kit-8 (CCK-8) assay and flow cytometry.

RESULTS

Atorvastatin protected PAH-induced increases in RVSP and Fulton's index in rats. Meanwhile, it inhibited vascular remodeling following PAH by downregulating HIF-1α and PDGF-BB. Hypoxia or PDGF-BB treatment in hPASMCs resulted in upregulation of p-ERK and p-Akt, and viability increase, which were partially abolished by Atorvastatin intervention. In addition, atorvastatin triggered apoptosis in hypoxia or PDGF-BB-induced hPASMCs.

CONCLUSIONS

Atorvastatin inhibits the activation of HIF-1α and proliferative ability, and triggers apoptosis in hPASMCs exposed to hypoxia or PDGF-BB treatment through inactivating the AKT/ERK pathway.

Mitochondrial oxygen sensing of acute hypoxia in specialized cells - Is there a unifying mechanism?

Acclimation to acute hypoxia through cardiorespiratory responses is mediated by specialized cells in the carotid body and pulmonary vasculature to optimize systemic arterial oxygenation and thus oxygen supply to the tissues. Acute oxygen sensing by these cells triggers hyperventilation and hypoxic pulmonary vasoconstriction which limits pulmonary blood flow through areas of low alveolar oxygen content. Oxygen sensing of acute hypoxia by specialized cells thus is a fundamental pre-requisite for aerobic life and maintains systemic oxygen supply. However, the primary oxygen sensing mechanism and the question of a common mechanism in different specialized oxygen sensing cells remains unresolved. Recent studies unraveled basic oxygen sensing mechanisms involving the mitochondrial cytochrome c oxidase subunit 4 isoform 2 that is essential for the hypoxia-induced release of mitochondrial reactive oxygen species and subsequent acute hypoxic responses in both, the carotid body and pulmonary vasculature. This review compares basic mitochondrial oxygen sensing mechanisms in the pulmonary vasculature and the carotid body.

Hypoxia and hemorheological properties in older individuals

Hypoxia is caused by insufficient oxygen availability for the organism leading to reduced oxygen delivery to tissues and cells. It has been regarded as a severe threat to human health and it is indeed implicated in pathophysiological mechanisms involved in the development and progression of many diseases. Nevertheless, the potential of controlled hypoxia interventions (i.e. hypoxia conditioning) for improving cardio-vascular health is gaining increased attention. However, blood rheology is often a forgotten factor for vascular health while aging and hypoxia exposure are both suspected to alter hemorheological properties. These changes in blood rheology may influence the benefits-risks balance of hypoxia exposure in older individuals. The benefits of hypoxia exposure for vascular health are mainly reported for healthy populations and the combined impact of aging and hypoxia on blood rheology could therefore be deleterious in older individuals. This review discusses evidence of hypoxia-related and aging-related changes in blood viscosity and its determinants. It draws upon an extensive literature search on the effects of hypoxia/altitude and aging on blood rheology. Aging increases blood viscosity mainly through a rise in plasma viscosity, red blood cell (RBC) aggregation and a decrease in RBC deformability. Hypoxia also causes an increase in RBC aggregation and plasma viscosity. In addition, hypoxia exposure may increase hematocrit and modulate RBC deformability, depending on the hypoxic dose, i.e, beneficial effect of intermittent hypoxia with moderate dose vs deleterious effect of chronic continuous or intermittent hypoxia or if the hypoxic dose is too high. Special attention is directed toward the risks vs. benefits of hemorheological changes during hypoxia exposure in older individuals, and its clinical relevance for vascular disorders.

Long Noncoding RNA Rps4l Mediates the Proliferation of Hypoxic Pulmonary Artery Smooth Muscle Cells

Pulmonary hypertension (PH) is a rare and fatal disorder involving the vascular remodeling of pulmonary arteries mediated by the enhanced proliferation of pulmonary artery smooth muscle cells (PASMCs). Long noncoding RNAs are a subclass of regulatory molecules with diverse cellular functions, but their role in PH remains largely unexplored. We aimed to identify and determine the functions of long noncoding RNAs involved in hypoxia-induced PH and PASMC proliferation. RNA sequencing in a hypoxic mouse model identified hypoxia-regulated long noncoding RNAs, including Rps4l. Rps4l expression was significantly reduced in PH-model mice and hypoxic PASMCs. The subcellular localization of Rps4l was detected by RNA fluorescence in situ hybridization and quantification of nuclear/cytoplasmic RNA. Rps4l overexpression rescued pulmonary arterial hypertension features, as demonstrated by right ventricle hypertrophy, right ventricular systolic pressure, hemodynamics, cardiac function, and vascular remodeling. At the cellular level, Rps4l overexpression weakened cell viability and proliferation and suppressed cell cycle progression. Potential Rps4l-binding proteins were identified via RNA pull-down followed by mass spectrometry, RNA immunoprecipitation, and microscale thermophoresis. These results indicated that Rps4l is associated with and affects the stabilization of ILF3 (interleukin enhancer-binding factor 3). Rps41 further regulates the levels of HIF-1α and consequently leads to hypoxia-induced PASMC proliferation and migration. Our results showed that in hypoxic PASMCs, Rps4l expression decreases due to regulation by hypoxia. This decrease affects the proliferation, migration, and cell cycle progression of PASMCs through ILF3/HIF-1α. These results provide a theoretical basis for further investigations into the pathological mechanism of hypoxic PH and may provide insight for the development of novel treatments.

Reactive Oxygen Species and Pulmonary Vasculature During Hypobaric Hypoxia

An increasing number of people are living or working at high altitudes (hypobaric hypoxia) and therefore suffering several physiological, biochemical, and molecular changes. Pulmonary vasculature is one of the main and first responses to hypoxia. These responses imply hypoxic pulmonary vasoconstriction (HPV), remodeling, and eventually pulmonary hypertension (PH). These events occur according to the type and extension of the exposure. There is also increasing evidence that these changes in the pulmonary vascular bed could be mainly attributed to a homeostatic imbalance as a result of increased levels of reactive oxygen species (ROS). The increase in ROS production during hypobaric hypoxia has been attributed to an enhanced activity and expression of nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase), though there is some dispute about which subunit is involved. This enzymatic complex may be directly induced by hypoxia-inducible factor-1α (HIF-1α). ROS has been found to be related to several pathways, cells, enzymes, and molecules in hypoxic pulmonary vasculature responses, from HPV to inflammation, and structural changes, such as remodeling and, ultimately, PH. Therefore, we performed a comprehensive review of the current evidence on the role of ROS in the development of pulmonary vasculature changes under hypoxic conditions, with a focus on hypobaric hypoxia. This review provides information supporting the role of oxidative stress (mainly ROS) in the pulmonary vasculature’s responses under hypobaric hypoxia and depicting possible future therapeutics or research targets. NADPH oxidase-produced oxidative stress is highlighted as a major source of ROS. Moreover, new molecules, such as asymmetric dimethylarginine, and critical inflammatory cells as fibroblasts, could be also involved. Several controversies remain regarding the role of ROS and the mechanisms involved in hypoxic responses that need to be elucidated.

Effects of human relaxin-2 (serelaxin) on hypoxic pulmonary vasoconstriction during acute hypoxia in a sheep model

Purpose

Hypoxia induces pulmonary vasoconstriction with a subsequent increase of pulmonary artery pressure (PAP), which can result in pulmonary hypertension. Serelaxin has shown an increase of pulmonary hemodynamic parameters after serelaxin injection. We therefore investigated the response of pulmonary hemodynamic parameters after serelaxin administration in a clinically relevant model.

Methods

Six controls and six sheep that received 30 μg/kg serelaxin underwent right heart catheterization during a 12-minute hypoxia period (inhalation of 5% oxygen and 95% nitrogen) and subsequent reoxygenation. Systolic, diastolic, and mean values of both PAP (respectively, PAPs, PAPd, and PAPm) and pulmonary capillary wedge pressure (respectively, PCWPs, PCWPd, and PCWPm), blood gases, heart rate (HR), and both peripheral and pulmonary arterial oxygen saturation were obtained. Cardiac output (CO), stroke volume (SV), pulmonary vascular resistance (PVR), pulmonary arterial compliance (PAcompl), and systemic vascular resistance (SVR) were calculated.

Results

The key findings of the current study are that serelaxin prevents the rise of PAPs (p≤0.001), PAPm, PCWPm, PCWPs (p≤0.03), and PAPd (p≤0.05) during hypoxia, while it simultaneously increases CO and SV (p≤0.001). Similar courses of decreases of PAPm, PAPd, PAPs, CO, SVR (p≤0.001), and PCWPd (p≤0.03) as compared to hypoxic values were observed during reoxygenation. In direct comparison, the experimental groups differed during hypoxia in regard to HR, PAPm, PVR, and SVR (p≤0.03), and during reoxygenation in regard to HR (p≤0.001), PAPm, PAPs, PAPd, PVR, SVR (p≤0.03), and PCWPd (p≤0.05).

Conclusion

The findings of this study suggest that serelaxin treatment improves pulmonary hemodynamic parameters during acute hypoxia.

Adenosine A(2A) receptor activation reverses hypoxia‑induced rat pulmonary artery smooth muscle cell proliferation via cyclic AMP‑mediated inhibition of the SDF1‑CXC4 signaling pathway

The occurrence and the subsequent development of pulmonary arterial hypertension (PAH) involve complicated mechanisms. Of these, the proliferation of pulmonary artery smooth muscle cells (PASMCs) has been indicated to be closely associated with its progression. Therefore, therapeutic methods targeting PASMCs to inhibit proliferation is an effective method for alleviating PAH. The present study was designed to determine the role of the adenosine A(2A) receptor (A2A receptor) in hypoxia‑induced rat PASMC (RPASMC) proliferation. Primary RPASMCs were isolated from the pulmonary artery of adult male SD rats, cultured and used for the following experiments. The mRNA level and protein expression of CXCR4 were measured by reverse transcription‑quantitative polymerase chain reaction and western blot analysis, respectively. The cell proliferation of RPASMCs was measured using a cell proliferation assay kit. In the present study, it was demonstrated that the proliferation of RPASMCs was partially mediated by activation of the stromal cell‑derived factor 1 (SDF1)‑CXC chemokine receptor 4 (CXCR4) axis under hypoxic conditions. In addition, SDF1‑α alone upregulated the mRNA and protein expression levels of CXCR4, and stimulated the proliferation of RPASMCs. The protein expression of CXCR4 and the cell proliferation were markedly inhibited by application of A2A receptor agonist CGS21680 or cyclic adenosine monophosphate (cAMP) under hypoxic conditions or treatment with SDF1‑α and was reversed by the A2A receptor antagonist SCH58261 or 8‑bromoadenosine‑3',5'‑cyclic monophosphorothioate. These results demonstrated that the inhibition of SDF1‑CXC4 signaling by the activation of A2A receptor and subsequent increase in the level of cAMP may be a potential method to ameliorate PAH.

The Fetus Can Teach Us: Oxygen and the Pulmonary Vasculature

Neonates suffering from pulmonary hypertension of the newborn (PPHN) continue to represent an important proportion of patients requiring intensive neonatal care, and have an increased risk of morbidity and mortality. The human fetus has evolved to maintain a high pulmonary vascular resistance (PVR) in utero to allow the majority of the fetal circulation to bypass the lungs, which do not participate in gas exchange, towards the low resistance placenta. At birth, oxygen plays a major role in decreasing PVR to enhance pulmonary blood flow and establish the lungs as the organ of gas exchange. The failure of PVR to fall following birth results in PPHN, and oxygen remains the mainstay therapeutic intervention in the management of PPHN. Knowledge gaps on what constitutes the optimal oxygenation target leads to a wide variation in practices, and often leads to excessive oxygen use. Owing to the risk of oxygen toxicity, avoiding hyperoxemia is as important as avoiding hypoxemia in the management of PPHN. Current evidence supports maintaining arterial oxygen tension in the range of 50–80 mm Hg, and oxygen saturation between 90–97% in term infants with hypoxemic respiratory failure. Clinical studies evaluating the optimal oxygenation in the treatment of PPHN will be enthusiastically awaited.

The principal pathways involved in the in vivo modulation of hypoxic pulmonary vasoconstriction, pulmonary arterial remodelling and pulmonary hypertension

Hypoxic pulmonary vasoconstriction (HPV) serves to optimize ventilation-perfusion matching in focal hypoxia and thereby enhances pulmonary gas exchange. During global hypoxia, however, HPV induces general pulmonary vasoconstriction, which may lead to pulmonary hypertension (PH), impaired exercise capacity, right-heart failure and pulmonary oedema at high altitude. In chronic hypoxia, generalized HPV together with hypoxic pulmonary arterial remodelling, contribute to the development of PH. The present article reviews the principal pathways in the in vivo modulation of HPV, hypoxic pulmonary arterial remodelling and PH with primary focus on the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways. In summary, endothelin-1 and thromboxane A₂ may enhance, whereas nitric oxide and prostacyclin may moderate, HPV as well as hypoxic pulmonary arterial remodelling and PH. The production of prostacyclin seems to be coupled primarily to cyclooxygenase-1 in acute hypoxia, but to cyclooxygenase-2 in chronic hypoxia. The potential role of adenine nucleotides in modulating HPV is unclear, but warrants further study. Additional modulators of the pulmonary vascular responses to hypoxia may include angiotensin II, histamine, serotonin/5-hydroxytryptamine, leukotrienes and epoxyeicosatrienoic acids. Drugs targeting these pathways may reduce acute and/or chronic hypoxic PH. Endothelin receptor antagonists and phosphodiesterase-5 inhibitors may additionally improve exercise capacity in hypoxia. Importantly, the modulation of the pulmonary vascular responses to hypoxia varies between species and individuals, with hypoxic duration and age. The review also define how drugs targeting the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways may improve pulmonary haemodynamics, but also impair pulmonary gas exchange by interference with HPV in chronic lung diseases.

Role of Transcription Factors in Pulmonary Artery Smooth Muscle Cells: An Important Link to Hypoxic Pulmonary Hypertension

Hypoxia, namely a lack of oxygen in the blood, induces pulmonary vasoconstriction and vasoremodeling, which serve as essential pathologic factors leading to pulmonary hypertension (PH). The underlying molecular mechanisms are uncertain; however, pulmonary artery smooth muscle cells (PASMCs) play an essential role in hypoxia-induced pulmonary vasoconstriction, vasoremodeling, and PH. Hypoxia causes oxidative damage to DNAs, proteins, and lipids. This damage (oxidative stress) modulates the activity of ion channels and elevates the intracellular calcium concentration ([Ca²⁺]_i, Ca²⁺ signaling) of PASMCs. The oxidative stress and increased Ca²⁺ signaling mutually interact with each other, and synergistically results in a variety of cellular responses. These responses include functional and structural abnormalities of mitochondria, sarcoplasmic reticulum, and nucleus; cell contraction, proliferation, migration, and apoptosis, as well as generation of vasoactive substances, inflammatory molecules, and growth factors that mediate the development of PH. A number of studies reveal that various transcription factors (TFs) play important roles in hypoxia-induced oxidative stress, disrupted PAMSC Ca²⁺ signaling and the development and progress of PH. It is believed that in the pathogenesis of PH, hypoxia facilitates these roles by mediating the expression of multiple genes. Therefore, the identification of specific genes and their transcription factors implicated in PH is necessary for the complete understanding of the underlying molecular mechanisms. Moreover, this identification may aid in the development of novel and effective therapeutic strategies for PH.

AMP-activated protein kinase inhibits Kv 1.5 channel currents of pulmonary arterial myocytes in response to hypoxia and inhibition of mitochondrial oxidative phosphorylation

KEY POINTS

Progression of hypoxic pulmonary hypertension is thought to be due, in part, to suppression of voltage-gated potassium channels (Kv ) in pulmonary arterial smooth muscle by hypoxia, although the precise molecular mechanisms have been unclear. AMP-activated protein kinase (AMPK) has been proposed to couple inhibition of mitochondrial metabolism by hypoxia to acute hypoxic pulmonary vasoconstriction and progression of pulmonary hypertension. Inhibition of complex I of the mitochondrial electron transport chain activated AMPK and inhibited Kv 1.5 channels in pulmonary arterial myocytes. AMPK activation by 5-aminoimidazole-4-carboxamide riboside, A769662 or C13 attenuated Kv 1.5 currents in pulmonary arterial myocytes, and this effect was non-additive with respect to Kv 1.5 inhibition by hypoxia and mitochondrial poisons. Recombinant AMPK phosphorylated recombinant human Kv 1.5 channels in cell-free assays, and inhibited K(+) currents when introduced into HEK 293 cells stably expressing Kv 1.5. These results suggest that AMPK is the primary mediator of reductions in Kv 1.5 channels following inhibition of mitochondrial oxidative phosphorylation during hypoxia and by mitochondrial poisons.

ABSTRACT

Progression of hypoxic pulmonary hypertension is thought to be due, in part, to suppression of voltage-gated potassium channels (Kv ) in pulmonary arterial smooth muscle cells that is mediated by the inhibition of mitochondrial oxidative phosphorylation. We sought to determine the role in this process of the AMP-activated protein kinase (AMPK), which is intimately coupled to mitochondrial function due to its activation by LKB1-dependent phosphorylation in response to increases in the cellular AMP:ATP and/or ADP:ATP ratios. Inhibition of complex I of the mitochondrial electron transport chain using phenformin activated AMPK and inhibited Kv currents in pulmonary arterial myocytes, consistent with previously reported effects of mitochondrial inhibitors. Myocyte Kv currents were also markedly inhibited upon AMPK activation by A769662, 5-aminoimidazole-4-carboxamide riboside and C13 and by intracellular dialysis from a patch-pipette of activated (thiophosphorylated) recombinant AMPK heterotrimers (α2β2γ1 or α1β1γ1). Hypoxia and inhibitors of mitochondrial oxidative phosphorylation reduced AMPK-sensitive K(+) currents, which were also blocked by the selective Kv 1.5 channel inhibitor diphenyl phosphine oxide-1 but unaffected by the presence of the BKCa channel blocker paxilline. Moreover, recombinant human Kv 1.5 channels were phosphorylated by AMPK in cell-free assays, and K(+) currents carried by Kv 1.5 stably expressed in HEK 293 cells were inhibited by intracellular dialysis of AMPK heterotrimers and by A769662, the effects of which were blocked by compound C. We conclude that AMPK mediates Kv channel inhibition by hypoxia in pulmonary arterial myocytes, at least in part, through phosphorylation of Kv 1.5 and/or an associated protein.

Analytic Reviews : Hypoxic Pulmonary Vasoconstriction: Physiology and Pathophysiology

Hypoxic pulmonary vasoconstriction is the unique re sponse of the lung circulation to ventilation with hy poxic gas, resulting in an increase in pulmonary arterial pressure and vascular resistance. The site of hypoxic vasoconstriction is the small pulmonary arteries. The hypoxic pressor response optimizes ventilation and per fusion matching and thus preserves arterial oxygen ten sion. The mechanism of this response is not clear, al though considerable knowledge has been gained about its modulation. We review the current state of under standing of the physiology and pathophysiology of hy poxic pulmonary vasoconstriction.

Oxygen sensing and signal transduction in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV), also known as the von Euler-Liljestrand mechanism, is an essential response of the pulmonary vasculature to acute and sustained alveolar hypoxia. During local alveolar hypoxia, HPV matches perfusion to ventilation to maintain optimal arterial oxygenation. In contrast, during global alveolar hypoxia, HPV leads to pulmonary hypertension. The oxygen sensing and signal transduction machinery is located in the pulmonary arterial smooth muscle cells (PASMCs) of the pre-capillary vessels, albeit the physiological response may be modulated in vivo by the endothelium. While factors such as nitric oxide modulate HPV, reactive oxygen species (ROS) have been suggested to act as essential mediators in HPV. ROS may originate from mitochondria and/or NADPH oxidases but the exact oxygen sensing mechanisms, as well as the question of whether increased or decreased ROS cause HPV, are under debate. ROS may induce intracellular calcium increase and subsequent contraction of PASMCs via direct or indirect interactions with protein kinases, phospholipases, sarcoplasmic calcium channels, transient receptor potential channels, voltage-dependent potassium channels and L-type calcium channels, whose relevance may vary under different experimental conditions. Successful identification of factors regulating HPV may allow development of novel therapeutic approaches for conditions of disturbed HPV.

Modulation of the LKB1-AMPK Signalling Pathway Underpins Hypoxic Pulmonary Vasoconstriction and Pulmonary Hypertension

Perhaps the defining characteristic of pulmonary arteries is the process of hypoxic pulmonary vasoconstriction (HPV) which, under physiological conditions, supports ventilation-perfusion matching in the lung by diverting blood flow away from oxygen deprived areas of the lung to oxygen rich regions. However, when alveolar hypoxia is more widespread, either at altitude or with disease (e.g., cystic fibrosis), HPV may lead to hypoxic pulmonary hypertension. HPV is driven by the intrinsic response to hypoxia of pulmonary arterial smooth muscle and endothelial cells, which are acutely sensitive to relatively small changes in pO2 and have evolved to monitor oxygen supply and thus address ventilation-perfusion mismatch. There is now a consensus that the inhibition by hypoxia of mitochondrial oxidative phosphorylation represents a key step towards the induction of HPV, but the precise nature of the signalling pathway(s) engaged thereafter remains open to debate. We will consider the role of the AMP-activated protein kinase (AMPK) and liver kinase B1 (LKB1), an upstream kinase through which AMPK is intimately coupled to changes in oxygen supply via mitochondrial metabolism. A growing body of evidence, from our laboratory and others, suggests that modulation of the LKB1-AMPK signalling pathway underpins both hypoxic pulmonary vasoconstriction and the development of pulmonary hypertension.

Hypoxia and exercise increase the transpulmonary passage of 99mTc-labeled albumin particles in humans

Intrapulmonary arteriovenous anastomoses (IPAVs) are large diameter connections that allow blood to bypass the lung capillaries and may provide a route for right-to-left embolus transmission. These anastomoses are recruited by exercise and catecholamines and hypoxia. Yet, whether IPAVs are recruited via direct, oxygen sensitive regulatory mechanisms or indirect effects secondary to redistribution pulmonary blood flow is unknown. Here, we hypothesized that the addition of exercise to hypoxic gas breathing, which increases cardiac output, would augment IPAVs recruitment in healthy humans. To test this hypothesis, we measured the transpulmonary passage of 99mTc-macroaggregated albumin particles (99mTc-MAA) in seven healthy volunteers, at rest and with exercise at 85% of volitional max, with normoxic (FIO2 = 0.21) and hypoxic (FIO2 = 0.10) gas breathing. We found increased 99mTc-MAA passage in both exercise conditions and resting hypoxia. However, contrary to our hypothesis, we found the greatest 99mTc-MAA passage with resting hypoxia. As an additional, secondary endpoint, we also noted that the transpulmonary passage of 99mTc-MAA was well-correlated with the alveolar-arterial oxygen difference (A-aDO2) during exercise. While increased cardiac output has been proposed as an important modulator of IPAVs recruitment, we provide evidence that the modulation of blood flow through these pathways is more complex and that increasing cardiac output does not necessarily increase IPAVs recruitment. As we discuss, our data suggest that the resistance downstream of IPAVs is an important determinant of their perfusion.

Cellular responses to hypoxia in the pulmonary circulation

Hypoxia can be defined as a reduction in available oxygen, whether in a whole organism or in a tissue or cell. It is a real life cause of pulmonary hypertension in humans both in terms of patients with chronic hypoxic lung disease and people living at high altitude. The effect of hypoxia on the pulmonary vasculature can be described in two ways; Hypoxic pulmonary vasoconstriction (HPV) (resulting from smooth muscle cell contraction) and pulmonary vascular remodelling (PVR) (resulting from pulmonary vascular cell proliferation). The pulmonary artery is made up of three resident cell types, the endothelial (intima), smooth muscle (media) and fibroblast (adventitia) cells. This review will examine the effects of hypoxia on the cells of the pulmonary vasculature and give an insight into the possible underlying mechanisms.

Nitric oxide transport in blood: a third gas in the respiratory cycle

The trapping, processing, and delivery of nitric oxide (NO) bioactivity by red blood cells (RBCs) have emerged as a conserved mechanism through which regional blood flow is linked to biochemical cues of perfusion sufficiency. We present here an expanded paradigm for the human respiratory cycle based on the coordinated transport of three gases: NO, O₂, and CO₂. By linking O₂ and NO flux, RBCs couple vessel caliber (and thus blood flow) to O₂ availability in the lung and to O₂ need in the periphery. The elements required for regulated O₂-based signal transduction via controlled NO processing within RBCs are presented herein, including S-nitrosothiol (SNO) synthesis by hemoglobin and O₂-regulated delivery of NO bioactivity (capture, activation, and delivery of NO groups at sites remote from NO synthesis by NO synthase). The role of NO transport in the respiratory cycle at molecular, microcirculatory, and system levels is reviewed. We elucidate the mechanism through which regulated NO transport in blood supports O₂ homeostasis, not only through adaptive regulation of regional systemic blood flow but also by optimizing ventilation-perfusion matching in the lung. Furthermore, we discuss the role of NO transport in the central control of breathing and in baroreceptor control of blood pressure, which subserve O₂ supply to tissue. Additionally, malfunctions of this transport and signaling system that are implicated in a wide array of human pathophysiologies are described. Understanding the (dys)function of NO processing in blood is a prerequisite for the development of novel therapies that target the vasoactive capacities of RBCs.

sGC stimulation totally reverses hypoxia-induced pulmonary vasoconstriction alone and combined with dual endothelin-receptor blockade in a porcine model

AIM

Stimulation of soluble guanylate cyclase (sGC) with BAY 41-8543 was hypothesized to attenuate acute hypoxic pulmonary vasoconstriction alone and combined with dual endothelin (ET)-receptor antagonist tezosentan.

METHODS

Measurements were taken in 18 anaesthetized pigs with a mean ± SEM weight of 31.1 ± 0.4 kg, in normoxia (FiO(2)~0.21) and hypoxia (FiO(2)~0.10) without (control protocol, n = 6), and with right atrial infusion of BAY 41-8543 at 1, 3, 6, 9 and 12 μg min(-1) per kg (protocol 2, n = 6) or tezosentan at 5 mg kg(-1) followed by BAY 41-8543 at 1, 3 and 6 μg min(-1) per kg (protocol 3, n = 6).

RESULTS

Hypoxia (n = 18) increased (P < 0.001) mean pulmonary artery pressure (MPAP) and pulmonary vascular resistance (PVR) by 14.2 ± 0.6 mmHg and 2.8 ± 0.3 WU respectively. During sustained hypoxia without treatment, MPAP and PVR remained stable. BAY 41-8543 (n = 6) dose-dependently decreased (P < 0.001) MPAP and PVR by 15.0 ± 1.2 mmHg and 4.7 ± 0.7 WU respectively. Tezosentan (n = 6) decreased (P < 0.001) MPAP and PVR by 11.8 ± 1.2 mmHg and 2.0 ± 0.2 WU, respectively, whereafter BAY 41-8543 (n = 6) further decreased (P < 0.001) MPAP and PVR by 6.6 ± 0.9 mmHg and 1.9 ± 0.4 WU respectively. Both BAY 41-8543 and tezosentan decreased (P < 0.001) systemic arterial pressure and systemic vascular resistance. Blood-O(2) consumption remained unaltered (P = ns) during all interventions.

CONCLUSION

BAY 41-8543 totally reverses the effects of acute hypoxia-induced pulmonary vasoconstriction, and enhances the attenuating effects of tezosentan, without affecting oxygenation. Thus, sGC stimulation, alone or combined with dual ET-receptor blockade, could offer a means to treat pulmonary hypertension related to hypoxia and potentially other causes.

Cyclooxygenase-2 inhibition and thromboxane A(2) receptor antagonism attenuate hypoxic pulmonary vasoconstriction in a porcine model

AIM

Hypoxic pulmonary vasoconstriction (HPV) causes pulmonary hypertension that may lead to right heart failure. We hypothesized that the COX-2 inhibitor nimesulide and the thromboxane A(2) receptor antagonist daltroban would attenuate HPV.

METHODS

Haemodynamic measurements and blood sampling were performed in 18 anaesthetized, mechanically ventilated pigs, with mean ± SEM weights of 31.3 ± 0.6 kg, in normoxia (F(i)O(2)~0.21) and hypoxia (F(i)O(2)~0.10), before and 5, 15 and 45 min after initiation of right atrial infusion of nimesulide (n = 6) or daltroban (n = 6), respectively, and in six control pigs.

RESULTS

Compared with normoxia, hypoxia (n = 18) increased mean pulmonary artery pressure by 15.8 ± 0.8 mmHg (P < 0.001), pulmonary vascular resistance (PVR) by 2.7 ± 0.3 WU (P < 0.05) and mean right atrial pressure by 2.3 ± 0.3 mmHg (P < 0.001). In the control pigs, mean pulmonary artery pressure, PVR and mean right atrial pressure remained stable (P = ns) throughout 45 min hypoxia, compared with hypoxia baseline. Nimesulide decreased mean pulmonary artery pressure by 3.7 ± 1.3 mmHg after 45 min (P < 0.013), as well as PVR by 0.8 ± 0.2 WU (P < 0.05), levelling off after 15 min. Daltroban transiently increased (P < 0.001) mean pulmonary artery pressure and mean right atrial pressure by 7.2 ± 1.2 and 2.7 ± 0.4 mmHg, respectively, but they returned to hypoxia baseline (P = ns) within 5 min. Daltroban then decreased mean pulmonary artery pressure to after 45 min be 4.2 ± 1.6 mmHg lower (P < 0.005) than at hypoxia baseline.

CONCLUSION

COX-2 inhibition and thromboxane A(2) receptor antagonism attenuate HPV by decreasing mean pulmonary artery pressure by approximately 10-11%, as measured 45 min after initiation of nimesulide or daltroban infusion respectively.

Optimal Initial Palliation for Patients With Functionally Univentricular Hearts

This review will outline the optimal, initial palliation for children who are born with a functionally univentricular heart. Optimizing the initial palliation is of critical importance in this patient population to prevent potential problems such as systemic outflow and pulmonary vein obstruction that may complicate further surgical intervention. The palliative techniques that are discussed include pulmonary artery banding, modified Blalock-Taussig shunt, Damus-Kaye-Stansel procedure, modified Norwood, hybrid, and early bidirectional Glenn. Our recommendations for optimal palliation for children with a univentricular heart are based on our experience with nearly 200 patients who had either a lateral tunnel or extracardiac Fontan procedure and 130 patients who had Fontan conversion with arrhythmia surgery.

Dual endothelin receptor blockade with tezosentan markedly attenuates hypoxia-induced pulmonary vasoconstriction in a porcine model

AIM

Our aim was to test the hypothesis that dual endothelin receptor blockade with tezosentan attenuates hypoxia-induced pulmonary vasoconstriction.

METHODS

Fourteen anaesthetized, ventilated pigs, with a mean ± SEM weight of 30.5 ± 0.6 kg, were studied, in normoxia (FiO(2) 0.21) and with tezosentan (5 mg kg(-1)) infusion during (n = 7) or before (n = 7) hypoxia (FiO(2) 0.10).

RESULTS

Compared to normoxia, hypoxia increased (P < 0.05) pulmonary vascular resistance (PVR) by 3.4 ± 0.7 WU, mean pulmonary artery pressure by 13.7 ± 1.3 mmHg, mean right atrial pressure by 1.9 ± 0.4 mmHg and decreased (P < 0.02) systemic vascular resistance (SVR) by 5.2 ± 2.1 WU. Pulmonary capillary wedge pressure (PCWP), mean aortic blood pressure, heart rate, cardiac output, stroke volume and blood-O(2)-consumption were unaltered (P = ns). Tezosentan infused during hypoxia, normalized PVR, decreased (P < 0.05) maximally mean pulmonary artery pressure by 7.5 ± 0.8 mmHg, SVR by 5.8 ± 0.7 WU, mean aortic blood pressure by 10.8 ± 3.0 mmHg and increased (P < 0.04) stroke volume by 8.5 ± 1.8 mL. Mean right atrial pressure, PCWP, heart rate, cardiac output and blood-O(2) -consumption were unaltered (P = ns). Tezosentan infused before hypoxia additionally attenuated approx. 70% of the initial mean pulmonary artery pressure increase and abolished the PVR increase, without additionally affecting the other parameters.

CONCLUSION

Dual endothelin receptor blockade during hypoxia attenuates the 'sustained' acute pulmonary vasoconstrictor response by reducing the mean pulmonary artery pressure increase by approx. 62% and by normalizing PVR. Pre-treatment with tezosentan before hypoxia, additionally attenuates the initial hypoxia-induced mean pulmonary artery pressure rise by approx. 70% and abolishes the PVR increase, during stable circulatory conditions, without affecting oxygenation.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Hypoxia-induced pulmonary arterial smooth muscle cell proliferation is controlled by forkhead box M1

Pulmonary arterial hypertension (PAH) is a devastating disease, and no effective treatments are available. Hypoxia-induced pulmonary artery remodeling, including smooth muscle cell proliferation, contributes to PAH, but the exact mechanisms underlying this abnormal process are largely undefined. The forkhead box M1 (FoxM1) transcription factor regulates cancer cell growth by modulating gene expression critical for cell cycle progression. Here, we report for the first time, to the best of our knowledge, a novel function of FoxM1 in the hypoxia-stimulated proliferation of human pulmonary artery smooth muscle cells (HPASMCs). Exposure to hypoxia caused a marked up-regulation of FoxM1 gene expression, mainly at the transcription level, and this induction correlated with HPASMC cell proliferation. The knockdown of FoxM1 inhibited the hypoxia-stimulated proliferation of HPASMCs. We found that the knockdown of HIF-2α, but not HIF-1α, diminished FoxM1 induction in response to hypoxia. However, the knockdown of FoxM1 did not alter expression levels of HIF-2α or HIF-1α, suggesting that HIF-2α is an upstream regulator of FoxM1. Furthermore, the knockdown of FoxM1 prevented the hypoxia-induced expression of aurora A kinase and cyclin D1. Collectively, our results suggest that hypoxia induces FoxM1 gene expression in an HIF-2α-dependent pathway, thereby promoting HPASMC proliferation.

Hypoxic pulmonary vasoconstriction: mechanisms of oxygen-sensing

PURPOSE OF REVIEW

Hypoxic pulmonary vasoconstriction (HPV) is driven by the intrinsic response to hypoxia of pulmonary arterial smooth muscle and endothelial cells. These are representatives of a group of specialized O2-sensing cells, defined by their acute sensitivity to relatively small changes in pO2, which have evolved to modulate respiratory and circulatory function in order to maintain O2 supply within physiological limits. The aim of this article is to discuss recent investigations into the mechanism(s) of hypoxia-response coupling and, in light of these, provide a critical assessment of current working hypotheses.

RECENT FINDINGS

Upon exposure to hypoxia state-of-the-art technologies have now confirmed that mitochondrial oxidative phosphorylation is inhibited in all O2-sensing cells, including pulmonary arterial smooth muscle cells. Thereafter, evidence has been presented to indicate a role as principal effector for the 'gasotransmitters' carbon monoxide and hydrogen sulphide, reactive oxygen species or, in marked contrast, reduced cellular redox couples. Considering recent evidence in favour and against these proposals we suggest that an alternative mechanism may be key, namely the activation of adenosine monophosphate-activated protein kinase consequent to inhibition of mitochondrial oxidative phosphorylation.

SUMMARY

HPV supports ventilation-perfusion matching in the lung by diverting blood flow away from oxygen-deprived areas towards regions rich in O2. However, in diseases such as emphysema and cystic fibrosis, widespread HPV leads to hypoxic pulmonary hypertension and ultimately right heart failure. Determining the precise mechanism(s) that underpins hypoxia-response coupling will therefore advance understanding of the fundamental processes contributing to related pathophysiology and provide for improved therapeutics.

Redox signaling and reactive oxygen species in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an essential physiological mechanism of the lung that matches blood perfusion with alveolar ventilation to optimize gas exchange. Perturbations of HPV, as may occur in pneumonia or adult respiratory distress syndrome, can cause life-threatening hypoxemia. Despite intensive research for decades, the molecular mechanisms of HPV have not been fully elucidated. Reactive oxygen species (ROS) and changes in the cellular redox state are proposed to link O2 sensing and pulmonary arterial smooth muscle cell contraction underlying HPV. In this regard, mitochondria and NAD(P)H oxidases are discussed as sources of ROS. However, there is controversy whether ROS levels decrease or increase during hypoxia. With this background we summarize the current knowledge on the role of ROS and redox state in HPV.

Extent to which pulmonary vascular responses to PCO2 and PO2 play a functional role within the healthy human lung

Regional blood flow in the lung is known to be influenced by the alveolar Pco₂ and alveolar Po₂. For the healthy lung, the extent to which this influence is of functional importance in limiting heterogeneity in alveolar gas composition by matching regional perfusion (q̇) to regional ventilation (v̇) remains unclear. To address this issue, the efficiency of regulation (E) was defined as the percent correction to an initial perturbation in regional alveolar gas composition generated by the pulmonary vascular response to the disturbance. This study develops the theory to calculate E from global measurements of vascular reactivity to CO₂ and O₂ in human volunteers. For O₂, these data were available from the literature. For CO₂, an experimental component of the present study used Doppler echocardiography to evaluate the magnitude of the global vascular response to hypercapnia and hypocapnia in 12 volunteers over a timescale of ∼0.5 h. The results suggest a value for E of ∼60% over a wide range of values for v̇-to-q̇ ratio (∼0.1–10) encompassing those found in normal lung. At low v̇/q̇ (<0.65), the vascular response to O₂ forms the dominant mechanism; however, at higher v̇/q̇ (>0.65), the response to CO₂ dominates. The values for E suggest that the pulmonary vascular responses to both CO₂ and O₂ play a significant role in ventilation-perfusion matching in the healthy human lung.

Structural organization of pulmonary veins in the rat lung, with special emphasis on the musculature consisting of cardiac and smooth muscles

Recent physiological studies have indicated the significant role of pulmonary veins in the total resistance of pulmonary vasculature. The structure of pulmonary veins in the rat was reinvestigated to clarify the different venous segments and their ultrastructure with regard to the musculature including cardiac muscles and smooth muscles with light and electron microscopy. The cardiac muscles were located in the axial and the primary branches of the pulmonary veins within a certain distance limit from the hilum (CM segment) and not in the peripheral region (non-CM segment). The smooth muscles were found indifferent to the presence of cardiac muscles as a continuous layer in segments larger than 180 microm (continuous SM segment) or as a discontinuous layer of circular smooth muscle cells in segments between 50 and 180 microm (partial SM segment). The smooth muscle layer was extremely thin in the CM segments, whereas it became conspicuously thick in the non-CM segment with an irregularly undulating luminal outline, especially in the partial SM segments. There were two elastic laminae in the CM segments: a conspicuous one on the interstitial side of the smooth muscles, and a weaker one between the endothelium and smooth muscles. In the non-CM segment, one elastic lamina was found on the interstitial side of the smooth muscles. Considering the limited range of contraction of cardiac muscles and the thinness of smooth muscle cells in the CM segments, it was concluded that vasoconstriction in the pulmonary veins is executed by smooth muscle cells in the non-CM segments thicker than 50 microm.

The role of intracellular ion channels in regulating cytoplasmic calciumin pulmonary arterial mmooth muscle: which store and where?

The mobilisation of intracellular Ca(2+) stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. Moreover, different spatiotemporal Ca(2+) signalling patterns may trigger differential gene expression while mediating the same functional response. These facts alone serve to highlight the importance of the growing body of evidence in support of the view that different Ca(2+) storing organelles may be selected by the discrete or co-ordinated actions of multiple Ca(2+) mobilising messengers. In this respect, it is generally accepted that sarcoplasmic reticulum stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca(2+) mobilising pyridine nucleotides in arterial smooth muscle, namely cyclic adenosine diphosphate-ribose and nicotinic acid adenine dinucleotide phosphate. This review will, therefore, focus on the role of these novel Ca(2+) mobilising messengers in pulmonary arterial smooth muscle, with particular reference to hypoxic pulmonary vasoconstriction.

Ion channel regulation by AMPK: the route of hypoxia-response coupling in thecarotid body and pulmonary artery

Vital homeostatic mechanisms monitor O2 supply and adjust respiratory and circulatory function to meet demand. The pulmonary arteries and carotid bodies are key systems in this respect. Hypoxic pulmonary vasoconstriction (HPV) aids ventilation-perfusion matching in the lung by diverting blood flow from areas with an O2 deficit to those rich in O2, while a fall in arterial pO2 increases sensory afferent discharge from the carotid body to elicit corrective changes in breathing patterns. We discuss here the new concept that hypoxia, by inhibiting oxidative phosphorylation, activates AMP-activated protein kinase (AMPK) leading to consequent phosphorylation of target proteins, such as ion channels, which initiate pulmonary artery constriction and carotid body activation. Consistent with this view, AMPK knockout mice exhibit an impaired ventilatory response to hypoxia. Thus, AMPK may be sufficient and necessary for hypoxia-response coupling and may regulate O2 and thereby energy (ATP) supply at the whole body as well as the cellular level.

Exaggerated pulmonary hypertension during mild exercise in chronic mountain sickness

BACKGROUND

Chronic mountain sickness (CMS) is an important public health problem and is characterized by exaggerated hypoxemia, erythrocytosis, and pulmonary hypertension. While pulmonary hypertension is a leading cause of morbidity and mortality in patients with CMS, it is relatively mild and its underlying mechanisms are not known. We speculated that during mild exercise associated with daily activities, pulmonary hypertension in CMS is much more pronounced.

METHODS

We estimated pulmonary artery pressure by using echocardiography at rest and during mild bicycle exercise at 50 W in 30 male patients with CMS and 32 age-matched, healthy control subjects who were born and living at an altitude of 3,600 m.

RESULTS

The modest, albeit significant difference of the systolic right-ventricular-to-right-atrial pressure gradient between patients with CMS and controls at rest (30.3 +/- 8.0 vs 25.4 +/- 4.5 mm Hg, P 5 .002) became more than three times larger during mild bicycle exercise (56.4 +/- 19.0 vs 39.8 +/- 8.0 mm Hg, P < .001).

CONCLUSIONS

Measurements of pulmonary artery pressure at rest greatly underestimate pulmonary artery pressure during daily activity in patients with CMS. The marked pulmonary hypertension during mild exercise associated with daily activity may explain why this problem is a leading cause of morbidity and mortality in patients with CMS.

Oxidant and redox signaling in vascular oxygen sensing: implications for systemic and pulmonary hypertension

It has been well known for >100 years that systemic blood vessels dilate in response to decreases in oxygen tension (hypoxia; low PO2), and this response appears to be critical to supply blood to the stressed organ. Conversely, pulmonary vessels constrict to a decrease in alveolar PO2 to maintain a balance in the ventilation-to-perfusion ratio. Currently, although little question exists that the PO2 affects vascular reactivity and vascular smooth muscle cells (VSMCs) act as oxygen sensors, the molecular mechanisms involved in modulating the vascular reactivity are still not clearly understood. Many laboratories, including ours, have suggested that the intracellular calcium concentration ([Ca2+]i), which regulates vasomotor function, is controlled by free radicals and redox signaling, including NAD(P)H and glutathione (GSH) redox. In this review article, therefore, we discuss the implications of redox and oxidant alterations seen in pulmonary and systemic hypertension, and how key targets that control [Ca2+]i, such as ion channels, Ca2+ release from internal stores and uptake by the sarcoplasmic reticulum, and the Ca2+ sensitivity to the myofilaments, are regulated by changes in intracellular redox and oxidants associated with vascular PO2sensing in physiologic or pathophysiologic conditions.

Hypoxic pulmonary vasoconstriction

HPV (hypoxic pulmonary vasoconstriction) is the critical and distinguishing characteristic of the arteries that feed the lung. In marked contrast, systemic arteries dilate in response to hypoxia to meet the metabolic demands of the tissues they supply. Physiologically, HPV contributes to ventilation-perfusion matching in the lung by diverting blood flow to oxygen-rich areas. However, when alveolar hypoxia is global, as in diseases such as emphysema and cystic fibrosis, HPV leads to HPH (hypoxic pulmonary hypertension) and right heart failure. HPV is driven by the intrinsic response to hypoxia of two different cell types, namely the pulmonary arterial smooth muscle and endothelial cells. These are representatives of a group of specialized cells, commonly referred to as oxygen-sensing cells, which are defined by their acute sensitivity to relatively small changes in PO(2) and have evolved to monitor oxygen supply and alter respiratory and circulatory function, as well as the capacity of the blood to transport oxygen. Upon exposure to hypoxia, mitochondrial oxidative phosphorylation is inhibited in all such cells and this, in part, mediates cell activation. In the case of pulmonary arteries, constriction is triggered via: (i) calcium release from the smooth muscle sarcoplasmic reticulum and consequent store-depletion-activated calcium entry into the smooth muscle cells and, (ii) the modulation of transmitter release from the pulmonary artery endothelium, which leads to further constriction of the smooth muscle by increasing the sensitivity of the contractile apparatus to calcium.

Clinical perspective of hypoxia-mediated pulmonary hypertension

Pulmonary hypertension is a condition associated with a variety of pulmonary disorders whose common denominator is alveolar hypoxia. Such disorders include chronic obstructive pulmonary disease, pulmonary fibrosis, sleep-disordered breathing, and exposure to high altitude. Acute hypoxia is characterized by vasoconstriction of small pulmonary arteries, a phenomenon called hypoxic pulmonary vasoconstriction. With prolonged hypoxia, thickening of the smooth vascular layer of the small pulmonary arteries occurs, a phenomenon described as pulmonary vascular remodeling. Although the core mechanisms of both vasoconstriction and remodeling are thought to reside in the smooth muscle cell layer, the endothelium modulates these two processes. The purpose of this review is briefly to (a) discuss the mechanisms of hypoxic pulmonary hypertension as it pertains to certain disease states, and (b) examine the pathways that have potential therapeutic applications for this condition.

Potassium channel diversity in the pulmonary arteries and pulmonary veins: implications for regulation of the pulmonary vasculature in health and during pulmonary hypertension

This review describes the ionic heterogeneity manifest in the pulmonary circulation, particularly as it pertains to hypoxic pulmonary vasoconstriction (HPV) and pulmonary arterial hypertension (PAH). Heterogeneity in potassium (K(+)) channels, key regulators of vascular tone, cell proliferation, and apoptosis rates, contribute to the diverse response of vascular segments to hypoxia and to the localization of pathological changes in PAH. Pulmonary artery (PA) and pulmonary vein (PV) smooth muscle cells (SMC) express several K(+) channel families, including calcium-sensitive (KCa), voltage-gated (K(v)), inward rectifier (Kir), and 2-pore channels. Diversity is created by heterogeneous occurrence of alternatively spliced, mRNA species, assembly of heterotetrameric channels from diverse alpha-subunits, and association of channels with regulatory beta-subunits. Local heterogeneity in transcription factor activity may underlie differences in channel expression. Enrichment of resistance PASMCs with O(2)-sensitive K(+) channels, such as K(v)1.5, partially explains the greater HPV in resistance versus conduit PAs. In addition, resistance PAs are unique in having mitochondria which dynamically alter production of reactive O(2) species (ROS) in proportion to PO(2), thereby regulating K(+) channel activity and controlling expression through transcription factors, such as HIF-1alpha. In intraparenchymal PVs, a coaxial layer of cardiomyocytes encompasses a media of typical vascular SMCs. PV cardiomyocytes have rhythmic contraction and their Kir-enriched channels may be relevant to genesis of atrial arrhythmias and pulmonary edema. K(v) channel expression is decreased in PAH, leading to elevations of cytosolic K(+) and Ca(2+) that impair apoptosis and increase proliferation. Understanding ionic diversity may allow development of therapies that locally increase K(+) channel current and expression to treat PHT.

AMP-activated protein kinase underpins hypoxic pulmonary vasoconstriction and carotid body excitation by hypoxia in mammals

In order to maintain tissue partial pressure of oxygen (P(O(2))) within physiological limits, vital homeostatic mechanisms monitor O(2) supply and respond to a fall in P(O(2)) by altering respiratory and circulatory function, and the capacity of the blood to transport O(2). Two systems that are key to this process in the acute phase are the pulmonary arteries and the carotid bodies. Hypoxic pulmonary vasoconstriction is driven by mechanisms intrinsic to the pulmonary arterial smooth muscle and endothelial cells, and aids ventilation-perfusion matching in the lung by diverting blood flow from areas with an O(2) deficit to those that are rich in O(2). By contrast, a fall in arterial P(O(2)) precipitates excitation-secretion coupling in carotid body type I cells, increases sensory afferent discharge from the carotid body and thereby elicits corrective changes in breathing patterns via the brainstem. There is a general consensus that hypoxia inhibits mitochondrial oxidative phosphorylation in these O(2)-sensing cells over a range of P(O(2)) values that has no such effect on other cell types. However, the question remains as to the identity of the mechanism that underpins hypoxia-response coupling in O(2)-sensing cells. Here, I lay out the case in support of a primary role for AMP-activated protein kinase in mediating chemotransduction by hypoxia.

Role of NO and prostaglandins in acute hypoxic vasoconstriction in sheep pulmonary veins

The aim of this study was to investigate the effect of hypoxia on and the role of nitric oxide (NO) and cyclooxgenase inhibition in hypoxia-induced vasoconstriction in sheep isolated pulmonary veins. We used the potent pulmonary vasoconstrictor U46619, a thromboxane analog, as a precontractile agent. Our results showed that hypoxia caused a vasoconstriction both under resting tone and in U46619 (10(-6) mol/l) precontracted pulmonary veins. In the presence of the nonselective NO synthase inhibitior Nomega-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-5) mol/l), the hypoxic pulmonary vasoconstriction (HPV) was significantly increased in veins under resting force. However, there was a decrease in HPV in pulmonary veins precontracted with U46619 in the presence of L-NAME. Moreover, L-NAME markedly augmented the U46619-induced pulmonary contractions under normoxic conditions. Cyclooxygenase inhibition with indomethacin (10(-5) mol/l) significantly reduced the HPV both under resting tone and in precontracted veins. Indomethacin also significantly decreased the U46619-induced pulmonary contractions prior to the induction of hypoxia. Our findings suggest that NO and prostaglandins can act as a modulators of the hypoxic vasoconstriction in isolated pulmonary veins.

AMP-activated protein kinase and the regulation of Ca2+ signalling in O2-sensing cells

All cells respond to metabolic stress. However, a variety of specialized cells, commonly referred to as O2-sensing cells, are acutely sensitive to relatively small changes in PO2. Within a variety of organisms such O2-sensing cells have evolved as vital homeostatic mechanisms that monitor O2 supply and alter respiratory and circulatory function, as well as the capacity of the blood to transport O2. Thereby, arterial PO2 may be maintained within physiological limits. In mammals, for example, two key tissues that contribute to this process are the pulmonary arteries and the carotid bodies. Constriction of pulmonary arteries by hypoxia optimizes ventilation-perfusion matching in the lung, whilst carotid body excitation by hypoxia initiates corrective changes in breathing patterns via increased sensory afferent discharge to the brain stem. Despite extensive investigation, the precise mechanism(s) by which hypoxia mediates these responses has remained elusive. It is clear, however, that hypoxia inhibits mitochondrial function in O2-sensing cells over a range of PO2 that has no such effect on other cell types. This raised the possibility that AMP-activated protein kinase might function to couple mitochondrial oxidative phosphorylation to Ca2+ signalling mechanisms in O2-sensing cells and thereby underpin pulmonary artery constriction and carotid body excitation by hypoxia. Our recent investigations have provided significant evidence in support of this view.

Therapeutic concepts for hypoxic pulmonary vasoconstriction involving ion regulation and the smooth muscle contractile apparatus

Hypoxic pulmonary vasoconstriction (HPV) and pulmonary hypertension present a common and formidable clinical problem for practicing intensivists, thoracic, transplant, and trauma surgeons. The Redox Theory for the mechanisms of HPV has provided researchers with a new understanding of the etiology behind HPV that has opened the door to many new avenues of therapy for the disease. Potassium channels have been proposed to be the main mediator contributing to HPV, and treatment concepts that attempt to manipulate the function and number of those channels have been explored. Additionally, attempts to transfer genes that express the formation of specific potassium channels directly into pulmonary hypertensive lungs have proven to be very promising. Finally, rho kinase (ROK) has been discovered to play a very central role in the formation of hypoxia-induced pulmonary hypertension, and the advent of very specific ROK inhibitors has shown positive clinical results. The purposes of this review are to: (1) briefly discuss some of the basic mechanisms that undergird HPV, including the Redox Theory for the mechanisms of HPV; (2) address current research involving treatments concepts related to ion channels; (3) report on research involving gene therapy to combat pulmonary hypertension; and (4) examine potential therapeutic avenues associated with inhibition of rho kinase.

An anesthesiologist's guide to hypoxic pulmonary vasoconstriction: implications for managing single-lung anesthesia and atelectasis

PURPOSE OF THE REVIEW

Hypoxic pulmonary vasoconstriction is the pulmonary circulation's homeostatic mechanism for matching regional perfusion to ventilation and optimizing systemic PaO2. The role of hypoxic pulmonary vasoconstriction in anesthesiology is reviewed.

RECENT FINDINGS

In hypoxic pulmonary vasoconstriction, airway hypoxia causes resistance pulmonary arteries to constrict, diverting blood to better-oxygenated alveoli. Hypoxic pulmonary vasoconstriction optimizes O2 uptake in atelectasis, pneumonia, asthma, and adult respiratory distress syndrome. During single-lung anesthesia, hypoxic pulmonary vasoconstriction helps maintain systemic oxygenation. When hypoxic pulmonary vasoconstriction is weak, systemic hypoxemia is exacerbated. Although not widely used, the peripheral chemoreceptor agonist almitrine enhances hypoxic pulmonary vasoconstriction and improves PaO2 during single-lung anesthesia. The mechanism of hypoxic pulmonary vasoconstriction involves a redox-based O2 sensor within pulmonary artery smooth muscle cells. Pulmonary artery smooth muscle cells mitochondria vary production of reactive O2 species in proportion to PaO2. Hypoxic withdrawal of these redox second messengers inhibits voltage-gated potassium channels, depolarizing the pulmonary artery smooth muscle cells. Depolarization activates L-type calcium channels, increasing cytosolic calcium and triggering hypoxic pulmonary vasoconstriction.

SUMMARY

An understanding of hypoxic pulmonary vasoconstriction is clinically relevant for anesthesiologists. Randomized clinical trials with robust endpoints are required to assess strategies for enhancing hypoxic pulmonary vasoconstriction in thoracic surgery patients.

EXPERIMENTAL THERAPIES FOR HYPOXIA-INDUCED PULMONARY HYPERTENSION DURING ACUTE LUNG INJURY

Hypoxic pulmonary vasoconstriction (HPV) and pulmonary hypertension present a common and formidable clinical problem for practicing thoracic, transplant, and trauma surgeons. The recent discovery of efficacious drugs that are selective for the pulmonary vasculature has brought about the potential for very powerful therapeutic agents. Inhaled nitric oxide (NO) therapy has already found broad clinical utility, yet its use is limited by potential toxicities. Rho kinase (ROK) has been discovered to play a very central role in the formation of hypoxia induced pulmonary hypertension, and the advent of very specific ROK inhibitors has shown positive clinical results. Finally, phosphodiesterase-5 inhibitors have been found to selectively vasodilate the pulmonary vasculature in the midst of HPV. The purposes of this review are to: 1) discuss the advantages and disadvantages of inhaled preparations of NO; 2) address experimental alternatives to inhaled preparations of NO to treat HPV; 3) explore potential therapeutic avenues associated with inhibition of Rho-kinase; and, 4) examine the use of phosphodiesterase-5 (PDE-5) inhibitors and combination therapy in the treatment of HPV.