Hypoxic pulmonary vasoconstriction: role of voltage-gated potassium channels

Nicardipine-induced acute respiratory failure: Case report and literature review

Hypoxic pulmonary vasoconstriction (HPV) is a major physiological mechanism that prevents the development of hypoxemia secondary to a regional decrease in the ventilation-perfusion ratio (the intrapulmonary shunt effect). Calcium plays a critical role in the cellular response to hypoxia and the regulation of the pulmonary vascular tone. Therefore, calcium channel antagonists such as nicardipine have the potential to interfere with the pulmonary response to hypoxia, increasing intrapulmonary blood shunt and thus worsening underlying hypoxemia. This article reports the case of a 40-year-old man suffering from lobar pneumonia, who developed a rapidly progressing hypoxemia after starting nicardipine infusion for blood pressure control. After ruling out all major causes of hypoxemic respiratory failure, the involvement of the calcium channel antagonist was strongly suspected. Hypoxemia caused by HPV release is an underreported side effect of calcium channel blockers. There are few clinical reports that describe the occurrence of this adverse event, and to our knowledge, only one other publication describes a patient suffering from infectious pneumopathy. In this article, we discuss the cellular mechanisms behind the HPV, as well as the pharmacology of calcium channel antagonists and their involvement in the development of acute respiratory failure. The purpose of this report is to remind clinicians dealing with patients affected by acute hypoxemia that pharmacologic HPV inhibition should be considered as part of the differential diagnosis, thus avoiding unnecessary costly and time-consuming assessments.

circNFXL1 Modulates the Kv2.1 Channel Function in Hypoxic Human Pulmonary Artery Smooth Muscle Cells via Sponging miR-29b-2-5p as a Competitive Endogenous RNA

ABSTRACT

Pulmonary arterial hypertension is characterized by abnormal pulmonary vasoconstriction and vascular remodeling caused by the dysregulation of K + channels in PA smooth muscle cells (PASMCs). However, how the K + channels are dysregulated is still unclear. Circular RNAs (circRNAs) are noncoding RNAs with a closed-loop structure capable of sponging microRNAs (miRs), thus regulating gene expression at the post-transcriptional level. Our previous studies have demonstrated the importance of one novel circRNA (hsa_circNFXL1_009, circNFXL1) in pulmonary arterial hypertension patients, playing as a critical regulator for K + channel activation in hypoxic human PASMCs (hPASMCs). Here, we explore the mechanisms underlying circNFXL1-regulated K + channel expression and functions in hypoxic hPASMCs. In cultured hPASMCs, the reduction of Kv current induced by hypoxia was significantly recovered by delivering exogenous circNFXL1. Moreover, luciferase, quantitative reverse transcription-quantitative polymerase chain reaction, western blot, and mutagenesis studies confirmed that circNFXL1 reversed hypoxia-induced inhibitory effects on the Kv2.1 channel via sponging hsa-miR-29b-2-5p (miR-29b-2). Furthermore, we found that circNFXL1 reversed the miR-29b-induced Kv2.1 channel dysfunction at the whole-cell and single-channel level in HEK cells using a patch-clamp. Finally, calcium imaging revealed that hypoxia also triggered a substantial rise in the cytosolic calcium concentration ([Ca2 + ]cyt) in hPASMCs, and this hypoxia-induced elevation of [Ca2 + ]cyt was reduced by circNFXL1 through miR-29b-2. These data suggested that circNFXL1-mediated regulation of the Kv2.1 channel activation and the related intracellular calcium concentration may contribute to the effects of hypoxic pulmonary vasoconstriction.

Health-Screening-Based Chronic Obstructive Pulmonary Disease and Its Effect on Cardiovascular Disease Risk

Chronic obstructive pulmonary disease (COPD) is considered a major cause of death worldwide, and various studies have been conducted for its early diagnosis. Our work developed a scoring system by predicting and validating COPD and performed predictive model implementations. Participants who underwent a health screening between 2017 and 2020 were extracted from the Korea National Health and Nutrition Examination Survey (KNHANES) database. COPD individuals were defined as aged 40 years or older with prebronchodilator forced expiratory volume in 1 s/forced vital capacity (FEV1/FVC < 0.7). The logistic regression model was performed, and the C-index was used for variable selection. Receiver operating characteristic (ROC) curves with area under the curve (AUC) values were generated for evaluation. Age, sex, waist circumference and diastolic blood pressure were used to predict COPD and to develop a COPD score based on a multivariable model. A simplified model for COPD was validated with an AUC value of 0.780 from the ROC curves. In addition, we evaluated the association of the derived score with cardiovascular disease (CVD). COPD scores showed significant performance in COPD prediction. The developed score also showed a good effect on the diagnostic ability for CVD risk. In the future, studies comparing the diagnostic accuracy of the derived scores with standard diagnostic tests are needed.

TRPC6, a therapeutic target for pulmonary hypertension

Idiopathic pulmonary arterial hypertension (PAH) is a fatal and progressive disease. Sustained vasoconstriction due to pulmonary arterial smooth muscle cell (PASMC) contraction and concentric arterial remodeling due partially to PASMC proliferation are the major causes for increased pulmonary vascular resistance and increased pulmonary arterial pressure in patients with precapillary pulmonary hypertension (PH) including PAH and PH due to respiratory diseases or hypoxemia. We and others observed upregulation of TRPC6 channels in PASMCs from patients with PAH. A rise in cytosolic Ca2+ concentration ([Ca2+]cyt) in PASMC triggers PASMC contraction and vasoconstriction, while Ca2+-dependent activation of PI3K/AKT/mTOR pathway is a pivotal signaling cascade for cell proliferation and gene expression. Despite evidence supporting a pathological role of TRPC6, no selective and orally bioavailable TRPC6 antagonist has yet been developed and tested for treatment of PAH or PH. In this study, we sought to investigate whether block of receptor-operated Ca2+ channels using a nonselective blocker of cation channels, 2-aminoethyl diphenylborinate (2-APB, administered intraperitoneally) and a selective blocker of TRPC6, BI-749327 (administered orally) can reverse established PH in mice. The results from the study show that intrapulmonary application of 2-APB (40 µM) or BI-749327 (3-10 µM) significantly and reversibly inhibited acute alveolar hypoxia-induced pulmonary vasoconstriction. Intraperitoneal injection of 2-APB (1 mg/kg per day) significantly attenuated the development of PH and partially reversed established PH in mice. Oral gavage of BI-749327 (30 mg/kg, every day, for 2 wk) reversed established PH by ∼50% via regression of pulmonary vascular remodeling. Furthermore, 2-APB and BI-749327 both significantly inhibited PDGF- and serum-mediated phosphorylation of AKT and mTOR in PASMC. In summary, the receptor-operated and mechanosensitive TRPC6 channel is a good target for developing novel treatment for PAH/PH. BI-749327, a selective TRPC6 blocker, is potentially a novel and effective drug for treating PAH and PH due to respiratory diseases or hypoxemia.

Kv7 channel inhibition increases hypoxic pulmonary vasoconstriction in endotoxemic mouse lungs

PURPOSE

Hypoxic pulmonary vasoconstriction (HPV) regulates regional pulmonary blood flow in order to match regional ventilation to preserve arterial oxygenation. HPV is impaired in patients with sepsis or acute respiratory distress syndrome (ARDS). Endotoxemic mice show reduced HPV and recent evidence suggests a central role of voltage gated potassium channel 7 (Kv7) in regulating HPV. Therefore, we tested the hypothesis if Kv7 is induced and inhibition of Kv7 increases HPV in endotoxemia.

MATERIALS AND METHODS

Isolated lungs of LPS-pretreated and untreated animals were perfused with and without specific inhibitors of Kv7 (linopirdine (LI) 0, 0.1, 1 and 10 µM) or Kv7.1 (HMR1556 100 nM). Pulmonary artery pressure (PAP) during normoxic (FiO₂ 0.21) as well as hypoxic (FiO₂ 0.01) ventilation were obtained. Expressions of Kv7 composing (KCNQ1-5) as well as auxiliary subunits (KCNE1-5) were measured in mouse lungs with and without endotoxemia.

RESULTS

HPV was impaired in lungs from LPS mice (16 ± 7% vs 105 ± 13% control, p < 0.05). Perfusion of control lungs with 10 µM LI or 100 nM HMR1556 did not affect HPV (LI 105 ± 12% vs 105 ± 13% vehicle, HMR1556 100 ± 6% vs 98 ± 26%, P = NS). In LPS mice perfusion with 10 µM LI (74.2 ± 7% vs. 16 ± 7% vehicle, P < 0.05) or HMR1556 100 nM augmented HPV (74 ± 28% vs. 15 ± 17% vehicle, P < 0.05). KCNQ1, 4 and 5 gene- and protein expressions as well as KCNE1, 2 and 4 gene expressions were unaltered in endotoxemic lungs. KCNE3 gene and protein expressions were increased in lungs of LPS treated mice (3.1 ± 1.3-fold and 1.8 ± 0.3-fold, respectively, P < 0.05 for both).

CONCLUSIONS

Endotoxemia does not alter KCNQ1, 4 and 5 gene and protein expressions but increases pulmonary KCNE3 gene and protein expression. In isolated perfused endotoxemic mouse lungs, perfusion with 10 µM LI or 100 nM HMR1556 augments HPV.

Biochemical and physiological properties of K+ channel-associated AKR6A (Kvβ) proteins

Voltage-gated potassium (Kv) channels play an essential role in the regulation of membrane excitability and thereby control physiological processes such as cardiac excitability, neural communication, muscle contraction, and hormone secretion. Members of the Kv1 and Kv4 families are known to associate with auxiliary intracellular Kvβ subunits, which belong to the aldo-keto reductase superfamily. Electrophysiological studies have shown that these proteins regulate the gating properties of Kv channels. Although the three gene products encoding Kvβ proteins are functional enzymes in that they catalyze the nicotinamide adenine dinucleotide phosphate (NAD[P]H)-dependent reduction of a wide range of aldehyde and ketone substrates, the physiological role for these proteins and how each subtype may perform unique roles in coupling membrane excitability with cellular metabolic processes remains unclear. Here, we discuss current knowledge of the enzymatic properties of Kvβ proteins from biochemical studies with their described and purported physiological and pathophysiological influences.

The effect of remote ischemic pre-conditioning on pulmonary vascular pressure and gas exchange in healthy humans during hypoxia

This study investigated whether rIPC alters the typical changes in pulmonary arterial pressure, pulmonary gas exchange associated with exercise in hypoxia.

METHODS

16 healthy adults were randomized to either rIPC treatment (n = 8) or control (n = 8). Afterward, subjects performed supine ergometry at constant load (30 W, 40˜50 rpm) for 25 min during hypoxia (12.5% O₂). Following a 90˜120 min rest, either rIPC or sham treatment was performed, which was then followed by post-assessment exercise. Throughout exercise, pulmonary arterial systolic pressure (PASP) and mean pulmonary arterial pressure (mPAP) were measured via echocardiography, while pulmonary gas exchange was being assessed.

RESULTS

The rICP group demonstrated improved PASP and mPAP (p < 0.05), whereas the control group did not. Additionally, breathing efficiency (V_E/V_CO2) and end-tidal CO₂ (PET_CO2) were improved in rIPC group (p < 0.05), but not in controls.

CONCLUSION

These data suggest that rIPC contributes to reduced pulmonary arterial pressure, and improved pulmonary gas exchange during hypoxic exercise. However, follow-up studies are needed to apply these findings to patient care settings.

Hypoxic Pulmonary Vasoconstriction in Humans: Tale or Myth

Hypoxic Pulmonary vasoconstriction (HPV) describes the physiological adaptive process of lungs to preserves systemic oxygenation. It has clinical implications in the development of pulmonary hypertension which impacts on outcomes of patients undergoing cardiothoracic surgery. This review examines both acute and chronic hypoxic vasoconstriction focusing on the distinct clinical implications and highlights the role of calcium and mitochondria in acute versus the role of reactive oxygen species and Rho GTPases in chronic HPV. Furthermore it identifies gaps of knowledge and need for further research in humans to clearly define this phenomenon and the underlying mechanism.

Indications for manual lung hyperinflation (MHI) in the mechanically ventilated patient with chronic obstructivepulmonary disease

Manual lung hyperinflation (MHI) can enhance secretion clearance, improve total lung/thorax compliance and assistin the resolution of acute atelectasis. To enhance secretion clearance in the intubated patient, the evidence highlights the need to maximize expiratory flow. Chronic pulmonary diseases such as chronic obstructive pulmonary disease(COPD) have often been cited as potential precautions and/or contra-indications to the use of manual lung hyperinflation (MHI). There is an absence of evidence on the effects of MHI in the patient with COPD. Research on the effects of mechanical ventilation in the patient with COPD providesa useful clinical examination of the effect of positive pressure on cardiac and pulmonary function. The potential effects of MHI in the COPD patient group were extrapolated on the basis of the MHI and mechanical ventilation literature. There is the potential for MHI to have both detrimental and beneficial effects on cardiac and pulmonary functionin patients with COPD. The potential detrimental effects of MHI may include either, increased intrinsic peep throughinadequate time for expiration by the breath delivery rate, tidal volume delivered or through the removal of appliedextemal PEEP thereby causing more dynamic airway compression compromising downward expiratory flow, which may also retard bronchial mucus transport. MHI may also increase right ventricular after load through raised intrathoracic pressures with lung hyperinflation, and may therefore impair right ventricular function in patients with evidence of cor pulmonale. There is the potential for beneficial effectsfrom MHI in the intubated COPD patient group (i.e., secretion clearance), but further research is required, especially on the effect of MHI on inspiratory and expiratory flowrate profiles in this patient group. The more controlled delivery of lung hyperinflation through the use of the mechanical ventilator may be a more optimal means of providinglunghyperinflation and shouldbe furtherinvestigated.

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 (K_v) 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 K_v1.5 channels in pulmonary arterial myocytes.

AMPK activation by 5‐aminoimidazole‐4‐carboxamide riboside, A769662 or C13 attenuated K_v1.5 currents in pulmonary arterial myocytes, and this effect was non‐additive with respect to K_v1.5 inhibition by hypoxia and mitochondrial poisons.

Recombinant AMPK phosphorylated recombinant human K_v1.5 channels in cell‐free assays, and inhibited K⁺ currents when introduced into HEK 293 cells stably expressing K_v1.5.

These results suggest that AMPK is the primary mediator of reductions in K_v1.5 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 (K_v) 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 K_v currents in pulmonary arterial myocytes, consistent with previously reported effects of mitochondrial inhibitors. Myocyte K_v 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 K_v1.5 channel inhibitor diphenyl phosphine oxide‐1 but unaffected by the presence of the BK_Ca channel blocker paxilline. Moreover, recombinant human K_v1.5 channels were phosphorylated by AMPK in cell‐free assays, and K⁺ currents carried by K_v1.5 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 K_v channel inhibition by hypoxia in pulmonary arterial myocytes, at least in part, through phosphorylation of K_v1.5 and/or an associated protein.

Progression of hypoxic pulmonary hypertension is thought to be due, in part, to suppression of voltage‐gated potassium channels (K_v) 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 K_v1.5 channels in pulmonary arterial myocytes.

AMPK activation by 5‐aminoimidazole‐4‐carboxamide riboside, A769662 or C13 attenuated K_v1.5 currents in pulmonary arterial myocytes, and this effect was non‐additive with respect to K_v1.5 inhibition by hypoxia and mitochondrial poisons.

Recombinant AMPK phosphorylated recombinant human K_v1.5 channels in cell‐free assays, and inhibited K⁺ currents when introduced into HEK 293 cells stably expressing K_v1.5.

These results suggest that AMPK is the primary mediator of reductions in K_v1.5 channels following inhibition of mitochondrial oxidative phosphorylation during hypoxia and by mitochondrial poisons.

Targeted inhibition of survivin with YM155 promotes apoptosis of hypoxic human pulmonary arterial smooth muscle cells via the upregulation of voltage-dependent K⁺ channels

Hypoxic pulmonary hypertension (PH) is a common disease characterized by a disturbance to the balance of apoptosis and cell proliferation in pulmonary artery smooth muscle cells (PASMCs). The anti-apoptotic protein, survivin, has been observed to be upregulated in pulmonary arteries (PAs) of chronic hypoxia-induced PH rats. The present study aimed to investigate the therapeutic potential of sepantronium bromide (YM155), a selective survivin inhibitor, on hypoxic human PASMCs and examine the potential underlying mechanisms. Cultured human PASMCs (HPASMCs) were randomly divided into the following groups: i) Normoxia (N); ii) normoxia + 100 nmol/l YM155 (NY100); iii) hypoxia (H); iv) hypoxia + 1 nmol/l YM155 (HY1); v) hypoxia + 10 nmol/l YM155 (HY10); and hypoxia + 100 nmol/l YM155 (HY100) groups. The cells were exposed to the different conditions for 24 h, according to the group. Cell viability was then determined using a Cell Counting Kit-8 assay, and apoptosis was detected using a terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling assay. The expression levels of survivin were determined using reverse transcription-quantitative polymerase chain reaction (RT-qPCR), immunocytochemistry and Western blot analyses. The expression levels of the voltage-dependent K⁺ (K_v) channels, K_v1.5 and K_v2.1, were measured using RT-qPCR and Western blotting. Cell proliferation in the hypoxic PASMCs was significantly increased by hypoxia, however, apoptosis of the HPASMCs was suppressed, the expression of survivin were upregulated and the expression levels of K_v1.5 and K_v2.1 were downregulated. YM155 treatment ameliorated the hypoxia-induced increase in cell proliferation and expression of survivin in a concentration-dependent manner, increased apoptosis, and increased the expression levels of K_v1.5 and K_v2.1 (P<0.05). By contrast, YM155 treatment in normoxic HPASMCs had no significant effects on proliferation, apop-tosis, or the expression levels of survivin and K_v channels in the PASMCs. The present study is the first, to the best of our knowledge, to demonstrate that YM155, a selective survivin inhibitor, has a beneficial therapeutic effect on hypoxic HPASMCs, and that YM155 induces a pro-apoptotic effect by downregulating the apoptosis inhibitor, survivin, possibly through a K_v channel-mediated mechanism.

Ca(2+) and ion channels in hypoxia-mediated pulmonary hypertension

Alveolar hypoxia, a consequence of many lung diseases, can have adverse effects on the pulmonary vasculature. The changes that occur in the pulmonary circulation with exposure to chronic hypoxia include reductions in the diameter of the pulmonary arteries due to structural remodeling of the vasculature. Although the structural and functional changes that occur in the development of pulmonary hypertension have been well investigated, less is known about the cellular and molecular mechanisms of this process. This review will discuss the role of several potassium and calcium channels in hypoxic pulmonary vasoconstriction, both in elevating calcium influx into pulmonary artery smooth muscle cells (PASMCs). In addition to other signal transduction pathways, Ca(2+) signaling in PASMCs plays an important role in the development and progression of pulmonary hypertension due to its central roles in vasoconstriction and vascular remodeling. This review will focus on the effect of chronic hypoxia on ion channels and the potential pathogenic role of Ca(2+) signaling and regulation in the progression of pulmonary hypertension.

MicroRNA-190 regulates hypoxic pulmonary vasoconstriction by targeting a voltage-gated K⁺ channel in arterial smooth muscle cells

Pulmonary arterial hypertension (PAH) is associated with sustained vasoconstriction, profound structural remodeling of vasculatures and alterations in Ca(2+) homeostasis in arterial smooth muscle cells (SMCs), while the underlying mechanisms are still elusive. By regulating the expression of proteins, microRNAs (miRNAs) are known to play an important role in cell fates including differentiation, apoptosis and proliferation, and may be involved in the development of PAH. Based on our previous study, hypoxia produced a significant increase of the miR-190 level in the pulmonary artery (PA), here, we used synthetic miR-190 to mimic the increase in hypoxic conditions and showed evidence for the effects of miR-190 on pulmonary arterial vasoconstriction and Ca(2+) influx in arterial SMCs. Synthetic miR-190 remarkably enhanced the vasoconstriction responses to phenylephrine (PE) and KCl. The voltage-gated K(+) channel subfamily member, Kcnq5, mRNA was shown to be a target for miR-190. Meanwhile, miR-190 antisense oligos can partially reverse the effects of miR-190 on PASMCs and PAs. Therefore, these results suggest that miR-190 appears to be a positive regulator of Ca(2+) influx, and plays an important role in hypoxic pulmonary vascular constriction.

Oxidative modulation of voltage-gated potassium channels

SIGNIFICANCE

Voltage-gated K+ channels are a large family of K+-selective ion channel protein complexes that open on membrane depolarization. These K+ channels are expressed in diverse tissues and their function is vital for numerous physiological processes, in particular of neurons and muscle cells. Potentially reversible oxidative regulation of voltage-gated K+ channels by reactive species such as reactive oxygen species (ROS) represents a contributing mechanism of normal cellular plasticity and may play important roles in diverse pathologies including neurodegenerative diseases.

RECENT ADVANCES

Studies using various protocols of oxidative modification, site-directed mutagenesis, and structural and kinetic modeling provide a broader phenomenology and emerging mechanistic insights.

CRITICAL ISSUES

Physicochemical mechanisms of the functional consequences of oxidative modifications of voltage-gated K+ channels are only beginning to be revealed. In vivo documentation of oxidative modifications of specific amino-acid residues of various voltage-gated K+ channel proteins, including the target specificity issue, is largely absent.

FUTURE DIRECTIONS

High-resolution chemical and proteomic analysis of ion channel proteins with respect to oxidative modification combined with ongoing studies on channel structure and function will provide a better understanding of how the function of voltage-gated K+ channels is tuned by ROS and the corresponding reducing enzymes to meet cellular needs.

Endothelial and smooth muscle cell ion channels in pulmonary vasoconstriction and vascular remodeling

The pulmonary circulation is a low resistance and low pressure system. Sustained pulmonary vasoconstriction and excessive vascular remodeling often occur under pathophysiological conditions such as in patients with pulmonary hypertension. Pulmonary vasoconstriction is a consequence of smooth muscle contraction. Many factors released from the endothelium contribute to regulating pulmonary vascular tone, while the extracellular matrix in the adventitia is the major determinant of vascular wall compliance. Pulmonary vascular remodeling is characterized by adventitial and medial hypertrophy due to fibroblast and smooth muscle cell proliferation, neointimal proliferation, intimal, and plexiform lesions that obliterate the lumen, muscularization of precapillary arterioles, and in situ thrombosis. A rise in cytosolic free Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary artery smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction, while increased release of mitogenic factors, upregulation (or downregulation) of ion channels and transporters, and abnormalities in intracellular signaling cascades are key to the remodeling of the pulmonary vasculature. Changes in the expression, function, and regulation of ion channels in PASMC and pulmonary arterial endothelial cells play an important role in the regulation of vascular tone and development of vascular remodeling. This article will focus on describing the ion channels and transporters that are involved in the regulation of pulmonary vascular function and structure and illustrating the potential pathogenic role of ion channels and transporters in the development of pulmonary vascular disease.

Disappearance of hypoxic pulmonary vasoconstriction and o2-sensitive nonselective cationic current in arterial myocytes of rats under ambient hypoxia

Acute hypoxia induces contraction of pulmonary artery (PA) to protect ventilation/perfusion mismatch in lungs. As for the cellular mechanism of hypoxic pulmonary vasoconstriction (HPV), hypoxic inhibition of voltage-gated K⁺ channel (Kv) in PA smooth muscle cell (PASMC) has been suggested. In addition, our recent study showed that thromboxane A₂ (TXA₂) and hypoxia-activated nonselective cation channel (I_NSC) is also essential for HPV. However, it is not well understood whether HPV is maintained in the animals exposed to ambient hypoxia for two days (2d-H). Specifically, the associated electrophysiological changes in PASMCs have not been studied. Here we investigate the effects of 2d-H on HPV in isolated ventilated/perfused lungs (V/P lungs) from rats. HPV was almost abolished without structural remodeling of PA in 2d-H rats, and the lost HPV was not recovered by Kv inhibitor, 4-aminopyridine. Patch clamp study showed that the hypoxic inhibition of Kv current in PASMC was similar between 2d-H and control. In contrast, hypoxia and TXA₂-activated I_NSC was not observed in PASMCs of 2d-H. From above results, it is suggested that the decreased I_NSC might be the primary functional cause of HPV disappearance in the relatively early period (2 d) of hypoxia.

Signal transduction in the development of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a unique disease. Properly speaking, it is not a disease of the lung. It can be seen more as a microvascular disease occurring mainly in the lungs and affecting the heart. At the cellular level, the PAH paradigm is characterized by inflammation, vascular tone imbalance, pulmonary arterial smooth muscle cell proliferation and resistance to apoptosis and the presence of in situ thrombosis. At a clinical level, the aforementioned abnormal vascular properties alter physically the pulmonary circulation and ventilation, which greatly influence the right ventricle function as it highly correlates with disease severity. Consequently, right heart failure remains the principal cause of death within this cohort of patients. While current treatment modestly improve patients' conditions, none of them are curative and, as of today, new therapies are lacking. However, the future holds potential new therapies that might have positive influence on the quality of life of the patient. This article will first review the clinical presentation of the disease and the different molecular pathways implicated in the pathobiology of PAH. The second part will review tomorrow's future putative therapies for PAH.

Hypoxia induces voltage-gated K+ (Kv) channel expression in pulmonary arterial smooth muscle cells through hypoxia-inducible factor-1 (HIF-1)

Hypoxia-inducible factor-1 (HIF-1) regulates the expression of hypoxia-inducible genes by binding erythropoietin (EPO) enhancer fragments. Of these genes, HIF-1 upregulates voltage-gated K+1.2 channels (Kv1.2) in rat PC12 cells. Whether HIF-1 regulates hypoxia-induced Kv channel expression in cultured pulmonary artery smooth muscle cells (PASMCs), however, has not been determined. In this study, we investigated the effects of hypoxia on the expression of Kv1.2 Kv1.5, Kv2.1, and Kv9.3 channels in PASMCs and examined the direct role of HIF-1 by transfecting either wild type or mutant EPO enhancer fragments. Our results showed that 18 h exposure to hypoxia significantly increased the expression of Kv1.2, Kv1.5, Kv2.1, and Kv9.3; and this hypoxia-induced upregulation was completely inhibited after transfection with the wild type but not mutant EPO enhancer fragment. These results indicate that HIF-1 regulates hypoxia-stimulated induction of Kv1.2 Kv1.5, Kv2.1, and Kv9.3 channels in cultured PASMCs.

STIM2 Contributes to Enhanced Store-operated Ca Entry in Pulmonary Artery Smooth Muscle Cells from Patients with Idiopathic Pulmonary Arterial Hypertension

Pulmonary vasoconstriction and vascular remodeling are two major causes for elevated pulmonary vascular resistance and pulmonary arterial pressure in patients with idiopathic pulmonary arterial hypertension (IPAH). An increase in cytosolic free Ca²⁺concentration ([Ca²⁺]_cyt) in pulmonary artery smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and an important stimulus for PASMC proliferation, which causes pulmonary vascular remodeling. Store-operated Ca²⁺ entry (SOCE), induced by depletion of stored Ca²⁺ in the sarcoplasmic reticulum (SR), can increase [Ca²⁺]_cyt in PASMC, independent of other means of Ca²⁺ entry. Stromal interaction molecule (STIM) proteins, STIM1 and STIM2, were both recently identified as sensors for store depletion and also signaling molecules to open store-operated Ca²⁺ channels. We previously reported that SOCE was significantly enhanced in PASMC from IPAH patients compared to PASMC from normotensive control subjects. Enhanced SOCE plays an important role in the pathophysiological changes in PASMC associated with pulmonary arterial hypertension. In this study, we examine whether the expression levels of STIM1 and STIM2 are altered in IPAH-PASMC compared to control PASMC, and whether these putative changes in the STIM1 and STIM2 expression levels are responsible for enhanced SOCE and proliferation in IPAH-PASMC. Compared to control PASMC, the protein expression level of STIM2 was significantly increased in IPAH-PASMC, whereas STIM1 protein expression was not significantly changed. In IPAH-PASMC, the small interfering RNA (siRNA)-mediated knockdown of STIM2 decreased SOCE and proliferation, while knockdown of STIM2 in control PASMC had no effect on either SOCE or proliferation. Overexpression of STIM2 in the control PASMC failed to enhance SOCE or proliferation. These data indicate that enhanced protein expression of STIM2 is necessary, but not sufficient, for enhanced SOCE and proliferation of IPAH-PASMC.

Reciprocal regulation of Rac1 and PAK-1 by HIF-1alpha: a positive-feedback loop promoting pulmonary vascular remodeling

Pulmonary vascular remodeling associated with pulmonary hypertension is characterized by media thickening, disordered proliferation, and in situ thrombosis. The p21-activated kinase-1 (PAK-1) can control growth, migration, and prothrombotic activity, and the hypoxia-inducible transcription factor HIF-1alpha was associated with pulmonary vascular remodeling. Here we studied whether PAK-1 and HIF-1alpha are linked in pulmonary vascular remodeling. PAK-1 was expressed in the media of remodeled pulmonary vessels from patients with pulmonary vasculopathy and was upregulated, together with its upstream regulator Rac1 and HIF-1alpha in lung tissue from lambs with pulmonary vascular remodeling. PAK-1 and Rac1 were activated by thrombin involving calcium, thus resulting in enhanced generation of reactive oxygen species (ROS) in human pulmonary artery smooth muscle cells (PASMCs). Activation of PAK-1 stimulated HIF activity and HIF-1alpha expression involving ROS and NF-kappaB, enhanced the expression of the HIF-1 target gene plasminogen activator inhibitor-1, and stimulated PASMC proliferation. Importantly, HIF-1 itself bound to the Rac1 promoter and enhanced Rac1 and PAK-1 transcription. Thus, PAK-1 and its activator Rac1 are novel HIF-1 targets that may constitute a positive-feedback loop for induction of HIF-1alpha by thrombin and ROS, thus explaining elevated levels of PAK-1, Rac1, and HIF-1alpha in remodeled pulmonary vessels.

Hypoxia-induced changes in pulmonary and systemic vascular resistance: where is the O2 sensor?

Pulmonary arteries (PA) constrict in response to alveolar hypoxia, whereas systemic arteries (SA) undergo dilation. These physiological responses reflect the need to improve gas exchange in the lung, and to enhance the delivery of blood to hypoxic systemic tissues. An important unresolved question relates to the underlying mechanism by which the vascular cells detect a decrease in oxygen tension and translate that into a signal that triggers the functional response. A growing body of work implicates the mitochondria, which appear to function as O2 sensors by initiating a redox-signaling pathway that leads to the activation of downstream effectors that regulate vascular tone. However, the direction of this redox signal has been the subject of controversy. Part of the problem has been the lack of appropriate tools to assess redox signaling in live cells. Recent advancements in the development of redox sensors have led to studies that help to clarify the nature of the hypoxia-induced redox signaling by reactive oxygen species (ROS). Moreover, these studies provide valuable insight regarding the basis for discrepancies in earlier studies of the hypoxia-induced mechanism of redox signaling. Based on recent work, it appears that the O2 sensing mechanism in both the PA and SA are identical, that mitochondria function as the site of O2 sensing, and that increased ROS release from these organelles leads to the activation of cell-specific, downstream vascular responses.

Ion channel trafficking: a new therapeutic horizon for atrial fibrillation

Atrial fibrillation (AF) is a common cardiac arrhythmia with potentially life-threatening complications. Drug therapies for treatment of AF that seek long-term maintenance of normal sinus rhythm remain elusive due in large part to proarrhythmic ventricular actions. Kv1.5, which underlies the atrial specific I(Kur) current, is a major focus of research efforts seeking new therapeutic strategies and targets. Recent work has shown a novel effect of antiarrhythmic drugs where compounds that block Kv1.5 channel current also can alter ion channel trafficking. This work further suggests that the pleiotropic effects of antiarrhythmic drugs may be separable. Although this finding highlights the therapeutic potential for selective manipulation of ion channel surface density, it also reveals an uncertainty regarding the specificity of modulating trafficking pathways without risk of off-target effects. Future studies may show that specific alteration of Kv1.5 trafficking can overcome the proarrhythmic limitations of current pharmacotherapy and provide an effective method for long-term cardioversion in AF.

The role of classical transient receptor potential channels in the regulation of hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an essential mechanism of the lung matching blood perfusion to ventilation during local alveolar hypoxia. HPV thus optimizes pulmonary gas exchange. In contrast chronic and generalized hypoxia leads to pulmonary vascular remodeling with subsequent pulmonary hypertension and right heart hypertrophy. Among other non-selective cation channels, the family of classical transient receptor potential channels (TRPC) has been shown to be expressed in pulmonary arterial smooth muscle cells. Among this family, TRPC6 is essential for the regulation of acute HPV in mice. Against this background, in this chapter we give an overview about the TRPC family and their role in HPV.

Hypoxia triggers subcellular compartmental redox signaling in vascular smooth muscle cells

RATIONALE

Recent studies have implicated mitochondrial reactive oxygen species (ROS) in regulating hypoxic pulmonary vasoconstriction (HPV), but controversy exists regarding whether hypoxia increases or decreases ROS generation.

OBJECTIVE

This study tested the hypothesis that hypoxia induces redox changes that differ among subcellular compartments in pulmonary (PASMCs) and systemic (SASMCs) smooth muscle cells.

METHODS AND RESULTS

We used a novel, redox-sensitive, ratiometric fluorescent protein sensor (RoGFP) to assess the effects of hypoxia on redox signaling in cultured PASMCs and SASMCs. Using genetic targeting sequences, RoGFP was expressed in the cytosol (Cyto-RoGFP), the mitochondrial matrix (Mito-RoGFP), or the mitochondrial intermembrane space (IMS-RoGFP), allowing assessment of oxidant signaling in distinct intracellular compartments. Superfusion of PASMCs or SASMCs with hypoxic media increased oxidation of both Cyto-RoGFP and IMS-RoGFP. However, hypoxia decreased oxidation of Mito-RoGFP in both cell types. The hypoxia-induced oxidation of Cyto-RoGFP was attenuated through the overexpression of cytosolic catalase in PASMCs.

CONCLUSIONS

These results indicate that hypoxia causes a decrease in nonspecific ROS generation in the matrix compartment, whereas it increases regulated ROS production in the IMS, which diffuses to the cytosol of both PASMCs and SASMCs.

Upregulation of vascular calcium channels in neonatal piglets with hypoxia-induced pulmonary hypertension

Inhibition of voltage-gated, L-type Ca(2+) (Ca(L)) channels by clinical calcium channel blockers provides symptomatic improvement to some pediatric patients with pulmonary arterial hypertension (PAH). The present study investigated whether abnormalities of vascular Ca(L) channels contribute to the pathogenesis of neonatal PAH using a newborn piglet model of hypoxia-induced PAH. Neonatal piglets exposed to chronic hypoxia (CH) developed PAH by 21 days, which was evident as a 2.1-fold increase in pulmonary vascular resistance in vivo compared with piglets raised in normoxia (N). Transpulmonary pressures (DeltaPtp) in the corresponding isolated perfused lungs were 20.5 +/- 2.1 mmHg (CH) and 11.6 +/- 0.8 mmHg (N). Nifedipine reduced the elevated DeltaPtp in isolated lungs of CH piglets by 6.4 +/- 1.3 mmHg but only reduced DeltaPtp in lungs of N piglets by 1.9 +/- 0.2 mmHg. Small pulmonary arteries from CH piglets also demonstrated accentuated Ca(2+)-dependent contraction, and Ca(2+) channel current was 3.94-fold higher in the resident vascular muscle cells. Finally, although the level of mRNA encoding the pore-forming alpha(1C)-subunit of the Ca(L) channel was similar between small pulmonary arteries from N and CH piglets, a profound and persistent upregulation of the vascular alpha(1C) protein was detected by 10 days in CH piglets at a time when pulmonary vascular resistance was only mildly elevated. Thus chronic hypoxia in the neonate is associated with the anomalous upregulation of Ca(L) channels in small pulmonary arteries in vivo and the resulting abnormal Ca(2+)-dependent resistance may contribute to the pathogenesis of PAH.

Inhibition effect of arachidonic acid on hypoxia-induced [Ca(2+)](i) elevation in PC12 cells and human pulmonary artery smooth muscle cells

[Ca(2+)](i) elevation is a key event when O(2) sensitive cells, e.g. PC12 cells and pulmonary artery smooth muscle cells, face hypoxia. Ca(2+) entry pathways in mediating hypoxia-induced [Ca(2+)](i) elevation include: voltage-gated Ca(2+) channels (VGCCs), transient receptor potential (TRP) channel and Na(+)-Ca(2+) ex-changer (NCX). In the pulmonary artery, accumulated evidence strongly suggests that prostaglandins (PGs) are involved in pulmonary inflammation and cause vasoconstriction during hypoxia. In this study, we investigated the effect of arachidonic acid (AA), the upstream substrate for PGs, on hypoxia response in O(2) sensitive cells. Exogenous application of AA significantly inhibited hypoxia-induced [Ca(2+)](i) elevation. This effect was due to AA itself rather than its degenerative products. The pharmacological modulation of endogenous AA showed that the prevention of AA generation by blockage of cPLA2, diacylglycerol (DAG) lipase and fatty acid hydrolysis (FAAH), augments hypoxia-induced [Ca(2+)](i) elevation, whereas prevention of AA degeneration attenuates hypoxia-induced [Ca(2+)](i) elevation. Over-expression of COX2 enhances hypoxia-induced [Ca(2+)](i) elevation and this enhancement is reversed by exogenous AA. Our results suggest that AA inhibits hypoxia response. The dynamic alterations in cellular lipids might determine cell response to hypoxia.

Oxygen sensing in hypoxic pulmonary vasoconstriction: using new tools to answer an age-old question

Hypoxic pulmonary vasoconstriction (HPV) becomes activated in response to alveolar hypoxia and, although the characteristics of HPV have been well described, the underlying mechanism of O(2) sensing which initiates the HPV response has not been fully established. Mitochondria have long been considered as a putative site of oxygen sensing because they consume O(2) and therefore represent the intracellular site with the lowest oxygen tension. However, two opposing theories have emerged regarding mitochondria-dependent O(2) sensing during hypoxia. One model suggests that there is a decrease in mitochondrial reactive oxygen species (ROS) levels during the transition from normoxia to hypoxia, resulting in the shift in cytosolic redox to a more reduced state. An alternative model proposes that hypoxia paradoxically increases mitochondrial ROS signalling in pulmonary arterial smooth muscle. Experimental resolution of the question of whether the mitochondrial ROS levels increase or decrease during hypoxia has been problematic owing to the technical limitations of the tools used to assess oxidant stress as well as the pharmacological agents used to inhibit the mitochondrial electron transport chain. However, recent developments in genetic techniques and redox-sensitive probes may allow us eventually to reach a consensus concerning the O(2) sensing mechanism underlying HPV.

Vascular adaptations to hypoxia: molecular and cellular mechanisms regulating vascular tone

Several molecular and cellular adaptive mechanisms to hypoxia exist within the vasculature. Many of these processes involve oxygen sensing which is transduced into mediators of vasoconstriction in the pulmonary circulation and vasodilation in the systemic circulation. A variety of oxygen-responsive pathways, such as HIF (hypoxia-inducible factor)-1 and HOs (haem oxygenases), contribute to the overall adaptive process during hypoxia and are currently an area of intense research. Generation of ROS (reactive oxygen species) may also differentially regulate vascular tone in these circulations. Potential candidates underlying the divergent responses between the systemic and pulmonary circulations may include Nox (NADPH oxidase)-derived ROS and mitochondrial-derived ROS. In addition to alterations in ROS production governing vascular tone in the hypoxic setting, other vascular adaptations are likely to be involved. HPV (hypoxic pulmonary vasoconstriction) and CH (chronic hypoxia)-induced alterations in cellular proliferation, ionic conductances and changes in the contractile apparatus sensitivity to calcium, all occur as adaptive processes within the vasculature.

Pathogenic mechanisms of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a complex disease that causes significant morbidity and mortality and is clinically characterized by an increase in pulmonary vascular resistance. The histopathology is marked by vascular proliferation/fibrosis, remodeling, and vessel obstruction. Development of PAH involves the complex interaction of multiple vascular effectors at all anatomic levels of the arterial wall. Subsequent vasoconstriction, thrombosis, and inflammation ensue, leading to vessel wall remodeling and cellular hyperproliferation as the hallmarks of severe disease. These processes are influenced by genetic predisposition as well as diverse endogenous and exogenous stimuli. Recent studies have provided a glimpse at certain molecular pathways that contribute to pathogenesis; these have led to the identification of attractive targets for therapeutic intervention. We will review our current understanding of the mechanistic underpinnings of the genetic and exogenous/acquired triggers of PAH. The resulting imbalance of vascular effectors provoking pathogenic vascular changes will also be discussed, with an emphasis on common and overarching regulatory pathways that may relate to the primary triggers of disease. The current conceptual framework should allow for future studies to refine our understanding of the molecular pathogenesis of PAH and improve the therapeutic regimen for this disease.

Pulmonary hypertension and chronic cor pulmonale in COPD

Hypoxia and endothelial dysfunction play a central role in the development of pulmonary hypertension. Cor pulmonale is a maladaptive response to pulmonary hypertension. The presence of peripheral edema in cor pulmonale is almost invariably associated with hypercapnia. Correction of abnormalities of gas exchange and ventilation can ameliorate pulmonary hypertension and improve survival. This review focuses on new information about the pathogenesis and treatment of pulmonary hypertension in COPD including information derived from lung volume reduction surgery, the role of brain natriuretic peptide, exhaled nitric oxide for diagnosis, and the treatment of cor pulmonale with recently available specific pulmonary vasodilators.

[Pathophysiology of the lung inhalation injuries]

<zakljucak> U ovom radu prikazani su neki od patofizioloskih aspekata inhalacionih povreda pluca, nezavisno od toga da li su bile udruzene s opekotinama ili nisu. Mada tacan mehanizam akutne inhalacione povrede pluca nije razjasnjen u potpunosti, na osnovu mnogobrojnih istrazivanja moze se zakljuciti da ogromnu ulogu u tim procesima imaju NO, PARP i opstrukcija vazdusnih puteva.

SUMO modification regulates inactivation of the voltage-gated potassium channel Kv1.5

The voltage-gated potassium (Kv) channel Kv1.5 mediates the I(Kur) repolarizing current in human atrial myocytes and regulates vascular tone in multiple peripheral vascular beds. Understanding the complex regulation of Kv1.5 function is of substantial interest because it represents a promising pharmacological target for the treatment of atrial fibrillation and hypoxic pulmonary hypertension. Herein we demonstrate that posttranslational modification of Kv1.5 by small ubiquitin-like modifier (SUMO) proteins modulates Kv1.5 function. We have identified two membrane-proximal and highly conserved cytoplasmic sequences in Kv1.5 that conform to established SUMO modification sites in transcription factors. We find that Kv1.5 interacts specifically with the SUMO-conjugating enzyme Ubc9 and is a target for modification by SUMO-1, -2, and -3 in vivo. In addition, purified recombinant Kv1.5 serves as a substrate in a minimal in vitro reconstituted SUMOylation reaction. The SUMO-specific proteases SENP2 and Ulp1 efficiently deconjugate SUMO from Kv1.5 in vivo and in vitro, and disruption of the two identified target motifs results in a loss of the major SUMO-conjugated forms of Kv1.5. In whole-cell patch-clamp electrophysiological studies, loss of Kv1.5 SUMOylation, by either disruption of the conjugation sites or expression of the SUMO protease SENP2, leads to a selective approximately 15-mV hyperpolarizing shift in the voltage dependence of steady-state inactivation. Reversible control of voltage-sensitive channels through SUMOylation constitutes a unique and likely widespread mechanism for adaptive tuning of the electrical excitability of cells.

Increases in mitochondrial reactive oxygen species trigger hypoxia-induced calcium responses in pulmonary artery smooth muscle cells

Mitochondria have been implicated as a potential site of O(2) sensing underlying hypoxic pulmonary vasoconstriction (HPV), but 2 disparate models have been proposed to explain their reaction to hypoxia. One model proposes that hypoxia-induced increases in mitochondrial reactive oxygen species (ROS) generation activate HPV through an oxidant-signaling pathway, whereas the other proposes that HPV is a result of decreased oxidant signaling. In an attempt to resolve this debate, we use a novel, ratiometric, redox-sensitive fluorescence resonance energy transfer (HSP-FRET) probe, in concert with measurements of reduced/oxidized glutathione (GSH/GSSG), to assess cytosolic redox responses in cultured pulmonary artery smooth muscle cells (PASMCs). Superfusion of PASMCs with hypoxic media increases the HSP-FRET ratio and decreases GSH/GSSG, indicating an increase in oxidant stress. The antioxidants pyrrolidinedithiocarbamate and N-acetyl-l-cysteine attenuated this response, as well as the hypoxia-induced increases in cytosolic calcium ([Ca(2+)](i)), assessed by the Ca(2+)-sensitive FRET sensor YC2.3. Adenoviral overexpression of glutathione peroxidase or cytosolic or mitochondrial catalase attenuated the hypoxia-induced increase in ROS signaling and [Ca(2+)](i). Adenoviral overexpression of cytosolic Cu, Zn-superoxide dismutase (SOD-I) had no effect on the hypoxia-induced increase in ROS signaling and [Ca(2+)](i), whereas mitochondrial matrix-targeted Mn-SOD (SOD-II) augmented [Ca(2+)](i). The mitochondrial inhibitor myxothiazol attenuated the hypoxia-induced changes in the ROS signaling and [Ca(2+)](i), whereas cyanide augmented the increase in [Ca(2+)](i). Finally, simultaneous measurement of ROS and Ca(2+) signaling in the same cell revealed that the initial increase in these 2 signals could not be distinguished temporally. These results demonstrate that hypoxia triggers increases in PASMC [Ca(2+)](i) by augmenting ROS signaling from the mitochondria.

Hypoxemic Fetoplacental Vasoconstriction: A Graduated Response to Reduced Oxygen Conditions in the Human Placenta

We investigated the characteristics of hypoxemic fetoplacental vasoconstriction (HFPV) in the dual perfused, single isolated human placental cotyledon. Fetal arterial blood pressures (FAP) were measured in four cotyledons (Group 1) equilibrated with 21% oxygen (O2), 5% carbon dioxide (CO2), and nitrogen (N2) [control] followed by 5% CO2 in N2 for 30 min. FAP (mean +/- sd) increased from 69.8 (+/- 6.4) to 105 (+/- 3.0) mm Hg (P < 0.05), confirming the utility of HFPV in the human placenta. Eight more cotyledons (Group 2) were exposed sequentially and alternately at 15-min intervals to the control gases and to gas blends containing 15%, 12%, 5%, and 0% O2 with 5% CO2 and N2. FAP increased significantly (P < 0.05) in a stepwise fashion from 68.7 (+/- 3.7) to 70.5 (+/- 3.3) mm Hg with 15% O2; from 69.3 (+/- 3.8) to 72.4 (+/- 4.3) mm Hg with 12% O2; from 67.8 (+/- 3.2) to 74.5 (+/- 3.4) mm Hg with 5% O2; and from 69.7 (+/- 3.4) to 77.9 (+/- 5.9) mm Hg with 0% O2, suggesting that HFPV is a graduated response to reduced O2 conditions in the human placenta.

Pulmonary arterial hypertension

Pulmonary arterial hypertension is a disease of the small pulmonary arteries characterized by vascular narrowing and increased pulmonary vascular resistance, which eventually leads to right ventricular failure. Vasoconstriction, vascular proliferation, remodeling of the pulmonary vessels, and thrombosis are all contributing factors to the increased vascular resistance seen in this disease. Pulmonary arterial hypertension develops as a sporadic disease (idiopathic), as an inherited disorder (familial), or in association with certain conditions (collagen vascular diseases, portal hypertension, human immunodeficiency virus infection, congenital systemic-to-pulmonary shunts, ingestion of drugs or dietary products, or persistent fetal circulation). The pathogenesis of pulmonary arterial hypertension is a complicated, multifactorial process. It seems doubtful that any one factor alone is sufficient to activate the necessary pathways leading to the development of this disease. Rather, clinically apparent pulmonary arterial hypertension most likely develops after a second insult occurs in an individual who is already susceptible owing to genetic factors, environmental exposures, or acquired disorders. Currently, there is no cure for pulmonary arterial hypertension but several novel therapeutic options are now available that can improve symptoms and increase survival.

BMP signaling controls PASMC KV channel expression in vitro and in vivo

Bone morphogenetic proteins (BMPs) have been implicated in the pathogenesis of familial pulmonary arterial hypertension. The type 2 receptor (BMPR2) is required for recognition of all BMPs. Transgenic mice with a smooth muscle cell-targeted mutation in this receptor (SM22-tet-BMPR2(delx4+)) developed increased pulmonary artery pressure, associated with a modest increase in arterial muscularization, after 8 wk of transgene activation (West J, Fagan K, Steudel W, Fouty B, Lane K, Harral J, Hoedt-Miller M, Tada Y, Ozimek J, Tuder R, and Rodman DM. Circ Res 94: 1109-1114, 2004). In the present study, we show that these transgenic mice developed increased right ventricular pressures after only 1 wk of transgene activation, without significant remodeling of the vasculature. We then tested the hypothesis that the increased pulmonary artery pressure due to loss of BMPR2 signaling was mediated by reduced K(V) channel expression. There was decreased expression of K(V)1.1, K(V)1.5, and K(V)4.3 mRNA isolated from whole lung. Western blot confirmed decreased K(V)1.5 protein in these lungs. Human pulmonary artery smooth muscle cells (PASMC) treated with recombinant BMP2 had increased K(V)1.5 protein and macroscopic K(V) current density, which was blocked by anti-K(V)1.5 antibody. In vivo, nifedipine, a selective L-type Ca(2+) channel blocker, reduced RV systolic pressure in these dominant-negative BMPR2 mice to levels seen in control animals. This suggests that activation of L-type Ca(2+) channels caused by reduced K(V)1.5 mediates increased pulmonary artery pressure in these animals. These studies suggest that BMP regulates K(V) channel expression and that loss of this signaling pathway in PASMC through a mutation in BMPR2 is sufficient to cause pulmonary artery vasoconstriction.

Redox Activation of Intracellular Calcium Release Channels (Ryanodine Receptors) in the Sustained Phase of Hypoxia-Induced Pulmonary Vasoconstriction

Hypoxia-induced pulmonary vasoconstriction (HPV) is an important adaptive process that remains incompletely understood. In preconstricted rat pulmonary arteries (inner diameter, 250 to 400 microm), hypoxia (pO2 approximately 10 mm Hg) induces an initial transient phase and a more slowly developing sustained phase of vasoconstriction. Since the release of calcium ions (Ca2+) from intracellular stores by redox-sensitive intracellular Ca2+ release channels known as ryanodine receptors (RyRs) in pulmonary arterial smooth-muscle cells (PASMCs) may play a role in HPV, and considerable evidence now supports that levels of reactive oxygen species (ROS) are paradoxically increased in PASMC under hypoxia, we investigated whether redox activation of RyRs by ROS may transduce HPV. By reverse transcriptase-polymerase chain reaction, we found that all three RyR isoforms are expressed in rat pulmonary arteries and in PASMCs. The sustained phase, but not the transient phase, of HPV can be prevented by pretreating pulmonary arteries with RyR inhibitors ryanodine (200 micromol/L) or dantrolene (50 micromol/L). The addition of dantrolene, ryanodine or the thiol-reducing agent dithiothreitol (1 mmol/L) during the sustained phase of HPV reversed the hypoxic vasoconstriction. In contrast, the superoxide scavenger nitroblue tetrazolium (500 nmol/L) prevented further hypoxic pulmonary vasoconstriction during the sustained phase of HPV but did not reverse it. Taken together, our data suggest that redox activation of RyRs by ROS has an important role in transducing the sustained contraction of pulmonary arteries under hypoxia.

Contribution of Kv channels to phenotypic remodeling of human uterine artery smooth muscle cells

Vascular smooth muscle cells (VSMCs) perform diverse functions that can be classified into contractile and synthetic (or proliferating). All of these functions can be fulfilled by the same cell because of its capacity of phenotypic modulation in response to environmental changes. The resting membrane potential is a key determinant for both contractile and proliferating functions. Here, we have explored the expression of voltage-dependent K+ (Kv) channels in contractile (freshly dissociated) and proliferating (cultured) VSMCs obtained from human uterine arteries to establish their contribution to the functional properties of the cells and their possible participation in the phenotypic switch. We have studied the expression pattern (both at the mRNA and at the protein level) of Kvalpha subunits in both preparations as well as their functional contribution to the K+ currents of VSMCs. Our results indicate that phenotypic remodeling associates with a change in the expression and distribution of Kv channels. Whereas Kv currents in contractile VSMCs are mainly performed by Kv1 channels, Kv3.4 is the principal contributor to K+ currents in cultured VSMCs. Furthermore, selective blockade of Kv3.4 channels resulted in a reduced proliferation rate, suggesting a link between Kv channels expression and phenotypic remodeling.

Hypoxic pulmonary vasoconstriction in reptiles: a comparative study of four species with different lung structures and pulmonary blood pressures

Low O2levels in the lungs of birds and mammals cause constriction of the pulmonary vasculature that elevates resistance to pulmonary blood flow and increases pulmonary blood pressure. This hypoxic pulmonary vasoconstriction (HPV) diverts pulmonary blood flow from poorly ventilated and hypoxic areas of the lung to more well-ventilated parts and is considered important for the local matching of ventilation to blood perfusion. In the present study, the effects of acute hypoxia on pulmonary and systemic blood flows and pressures were measured in four species of anesthetized reptiles with diverse lung structures and heart morphologies: varanid lizards ( Varanus exanthematicus), caimans ( Caiman latirostris), rattlesnakes ( Crotalus durissus), and tegu lizards ( Tupinambis merianae). As previously shown in turtles, hypoxia causes a reversible constriction of the pulmonary vasculature in varanids and caimans, decreasing pulmonary vascular conductance by 37 and 31%, respectively. These three species possess complex multicameral lungs, and it is likely that HPV would aid to secure ventilation-perfusion homogeneity. There was no HPV in rattlesnakes, which have structurally simple lungs where local ventilation-perfusion inhomogeneities are less likely to occur. However, tegu lizards, which also have simple unicameral lungs, did exhibit HPV, decreasing pulmonary vascular conductance by 32%, albeit at a lower threshold than varanids and caimans (6.2 kPa oxygen in inspired air vs. 8.2 and 13.9 kPa, respectively). Although these observations suggest that HPV is more pronounced in species with complex lungs and functionally divided hearts, it is also clear that other components are involved.