New mechanisms of pulmonary arterial hypertension: role of Ca²⁺ signaling

2-AMINOETHYLDIPHENYLBORINATE MODIFIES THE PULMONARY CIRCULATION IN PULMONARY HYPERTENSIVE NEWBORN LAMBS WITH PARTIAL GESTATION AT HIGH ALTITUDE

Calcium signaling through store operated channels (SOC) is involved in hypoxic pulmonary hypertension. We determined whether a treatment with 2-aminoethyldiphenylborinate (2-APB), a compound with SOC blocker activity, reduces pulmonary hypertension and vascular remodeling. Twelve newborn lambs exposed to perinatal chronic hypoxia were studied, 6 of them received a 2-APB treatment and the other 6 received vehicle treatment, for 10 days in both cases. Throughout this period, we recorded cardiopulmonary variables and on day 11 we evaluated the response to an acute hypoxic challenge. Additionally, we assessed the vasoconstrictor and vasodilator function in isolated pulmonary arteries as well as their remodeling in lung slices. 2-APB reduced pulmonary arterial pressure at the third and tenth days, cardiac output between the fourth and eighth days, and pulmonary vascular resistance at the tenth day of treatment. The pulmonary vasoconstrictor response to acute hypoxia was reduced by the end of treatment. 2-APB also decreased maximal vasoconstrictor response to the thromboxane mimetic U46619 and endothelin-1 and increased maximal relaxation to 8-Br-cGMP. The maximal relaxation and potency to phosphodiesterase-5 and Rho-kinase inhibition with sildenafil and fasudil respectively, were also increased. Finally, 2-APB reduced the medial and adventitial layers' thickness, the expression of α-actin and the percentage of Ki67+ nuclei of small pulmonary arteries. Taken together, our results indicate that 2-APB reduces pulmonary hypertension, vasoconstrictor responses and pathological remodeling in pulmonary hypertensive lambs. We conclude that SOC targeting may be a useful strategy for the treatment of neonatal pulmonary hypertension, however, further testing of specific blockers is needed.

Sodium tanshinone IIA sulfonate inhibits hypoxia-induced enhancement of SOCE in pulmonary arterial smooth muscle cells via the PKG-PPAR-γ signaling axis

Our laboratory previously showed that sodium tanshinone IIA sulfonate (STS) inhibited store-operated Ca(2+) entry (SOCE) through store-operated Ca(2+) channels (SOCC) via downregulating the expression of transient receptor potential canonical proteins (TRPC), which contribute to the formation of SOCC (Wang J, Jiang Q, Wan L, Yang K, Zhang Y, Chen Y, Wang E, Lai N, Zhao L, Jiang H, Sun Y, Zhong N, Ran P, Lu W. Am J Respir Cell Mol Biol 48: 125-134, 2013). The detailed molecular mechanisms by which STS inhibits SOCE and downregulates TRPC, however, remain largely unknown. We have previously shown that, under hypoxic conditions, inhibition of protein kinase G (PKG) and peroxisome proliferator-activated receptor-γ (PPAR-γ) signaling axis results in the upregulation of TRPC (Wang J, Yang K, Xu L, Zhang Y, Lai N, Jiang H, Zhang Y, Zhong N, Ran P, Lu W. Am J Respir Cell Mol Biol 49: 231-240, 2013). This suggests that strategies targeting the restoration of this signaling pathway may be an effective treatment strategy for pulmonary hypertension. In this study, our results demonstrated that STS treatment can effectively prevent the hypoxia-mediated inhibition of the PKG-PPAR-γ signaling axis in rat distal pulmonary arterial smooth muscle cells (PASMCs) and distal pulmonary arteries. These effects of STS treatment were blocked by pharmacological inhibition or specific small interfering RNA knockdown of either PKG or PPAR-γ. Moreover, targeted PPAR-γ agonist markedly enhanced the beneficial effects of STS. These results comprehensively suggest that STS treatment can prevent hypoxia-mediated increases in intracellular calcium homeostasis and cell proliferation, by targeting and restoring the hypoxia-inhibited PKG-PPAR-γ signaling pathway in PASMCs.

Intratracheal Gene Delivery of SERCA2a Ameliorates Chronic Post-Capillary Pulmonary Hypertension: A Large Animal Model

BACKGROUND

Pulmonary hypertension (PH) is characterized by pulmonary arterial remodeling that results in increased pulmonary vascular resistance, right ventricular (RV) failure, and premature death. Down-regulation of sarcoplasmic reticulum Ca(2+)-ATPase 2a (SERCA2a) in the pulmonary vasculature leads to perturbations in calcium ion (Ca(2+)) homeostasis and transition of pulmonary artery smooth muscle cells to a proliferative phenotype.

OBJECTIVES

We assessed the feasibility of sustained pulmonary vascular SERCA2a gene expression using aerosolized delivery of adeno-associated virus type 1 (AAV1) in a large animal model of chronic PH and evaluated the efficacy of gene transfer regarding progression of pulmonary vascular and RV remodeling.

METHODS

A model of chronic post-capillary PH was created in Yorkshire swine by partial pulmonary vein banding. Development of chronic PH was confirmed hemodynamically, and animals were randomized to intratracheal administration of aerosolized AAV1 carrying the human SERCA2a gene (n = 10, AAV1.SERCA2a group) or saline (n = 10). Therapeutic efficacy was evaluated 2 months after gene delivery.

RESULTS

Transduction efficacy after intratracheal delivery of AAV1 was confirmed by β-galactosidase detection in the distal pulmonary vasculature. Treatment with aerosolized AAV1.SERCA2a prevented disease progression as evaluated by mean pulmonary artery pressure, vascular resistance, and limited vascular remodeling quantified by histology. Therapeutic efficacy was supported further by the preservation of RV ejection fraction (p = 0.014) and improvement of the RV end-diastolic pressure-volume relationship in PH pigs treated with aerosolized AAV1.SERCA2a.

CONCLUSIONS

Airway-based delivery of AAV vectors to the pulmonary arteries was feasible, efficient, and safe in a clinically relevant chronic PH model. Vascular SERCA2a overexpression resulted in beneficial effects on pulmonary arterial remodeling, with attendant improvements in pulmonary hemodynamics and RV performance, and might offer therapeutic benefit by modifying fundamental pathophysiology in pulmonary vascular diseases.

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.

Pathogenic role of calcium-sensing receptors in the development and progression of pulmonary hypertension

An increase in cytosolic free Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary arterial smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and a critical stimulation for PASMC proliferation and migration. Previously, we demonstrated that expression and function of calcium sensing receptors (CaSR) in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH) and animals with experimental pulmonary hypertension (PH) were greater than in PASMC from normal subjects and control animals. However, the mechanisms by which CaSR triggers Ca(2+) influx in PASMC and the implication of CaSR in the development of PH remain elusive. Here, we report that CaSR functionally interacts with TRPC6 to regulate [Ca(2+)]cyt in PASMC. Downregulation of CaSR or TRPC6 with siRNA inhibited Ca(2+)-induced [Ca(2+)]cyt increase in IPAH-PASMC (in which CaSR is upregulated), whereas overexpression of CaSR or TRPC6 enhanced Ca(2+)-induced [Ca(2+)]cyt increase in normal PASMC (in which CaSR expression level is low). The upregulated CaSR in IPAH-PASMC was also associated with enhanced Akt phosphorylation, whereas blockade of CaSR in IPAH-PASMC attenuated cell proliferation. In in vivo experiments, deletion of the CaSR gene in mice (casr(-/-)) significantly inhibited the development and progression of experimental PH and markedly attenuated acute hypoxia-induced pulmonary vasoconstriction. These data indicate that functional interaction of upregulated CaSR and upregulated TRPC6 in PASMC from IPAH patients and animals with experimental PH may play an important role in the development and progression of sustained pulmonary vasoconstriction and pulmonary vascular remodeling. Blockade or downregulation of CaSR and/or TRPC6 with siRNA or miRNA may be a novel therapeutic strategy to develop new drugs for patients with pulmonary arterial hypertension.

A Novel Selectable Islet 1 Positive Progenitor Cell Reprogrammed to Expandable and Functional Smooth Muscle Cells

Disorders affecting smooth muscle structure/function may require technologies that can generate large scale, differentiated and contractile smooth muscle cells (SMC) suitable for cell therapy. To date no clonal precursor population that provides large numbers of differentiated SMC in culture has been identified in a rodent. Identification of such cells may also enhance insight into progenitor cell fate decisions and the relationship between smooth muscle precursors and disease states that implicate differentiated SMC.  In this study, we used classic clonal expansion techniques to identify novel self-renewing Islet 1 (Isl-1) positive primitive progenitor cells (PPC) within rat bone marrow that exhibited canonical stem cell markers and preferential differentiation towards a smooth muscle-like fate. We subsequently used molecular tagging to select Isl-1 positive clonal populations from expanded and de novo marrow cell populations. We refer to these previously undescribed cells as the PPC given its stem cell marker profile, and robust self-renewal capacity. PPC could be directly converted into induced smooth muscle cells (iSMC) using single transcription factor (Kruppel-like factor 4) knockdown or transactivator (myocardin) overexpression in contrast to three control cells (HEK 293, endothelial cells and mesenchymal stem cells) where such induction was not possible. iSMC exhibited immuno- and cytoskeletal-phenotype, calcium signaling profile and contractile responses similar to bona fide SMC. Passaged iSMC could be expanded to a scale sufficient for large scale tissue replacement.  PPC and reprogramed iSMC so derived may offer future opportunities to investigate molecular, structure/function and cell-based replacement therapy approaches to diverse cardiovascular, respiratory, gastrointestinal, and genitourinary diseases that have as their basis smooth muscle cell functional aberrancy or numerical loss. Stem Cells 2016;34:1354-1368.

Reconciling paradigms of abnormal pulmonary blood flow and quasi-malignant cellular alterations in pulmonary arterial hypertension

In pulmonary arterial hypertension (PAH) structural and functional abnormalities of the small lung vessels interact and lead to a progressive increase in pulmonary vascular resistance and right heart failure. A current pathobiological concept characterizes PAH as a 'quasi-malignant' disease focusing on cancer-like alterations in endothelial cells (EC) and the importance of their acquired apoptosis-resistant, hyper-proliferative phenotype in the process of vascular remodeling. While changes in pulmonary blood flow (PBF) have been long-since recognized and linked to the development of PAH, little is known about a possible relationship between an altered PBF and the quasi-malignant cell phenotype in the pulmonary vascular wall. This review summarizes recognized and hypothetical effects of an abnormal PBF on the pulmonary vascular bed and links these to quasi-malignant changes found in the pulmonary endothelium. Here we describe that abnormal PBF does not only trigger a pulmonary vascular cell growth program, but may also maintain the cancer-like phenotype of the endothelium. Consequently, normalization of PBF and EC response to abnormal PBF may represent a treatment strategy in patients with established PAH.

Potassium channels in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a devastating cardiopulmonary disorder with various origins. All forms of PAH share a common pulmonary arteriopathy characterised by vasoconstriction, remodelling of the pre-capillary pulmonary vessel wall, and in situ thrombosis. Although the pathogenesis of PAH is recognised as a complex and multifactorial process, there is growing evidence that potassium channels dysfunction in pulmonary artery smooth muscle cells is a hallmark of PAH. Besides regulating many physiological functions, reduced potassium channels expression and/or activity have significant effects on PAH establishment and progression. This review describes the molecular mechanisms and physiological consequences of potassium channel modulation. Special emphasis is placed on KCNA5 (Kv1.5) and KCNK3 (TASK1), which are considered to play a central role in determining pulmonary vascular tone and may represent attractive therapeutic targets in the treatment of PAH.

Role of Hypoxia-Induced Brain Derived Neurotrophic Factor in Human Pulmonary Artery Smooth Muscle

BACKGROUND

Hypoxia effects on pulmonary artery structure and function are key to diseases such as pulmonary hypertension. Recent studies suggest that growth factors called neurotrophins, particularly brain-derived neurotrophic factor (BDNF), can influence lung structure and function, and their role in the pulmonary artery warrants further investigation. In this study, we examined the effect of hypoxia on BDNF in humans, and the influence of hypoxia-enhanced BDNF expression and signaling in human pulmonary artery smooth muscle cells (PASMCs).

METHODS AND RESULTS

48h of 1% hypoxia enhanced BDNF and TrkB expression, as well as release of BDNF. In arteries of patients with pulmonary hypertension, BDNF expression and release was higher at baseline. In isolated PASMCs, hypoxia-induced BDNF increased intracellular Ca2+ responses to serotonin: an effect altered by HIF1α inhibition or by neutralization of extracellular BDNF via chimeric TrkB-Fc. Enhanced BDNF/TrkB signaling increased PASMC survival and proliferation, and decreased apoptosis following hypoxia.

CONCLUSIONS

Enhanced expression and signaling of the BDNF-TrkB system in PASMCs is a potential mechanism by which hypoxia can promote changes in pulmonary artery structure and function. Accordingly, the BDNF-TrkB system could be a key player in the pathogenesis of hypoxia-induced pulmonary vascular diseases, and thus a potential target for therapy.

Molecular and functional significance of Ca(2+)-activated Cl(-) channels in pulmonary arterial smooth muscle

Increased peripheral resistance of small distal pulmonary arteries is a hallmark signature of pulmonary hypertension (PH) and is believed to be the consequence of enhanced vasoconstriction to agonists, thickening of the arterial wall due to remodeling, and increased thrombosis. The elevation in arterial tone in PH is attributable, at least in part, to smooth muscle cells of PH patients being more depolarized and displaying higher intracellular Ca(2+) levels than cells from normal subjects. It is now clear that downregulation of voltage-dependent K(+) channels (e.g., Kv1.5) and increased expression and activity of voltage-dependent (Cav1.2) and voltage-independent (e.g., canonical and vanilloid transient receptor potential [TRPC and TRPV]) Ca(2+) channels play an important role in the functional remodeling of pulmonary arteries in PH. This review focuses on an anion-permeable channel that is now considered a novel excitatory mechanism in the systemic and pulmonary circulations. It is permeable to Cl(-) and is activated by a rise in intracellular Ca(2+) concentration (Ca(2+)-activated Cl(-) channel, or CaCC). The first section outlines the biophysical and pharmacological properties of the channel and ends with a description of the molecular candidate genes postulated to encode for CaCCs, with particular emphasis on the bestrophin and the newly discovered TMEM16 and anoctamin families of genes. The second section provides a review of the various sources of Ca(2+) activating CaCCs, which include stimulation by mobilization from intracellular Ca(2+) stores and Ca(2+) entry through voltage-dependent and voltage-independent Ca(2+) channels. The third and final section summarizes recent findings that suggest a potentially important role for CaCCs and the gene TMEM16A in PH.

Redox regulation of ion channels in the pulmonary circulation

SIGNIFICANCE

The pulmonary circulation is a low-pressure, low-resistance, highly compliant vasculature. In contrast to the systemic circulation, it is not primarily regulated by a central nervous control mechanism. The regulation of resting membrane potential due to ion channels is of integral importance in the physiology and pathophysiology of the pulmonary vasculature.

RECENT ADVANCES

Redox-driven ion conductance changes initiated by direct oxidation, nitration, and S-nitrosylation of the cysteine thiols and indirect phosphorylation of the threonine and serine residues directly affect pulmonary vascular tone.

CRITICAL ISSUES

Molecular mechanisms of changes in ion channel conductance, especially the identification of the sites of action, are still not fully elucidated.

FUTURE DIRECTIONS

Further investigation of the interaction between redox status and ion channel gating, especially the physiological significance of S-glutathionylation and S-nitrosylation, could result in a better understanding of the physiological and pathophysiological importance of these mediators in general and the implications of such modifications in cellular functions and related diseases and their importance for targeted treatment strategies.

Pulmonary hypertension caused by pulmonary venous hypertension

The effect of pulmonary venous hypertension (PVH) on the pulmonary circulation is extraordinarily variable, ranging from no impact on pulmonary vascular resistance (PVR) to a marked increase. The reasons for this are unknown. Both acutely reversible pulmonary vasoconstriction and pathological remodeling (especially medial hypertrophy and intimal hyperplasia) account for increased PVR when present. The mechanisms involved in vasoconstriction and remodeling are not clearly defined, but increased wall stress, especially in small pulmonary arteries, presumably plays an important role. Myogenic contraction may account for increased vascular tone and also indirectly stimulate remodeling of the vessel wall. Increased wall stress may also directly cause smooth muscle growth, migration, and intimal hyperplasia. Even long-standing and severe pulmonary hypertension (PH) usually abates with elimination of PVH, but PVH-PH is an important clinical problem, especially because PVH due to left ventricular noncompliance lacks definitive therapy. The role of targeted PH therapy in patients with PVH-PH is unclear at this time. Most prospective studies indicate that these medications are not helpful or worse, but there is ample reason to think that a subset of patients with PVH-PH may benefit from phosphodiesterase inhibitors or other agents. A different approach to evaluating possible pharmacologic therapy for PVH-PH may be required to better define its possible utility.

microRNA and Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is a lethal vasculopathy associated with complex etiology that involves remodeling of distal pulmonary arteries leading to elevation of pulmonary vascular resistance. This process results in right ventricular (RV) hypertrophy and ultimately RV failure. In addition, PAH is associated with systemic impairment in the skeletal muscle contributing to exercise intolerance. It has only been a few decades since microRNAs (miRNAs) have been implied in the development and progression of PAH regarding every organ affected by the disease. Indeed, impairment of miRNA's expression has been involved in vascular cell remodeling processes such as adventitial fibroblast (AdvFB) migration; pulmonary arterial smooth muscle cell (PASMC) proliferation and pulmonary arterial endothelial cell (PAEC) dysfunction observed in PAH. At the molecular level miRNAs have been described in the control of ion channels and mitochondrial function as well as the regulation of the BMPR2 signaling pathways contributing to PAH lung impairment. Recently miRNAs have also been specifically implicated in RV dysfunction and systemic angiogenic impairment, observed in PAH. In this chapter, we will summarize the knowledge on miRNA in PAH and highlight their crucial role in the etiology of this disease.

Antioxidant mechanism of Rutin on hypoxia-induced pulmonary arterial cell proliferation

Reactive oxygen species (ROS) are involved in the pathologic process of pulmonary arterial hypertension as either mediators or inducers. Rutin is a type of flavonoid which exhibits significant scavenging properties on oxygen radicals both in vitro and in vivo. In this study, we proposed that rutin attenuated hypoxia-induced pulmonary artery smooth muscle cell (PASMC) proliferation by scavenging ROS. Immunofluorescence data showed that rutin decreased the production of ROS, which was mainly generated through mitochondria and NADPH oxidase 4 (Nox4) in pulmonary artery endothelial cells (PAECs). Western blot results provided further evidence on rutin increasing expression of Nox4 and hypoxia-inducible factor-1α (HIF-1α). Moreover, cell cycle analysis by flow cytometry indicated that proliferation of PASMCs triggered by hypoxia was also repressed by rutin. However, N-acetyl-L-cysteine (NAC), a scavenger of ROS, abolished or diminished the capability of rutin in repressing hypoxia-induced cell proliferation. These data suggest that rutin shows a potential benefit against the development of hypoxic pulmonary arterial hypertension by inhibiting ROS, subsequently preventing hypoxia-induced PASMC proliferation.

Chronic hypoxia increases TRPC6 expression and basal intracellular Ca2+ concentration in rat distal pulmonary venous smooth muscle

BACKGROUND

Hypoxia causes remodeling and contractile responses in both pulmonary artery (PA) and pulmonary vein (PV). Here we explore the effect of hypoxia on PV and pulmonary venous smooth muscle cells (PVSMCs).

METHODS

Chronic hypoxic pulmonary hypertension (CHPH) model was established by exposing rats to 10% O2 for 21 days. Rat distal PVSMCs were isolated and cultured for in vitro experiments. The fura-2 based fluorescence calcium imaging was used to measure the basal intracellular Ca2+ concentration ([Ca2+]i) and store-operated Ca2+ entry (SOCE). Quantitative RT-PCR and western blotting were performed to measure the expression of mRNA and levels of canonical transient receptor potential (TRPC) protein respectively.

RESULTS

Hypoxia increased the basal [Ca2+]i and SOCE in both freshly dissociated and serum cultured distal PVSMCs. Moreover, hypoxia increased TRPC6 expression at mRNA and protein levels in both cultured PVSMCs exposed to prolonged hypoxia (4% O2, 60 h) and distal PV isolated from CHPH rats. Hypoxia also enhanced proliferation and migration of rat distal PVSMCs.

CONCLUSIONS

Hypoxia induces elevation of SOCE in distal PVSMCs, leading to enhancement of basal [Ca2+]i in PVSMCs. This enhancement is potentially correlated with the increased expression of TRPC6. Hypoxia triggered intracellular calcium contributes to promoted proliferation and migration of PVSMCs.

Melatonin attenuates hypoxic pulmonary hypertension by inhibiting the inflammation and the proliferation of pulmonary arterial smooth muscle cells

Hypoxia-induced inflammation and excessive proliferation of pulmonary artery smooth muscle cells (PASMCs) play important roles in the pathological process of hypoxic pulmonary hypertension (HPH). Melatonin possesses anti-inflammatory and antiproliferative properties. However, the effect of melatonin on HPH remains unclear. In this study, adult Sprague-Dawley rats were exposed to intermittent chronic hypoxia for 4 wk to mimic a severe HPH condition. Hemodynamic and pulmonary pathomorphology data showed that chronic hypoxia significantly increased right ventricular systolic pressures (RVSP), weight of the right ventricle/left ventricle plus septum (RV/LV+S) ratio, and median width of pulmonary arterioles. Melatonin attenuated the elevation of RVSP, RV/LV+S, and mitigated the pulmonary vascular structure remodeling. Melatonin also suppressed the hypoxia-induced high expression of proliferating cell nuclear antigen (PCNA), hypoxia-inducible factor-1α (HIF-1α), and nuclear factor-κB (NF-κB). In vitro, melatonin concentration-dependently inhibited the proliferation of PASMCs and the levels of phosphorylation of Akt and extracellular signal-regulated kinases1/2 (ERK1/2) caused by hypoxia. These results suggested that melatonin might potentially prevent HPH via anti-inflammatory and antiproliferative mechanisms.

Adaptation and remodelling of the pulmonary circulation in pulmonary hypertension

Pulmonary arterial hypertension (PAH) is characterized by remodelling of pulmonary arteries caused by a proliferation/apoptosis imbalance within the vascular wall. This pathological phenotype seems to be triggered by different environmental stress and injury events such as increased inflammation, DNA damage, and epigenetic deregulation. It appears that one of the first hit to occur is endothelial cells (ECs) injury and apoptosis, which leads to paracrine signalling to other ECs, pulmonary artery smooth muscle cells (PASMCs), and fibroblasts. These signals promote a phenotypic change of surviving ECs by disturbing different signalling pathways leading to sustained vasoconstriction, proproliferative and antiapoptotic phenotype, deregulated angiogenesis, and formation of plexiform lesions. EC signalling also recruits proinflammatory cells, leading to pulmonary infiltration of lymphocytes, macrophages, and dendritic cells, sustaining the inflammatory environment and autoimmune response. Finally, EC signalling promotes proliferative and antiapoptotic PAH-PASMC phenotypes, which acquire migratory capacities, resulting in increased vascular wall thickness and muscularization of small pulmonary arterioles. Adaptation and remodelling of pulmonary circulation also involves epigenetic components, such as microRNA deregulation, DNA methylation, and histone modification. This review will focus on the different cellular and epigenetic aspects including EC stress response, molecular mechanisms contributing to PAH-PASMC and PAEC proliferation and resistance to apoptosis, as well as epigenetic control involved in adaptation and remodelling of the pulmonary circulation in PAH.

TWIK-2 channel deficiency leads to pulmonary hypertension through a rho-kinase-mediated process

TWIK-2 (KCNK6) is a member of the 2-pore domain (K2P) family of potassium channels, which are highly expressed in the vascular system. We tested the hypothesis that TWIK-2 deficiency leads to pulmonary hypertension. TWIK-2 knockout mice and their wildtype littermates at 8 weeks of age had similar mean right ventricular systolic pressures (24±3 and 21±3 mm Hg, respectively.) Significantly, by 20 weeks of age, the mean right ventricular systolic pressures in TWIK-2 knockout mice increased to 35±3 mm Hg (P≤0.036), whereas mean right ventricular systolic pressures in wildtype littermates remained at 22±3 mm Hg. Elevated mean right ventricular systolic pressures in the TWIK-2 knockout mice was accompanied by pulmonary vascular remodeling as determined by a 25% increase in the cross-sectional area of the vessels occupied by the vessel wall. Additionally, secondary branches of the pulmonary artery from 20-week-old TWIK-2 knockout mice showed an enhanced contractile response to U46619 (10(-6) moles/L), a thromboxane A2 mimetic, which was completely abolished with the Rho-kinase inhibitor, Y27632 (10(-6) and 10(-5) moles/L). Treatment of TWIK-2 knockout mice with the Rho-kinase inhibitor, fasudil, in the drinking water for 12 weeks, abolished the development of pulmonary hypertension and attenuated the vessel remodeling. We concluded that mice deficient in the TWIK-2 channel develop pulmonary hypertension between 8 and 20 weeks of age through a mechanism involving Rho-kinase. Our results suggest that downregulation of TWIK-2 in the pulmonary vasculature may be an underlying mechanism in the development of pulmonary hypertension.

The genetics of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive and fatal disease for which there is an ever-expanding body of genetic and related pathophysiological information on disease pathogenesis. Many germline gene mutations have now been described, including mutations in the gene coding bone morphogenic protein receptor type 2 (BMPR2) and related genes. Recent advanced gene-sequencing methods have facilitated the discovery of additional genes with mutations among those with and those without familial forms of PAH (CAV1, KCNK3, EIF2AK4). The reduced penetrance, variable expressivity, and female predominance of PAH suggest that genetic, genomic, and other factors modify disease expression. These multi-faceted variations are an active area of investigation in the field, including but not limited to common genetic variants and epigenetic processes, and may provide novel opportunities for pharmacological intervention in the near future. They also highlight the need for a systems-oriented multi-level approach to incorporate the multitude of biological variations now associated with PAH. Ultimately, an in-depth understanding of the genetic factors relevant to PAH provides the opportunity for improved patient and family counseling about this devastating disease.

T-type calcium channels are involved in hypoxic pulmonary hypertension

AIMS

Pulmonary hypertension (PH) is the main disease of pulmonary circulation. Alteration in calcium homeostasis in pulmonary artery smooth muscle cells (PASMCs) is recognized as a key feature in PH. The present study was undertaken to investigate the involvement of T-type voltage-gated calcium channels (T-VGCCs) in the control of the pulmonary vascular tone and thereby in the development of PH.

METHODS AND RESULTS

Experiments were conducted in animals (rats and mice) kept 3-4 weeks in either normal (normoxic) or hypoxic environment (hypobaric chamber) to induce chronic hypoxia (CH) PH. In vivo, chronic treatment of CH rats with the T-VGCC blocker, TTA-A2, prevented PH and the associated vascular hyperreactivity, pulmonary arterial remodelling, and right cardiac hypertrophy. Deletion of the Cav3.1 gene (a T-VGCC isoform) protected mice from CH-PH. In vitro, patch-clamp and PCR experiments revealed the presence of T-VGCCs (mainly Cav3.1 and Cav3.2) in PASMCs. Mibefradil, NNC550396, and TTA-A2 inhibited, in a concentration-dependent manner, T-VGCC current, KCl-induced contraction, and PASMC proliferation.

CONCLUSION

The present study demonstrates that T-VGCCs contribute to intrapulmonary vascular reactivity and is implicated in the development of hypoxic PH. Specific blockers of T-VGCCs may thus prove useful for the therapeutic management of PH.

Activation of Notch signaling by short-term treatment with Jagged-1 enhances store-operated Ca(2+) entry in human pulmonary arterial smooth muscle cells

Notch signaling plays a critical role in controlling proliferation and differentiation of pulmonary arterial smooth muscle cells (PASMC). Upregulated Notch ligands and Notch3 receptors in PASMC have been reported to promote the development of pulmonary vascular remodeling in patients with pulmonary arterial hypertension (PAH) and in animals with experimental pulmonary hypertension. Activation of Notch receptors by their ligands leads to the cleavage of the Notch intracellular domain (NICD) to the cytosol by γ-secretase; NICD then translocates into the nucleus to regulate gene transcription. In this study, we examined whether short-term activation of Notch functionally regulates store-operated Ca(2+) entry (SOCE) in human PASMC. Treatment of PASMC with the active fragment of human Jagged-1 protein (Jag-1) for 15-60 min significantly increased the amplitude of SOCE induced by passive deletion of Ca(2+) from the intracellular stores, the sarcoplasmic reticulum (SR). The Jag-1-induced enhancement of SOCE was time dependent: the amplitude was maximized at 30 min of treatment with Jag-1, which was closely correlated with the time course of Jag-1-mediated increase in NICD protein level. The scrambled peptide of Jag-1 active fragment had no effect on SOCE. Inhibition of γ-secretase by N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester (DAPT) significantly attenuated the Jag-1-induced augmentation of SOCE. In addition to the short-term effect, prolonged treatment of PASMC with Jag-1 for 48 h also markedly enhanced the amplitude of SOCE. These data demonstrate that short-term activation of Notch signaling enhances SOCE in PASMC; the NICD-mediated functional interaction with store-operated Ca(2+) channels (SOC) may be involved in the Jag-1-mediated enhancement of SOCE in human PASMC.

Role of ASIC1 in the development of chronic hypoxia-induced pulmonary hypertension

Chronic hypoxia (CH) associated with respiratory disease results in elevated pulmonary vascular intracellular Ca(2+) concentration, which elicits enhanced vasoconstriction and promotes vascular arterial remodeling and thus has important implications in the development of pulmonary hypertension (PH). Store-operated Ca(2+) entry (SOCE) contributes to this elevated intracellular Ca(2+) concentration and has also been linked to acute hypoxic pulmonary vasoconstriction (HPV). Since our laboratory has recently demonstrated an important role for acid-sensing ion channel 1 (ASIC1) in mediating SOCE, we hypothesized that ASIC1 contributes to both HPV and the development of CH-induced PH. To test this hypothesis, we examined responses to acute hypoxia in isolated lungs and assessed the effects of CH on indexes of PH, arterial remodeling, and vasoconstrictor reactivity in wild-type (ASIC1(+/+)) and ASIC1 knockout (ASIC1(-/-)) mice. Restoration of ASIC1 expression in pulmonary arterial smooth muscle cells from ASIC1(-/-) mice rescued SOCE, confirming the requirement for ASIC1 in this response. HPV responses were blunted in lungs from ASIC1(-/-) mice. Both SOCE and receptor-mediated Ca(2+) entry, along with agonist-dependent vasoconstrictor responses, were diminished in small pulmonary arteries from control ASIC(-/-) mice compared with ASIC(+/+) mice. The effects of CH to augment receptor-mediated vasoconstrictor and SOCE responses in vessels from ASIC1(+/+) mice were not observed after CH in ASIC1(-/-) mice. In addition, ASIC1(-/-) mice exhibited diminished right ventricular systolic pressure, right ventricular hypertrophy, and arterial remodeling in response to CH compared with ASIC1(+/+) mice. Taken together, these data demonstrate an important role for ASIC1 in both HPV and the development of CH-induced PH.

Classical transient receptor potential 1 and 6 contribute to hypoxic pulmonary hypertension through differential regulation of pulmonary vascular functions

Hypoxic pulmonary hypertension is characterized by increased vascular tone, altered vasoreactivity, and vascular remodeling, which are associated with alterations in Ca(2+) homeostasis in pulmonary arterial smooth muscle cells. We have previously shown that classical transient receptor potential 1 and 6 (TRPC1 and TRPC6) are upregulated in pulmonary arteries (PAs) of chronic hypoxic rats, but it is unclear whether these channels are essential for the development of pulmonary hypertension. Here we found that pulmonary hypertension was suppressed in TRPC1 and TRPC6 knockout (Trpc1(-/-) and Trpc6(-/-)) mice compared with wild-type after exposure to 10% O(2) for 1 and 3 weeks. Muscularization of pulmonary microvessels was inhibited, but rarefaction was unaltered in hypoxic Trpc1(-/-) and Trpc6(-/-) mice. Small PAs of normoxic wild-type mice exhibited vasomotor tone, which was significantly enhanced by chronic hypoxia. Similar vasomotor tone was found in normoxic Trpc1(-/-) PAs, but the hypoxia-induced enhancement was blunted. In contrast, there was minimal vascular tone in normoxic Trpc6(-/-) PAs, but the hypoxia-enhanced tone was preserved. Chronic hypoxia caused significant increase in serotonin-induced vasoconstriction; the augmented vasoreactivity was attenuated in Trpc1(-/-) and eliminated in Trpc6(-/-) PAs. Moreover, the effects of 3-week hypoxia on pulmonary arterial pressure, right ventricular hypertrophy, and muscularization of microvessels were further suppressed in TRPC1-TRPC6 double-knockout mice. Our results, therefore, provide clear evidence that TRPC1 and TRPC6 participate differentially in various pathophysiological processes, and that the presence of TRPC1 and TRPC6 is essential for the full development of hypoxic pulmonary hypertension in the mouse model.

TRPC4 inactivation confers a survival benefit in severe pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterized by elevated pulmonary arterial pressure with lumen-occluding neointimal and plexiform lesions. Activation of store-operated calcium entry channels promotes contraction and proliferation of lung vascular cells. TRPC4 is a ubiquitously expressed store-operated calcium entry channel, but its role in PAH is unknown. We tested the hypothesis that TRPC4 promotes pulmonary arterial constriction and occlusive remodeling, leading to right ventricular failure in severe PAH. Severe PAH was induced in Sprague-Dawley rats and in wild-type and TRPC4-knockout Fischer 344 rats by a single subcutaneous injection of SU5416 [SU (semaxanib)], followed by hypoxia exposure (Hx; 10% O2) for 3 weeks and then a return to normoxia (Nx; 21% O2) for 3 to 10 additional weeks (SU/Hx/Nx). Although rats of both backgrounds exhibited indistinguishable pulmonary hypertensive responses to SU/Hx/Nx, Fischer 344 rats died within 6 to 8 weeks. Normoxic and hypertensive TRPC4-knockout rats recorded hemodynamic parameters similar to those of their wild-type littermates. However, TRPC4 inactivation conferred a striking survival benefit, due in part to preservation of cardiac output. Histological grading of vascular lesions revealed a reduction in the density of severely occluded small pulmonary arteries and in the number of plexiform lesions in TRPC4-knockout rats. TRPC4 inactivation therefore provides a survival benefit in severe PAH, associated with a decrease in the magnitude of occlusive remodeling.

Heritable forms of pulmonary arterial hypertension

Tremendous progress has been made in understanding the genetics of heritable pulmonary arterial hypertension (HPAH) since its description in the 1950s. Germline mutations in the gene coding bone morphogenetic receptor type 2 (BMPR2) are detectable in the majority of cases of HPAH, and in a small proportion of cases of idiopathic pulmonary arterial hypertension (IPAH). Recent advancements in gene sequencing methods have facilitated the discovery of additional genes with mutations among those with and without familial PAH (CAV1, KCNK3). HPAH is an autosomal dominant disease characterized by reduced penetrance, variable expressivity, and female predominance. These characteristics suggest that genetic and nongenetic factors modify disease expression, highlighting areas of active investigation. The reduced penetrance makes genetic counseling complex, as the majority of carriers of PAH-related mutations will never be diagnosed with the disease. This issue is increasingly important, as clinical testing for BMPR2 and other mutations is now available for the evaluation of patients and their at-risk kin. The possibilities to avoid mutation transmission, such as the rapidly advancing field of preimplantation genetic testing, highlight the need for all clinicians to understand the genetic features of PAH risk.

Chronic hypoxia selectively enhances L- and T-type voltage-dependent Ca2+ channel activity in pulmonary artery by upregulating Cav1.2 and Cav3.2

Hypoxia-induced pulmonary hypertension (HPH) is characterized by sustained pulmonary vasoconstriction and vascular remodeling, both of which are mediated by pulmonary artery smooth muscle cell (PASMC) contraction and proliferation, respectively. An increase in cytosolic Ca²⁺ concentration ([Ca²⁺]cyt) is a major trigger for pulmonary vasoconstriction and an important stimulus for cell proliferation in PASMCs. Ca²⁺ influx through voltage-dependent Ca²⁺ channels (VDCC) is an important pathway for the regulation of [Ca²⁺]cyt. The potential role for L- and T-type VDCC in the development of HPH is still unclear. Using a hypoxic-induced pulmonary hypertension mouse model, we undertook this study to identify if VDCC in pulmonary artery (PA) are functionally upregulated and determine which type of VDCC are altered in HPH. Mice subjected to chronic hypoxia developed pulmonary hypertension within 4 wk, and high-K⁺- and U-46619-induced contraction of PA was greater in chronic hypoxic mice than that in normoxic control mice. Additionally, we demonstrate that high-K⁺- and U-46619-induced Ca²⁺ influx in PASMC is significantly increased in the hypoxic group. The VDCC activator, Bay K8864, induced greater contraction of the PA of hypoxic mice than in that of normoxic mice in isometric force measurements. L-type and T-type VDCC blockers significantly attenuated absolute contraction of the PA in hypoxic mice. Chronic hypoxia did not increase high-K⁺- and U-46619-induced contraction of mesenteric artery (MA). Compared with MA, PA displayed higher expression of calcium channel voltage-dependent L-type α1C-subunit (Cav1.2) and T-type α1H-subunit (Cav3.2) upon exposure to chronic hypoxia. In conclusion, both L-type and T-type VDCC were functionally upregulated in PA, but not MA, in HPH mice, which could result from selectively increased expression of Cav1.2 and Cav3.2.

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.

Regulation of ca(2+) signaling in pulmonary hypertension

Understanding the cellular and molecular mechanisms involved in the development and progression of pulmonary hypertension (PH) remains imperative if we are to successfully improve the quality of life and life span of patients with the disease. A whole plethora of mechanisms are associated with the development and progression of PH. Such complexity makes it difficult to isolate one particular pathway to target clinically. Changes in intracellular free calcium concentration, the most common intracellular second messenger, can have significant impact in defining the pathogenic mechanisms leading to its development and persistence. Signaling pathways leading to the elevation of [Ca(2+)](cyt) contribute to pulmonary vasoconstriction, excessive proliferation of smooth muscle cells and ultimately pulmonary vascular remodeling. This current review serves to summarize the some of the most recent advances in the regulation of calcium during pulmonary hypertension.

A process-based review of mouse models of pulmonary hypertension

Genetically modified mouse models have unparalleled power to determine the mechanisms behind different processes involved in the molecular and physiologic etiology of various classes of human pulmonary hypertension (PH). Processes known to be involved in PH for which there are extensive mouse models available include the following: (1) Regulation of vascular tone through secreted vasoactive factors; (2) regulation of vascular tone through potassium and calcium channels; (3) regulation of vascular remodeling through alteration in metabolic processes, either through alteration in substrate usage or through circulating factors; (4) spontaneous vascular remodeling either before or after development of elevated pulmonary pressures; and (5) models in which changes in tone and remodeling are primarily driven by inflammation. PH development in mice is of necessity faster and with different physiologic ramifications than found in human disease, and so mice make poor models of natural history of PH. However, transgenic mouse models are a perfect tool for studying the processes involved in pulmonary vascular function and disease, and can effectively be used to test interventions designed against particular molecular pathways and processes involved in disease.

Altered expression and signal transduction of endothelin-1 receptors in heritable and idiopathic pulmonary arterial hypertension

Human pulmonary arterial smooth muscle cells (PASMC) were isolated from elastic pulmonary arteries dissected from lungs of individuals with and without pulmonary arterial hypertension (PAH). Reflecting increased smooth muscle constriction in cells from PAH subject, Ca(2+) influx in response to endothelin-1 (ET-1) increased in all the PAH PASMC populations relative to the normal donor control cells. The ETA receptor mRNA levels remained unchanged, whereas the ETB receptor mRNA levels decreased in both heritable and idiopathic PAH-derived PASMC. All the PASMC populations expressed considerably higher ETA compared to ETB receptor number. Both ETA and ETB receptor numbers were reduced in bone morphogenetic protein receptor type II (BMPR2) mutation PAH. ETB receptors showed a particular reduction in number. Phospho-antibody array analysis of normal and BMPR2 deletion PASMC illustrated ERK and Akt activation to be the most prominent and to be taking place principally through ETB receptors in normal PASMC, but primarily through ETA receptors in PASMC from BMPR2 PAH subjects. Additionally in the PAH cells the total relative ET-1 signal response was markedly reduced. Western analysis from the BMPR2 PASMC duplicated the array results, whereas PASMC from iPAH subjects showed variability with most samples continuing to signal through ETB. In sum, these results indicate that generally both receptors are reduced in PAH particularly ETB, and that ETB signaling through protein kinases becomes markedly reduced in BMPR2 PASMC, while it continues in IPAH. Importantly, the data suggest that caution must be taken when applying ET-1 receptor antagonist therapy to PAH patients.

Vascular remodeling in pulmonary hypertension

Pulmonary hypertension is a complex, progressive condition arising from a variety of genetic and pathogenic causes. Patients present with a spectrum of histologic and pathophysiological features, likely reflecting the diversity in underlying pathogenesis. It is widely recognized that structural alterations in the vascular wall contribute to all forms of pulmonary hypertension. Features characteristic of the remodeled vasculature in patients with pulmonary hypertension include increased stiffening of the elastic proximal pulmonary arteries, thickening of the intimal and/or medial layer of muscular arteries, development of vaso-occlusive lesions, and the appearance of cells expressing smooth muscle-specific markers in normally non-muscular small diameter vessels, resulting from proliferation and migration of pulmonary arterial smooth muscle cells and cellular transdifferentiation. The development of several animal models of pulmonary hypertension has provided the means to explore the mechanistic underpinnings of pulmonary vascular remodeling, although none of the experimental models currently used entirely replicates the pulmonary arterial hypertension observed in patients. Herein, we provide an overview of the histological abnormalities observed in humans with pulmonary hypertension and in preclinical models and discuss insights gained regarding several key signaling pathways contributing to the remodeling process. In particular, we will focus on the roles of ion homeostasis, endothelin-1, serotonin, bone morphogenetic proteins, Rho kinase, and hypoxia-inducible factor 1 in pulmonary arterial smooth muscle and endothelial cells, highlighting areas of cross-talk between these pathways and potentials for therapeutic targeting.

Emerging roles for native Orai Ca2+ channels in cardiovascular disease

Orai proteins form highly calcium (Ca(2+))-selective channels located in the plasma membrane of both nonexcitable and excitable cells, where they make important contributions to many cellular processes. The well-characterized Ca(2+) release-activated Ca(2+) current is mediated by Orai1 multimers and is activated, upon depletion of inositol 1,4,5-trisphosphate-sensitive stores, by direct interaction of Orai1 with the endoplasmic reticulum Ca(2+) sensor, stromal interaction molecule 1 (STIM1). This pathway is known as capacitative Ca(2+) entry or store-operated Ca(2+) entry. While most investigations have focused on STIM1 and Orai1 in their store-dependent mode, emerging evidence suggests that Orai1 and Orai3 heteromultimeric channels can form store-independent Ca(2+)-selective channels. The role of store-dependent and store-independent channels in excitation-transcription coupling and the pathological remodeling of the cardiovascular system are beginning to come forth. Recent evidence suggests that STIM/Orai-generated Ca(2+) signaling couples to gene transcription and subsequent phenotypic changes associated with the processes of cardiac and vascular remodeling. This short review will explore the contributions of native Orai channels to heart and vessel physiology and their role in cardiovascular diseases.

Slow receptor dissociation kinetics differentiate macitentan from other endothelin receptor antagonists in pulmonary arterial smooth muscle cells

Two endothelin receptor antagonists (ERAs), bosentan and ambrisentan, are currently approved for the treatment of pulmonary arterial hypertension (PAH), a devastating disease involving an activated endothelin system and aberrant contraction and proliferation of pulmonary arterial smooth muscle cells (PASMC). The novel ERA macitentan has recently concluded testing in a Phase III morbidity/mortality clinical trial in PAH patients. Since the association and dissociation rates of G protein-coupled receptor antagonists can influence their pharmacological activity in vivo, we used human PASMC to characterize inhibitory potency and receptor inhibition kinetics of macitentan, ambrisentan and bosentan using calcium release and inositol-1-phosphate (IP(1)) assays. In calcium release assays macitentan, ambrisentan and bosentan were highly potent ERAs with K(b) values of 0.14 nM, 0.12 nM and 1.1 nM, respectively. Macitentan, but not ambrisentan and bosentan, displayed slow apparent receptor association kinetics as evidenced by increased antagonistic potency upon prolongation of antagonist pre-incubation times. In compound washout experiments, macitentan displayed a significantly lower receptor dissociation rate and longer receptor occupancy half-life (ROt(1/2)) compared to bosentan and ambrisentan (ROt(1/2):17 minutes versus 70 seconds and 40 seconds, respectively). Because of its lower dissociation rate macitentan behaved as an insurmountable antagonist in calcium release and IP(1) assays, and unlike bosentan and ambrisentan it blocked endothelin receptor activation across a wide range of endothelin-1 (ET-1) concentrations. However, prolongation of the ET-1 stimulation time beyond ROt(1/2) rendered macitentan a surmountable antagonist, revealing its competitive binding mode. Bosentan and ambrisentan behaved as surmountable antagonists irrespective of the assay duration and they lacked inhibitory activity at high ET-1 concentrations. Thus, macitentan is a competitive ERA with significantly slower receptor dissociation kinetics than the currently approved ERAs. Slow dissociation caused insurmountable antagonism in functional PASMC-based assays and this could contribute to an enhanced pharmacological activity of macitentan in ET-1-dependent pathologies.

Slow receptor dissociation kinetics differentiate macitentan from other endothelin receptor antagonists in pulmonary arterial smooth muscle cells

Two endothelin receptor antagonists (ERAs), bosentan and ambrisentan, are currently approved for the treatment of pulmonary arterial hypertension (PAH), a devastating disease involving an activated endothelin system and aberrant contraction and proliferation of pulmonary arterial smooth muscle cells (PASMC). The novel ERA macitentan has recently concluded testing in a Phase III morbidity/mortality clinical trial in PAH patients. Since the association and dissociation rates of G protein-coupled receptor antagonists can influence their pharmacological activity in vivo, we used human PASMC to characterize inhibitory potency and receptor inhibition kinetics of macitentan, ambrisentan and bosentan using calcium release and inositol-1-phosphate (IP₁) assays. In calcium release assays macitentan, ambrisentan and bosentan were highly potent ERAs with K_b values of 0.14 nM, 0.12 nM and 1.1 nM, respectively. Macitentan, but not ambrisentan and bosentan, displayed slow apparent receptor association kinetics as evidenced by increased antagonistic potency upon prolongation of antagonist pre-incubation times. In compound washout experiments, macitentan displayed a significantly lower receptor dissociation rate and longer receptor occupancy half-life (ROt_1/2) compared to bosentan and ambrisentan (ROt_1/2∶17 minutes versus 70 seconds and 40 seconds, respectively). Because of its lower dissociation rate macitentan behaved as an insurmountable antagonist in calcium release and IP₁ assays, and unlike bosentan and ambrisentan it blocked endothelin receptor activation across a wide range of endothelin-1 (ET-1) concentrations. However, prolongation of the ET-1 stimulation time beyond ROt_1/2 rendered macitentan a surmountable antagonist, revealing its competitive binding mode. Bosentan and ambrisentan behaved as surmountable antagonists irrespective of the assay duration and they lacked inhibitory activity at high ET-1 concentrations. Thus, macitentan is a competitive ERA with significantly slower receptor dissociation kinetics than the currently approved ERAs. Slow dissociation caused insurmountable antagonism in functional PASMC-based assays and this could contribute to an enhanced pharmacological activity of macitentan in ET-1-dependent pathologies.

Activation of Ca2+-activated Cl- channels by store-operated Ca2+ entry in arterial smooth muscle cells does not require reverse-mode Na+/Ca2+ exchange

The main purpose of this study was to characterize the stimulation of Ca(2+)-activated Cl(-) (Cl(Ca)) by store-operated Ca(2+) entry (SOCE) channels in rabbit pulmonary arterial smooth muscle cells (PASMCs) and determine if this process requires reverse-mode Na(+)/Ca(2+) exchange (NCX). In whole-cell voltage clamped PASMCs incubated with 1 μmol/L nifedipine (Nif) to inhibit Ca(2+) channels, 30 μmol/L cyclopiazonic acid (CPA), a SERCA pump inhibitor, activated a nonselective cation conductance permeable to Na(+) (I(SOC)) during an initial 1-3 s step, ranging from-120 to +60 mV, and Ca(2+)-activated Cl(-) current (I(Cl(Ca))) during a second step to +90 mV that increased with the level of the preceding hyperpolarizing step. Niflumic acid (100 μmol/L), a Cl(Ca) channel blocker, abolished I(Cl(Ca)) but had no effect on I(SOC), whereas the I(SOC) blocker SKF-96365 (50 μmol/L) suppressed both currents. Dual patch clamp and Fluo-4 fluorescence measurements revealed the appearance of CPA-induced Ca(2+) transients of increasing magnitude with increasing hyperpolarizing steps, which correlated with I(Cl(Ca)) amplitude. The absence of Ca(2+) transients at positive potentials following a hyperpolarizing step combined with the observation that SOCE-stimulated I(Cl(Ca)) was unaffected by the NCX blocker KB-R7943 (1 μmol/L) suggest that the SOCE/Cl(Ca) interaction does not require reverse-mode NCX in our conditions.