Capacitative Ca(2+) entry in agonist-induced pulmonary vasoconstriction

Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

Magnolol and honokiol target TRPC4 to regulate extracellular calcium influx and relax intestinal smooth muscle

ETHNOPHARMACOLOGICAL RELEVANCE

Magnolia officinalis Cortex (M. officinalis) is a classical traditional Chinese medicine (TCM) widely used to treat digestive system diseases. It effectively regulates gastrointestinal motility to improve abdominal pain, abdominal distension and other symptoms. Magnolol (MAG) and honokiol (HON) are the main pharmacodynamic components responsible for the gastrointestinal activity of M. officinalis.

AIM OF THE STUDY

The transient receptor potential (TRP) family is highly expressed in the gastrointestinal tract and participates in the regulation of gastrointestinal motility, visceral hypersensitivity, visceral secretion and other physiological activities. In this study, the calcium-lowering mechanisms of MAG and HON contributing to the smooth muscle relaxation associated with TRP are discussed.

MATERIALS AND METHODS

The relaxation smooth muscle effects of MAG and HON were tested by the isolated intestine tone tests. A synthetic MAG probe (MAG-P) was used to target fishing for their possible target. The distribution of MAG on the smooth muscle was identified by a molecular tracer based on chemical biology. Ca²⁺ imaging and dual-luciferase reporter assays were used to determine the effects on the target proteins. Finally, the calcium-mediating effects of MAG and HON on smooth muscle cells and TRPC4-knockdown cells were compared to verify the potential mechanism.

RESULTS

After confirming the smooth muscle relaxation in the small intestine induced by MAG and HON, the relaxation effect was identified mainly due to the downregulation of intracellular calcium by controlling external calcium influx. Although MAG and HON inhibited both TRPV4 and TRPC4 channels to reduce calcium levels, the inhibitory effect on TRPC4 channels is an important mechanism of their smooth muscle relaxation effect, since TRPC4 is widely expressed in the small intestinal smooth muscle cells.

CONCLUSIONS

The inhibition of MAG and HON on TRPC4 channels contributes to the relaxation of intestinal smooth muscle.

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.

Role of Store-Operated Ca2+ Entry in the Pulmonary Vascular Remodeling Occurring in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a severe and multifactorial disease. PAH pathogenesis mostly involves pulmonary arterial endothelial and pulmonary arterial smooth muscle cell (PASMC) dysfunction, leading to alterations in pulmonary arterial tone and distal pulmonary vessel obstruction and remodeling. Unfortunately, current PAH therapies are not curative, and therapeutic approaches mostly target endothelial dysfunction, while PASMC dysfunction is under investigation. In PAH, modifications in intracellular Ca2+ homoeostasis could partly explain PASMC dysfunction. One of the most crucial actors regulating Ca2+ homeostasis is store-operated Ca2+ channels, which mediate store-operated Ca2+ entry (SOCE). This review focuses on the main actors of SOCE in human and experimental PASMC, their contribution to PAH pathogenesis, and their therapeutic potential in PAH.

Mechanosensitivity in Pulmonary Circulation: Pathophysiological Relevance of Stretch-Activated Channels in Pulmonary Hypertension

A variety of cell types in pulmonary arteries (endothelial cells, fibroblasts, and smooth muscle cells) are continuously exposed to mechanical stimulations such as shear stress and pulsatile blood pressure, which are altered under conditions of pulmonary hypertension (PH). Most functions of such vascular cells (e.g., contraction, migration, proliferation, production of extracellular matrix proteins, etc.) depend on a key event, i.e., the increase in intracellular calcium concentration ([Ca²⁺]_i) which results from an influx of extracellular Ca²⁺ and/or a release of intracellular stored Ca²⁺. Calcium entry from the extracellular space is a major step in the elevation of [Ca²⁺]_i, involving a variety of plasmalemmal Ca²⁺ channels including the superfamily of stretch-activated channels (SAC). A common characteristic of SAC is that their gating depends on membrane stretch. In general, SAC are non-selective Ca²⁺-permeable cation channels, including proteins of the TRP (Transient Receptor Potential) and Piezo channel superfamily. As membrane mechano-transducers, SAC convert physical forces into biological signals and hence into a cell response. Consequently, SAC play a major role in pulmonary arterial calcium homeostasis and, thus, appear as potential novel drug targets for a better management of PH.

Halofuginone, a promising drug for treatment of pulmonary hypertension

BACKGROUND AND PURPOSE

Halofuginone is a febrifugine derivative originally isolated from Chinese traditional herb Chang Shan that exhibits anti-hypertrophic, anti-fibrotic and anti-proliferative effects. We sought to investigate whether halofuginone induced pulmonary vasodilation and attenuates chronic hypoxia-induced pulmonary hypertension (HPH).

EXPERIMENTAL APPROACH

Patch-clamp experiments were conducted to examine the activity of voltage-dependent Ca²⁺ channels (VDCCs) in pulmonary artery smooth muscle cells (PASMCs). Digital fluorescence microscopy was used to measure intracellular Ca²⁺ concentration in PASMCs. Isolated perfused and ventilated mouse lungs were used to measure pulmonary artery pressure (PAP). Mice exposed to hypoxia (10% O₂ ) for 4 weeks were used as model of HPH for in vivo experiments.

KEY RESULTS

Halofuginone increased voltage-gated K⁺ (K_v ) currents in PASMCs and K⁺ currents through KCNA5 channels in HEK cells transfected with KCNA5 gene. HF (0.03-1 μM) inhibited receptor-operated Ca²⁺ entry in HEK cells transfected with calcium-sensing receptor gene and attenuated store-operated Ca²⁺ entry in PASMCs. Acute (3-5 min) intrapulmonary application of halofuginone significantly and reversibly inhibited alveolar hypoxia-induced pulmonary vasoconstriction dose-dependently (0.1-10 μM). Intraperitoneal administration of halofuginone (0.3 mg·kg^-1 , for 2 weeks) partly reversed established PH in mice.

CONCLUSION AND IMPLICATIONS

Halofuginone is a potent pulmonary vasodilator by activating K_v channels and blocking VDCC and receptor-operated and store-operated Ca²⁺ channels in PASMCs. The therapeutic effect of halofuginone on experimental PH is probably due to combination of its vasodilator effects, via inhibition of excitation-contraction coupling and anti-proliferative effects, via inhibition of the PI3K/Akt/mTOR signalling pathway.

NAADP-induced intracellular calcium ion is mediated by the TPCs (two-pore channels) in hypoxia-induced pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a form of obstructive vascular disease. Chronic hypoxic exposure leads to excessive proliferation of pulmonary arterial smooth muscle cells and pulmonary arterial endothelial cells. This condition can potentially be aggravated by [Ca²⁺] _i mobilization. In the present study, hypoxia exposure of rat's model was established. Two‐pore segment channels (TPCs) silencing was achieved in rats' models by injecting Lsh‐TPC1 or Lsh‐TPC2. The effects of TPC1/2 silencing on PAH were evaluated by H&E staining detecting pulmonary artery wall thickness and ELISA assay kit detecting NAADP concentrations in lung tissues. TPC1/2 silencing was achieved in PASMCs and PAECs, and cell proliferation was detected by MTT and BrdU incorporation assays. As the results shown, NAADP‐activated [Ca²⁺]_i shows to be mediated via two‐pore segment channels (TPCs) in PASMCs, with TPC1 being the dominant subtype. NAADP generation and TPC1/2 mRNA and protein levels were elevated in the hypoxia‐induced rat PAH model; NAADP was positively correlated with TPC1 and TPC2 expression, respectively. In vivo, Lsh‐TPC1 or Lsh‐TPC2 infection significantly improved the mean pulmonary artery pressure and PAH morphology. In vitro, TPC1 silencing inhibited NAADP‐AM‐induced PASMC proliferation and [Ca²⁺]_i in PASMCs, whereas TPC2 silencing had minor effects during this process; TPC2 silencing attenuated NAADP‐AM‐ induced [Ca²⁺]_i and ECM in endothelial cells, whereas TPC1 silencing barely ensued any physiological changes. In conclusion, TPC1/2 might provide a unifying mechanism within pulmonary arterial hypertension, which can potentially be regarded as a therapeutic target.

Revisiting the mechanism of hypoxic pulmonary vasoconstriction using isolated perfused/ventilated mouse lung

Hypoxic Pulmonary Vasoconstriction (HPV) is an important physiological mechanism of the lungs that matches perfusion to ventilation thus maximizing O2 saturation of the venous blood within the lungs. This study emphasizes on principal pathways in the initiation and modulation of hypoxic pulmonary vasoconstriction with a primary focus on the role of Ca2+ signaling and Ca2+ influx pathways in hypoxic pulmonary vasoconstriction. We used an ex vivo model, isolated perfused/ventilated mouse lung to evaluate hypoxic pulmonary vasoconstriction. Alveolar hypoxia (utilizing a mini ventilator) rapidly and reversibly increased pulmonary arterial pressure due to hypoxic pulmonary vasoconstriction in the isolated perfused/ventilated lung. By applying specific inhibitors for different membrane receptors and ion channels through intrapulmonary perfusion solution in isolated lung, we were able to define the targeted receptors and channels that regulate hypoxic pulmonary vasoconstriction. We show that extracellular Ca2+ or Ca2+ influx through various Ca2+-permeable channels in the plasma membrane is required for hypoxic pulmonary vasoconstriction. Removal of extracellular Ca2+ abolished hypoxic pulmonary vasoconstriction, while blockade of L-type voltage-dependent Ca2+ channels (with nifedipine), non-selective cation channels (with 30 µM SKF-96365), and TRPC6/TRPV1 channels (with 1 µM SAR-7334 and 30 µM capsazepine, respectively) significantly and reversibly inhibited hypoxic pulmonary vasoconstriction. Furthermore, blockers of Ca2+-sensing receptors (by 30 µM NPS2143, an allosteric Ca2+-sensing receptors inhibitor) and Notch (by 30 µM DAPT, a γ-secretase inhibitor) also attenuated hypoxic pulmonary vasoconstriction. These data indicate that Ca2+ influx in pulmonary arterial smooth muscle cells through voltage-dependent, receptor-operated, and store-operated Ca2+ entry pathways all contribute to initiation of hypoxic pulmonary vasoconstriction. The extracellular Ca2+-mediated activation of Ca2+-sensing receptors and the cell-cell interaction via Notch ligands and receptors contribute to the regulation of hypoxic pulmonary vasoconstriction.

Reduced store-operated Ca2+ entry impairs mesenteric artery function in response to high external glucose in type 2 diabetic ZDF rats

Diabetes is a major risk factor for cardiovascular disease, affecting both endothelial and smooth muscle cells. Store-operated Ca2+ channels (SOCCs) have been implicated in many diabetic complications. Vascular dysfunction is common in patients with diabetes, but the role of SOCCs in diabetic vasculopathy is still unclear. Our research aimed to investigate the effects of high glucose (HG) on store-operated Ca2+ entry (SOCE) in small arteries. Small mesenteric arteries from type 2 diabetic Zucker fatty rats (ZDF) versus their non-diabetic controls (Zucker lean, ZL) were examined in a pressurized myograph. Vascular smooth muscle cells (VSMC) were isolated and intracellular Ca2+ was measured (Fura 2-AM). A specific protocol to deplete intracellular Ca2+ stores and thereby open SOCCs, as well as pharmacological SOCE inhibitors (SKF-96365, BTP-2), were used to artificially activate and inhibit SOCE, respectively. High glucose (40 mmol/L) relaxed arteries in a SKF-sensitive manner. Diabetic arteries exhibited reduced HG-induced relaxation, as well as reduced contraction after Ca2+ replenishment. Further, the rise in intracellular Ca2+ on account of SOCE is diminished in diabetic versus non-diabetic VSMCs and was insensitive to HG in diabetic VSMCs. The expression of SOCC proteins was measured, detecting a downregulation of Orai1 in diabetes. In conclusion, diabetes leads to a reduction of SOCE and SOCE-induced contraction, which is unresponsive to HG-mediated inhibition. The reduced expression of Orai1 in diabetic arteries could account for the observed reduction in SOCE.

The Role of Transient Receptor Potential Channel 6 Channels in the Pulmonary Vasculature

Canonical or classical transient receptor potential channel 6 (TRPC6) is a Ca²⁺-permeable non-selective cation channel that is widely expressed in the heart, lung, and vascular tissues. The use of TRPC6-deficient (“knockout”) mice has provided important insights into the role of TRPC6 in normal physiology and disease states of the pulmonary vasculature. Evidence indicates that TRPC6 is a key regulator of acute hypoxic pulmonary vasoconstriction. Moreover, several studies implicated TRPC6 in the pathogenesis of pulmonary hypertension. Furthermore, a unique genetic variation in the TRPC6 gene promoter has been identified, which might link the inflammatory response to the upregulation of TRPC6 expression and ultimate development of pulmonary vascular abnormalities in idiopathic pulmonary arterial hypertension. Additionally, TRPC6 is critically involved in the regulation of pulmonary vascular permeability and lung edema formation during endotoxin or ischemia/reperfusion-induced acute lung injury. In this review, we will summarize latest findings on the role of TRPC6 in the pulmonary vasculature.

Store-Operated Ca2+ Channel in Renal Microcirculation and Glomeruli

Store-operated Ca2+ channel (SOC) is defined as a channel that opens in response to depletion of the internal Ca2+ stores. During the last decade, many investigators have made a great effort to identify and characterize SOC, and to evaluate its physiologic function and pathophysiologic relevance in a variety of cell lines, primary cultures, and native tissues. To date, accumulating evidence has demonstrated that SOC is an essential Ca2+ entry mechanism in vascular smooth-muscle cells of renal microvasculature and glomerular mesangial cells, both of which tightly control glomerular hemodynamics and filtration. Store-operated Ca2+, combined with other types of Ca2+ entry channels, constitutes a profile of Ca2+ changes in response to physiologic vasoconstrictors and, thereby, regulates renal microcirculation and mesangial function. In addition, SOC is associated with altered Ca2+ signaling occurring in diseased kidneys, such as diabetic nephropathy. Although the gating mechanism and molecular identity of SOC are still enigmatic and may be cell-type and tissue specific, data from several independent groups suggest that protein kinase C plays an important role in SOC activation and that certain isoforms of canonical transient receptor potential (TRPC) proteins are candidates of SOC in renal mlcrovessels and mesangial cells.

Impaired nitric oxide production and increased blood pressure in systemic heterozygous ATP2B1 null mice

BACKGROUND

In the 'Millennium Genome Project', we identified ATP2B1 as a gene responsible for hypertension through single-nucleotide polymorphism analysis. The ATP2B1 gene encodes the plasma membrane calcium ATPase isoform 1, which contributes to the maintenance of intracellular calcium homeostasis by removing calcium ions.

METHOD

Since ATP2B1 knockout mice are reported to be embryo-lethal, we generated systemic heterozygous ATP2B1 null (ATP2B1(+/-)) mice, and evaluated the implication of ATP2B1 in blood pressure.

RESULTS

ATP2B1(+/-) mice revealed significantly higher SBP as measured by a radiotelemetric method. Phenylephrine-induced vasoconstriction was significantly increased in vascular rings from ATP2B1(+/-) mice, and the difference in this contraction disappeared in the presence of a nitric oxide synthase (NOS) inhibitor. Vasorelaxation to acetylcholine was significantly attenuated in vascular rings from ATP2B1(+/-) mice. In addition, cultured endothelial cells of ATP2B1(+/-) mice showed that the phosphorylation (Ser-1177) level of endothelial NOS protein was significantly lower, and nitric oxide production in endothelial cells and aorta was lower compared with those in control mice. In contrast, neural NOS expression in vascular smooth muscle cells from ATP2B1(+/-) mice and control mice were not significantly different.

CONCLUSION

These results suggest that decreased ATP2B1 gene expression is associated with impaired endothelial NOS activity and nitric oxide production, and the ATP2B1 gene plays a crucial role in the regulation of blood pressure.

BMP4 increases the expression of TRPC and basal [Ca2+]i via the p38MAPK and ERK1/2 pathways independent of BMPRII in PASMCs

Multiple abnormalities of bone morphogenetic protein (BMPs) signaling are implicated in the process of pulmonary arterial hypertension (PAH). BMP4 plays an important role during the process of pulmonary arterial remodeling and mutant of the principle BMP4 receptor, BMP receptors II (BMPRII), is found to associate with the development of PAH. However, the likely mechanism defining the contribution of BMPRII to BMP4 mediated signaling in pulmonary arterial smooth muscle cells (PASMCs) remains comprehensively unclear. We previously found that enhanced store operated calcium entry (SOCE) and basal intracellular calcium concentration [Ca2+]i were induced by BMP4 via upregulation of TRPC1, 4 and 6 expression in PASMCs, and that BMP4 modulated TRPC channel expression through activating p38MAPK and ERK1/2 signaling pathways. In this study, BMPRII siRNA was used to knockdown BMPRII expression to investigate whether BMP4 upregulates the expression of TRPC and activating Smad1/5/8, ERK1/2 and p38MAPK pathway via BMPRII in distal PASMCs. Our results showed that knockdown of BMPRII: 1) attenuated BMP4 induced activation of P-Smad1/5/8, without altering BMP4 induced P-p38MAPK and P-ERK1/2 activation in PASMCs; 2) did not attenuate the BMP4-induced TRPC1, 4 and 6 expression; 3) did not affect BMP4-enhanced SOCE and basal [Ca2+]i. Thus, we concluded that BMP4 activated Smad1/5/8 pathway is BMPRII-dependent, while the BMP4 - ERK/p-P38 - TRPC - SOCE signaling axis are likely mediated through other receptor rather than BMPRII.

Role of hypoxia-induced transglutaminase 2 in pulmonary artery smooth muscle cell proliferation

We previously reported that transglutaminase 2 (TG2) activity is markedly elevated in lungs of hypoxia-exposed rodent models of pulmonary hypertension (PH). Since vascular remodeling of pulmonary artery smooth muscle cells (PASMCs) is important in PH, we undertook the present study to determine whether TG2 activity is altered in PASMCs with exposure to hypoxia and whether that alteration participates in their proliferative response to hypoxia. Cultured distal bovine (b) and proximal human (h) PASMCs were exposed to hypoxia (3% O2) or normoxia (21% O2). mRNA and protein expression were determined by PCR and Western blot analyses. TG2 activity and function were visualized and determined by fluorescent labeled 5-pentylamine biotin incorporation and immunoblotting of serotonylated fibronectin. Cell proliferation was assessed by [(3)H]thymidine incorporation assay. At 24 h, both TG2 expression and activity were stimulated by hypoxia in bPASMCs. Activation of TG2 by hypoxia was blocked by inhibition of the extracellular calcium-sensing receptor or the transient receptor potential channel V4. In contrast, TG2 expression was blocked by inhibition of the transcription factor hypoxia-inducible factor-1α, supporting the presence of separate mechanisms for stimulation of activity and expression of TG2. Pulmonary arterial hypertension patient-derived hPASMCs were found to proliferate significantly more rapidly and respond to hypoxia more strongly than control-derived hPASMCs. Similar to bovine cells, hypoxia-induced proliferation of patient-derived cells was blocked by inhibition of TG2 activity. Our results suggest an important role for TG2, mediated by intracellular calcium fluxes and HIF-1α, in hypoxia-induced PASMC proliferation and possibly in vascular remodeling in PH.

Ion channels and transporters as therapeutic targets in the pulmonary circulation

Pulmonary circulation is a low pressure, low resistance, high flow system. The low resting vascular tone is maintained by the concerted action of ion channels, exchangers and pumps. Under physiological as well as pathophysiological conditions, they are targets of locally secreted or circulating vasodilators and/or vasoconstrictors, leading to changes in expression or to posttranslational modifications. Both structural changes in the pulmonary arteries and a sustained increase in pulmonary vascular tone result in pulmonary vascular remodeling contributing to morbidity and mortality in pediatric and adult patients. There is increasing evidence demonstrating the pivotal role of ion channels such as K(+) and Cl(-) or transient receptor potential channels in different cell types which are thought to play a key role in vasoconstrictive remodeling. This review focuses on ion channels, exchangers and pumps in the pulmonary circulation and summarizes their putative pathophysiological as well as therapeutic role in pulmonary vascular remodeling. A better understanding of the mechanisms of their actions may allow for the development of new options for attenuating acute and chronic pulmonary vasoconstriction and remodeling treating the devastating disease pulmonary hypertension.

Acute hypoxia activates store-operated Ca(2+) entry and increases intracellular Ca(2+) concentration in rat distal pulmonary venous smooth muscle cells

RATIONALE

Exposure to acute hypoxia causes vasoconstriction in both pulmonary arteries (PA) and pulmonary veins (PV). The mechanisms on the arterial side have been studied extensively. However, bare attention has been paid to the venous side.

OBJECTIVES

To investigate if acute hypoxia caused the increase of intracellular Ca(2+) concentration ([Ca(2+)]i), and Ca(2+) influx through store-operated calcium channels (SOCC) in pulmonary venous smooth muscle cells (PVSMCs).

METHODS

Fluorescent microscopy and fura-2 were used to measure effects of 4% O2 on [Ca(2+)]i and store-operated Ca(2+) entry (SOCE) in isolated rat distal PVSMCs.

MEASUREMENTS AND MAIN RESULTS

In PVSMCs perfused with Ca(2+)-free Krebs Ringer bicarbonate solution (KRBS) containing cyclopiazonic acid to deplete Ca(2+) stores in the sarcoplasmic reticulum (SR) and nifedipine to prevent Ca(2+) entry through L-type voltage-depended Ca(2+) channels (VDCC), hypoxia markedly enhanced both the increase in [Ca(2+)]i caused by restoration of extracellular [Ca(2+)] and the rate at which extracellular Mn(2+) quenched fura-2 fluorescence. Moreover, the increased [Ca(2+)]i in PVSMCs perfused with normal salt solution was completely blocked by SOCC antagonists SKF-96365 and NiCl2 at concentrations that SOCE >85% was inhibited but [Ca(2+)]i responses to 60 mM KCl were not altered. On the contrary, L-type VDCC antagonist nifedipine inhibited increase in [Ca(2+)]i to hypoxia by only 50% at concentrations that completely blocked responses to KCl. The increased [Ca(2+)]i caused by hypoxia was completely abolished by perfusion with Ca(2+)-free KRBS.

CONCLUSIONS

These results suggest that acute hypoxia enhances SOCE via activating SOCCs, leading to increased [Ca(2+)]i in distal PVSMCs.

Sildenafil inhibits hypoxia-induced transient receptor potential canonical protein expression in pulmonary arterial smooth muscle via cGMP-PKG-PPARγ axis

Transient receptor potential canonical (TRPC) proteins play important roles in chronically hypoxic pulmonary hypertension (CHPH). Previous results indicated that sildenafil inhibited TRPC1 and TRPC6 expression in rat distal pulmonary arteries (PAs). However, the underlying mechanisms remain unknown. We undertook this study to investigate the downstream signaling of sildenafil's regulation on TRPC1 and TRPC6 expression in pulmonary arterial smooth muscle cells (PASMCs). Hypoxia-exposed rats (10% O2 for 21 d) and rat distal PASMCs (4% O2 for 60 h) were taken as models to mimic CHPH. Real-time PCR, Western blotting, and Fura-2-based fluorescent microscopy were performed for mRNA, protein, and Ca(2+) measurements, respectively. The cellular cyclic guanosine monophosphate (cGMP) analogue 8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate sodium salt (CPT-cGMP) (100 μM) inhibited TRPC1 and TRPC6 expression, store-operated Ca(2+) entry (SOCE), and the proliferation and migration of PASMCs exposed to prolonged hypoxia. The inhibition of CPT-cGMP on TRPC1 and TRPC6 expression in PASMCs was relieved by either the inhibition or knockdown of cGMP-dependent protein kinase (PKG) and peroxisome proliferator-activated receptor γ (PPARγ) expression. Under hypoxic conditions, CPT-cGMP increased PPARγ expression. This increase was abolished by the PKG antagonists Rp8 or KT5823. PPARγ agonist GW1929 significantly decreased TRPC1 and TRPC6 expression in PASMCs. Moreover, hypoxia exposure decreased, whereas sildenafil treatment increased, PKG and PPARγ expression in PASMCs ex vivo, and in rat distal PAs in vivo. The suppressive effects of sildenafil on TRPC1 and TRPC6 in rat distal PAs and on the hemodynamic parameters of CHPH were inhibited by treatment with the PPARγ antagonist T0070907. We conclude that sildenafil inhibits TRPC1 and TRPC6 expression in PASMCs via cGMP-PKG-PPARγ-dependent signaling during CHPH.

Bone morphogenetic protein 2 decreases TRPC expression, store-operated Ca(2+) entry, and basal [Ca(2+)]i in rat distal pulmonary arterial smooth muscle cells

Recent studies indicate that multiple bone morphogenetic protein (BMP) family ligands and receptors are involved in the development of pulmonary arterial hypertension, yet the underlying mechanisms are incompletely understood. Although BMP2 and BMP4 share high homology in amino acid sequence, they appear to exert divergent effects on chronic hypoxic pulmonary hypertension (CHPH). While BMP4 promotes vascular remodeling, BMP2 prevents CHPH. We previously demonstrated that BMP4 upregulates the expression of canonical transient receptor potential channel (TRPC) proteins and, thereby, enhances store-operated Ca(2+) entry (SOCE) and elevates intracellular Ca(2+) concentration ([Ca(2+)]i) in pulmonary arterial smooth muscle cells (PASMCs). In this study, we investigated the effects of BMP2 on these variables in rat distal PASMCs. We found that treatment with BMP2 (50 ng/ml, 60 h) inhibited TRPC1, TRPC4, and TRPC6 mRNA and protein expression. Moreover, BMP2 treatment led to reduced SOCE and decreased basal [Ca(2+)]i in PASMCs. These alterations were associated with decreased PASMC proliferation and migration. Conversely, knockdown of BMP2 with specific small interference RNA resulted in increased cellular levels of TRPC1, TRPC4, and TRPC6 mRNA and protein, enhanced SOCE, elevated basal [Ca(2+)]i, and increased proliferation and migration of PASMCs. Together, these results indicate that BMP2 participates in regulating Ca(2+) signaling in PASMCs by inhibiting TRPC1, TRPC4, and TRPC6 expression, thus leading to reduced SOCE and basal [Ca(2+)]i and inhibition of cell proliferation and migration.

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.

Cross talk between plasma membrane Na(+)/Ca (2+) exchanger-1 and TRPC/Orai-containing channels: key players in arterial hypertension

Arterial smooth muscle (ASM) Na(+)/Ca(2+) exchanger type 1 (NCX1) and TRPC/Orai-containing receptor/store-operated cation channels (ROC/SOC) are clustered with α2 Na(+) pumps in plasma membrane microdomains adjacent to the underlying junctional sarcoplasmic reticulum. This arrangement enables these transport proteins to function as integrated units to help regulate local Na(+) metabolism, Ca(2+) signaling, and arterial tone. They thus influence vascular resistance and blood pressure (BP). For instance, upregulation of NCX1 and TRPC6 has been implicated in the pathogenesis of high BP in several models of essential hypertension. The models include ouabain-induced hypertensive rats, Milan hypertensive rats, and Dahl salt-sensitive hypertensive rats, all of which exhibit elevated plasma ouabain levels. We suggest that these molecular mechanisms are key contributors to the increased vascular resistance ("whole body autoregulation") that elevates BP in essential hypertension. Enhanced expression and function of ASM NCX1 and TRPC/Orai1-containing channels in hypertension implies that these proteins are potential targets for pharmacological intervention.

The vasorelaxant effects of 1-nitro-2-phenylethane involve stimulation of the soluble guanylate cyclase-cGMP pathway

1-Nitro-2-phenylethane is the first organic NO₂-containing molecule isolated from plants. It possesses interesting hypotensive, bradycardic, and vasodilator properties, but the mode by which it induces vasorelaxation is still unknown. The underlying mechanism involved in the vasodilator effect of 1-nitro-2-phenylethane was investigated in rat aorta. The vasorelaxant effects of 1-nitro-2-phenylethane did not depend on endothelial layer integrity, and the effects were refractory to L-N(G)-nitroarginine methyl ester (L-NAME)-induced nitric oxide synthase inhibition. Vasorelaxation was similarly resistant to treatment with indomethacin, cis-N-(2-phenylcyclopentyl)-azacyclotridec-1-en-2-amine hydrochloride (MDL-12330A), and KT5720, indicating that neither prostaglandin release nor adenylyl cyclase activation is involved. Conversely, methylene blue- and ODQ-induced guanylate cyclase inhibition reduced the vasorelaxation induced by 1-nitro-2-phenylethane. The pharmacological blockade of K(+) channels with tetraethylammonium, glybenclamide, and 4-aminopyridine also blunted vasorelaxation induced by 1-nitro-2-phenylethane. The effects of 1-nitro-2-phenylethane were reversed by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and comparable to the effects induced by sodium nitroprusside. In silico analysis using an Ns H-NOX subunit of guanylate cyclase revealed a pocket on the macromolecule surface where 1-nitro-2-phenylethane preferentially docked. In vitro, 1-nitro-2-phenylethane increased cyclic guanosine 3',5'-monophosphate (cGMP) levels in rat aortic rings, an effect also reversed by ODQ. In conclusion, 1-nitro-2-phenylethane produces vasodilator effects by stimulating the soluble guanylate cyclase-cGMP pathway.

Pathogenic role of store-operated and receptor-operated ca(2+) channels in pulmonary arterial hypertension

Pulmonary circulation is an important circulatory system in which the body brings in oxygen. Pulmonary arterial hypertension (PAH) is a progressive and fatal disease that predominantly affects women. Sustained pulmonary vasoconstriction, excessive pulmonary vascular remodeling, in situ thrombosis, and increased pulmonary vascular stiffness are the major causes for the elevated pulmonary vascular resistance (PVR) in patients with PAH. The elevated PVR causes an increase in afterload in the right ventricle, leading to right ventricular hypertrophy, right heart failure, and eventually death. Understanding the pathogenic mechanisms of PAH is important for developing more effective therapeutic approach for the disease. An increase in cytosolic free Ca²⁺ concentration ([Ca²⁺]_cyt) in pulmonary arterial smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and an important stimulus for PASMC migration and proliferation which lead to pulmonary vascular wall thickening and remodeling. It is thus pertinent to define the pathogenic role of Ca²⁺ signaling in pulmonary vasoconstriction and PASMC proliferation to develop new therapies for PAH. [Ca²⁺]_cyt in PASMC is increased by Ca²⁺ influx through Ca²⁺ channels in the plasma membrane and by Ca²⁺ release or mobilization from the intracellular stores, such as sarcoplasmic reticulum (SR) or endoplasmic reticulum (ER). There are two Ca²⁺ entry pathways, voltage-dependent Ca²⁺ influx through voltage-dependent Ca²⁺ channels (VDCC) and voltage-independent Ca²⁺ influx through store-operated Ca²⁺ channels (SOC) and receptor-operated Ca²⁺ channels (ROC). This paper will focus on the potential role of VDCC, SOC, and ROC in the development and progression of sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in PAH.

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

Pulmonary arterial hypertension (PAH) is a severe and progressive disease that usually culminates in right heart failure and death if left untreated. Although there have been substantial improvements in our understanding and significant advances in the management of this disease, there is a grim prognosis for patients in the advanced stages of PAH. A major cause of PAH is increased pulmonary vascular resistance, which results from sustained vasoconstriction, excessive pulmonary vascular remodeling, in situ thrombosis, and increased pulmonary vascular stiffness. In addition to other signal transduction pathways, Ca(2+) signaling in pulmonary artery smooth muscle cells (PASMCs) plays a central role in the development and progression of PAH because of its involvement in both vasoconstriction, through its pivotal effect of PASMC contraction, and vascular remodeling, through its stimulatory effect on PASMC proliferation. Altered expression, function, and regulation of ion channels and transporters in PASMCs contribute to an increased cytosolic Ca(2+) concentration and enhanced Ca(2+) signaling in patients with PAH. This review will focus on the potential pathogenic role of Ca(2+) mobilization, regulation, and signaling in the development and progression of PAH.

Hypoxic pulmonary vasoconstriction

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

Orai1 and Ca2+-independent phospholipase A2 are required for store-operated Icat-SOC current, Ca2+ entry, and proliferation of primary vascular smooth muscle cells

Store-operated Ca(2+) entry (SOCE) is important for multiple functions of vascular smooth muscle cells (SMC), which, depending of their phenotype, can resemble excitable and nonexcitable cells. Similar to nonexcitable cells, Orai1 was found to mediate Ca(2+)-selective (CRAC-like) current and SOCE in dedifferentiated cultured SMC and smooth muscle-derived cell lines. However, the role of Orai1 in cation-selective store-operated channels (cat-SOC), which are responsible for SOCE in primary SMC, remains unclear. Here we focus on primary SMC, and assess the role of Orai1 and Ca(2+)-independent phospholipase A(2) (iPLA(2)β, or PLA2G6) in activation of cat-SOC current (I(cat-SOC)), SOCE, and SMC proliferation. Using molecular, electrophysiological, imaging, and functional approaches, we demonstrate that molecular knockdown of either Orai1 or iPLA(2)β leads to similar inhibition of the whole cell cat-SOC current and SOCE in primary aortic SMC and results in significant reduction in DNA synthesis and impairment of SMC proliferation. This is the first demonstration that Orai1 and iPLA(2)β are equally important for cat-SOC, SOCE, and proliferation of primary aortic SMC.

Activity of Ca -activated Cl channels contributes to regulating receptor- and store-operated Ca entry in human pulmonary artery smooth muscle cells

Intracellular Ca²⁺ plays a fundamental role in regulating cell functions in pulmonary arterial smooth muscle cells (PASMCs). A rise in cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) triggers pulmonary vasoconstriction and stimulates PASMC proliferation. [Ca²⁺]_cyt is increased mainly by Ca²⁺ release from intracellular stores and Ca²⁺ influx through plasmalemmal Ca²⁺-permeable channels. Given the high concentration of intracellular Cl^- in PASMCs, Ca²⁺-activated Cl^-(Cl_Ca) channels play an important role in regulating membrane potential and cell excitability of PASMCs. In this study, we examined whether activity of Cl_Ca channels was involved in regulating [Ca²⁺]_cyt in human PASMCs via regulating receptor- (ROCE) and store- (SOCE) operated Ca²⁺ entry. The data demonstrated that an angiotensin II (100 nM)-mediated increase in [Ca²⁺]_cyt via ROCE was markedly attenuated by the Cl_Ca channel inhibitors, niflumic acid (100 μM), flufenamic acid (100 μM), and 4,4’-diisothiocyanatostilbene-2,2’-disulfonic acid (100 μM). The inhibition of Cl_Ca channels by niflumic acid and flufenamic acid significantly reduced both transient and plateau phases of SOCE that was induced by passive depletion of Ca²⁺ from the sarcoplasmic reticulum by 10 μM cyclopiazonic acid. In addition, ROCE and SOCE were abolished by SKF-96365 (50 μM) and 2-aminoethyl diphenylborinate (100 μM), and were slightly decreased in the presence of diltiazem (10 μM). The electrophysiological and immunocytochemical data indicate that Cl_Ca currents were present and TMEM16A was functionally expressed in human PASMCs. The results from this study suggest that the function of Cl_Ca channels, potentially formed by TMEM16A proteins, contributes to regulating [Ca²⁺]_cyt by affecting ROCE and SOCE in human PASMCs.

Role of the gap junctions in the contractile response to agonists in pulmonary artery from two rat models of pulmonary hypertension

BACKGROUND

Pulmonary hypertension (PH) is characterized by arterial vascular remodelling and alteration in vascular reactivity. Since gap junctions are formed with proteins named connexins (Cx) and contribute to vasoreactivity, we investigated both expression and role of Cx in the pulmonary arterial vasoreactivity in two rat models of PH.

METHODS

Intrapulmonary arteries (IPA) were isolated from normoxic rats (N), rats exposed to chronic hypoxia (CH) or treated with monocrotaline (MCT). RT-PCR, Western Blot and immunofluorescent labelling were used to study the Cx expression. The role of Cx in arterial reactivity was assessed by using isometric contraction and specific gap junction blockers. Contractile responses were induced by agonists already known to be involved in PH, namely serotonin, endothelin-1 and phenylephrine.

RESULTS

Cx 37, 40 and 43 were expressed in all rat models and Cx43 was increased in CH rats. In IPA from N rats only, the contraction to serotonin was decreased after treatment with 37-43Gap27, a specific Cx-mimetic peptide blocker of Cx 37 and 43. The contraction to endothelin-1 was unchanged after incubation with 40Gap27 (a specific blocker of Cx 40) or 37-43Gap27 in N, CH and MCT rats. In contrast, the contraction to phenylephrine was decreased by 40Gap27 or 37-43Gap27 in CH and MCT rats. Moreover, the contractile sensitivity to high potassium solutions was increased in CH rats and this hypersensitivity was reversed following 37-43Gap27 incubation.

CONCLUSION

Altogether, Cx 37, 40 and 43 are differently expressed and involved in the vasoreactivity to various stimuli in IPA from different rat models. These data may help to understand alterations of pulmonary arterial reactivity observed in PH and to improve the development of innovative therapies according to PH aetiology.

Effects of atorvastatin and losartan on monocrotaline-induced pulmonary artery remodeling in rats

Structural remodeling of pulmonary artery plays an important role in maintaining sustained pulmonary arterial hypertension (PAH). The anti-remodeling effects of statins have been reported in systemic hypertension. In this study, we studied the effects of atovastatin (Ato) or losartan (Los) in monocrotaline (MCL)-induced pulmonary artery remodeling using a rat model. Forty Sprague-Dawley (SD) rats were randomly assigned into four groups (n = 10): normal control (Ctr), PAH, PAH treated with Los, and PAH treated with Ato. We found that in the Los- or Ato-treated group, the mean pulmonary arterial pressure, right heart hypertrophy index, ratio of wall/lumen thickness (WT%), as well as the wall/lumen area (WA%) were significantly reduced compared to the PAH group. Also in pulmonary arteries dissected from rats in the Ato- or Los-treated group, in both mRNA and protein levels, the expression of α1C subunit of voltage-gated calcium channel (Ca(v)α1c) was downregulated, while sarcoplasmic/endoplasmic reticulum calcium-ATPase (SERCA-2a) and inositol 1,4,5 triphosphate receptor 1 (IP3R-1) upregulated. However, the mRNA level of RyR-3 subunit of calcium regulating channel was increased, whereas its protein level was reduced in the treated groups. Our results suggest that atorvastatin or losartan may regress the remodeling of the pulmonary artery in pulmonary hypertensive rats, with differential expression of calcium regulating channels.

Diacylglycerol regulates acute hypoxic pulmonary vasoconstriction via TRPC6

Background

Hypoxic pulmonary vasoconstriction (HPV) is an essential mechanism of the lung that matches blood perfusion to alveolar ventilation to optimize gas exchange. Recently we have demonstrated that acute but not sustained HPV is critically dependent on the classical transient receptor potential 6 (TRPC6) channel. However, the mechanism of TRPC6 activation during acute HPV remains elusive. We hypothesize that a diacylglycerol (DAG)-dependent activation of TRPC6 regulates acute HPV.

Methods

We investigated the effect of the DAG analog 1-oleoyl-2-acetyl-sn-glycerol (OAG) on normoxic vascular tone in isolated perfused and ventilated mouse lungs from TRPC6-deficient and wild-type mice. Moreover, the effects of OAG, the DAG kinase inhibitor R59949 and the phospholipase C inhibitor U73122 on the strength of HPV were investigated compared to those on non-hypoxia-induced vasoconstriction elicited by the thromboxane mimeticum U46619.

Results

OAG increased normoxic vascular tone in lungs from wild-type mice, but not in lungs from TRPC6-deficient mice. Under conditions of repetitive hypoxic ventilation, OAG as well as R59949 dose-dependently attenuated the strength of acute HPV whereas U46619-induced vasoconstrictions were not reduced. Like OAG, R59949 mimicked HPV, since it induced a dose-dependent vasoconstriction during normoxic ventilation. In contrast, U73122, a blocker of DAG synthesis, inhibited acute HPV whereas U73343, the inactive form of U73122, had no effect on HPV.

Conclusion

These findings support the conclusion that the TRPC6-dependency of acute HPV is induced via DAG.

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.

Bone morphogenetic protein 4 enhances canonical transient receptor potential expression, store-operated Ca2+ entry, and basal [Ca2+]i in rat distal pulmonary arterial smooth muscle cells

Recent advances have identified an important role of bone morphogenetic protein 4 (BMP4) in pulmonary vascular remodeling, yet the underlying mechanisms remain largely unexplored. We have previously found that Ca(2+) influx through store-operated calcium channels (SOCC), which are mainly thought to be composed of canonical transient receptor potential (TRPC) proteins, likely contribute to the pathogenic development of chronic hypoxic pulmonary hypertension. In this study, we investigated the effect of BMP4 on expression of TRPC and store-operated Ca(2+) entry (SOCE) in pulmonary arterial smooth muscle cells (PASMCs). Real-time quantitative PCR and Western blotting revealed that treatment with BMP4 (50 ng/ml, 60 h) increased TRPC1, TRPC4, and TRPC6 mRNA and protein expression in growth-arrested rat distal PASMCs. Moreover, in comparison to vehicle control, cells treated with BMP4 also exhibited enhanced SOCE, and elevated basal intracellular calcium concentration ([Ca(2+)](i)) as determined by fluorescent microscopy using the Ca(2+) indicator Fura-2 AM. Perfusing cells with Ca(2+)-free Krebs-Ringer bicarbonate solution (KRBS) or KRBS containing SOCC antagonists SKF-96365 or NiCl(2) attenuated the increases in basal [Ca(2+)](i) caused by BMP4. Specific knockdown of BMP4 by small interference RNA significantly decreased the mRNA and protein expression of TRPC1, TRPC4, and TRPC6 and reduced SOCE and basal [Ca(2+)](i) in serum-stimulated PASMCs. We conclude that BMP4 regulates calcium signaling in PASMCs likely via upregulation of TRPC expression, leading to enhanced SOCE and basal [Ca(2+)](i) in PASMCs, and by this mechanism contributes to pulmonary vascular remodeling during pulmonary arterial hypertension.

Interactions between calcium and reactive oxygen species in pulmonary arterial smooth muscle responses to hypoxia

In contrast to the systemic vasculature, where hypoxia causes vasodilation, pulmonary arteries constrict in response to hypoxia. The mechanisms underlying this unique response have been the subject of investigation for over 50 years, and still remain a topic of great debate. Over the last 20 years, there has emerged a general consensus that both increases in intracellular calcium concentration and changes in reactive oxygen species (ROS) generation play key roles in the pulmonary vascular response to hypoxia. Controversy exists, however, regarding whether ROS increase or decrease during hypoxia, the source of ROS, and the mechanisms by which changes in ROS might impact intracellular calcium, and vice versa. This review will discuss the mechanisms regulating [Ca2+]i and ROS in PASMCs, and the interaction between ROS and Ca2+ signaling during exposure to acute hypoxia.

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.

Expression of store-operated Ca2+ entry and transient receptor potential canonical and vanilloid-related proteins in rat distal pulmonary venous smooth muscle

Chronic hypoxia causes remodeling and alters contractile responses in both pulmonary arteries and pulmonary veins. Although pulmonary arteries have been studied extensively in these disorders, the mechanisms by which pulmonary veins respond to hypoxia and whether these responses contribute to chronic hypoxic pulmonary hypertension remain poorly understood. In pulmonary arterial smooth muscle, we have previously demonstrated that influx of Ca(2+) through store-operated calcium channels (SOCC) thought to be composed of transient receptor potential (TRP) proteins is likely to play an important role in development of chronic hypoxic pulmonary hypertension. To determine whether this mechanism could also be operative in pulmonary venous smooth muscle, we measured intracellular Ca(2+) concentration ([Ca(2+)](i)) by fura-2 fluorescence microscopy in primary cultures of pulmonary venous smooth muscle cells (PVSMC) isolated from rat distal pulmonary veins. In cells perfused with Ca(2+)-free media containing cyclopiazonic acid (10 μM) and nifedipine (5 μM) to deplete sarcoplasmic reticulum Ca(2+) stores and block voltage-dependent Ca(2+) channels, restoration of extracellular Ca(2+) (2.5 mM) caused marked increases in [Ca(2+)](i), whereas MnCl(2) (200 μM) quenched fura-2 fluorescence, indicating store-operated Ca(2+) entry (SOCE). SKF-96365 and NiCl(2), antagonists of SOCC, blocked SOCE at concentrations that did not alter Ca(2+) responses to 60 mM KCl. Of the seven known canonical TRP (TRPC1-7) and six vanilloid-related TRP channels (TRPV1-6), real-time PCR revealed mRNA expression of TRPC1 > TRPC6 > TRPC4 > TRPC2 ≈ TRPC5 > TRPC3, TRPV2 > TRPV4 > TRPV1 in distal PVSMC, and TRPC1 > TRPC6 > TRPC3 > TRPC4 ≈ TRPC5, TRPV2 ≈ TRPV4 > TRPV1 in rat distal pulmonary vein (PV) smooth muscle. Western blotting confirmed protein expression of TRPC1, TRPC6, TRPV2, and TRPV4 in both PVSMC and PV. Our results suggest that SOCE through Ca(2+) channels composed of TRP proteins may contribute to Ca(2+) signaling in rat distal PV smooth muscle.

15-HETE mediates sub-acute hypoxia-induced TRPC1 expression and enhanced capacitative calcium entry in rat distal pulmonary arterial myocytes

Sub-acute hypoxia causes pulmonary vasoconstriction (HPV) is associated with increased intracellular Ca(2+) concentration ([Ca(2+)](i)) and contraction of pulmonary arterial smooth muscle cells (PASMCs). We previous have demonstrated that 15-hydroxyeicosatetraenoic acid (15-HETE), a metabolite of arachidonic acid by 15-lipoxygenase (15-LO), causes elevated [Ca(2+)](i) in PASMCs partly through Ca(2+) entry via other than L-type Ca(2+) channels. In this study, we used SKF96365/La(3+) (SOCC antagonists) and Nordihydro-guiairetic acid (NDGA, a blockage of 15-LO) to examine the effect of 15-HETE on capacitative Ca(2+) entry and activity/expression of store-operated Ca(2+) channels (SOCCs) during sub-acute hypoxic procedure and the contribution of SOCCs on the maintenance of vascular tones. The results showed that the 15-HETE induced constriction of PA rings from normoxic and sub-acute hypoxic rats can be abolished by SKF96365 and La(3+). Capacitative Ca(2+) entry (CCE) was also enhanced in PASMCs cultured with 15-HETE under sub-acute hypoxic condition (3% O(2), 48h) and incubation with NDGA in PASMCs can greatly suppress this enhancement. Moreover, TRPC1, not TRPC4 and TRPC6, mRNA and protein expression were increased in PASMCs during these procedures. Meanwhile, the effect of 15-HETE on CCE and TRPC1 expression under sub-acute hypoxic cultivation were greatly suppressed in 15-LO knockdown PASMCs and PAs. These results suggest that 15-HETE mediated HPV through increased TRPC1 expression, leading to enhanced CCE, contributing to the maintenance of vascular tone.

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.

Intricate interaction between store-operated calcium entry and calcium-activated chloride channels in pulmonary artery smooth muscle cells

Ca(2+)-activated Cl-() channels (Cl(Ca)) represent an important excitatory mechanism in vascular smooth muscle cells. Active accumulation of Cl-() by several classes of anion transporters results in an equilibrium potential for this ion about 30 mV more positive than the resting potential. Stimulation of Cl(Ca) channels leads to membrane depolarization, which enhances Ca(2+) entry through voltage-gated Ca(2+) channels and leads to vasoconstriction. Cl(Ca) channels can be activated by distinct sources of Ca(2+) that include (1) mobilization from intracellular Ca(2+) stores (ryanodine or inositol 1,4,5-trisphosphate [InsP(3)]) and (2) Ca(2+) entry through voltage-gated Ca(2+) channels or reverse-mode Na(+)/Ca(2+) exchange. The present study was undertaken to determine whether Ca(2+) influx triggered by store depletion (store-operated calcium entry, SOCE) activates Cl(Ca) channels in rabbit pulmonary artery (PA) smooth muscle. Classical store depletion protocols involving block of sarcoplasmic reticular Ca(2+) reuptake with thapsigargin (TG; 1 microM) or cyclopiazonic acid (CPA; 30 microM) led to a consistent nifedipine-insensitive contraction of intact PA rings and rise in intracellular Ca(2+) concentration in single PA myocytes that required the presence of extracellular Ca(2+). In patch clamp experiments, TG or CPA activated a time-independent nonselective cation current (I (SOC)) that (1) reversed between -10 and 0 mV; (2) displayed the typical "N"-shaped current-voltage relationship; and (3) was sensitive to the (I (SOC)) blocker by SKF-96365 (50 microM). In double-pulse protocol experiments, the amplitude of I (SOC) was varied by altering membrane potential during an initial step that was followed by a second constant step to +90 mV to register Ca(2+)-activated Cl(-) current, I (Cl(Ca)). The niflumic acid-sensitive time-dependent I (Cl(Ca)) at +90 mV increased in proportion to the magnitude of the preceding hyperpolarizing step, an effect attributed to graded membrane potential-dependent Ca(2+) entry through I (SOC) and confirmed in dual patch clamp and Fluo-5 experiments to record membrane current and free intracellular Ca(2+) concentration simultaneously. Reverse-transcription polymerase chain reaction (RT-PCR) experiments confirmed the expression of several molecular determinants of SOCE, including transient receptor potential canonical (TRPC) 1, TRPC4, and TRPC6; stromal interacting molecule (STIM) 1 and 2; and Orai1 and 2, as well as the novel and probable molecular candidates thought to encode for Cl(Ca) channels transmembrane protein 16A (TMEM16A) Anoctamin 1 (ANO1) and B (ANO2). Ourpreliminary investigation provides new evidence for a Ca(2+) entry pathway consistent with store-operated Ca(2+) entry signaling that can activate Ca(2+)-activated Cl-() channels in rabbit PA myocytes. We hypothesize that this mechanism may be important in the regulation of membrane potential, Ca(2+) influx, and tone in these cells under physiological and pathophysiological conditions.

Physiological functions of transient receptor potential channels in pulmonary arterial smooth muscle cells

The transient receptor potential (TRP) gene superfamily, which consists of 7 subfamilies with at least 28 mammalian homologues, is known to encode a wide variety of cation channels with diverse biophysical properties, activation mechanisms, and physiological functions. Recent studies have identified multiple TRP channel subtypes, belonging to the canonical (TRPC), melastatin-related (TRPM), and vanilloid-related (TRPV) subfamilies, in pulmonary arterial smooth muscle cells (PASMCs). They operate as specific Ca(2+) pathways responsive to stimuli, including Ca(2+) store depletion, receptor activation, reactive oxygen species, growth factors, and mechanical stress. Increasing evidence suggests that these channels play crucial roles in agonist-induced pulmonary vasoconstriction, hypoxic pulmonary vasoconstriction, smooth muscle cell proliferation, vascular remodeling, and pulmonary arterial hypertension. This chapter highlighted and discussed these putative physiological functions of TRP channels in pulmonary vasculatures. Since Ca(2+) ions regulate many cellular processes via specific Ca(2+) signals, future investigations of these novel channels will likely uncover more important regulatory mechanisms of pulmonary vascular functions in health and in disease states.

Evidence for STIM1- and Orai1-dependent store-operated calcium influx through ICRAC in vascular smooth muscle cells: role in proliferation and migration

The identity of store-operated calcium (Ca(2+)) entry (SOCE) channels in vascular smooth muscle cells (VSMCs) remains a highly contentious issue. Whereas previous studies have suggested that SOCE in VSMCs is mediated by the nonselective transient receptor potential canonical (TRPC) 1 protein, the identification of STIM1 and Orai1 as essential components of I(CRAC), a highly Ca(2+)-selective SOCE current in leukocytes, has challenged that view. Here we show that cultured proliferative migratory VSMCs isolated from rat aorta (called "synthetic") display SOCE with classic features, namely inhibition by 2-aminoethoxydiphenyl borate, ML-9, and low concentrations of lanthanides. On store depletion, synthetic VSMCs and A7r5 cells display currents with characteristics of I(CRAC). Protein knockdown of either STIM1 or Orai1 in synthetic VSMCs greatly reduced SOCE, whereas Orai2, Orai3, TRPC1, TRPC4, and TRPC6 knockdown had no effect. Orai1 knockdown reduced I(CRAC) in synthetic VSMCs and A7r5 cells. Synthetic VSMCs showed up-regulated STIM1/Orai1 proteins and SOCE compared with quiescent freshly isolated VSMC. Knockdown of STIM1 and Orai1 inhibited synthetic VSMC proliferation and migration, whereas STIM2, Orai2, and Orai3 knockdown had no effect. To our knowledge, these results are the first to show I(CRAC) in VSMCs and resolve a long-standing controversy by identifying CRAC as the elusive VSMC SOCE channel important for proliferation and migration.

TRPC1 and STIM1 mediate capacitative Ca2+ entry in mouse pulmonary arterial smooth muscle cells

Previous studies in pulmonary arterial smooth muscle cells (PASMCs) showed that the TRPC1 channel mediates capacitative Ca(2+) entry (CCE), but the molecular signal(s) that activate TRPC1 in PASMCs remains unknown. The aim of the present study was to determine if TRPC1 mediates CCE through activation of STIM1 protein in mouse PASMCs. In primary cultured mouse PASMCs loaded with fura-2, cyclopiazonic acid (CPA) caused a transient followed by a sustained rise in intracellular Ca(2+) concentration ([Ca(2+)](i)). The transient but not the sustained rise in [Ca(2+)](i) was partially inhibited by nifedipine. In addition, CPA increased the rate of Mn(2+) quench of fura-2 fluorescence that was inhibited by SKF 96365, Ni(2+), La(3+) and Gd(3+), exhibiting pharmacological properties characteristic of CCE. The nifedipine-insensitive sustained rise in [Ca(2+)](i) and the increase in Mn(2+) quench of fura-2 fluorescence caused by CPA were both inhibited in cells pretreated with antibody raised against an extracellular epitope of TRPC1. Moreover, STIM1 siRNA reduced the rise in [Ca(2+)](i) and Mn(2+) quench of fura-2 fluorescence caused by CPA, whereas overexpression of STIM1 resulted in a marked increase in these responses. RT-PCR revealed TRPC1 and STIM1 mRNAs, and Western blot analysis identified TRPC1 and STIM1 proteins in mouse PASMCs. Furthermore, TRPC1 was found to co-immunoprecipitate with STIM1, and the precipitation level of TRPC1 was increased in cells subjected to store depletion. Taken together, store depletion causes activation of voltage-operated Ca(2+) entry and CCE. These data provide direct evidence that CCE is mediated by TRPC1 channel through activation of STIM1 in mouse PASMCs.

p75 neurotrophin receptor regulates agonist-induced pulmonary vasoconstriction

A member of the TNF receptor family, the p75 neurotrophin receptor (p75(NTR)) has been previously shown to play a role in the regulation of fibrin deposition in the lung. However, the role of p75(NTR) in the regulation of pulmonary vascular tone in the lung is unknown. In the present study, we evaluated the expression of p75(NTR) in mouse pulmonary arteries and the putative role of p75(NTR) in modulating pulmonary vascular tone and agonist responsiveness using wild-type (WT) and p75(NTR) knockout (p75(-/-)) mice. Our data indicated that p75(NTR) is expressed in both smooth muscle and endothelial cells within the pulmonary vascular wall in WT mice. Pulmonary artery rings from p75(-/-) mice exhibited significantly elevated active tension due to endothelin-1-mediated Ca(2+) influx. Furthermore, the contraction due to capacitative Ca(2+) entry (CCE) in response to phenylephrine-mediated active depletion of intracellular Ca(2+) stores was significantly enhanced compared with WT rings. The contraction due to CCE induced by passive store depletion, however, was comparable between WT and p75(-/-) rings. Active tension induced by serotonin, U-46619 (a thromboxane A(2) analog), thrombin, 4-aminopyridine (a K(+) channel blocker), and high extracellular K(+) in p75(-/-) rings was similar to that in WT rings. Deletion of p75(NTR) did not alter pulmonary vasodilation to sodium nitroprusside (a nitric oxide donor). These data suggest that intact p75(NTR) signaling may play a role in modulating pulmonary vasoconstriction induced by endothelin-1 and by active store depletion.

Maturation of intracellular calcium homeostasis in sheep pulmonary arterial smooth muscle cells

Cytosolic Ca(2+) signaling dynamics are important to pulmonary arterial reactivity, and alterations are implicated in pulmonary vascular disorders. Yet, adaptations in cellular Ca(2+) homeostasis and receptor-mediated Ca(2+) signaling with maturation from fetal to adult life in pulmonary arterial smooth muscle cells (PASMCs) are not known. The present study tested the hypothesis that cytosolic Ca(2+) homeostasis and receptor-generated Ca(2+) signaling adapt with maturation in sheep PASMCs. Digitalized fluorescence microscopy was performed using isolated PASMCs from fetal and adult sheep that were loaded with the Ca(2+) indicator fura 2. The results show that basal cytosolic and sarcoplasmic reticulum Ca(2+) levels are attained before birth. Similarly, Ca(2+) efflux pathways from the cytosol and basal as well as capacitative Ca(2+) entry (CCE) are also developed before birth. However, receptor-mediated Ca(2+) signaling adapts with maturation. Prominently, serotonin stimulation elicited Ca(2+) elevations in very few fetal compared with adult PASMCs; in contrast, phenylephrine elevated Ca(2+) in a similar percentage of fetal and adult PASMCs. Serotonin and phenylephrine elicited Ca(2+) increases of a similar magnitude in reactive cells of fetus and adult, supporting the assertion that inositol trisphosphate signaling is intact. Caffeine and ATP elevated Ca(2+) in equivalent numbers of fetal and adult PASMCs. However, the caffeine-induced cytosolic Ca(2+) increase was significantly greater in fetal PASMCs, whereas the ATP-elicited increase was greater in adult cells. Overall, the results of this study demonstrate selective adaptations in receptor-mediated Ca(2+) signaling, but not in cellular Ca(2+) homeostasis.

Regulation of pulmonary vasoconstriction by agonists and caveolae

Sustained pulmonary vasoconstriction contributes to the elevated pulmonary vascular resistance observed in pulmonary arterial hypertension. A rise in cytosolic Ca(2 +) in pulmonary artery smooth muscle cells (PASMCs) is major trigger for pulmonary vasoconstriction. One family of drugs currently being pursued as a potential treatment for pulmonary hypertension are the statins, which act by depleting cholesterol and reducing the number of caveolae. This study aimed at investigating the role of caveolae, membrane receptors, and ion channels (that are potentially located in the caveolae) in agonist-mediated pulmonary vasoconstriction in order to gain a greater understanding of the signaling mechanisms involved in the regulation of pulmonary vascular tone. Chronic treatment of PASMCs with the cholesterol-depleting agent, methyl-beta -cyclodextrin (Mbeta CD), significantly reduced the number of cholesterol rich caveolae regions in the membrane. This disruption of cholesterol in caveolae significantly inhibited pharmacomechanical (induced by phenylephrine), but not electromechanical (induced by elevated extracellular potassium concentration), rat pulmonary artery contraction. These results indicate that receptors may functionally colocalize in caveolae in PASMCs and coordinate to regulate pulmonary vascular tone.