Hypoxic release of calcium from the sarcoplasmic reticulum of pulmonary artery smooth muscle

AMPK and the Challenge of Treating Hypoxic Pulmonary Hypertension

Hypoxic pulmonary hypertension (HPH) is characterized by sustained elevation of pulmonary artery pressure produced by vasoconstriction and hyperproliferative remodeling of the pulmonary artery and subsequent right ventricular hypertrophy (RVH). The search for therapeutic targets for cardiovascular pathophysiology has extended in many directions. However, studies focused on mitigating high-altitude pulmonary hypertension (HAPH) have been rare. Because AMP-activated protein kinase (AMPK) is involved in cardiovascular and metabolic pathology, AMPK is often studied as a potential therapeutic target. AMPK is best characterized as a sensor of cellular energy that can also restore cellular metabolic homeostasis. However, AMPK has been implicated in other pathways with vasculoprotective effects. Notably, cellular metabolic stress increases the intracellular ADP/ATP or AMP/ATP ratio, and AMPK activation restores ATP levels by activating energy-producing catabolic pathways and inhibiting energy-consuming anabolic pathways, such as cell growth and proliferation pathways, promoting cardiovascular protection. Thus, AMPK activation plays an important role in antiproliferative, antihypertrophic and antioxidant pathways in the pulmonary artery in HPH. However, AMPK plays contradictory roles in promoting HPH development. This review describes the main findings related to AMPK participation in HPH and its potential as a therapeutic target. It also extrapolates known AMPK functions to discuss the less-studied HAPH context.

Acetazolamide prevents hypoxia-induced reactive oxygen species generation and calcium release in pulmonary arterial smooth muscle

Upon sensing a reduction in local oxygen partial pressure, pulmonary vessels constrict, a phenomenon known as hypoxic pulmonary vasoconstriction. Excessive hypoxic pulmonary vasoconstriction can occur with ascent to high altitude and is a contributing factor to the development of high-altitude pulmonary edema. The carbonic anhydrase inhibitor, acetazolamide, attenuates hypoxic pulmonary vasoconstriction through stimulation of alveolar ventilation via modulation of acid-base homeostasis and by direct effects on pulmonary vascular smooth muscle. In pulmonary arterial smooth muscle cells (PASMCs), acetazolamide prevents hypoxia-induced increases in intracellular calcium concentration ([Ca²⁺]_i), although the exact mechanism by which this occurs is unknown. In this study, we explored the effect of acetazolamide on various calcium-handling pathways in PASMCs. Using fluorescent microscopy, we tested whether acetazolamide directly inhibited store-operated calcium entry or calcium release from the sarcoplasmic reticulum, two well-documented sources of hypoxia-induced increases in [Ca²⁺]_i in PASMCs. Acetazolamide had no effect on calcium entry stimulated by store-depletion, nor on calcium release from the sarcoplasmic reticulum induced by either phenylephrine to activate inositol triphosphate receptors or caffeine to activate ryanodine receptors. In contrast, acetazolamide completely prevented Ca²⁺-release from the sarcoplasmic reticulum induced by hypoxia (4% O₂). Since these results suggest the acetazolamide interferes with a mechanism upstream of the inositol triphosphate and ryanodine receptors, we also determined whether acetazolamide might prevent hypoxia-induced changes in reactive oxygen species production. Using roGFP, a ratiometric reactive oxygen species-sensitive fluorescent probe, we found that hypoxia caused a significant increase in reactive oxygen species in PASMCs that was prevented by 100 μM acetazolamide. Together, these results suggest that acetazolamide prevents hypoxia-induced changes in [Ca²⁺]_i by attenuating reactive oxygen species production and subsequent activation of Ca²⁺-release from sarcoplasmic reticulum stores.

Repeated intermittent hypoxic stimuli to operative lung reduce hypoxemia during subsequent one-lung ventilation for thoracoscopic surgery: A randomized controlled trial

Background

An intervention to potentiate hypoxic pulmonary vasoconstriction may reduce intrapulmonary shunt and hypoxemia during one-lung ventilation. Previous animal studies reported that repeated intermittent hypoxic stimuli potentiated hypoxic pulmonary vasoconstriction, but no clinical study has examined the effects of this intervention on hypoxemia during one-lung ventilation. We thus performed a single-center, parallel-group, double-blind, randomized controlled trial to investigate whether repeated intermittent hypoxic stimuli to the operative lung reduce hypoxemia during the subsequent one-lung ventilation for thoracoscopic surgery.

Methods

Patients undergoing one-lung ventilation were randomized into two groups (n = 68 each). Before one-lung ventilation, in the intermittent hypoxia group, the nondependent lung was not ventilated for 2 min and then ventilated for 2 min while the dependent lung was continuously ventilated. This was repeated five times. In the continuous normoxia group, both lungs were ventilated for 20 min. We measured SpO₂, PaO₂, FiO₂, PaCO₂, SaO₂, and central venous oxygen saturation during one-lung ventilation. The primary outcome was the number of patients with hypoxemia defined as a SpO₂ <95% during one-lung ventilation, which was analyzed with a chi-squared test.

Results

Hypoxemia was less frequent in the intermittent hypoxia group than in the continuous normoxia group during OLV [6/68 (8.8%) vs 17/68 (25.0%), risk ratio (95% CI) 0.35 (0.15–0.84), p = 0.012]. The PaO₂ (p = 0.008 for 30 min and 0.007 for 60 min) and PaO₂/FiO₂ (p = 0.008 for both) were higher 30 and 60 min after starting one-lung ventilation, and the alveolar-arterial pressure gradient (p = 0.010) and shunt index (p = 0.008) were lower 30 min after starting one-lung ventilation in the intermittent hypoxia group than in the continuous normoxia group. Postoperative adverse events did not differ significantly between groups.

Conclusions

Repeated intermittent hypoxic stimuli to the operative lung seemed to potentiate hypoxic pulmonary vasoconstriction, and thus reduced hypoxemia during the subsequent one-lung ventilation.

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

A plethora of cellular functions are controlled by calcium signals, that are greatly coordinated by calcium release from intracellular stores, the principal component of which is the sarco/endooplasmic reticulum (S/ER). In 1997 it was generally accepted that activation of various G protein-coupled receptors facilitated inositol-1,4,5-trisphosphate (IP3) production, activation of IP3 receptors and thus calcium release from S/ER. Adding to this, it was evident that S/ER resident ryanodine receptors (RyRs) could support two opposing cellular functions by delivering either highly localised calcium signals, such as calcium sparks, or by carrying propagating, global calcium waves. Coincidentally, it was reported that RyRs in mammalian cardiac myocytes might be regulated by a novel calcium mobilising messenger, cyclic adenosine diphosphate-ribose (cADPR), that had recently been discovered by HC Lee in sea urchin eggs. A reputedly selective and competitive cADPR antagonist, 8-bromo-cADPR, had been developed and was made available to us. We used 8-bromo-cADPR to further explore our observation that S/ER calcium release via RyRs could mediate two opposing functions, namely pulmonary artery dilation and constriction, in a manner seemingly independent of IP3Rs or calcium influx pathways. Importantly, the work of others had shown that, unlike skeletal and cardiac muscles, smooth muscles might express all three RyR subtypes. If this were the case in our experimental system and cADPR played a role, then 8-bromo-cADPR would surely block one of the opposing RyR-dependent functions identified, or the other, but certainly not both. The latter seemingly implausible scenario was confirmed. How could this be, do cells hold multiple, segregated SR stores that incorporate different RyR subtypes in receipt of spatially segregated signals carried by cADPR? The pharmacological profile of 8-bromo-cADPR action supported not only this, but also indicated that intracellular calcium signals were delivered across intracellular junctions formed by the S/ER. Not just one, at least two. This article retraces the steps along this journey, from the curious pharmacological profile of 8-bromo-cADPR to the discovery of the cell-wide web, a diverse network of cytoplasmic nanocourses demarcated by S/ER nanojunctions, which direct site-specific calcium flux and may thus coordinate the full panoply of cellular processes.

AMPK and the Need to Breathe and Feed: What's the Matter with Oxygen?

We live and to do so we must breathe and eat, so are we a combination of what we eat and breathe? Here, we will consider this question, and the role in this respect of the AMP-activated protein kinase (AMPK). Emerging evidence suggests that AMPK facilitates central and peripheral reflexes that coordinate breathing and oxygen supply, and contributes to the central regulation of feeding and food choice. We propose, therefore, that oxygen supply to the body is aligned with not only the quantity we eat, but also nutrient-based diet selection, and that the cell-specific expression pattern of AMPK subunit isoforms is critical to appropriate system alignment in this respect. Currently available information on how oxygen supply may be aligned with feeding and food choice, or vice versa, through our motivation to breathe and select particular nutrients is sparse, fragmented and lacks any integrated understanding. By addressing this, we aim to provide the foundations for a clinical perspective that reveals untapped potential, by highlighting how aberrant cell-specific changes in the expression of AMPK subunit isoforms could give rise, in part, to known associations between metabolic disease, such as obesity and type 2 diabetes, sleep-disordered breathing, pulmonary hypertension and acute respiratory distress syndrome.

Role of Ryanodine Type 2 Receptors in Elementary Ca2+ Signaling in Arteries and Vascular Adaptive Responses

Background Hypertension is the major risk factor for cardiovascular disease, the most common cause of death worldwide. Resistance arteries are capable of adapting their diameter independently in response to pressure and flow-associated shear stress. Ryanodine receptors (RyRs) are major Ca2+-release channels in the sarcoplasmic reticulum membrane of myocytes that contribute to the regulation of contractility. Vascular smooth muscle cells exhibit 3 different RyR isoforms (RyR1, RyR2, and RyR3), but the impact of individual RyR isoforms on adaptive vascular responses is largely unknown. Herein, we generated tamoxifen-inducible smooth muscle cell-specific RyR2-deficient mice and tested the hypothesis that vascular smooth muscle cell RyR2s play a specific role in elementary Ca2+ signaling and adaptive vascular responses to vascular pressure and/or flow. Methods and Results Targeted deletion of the Ryr2 gene resulted in a complete loss of sarcoplasmic reticulum-mediated Ca2+-release events and associated Ca2+-activated, large-conductance K+ channel currents in peripheral arteries, leading to increased myogenic tone and systemic blood pressure. In the absence of RyR2, the pulmonary artery pressure response to sustained hypoxia was enhanced, but flow-dependent effects, including blood flow recovery in ischemic hind limbs, were unaffected. Conclusions Our results establish that RyR2-mediated Ca2+-release events in VSCM s specifically regulate myogenic tone (systemic circulation) and arterial adaptation in response to changes in pressure (hypoxic lung model), but not flow. They further suggest that vascular smooth muscle cell-expressed RyR2 deserves scrutiny as a therapeutic target for the treatment of vascular responses in hypertension and chronic vascular diseases.

PLCγ1-PKCε-IP3R1 signaling plays an important role in hypoxia-induced calcium response in pulmonary artery smooth muscle cells

Hypoxia-induced pulmonary vasoconstriction (HPV) is attributed to an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in pulmonary artery smooth muscle cells (PASMCs). We have reported that phospholipase C-γ1 (PLCγ1) plays a significant role in the hypoxia-induced increase in [Ca²⁺]_i in PASMCs and attendant HPV. In this study, we intended to determine molecular mechanisms for hypoxic Ca²⁺ and contractile responses in PASMCs. Our data reveal that hypoxic vasoconstriction occurs in pulmonary arteries, but not in mesenteric arteries. Hypoxia caused a large increase in [Ca²⁺]_i in PASMCs, which is diminished by the PLC inhibitor U73122 and not by its inactive analog U73433 . Hypoxia augments PLCγ1-dependent inositol 1,4,5-trisphosphate (IP₃) generation. Exogenous ROS, hydrogen peroxide (H₂O₂), increases PLCγ1 phosphorylation at tyrosine-783 and IP₃ production. IP₃ receptor-1 (IP₃R1) knock-down remarkably diminishes hypoxia- or H₂O₂-induced increase in [Ca²⁺]_i. Hypoxia or H₂O₂ increases the activity of IP₃Rs, which is significantly reduced in protein kinase C-ε (PKCε) knockout PASMCs. A higher PLCγ1 expression, activity, and basal [Ca²⁺]_i are found in PASMCs, but not in mesenteric artery smooth muscle cells from mice exposed to chronic hypoxia (CH) for 21 days. CH enhances H₂O₂- and ATP-induced increase in [Ca²⁺]_i in PASMCs and PLC-dependent, norepinephrine-evoked pulmonary vasoconstriction. In conclusion, acute hypoxia uniquely causes ROS-dependent PLCγ1 activation, IP₃ production, PKCε activation, IP₃R1 opening, Ca²⁺ release, and contraction in mouse PASMCs; CH enhances PASM PLCγ1 expression, activity, and function, playing an essential role in pulmonary hypertension in mice.

Recent advances in oxygen sensing and signal transduction in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a physiological reaction, which adapts lung perfusion to regional ventilation and optimizes gas exchange. Impaired HPV may cause systemic hypoxemia, while generalized HPV contributes to the development of pulmonary hypertension. The triggering mechanisms underlying HPV are still not fully elucidated. Several hypotheses are currently under debate, including a possible decrease as well as an increase in reactive oxygen species as a triggering event. Recent findings suggest an increase in the production of reactive oxygen species in pulmonary artery smooth muscle cells by complex III of the mitochondrial electron transport chain and occurrence of oxygen sensing at complex IV. Other essential components are voltage-dependent potassium and possibly L-type, transient receptor potential channel 6, and transient receptor potential vanilloid 4 channels. The release of arachidonic acid metabolites appears also to be involved in HPV regulation. Further investigation of the HPV mechanisms will facilitate the development of novel therapeutic strategies for the treatment of HPV-related disorders.

Nanojunctions of the Sarcoplasmic Reticulum Deliver Site- and Function-Specific Calcium Signaling in Vascular Smooth Muscles

Vasoactive agents may induce myocyte contraction, dilation, and the switch from a contractile to a migratory-proliferative phenotype(s), which requires changes in gene expression. These processes are directed, in part, by Ca²⁺ signals, but how different Ca²⁺ signals are generated to select each function is enigmatic. We have previously proposed that the strategic positioning of Ca²⁺ pumps and release channels at membrane-membrane junctions of the sarcoplasmic reticulum (SR) demarcates cytoplasmic nanodomains, within which site- and function-specific Ca²⁺ signals arise. This chapter will describe how nanojunctions of the SR may: (1) define cytoplasmic nanospaces about the plasma membrane, mitochondria, contractile myofilaments, lysosomes, and the nucleus; (2) provide for functional segregation by restricting passive diffusion and by coordinating active ion transfer within a given nanospace via resident Ca²⁺ pumps and release channels; (3) select for contraction, relaxation, and/or changes in gene expression; and (4) facilitate the switch in myocyte phenotype through junctional reorganization. This should serve to highlight the need for further exploration of cellular nanojunctions and the mechanisms by which they operate, that will undoubtedly open up new therapeutic horizons.

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

KEY POINTS

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

ABSTRACT

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

From contraction to gene expression: nanojunctions of the sarco/endoplasmic reticulum deliver site- and function-specific calcium signals

Calcium signals determine, for example, smooth muscle contraction and changes in gene expression. How calcium signals select for these processes is enigmatic. We build on the "panjunctional sarcoplasmic reticulum" hypothesis, describing our view that different calcium pumps and release channels, with different kinetics and affinities for calcium, are strategically positioned within nanojunctions of the SR and help demarcate their respective cytoplasmic nanodomains. SERCA2b and RyR1 are preferentially targeted to the sarcoplasmic reticulum (SR) proximal to the plasma membrane (PM), i.e., to the superficial buffer barrier formed by PM-SR nanojunctions, and support vasodilation. In marked contrast, SERCA2a may be entirely restricted to the deep, perinuclear SR and may supply calcium to this sub-compartment in support of vasoconstriction. RyR3 is also preferentially targeted to the perinuclear SR, where its clusters associate with lysosome-SR nanojunctions. The distribution of RyR2 is more widespread and extends from this region to the wider cell. Therefore, perinuclear RyR3s most likely support the initiation of global calcium waves at L-SR junctions, which subsequently propagate by calcium-induced calcium release via RyR2 in order to elicit contraction. Data also suggest that unique SERCA and RyR are preferentially targeted to invaginations of the nuclear membrane. Site- and function-specific calcium signals may thus arise to modulate stimulus-response coupling and transcriptional cascades.

Involvement of gap junctions between smooth muscle cells in sustained hypoxic pulmonary vasoconstriction development: a potential role for 15-HETE and 20-HETE

In response to hypoxia, the pulmonary artery normally constricts to maintain optimal ventilation-perfusion matching in the lung, but chronic hypoxia leads to the development of pulmonary hypertension. The mechanisms of sustained hypoxic pulmonary vasoconstriction (HPV) remain unclear. The aim of this study was to determine the role of gap junctions (GJs) between smooth muscle cells (SMCs) in the sustained HPV development and involvement of arachidonic acid (AA) metabolites in GJ-mediated signaling. Vascular tone was measured in bovine intrapulmonary arteries (BIPAs) using isometric force measurement technique. Expression of contractile proteins was determined by Western blot. AA metabolites in the bath fluid were analyzed by mass spectrometry. Prolonged hypoxia elicited endothelium-independent sustained HPV in BIPAs. Inhibition of GJs by 18β-glycyrrhetinic acid (18β-GA) and heptanol, nonspecific blockers, and Gap-27, a specific blocker, decreased HPV in deendothelized BIPAs. The sustained HPV was not dependent on Ca(2+) entry but decreased by removal of Ca(2+) and by Rho-kinase inhibition with Y-27632. Furthermore, inhibition of GJs decreased smooth muscle myosin heavy chain (SM-MHC) expression and myosin light chain phosphorylation in BIPAs. Interestingly, inhibition of 15- and 20-hydroxyeicosatetraenoic acid (HETE) synthesis decreased HPV in deendothelized BIPAs. 15-HETE- and 20-HETE-stimulated constriction of BIPAs was inhibited by 18β-GA and Gap-27. Application of 15-HETE and 20-HETE to BIPAs increased SM-MHC expression, which was also suppressed by 18β-GA and by inhibitors of lipoxygenase and cytochrome P450 monooxygenases. More interestingly, 15,20-dihydroxyeicosatetraenoic acid and 20-OH-prostaglandin E2, novel derivatives of 20-HETE, were detected in tissue bath fluid and synthesis of these derivatives was almost completely abolished by 18β-GA. Taken together, our novel findings show that GJs between SMCs are involved in the sustained HPV in BIPAs, and 15-HETE and 20-HETE, through GJs, appear to mediate SM-MHC expression and contribute to the sustained HPV development.

Oxygen sensing and signal transduction in hypoxic pulmonary vasoconstriction

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

Hypoxic pulmonary vasoconstriction, carotid body function and erythropoietin production in adult rats perinatally exposed to hyperoxia

KEY POINTS

Adult animals that have been perinatally exposed to oxygen-rich atmospheres (hyperoxia), recalling those used for oxygen therapy in infants, exhibit a loss of hypoxic pulmonary vasoconstriction, whereas vasoconstriction elicited by depolarizing agents is maintained. Loss of pulmonary hypoxic vasoconstriction is not linked to alterations in oxygen-sensitive K(+) currents in pulmonary artery smooth muscle cells. Loss of hypoxic vasoconstriction is associated with early postnatal oxidative damage and corrected by an antioxidant diet. Perinatal hyperoxia damages carotid body chemoreceptor cell function and the antioxidant diet does not reverse it. The hypoxia-elicited increase in erythropoietin plasma levels is not affected by perinatal hyperoxia. The potential clinical significance of the findings in clinical situations such as pneumonia, chronic obstructive pulmonary disease or general anaesthesia is considered.

ABSTRACT

Adult mammalians possess three cell systems that are activated by acute bodily hypoxia: pulmonary artery smooth muscle cells (PASMC), carotid body chemoreceptor cells (CBCC) and erythropoietin (EPO)-producing cells. In rats, chronic perinatal hyperoxia causes permanent carotid body (CB) atrophy and functional alterations of surviving CBCC. There are no studies on PASMC or EPO-producing cells. Our aim is to define possible long-lasting functional changes in PASMC or EPO-producing cells (measured as EPO plasma levels) and, further, to analyse CBCC functional alterations. We used 3- to 4-month-old rats born and reared in a normal atmosphere or exposed to perinatal hyperoxia (55-60% O2 for the last 5-6 days of pregnancy and 4 weeks after birth). Perinatal hyperoxia causes an almost complete loss of hypoxic pulmonary vasoconstriction (HPV), which was correlated with lung oxidative status in early postnatal life and prevented by antioxidant supplementation in the diet. O2 -sensitivity of K(+) currents in the PASMC of hyperoxic animals is normal, indicating that their inhibition is not sufficient to trigger HPV. Perinatal hyperoxia also abrogated responses elicited by hypoxia on catecholamine and cAMP metabolism in the CB. An increase in EPO plasma levels elicited by hypoxia was identical in hyperoxic and control animals, implying a normal functioning of EPO-producing cells. The loss of HPV observed in adult rats and caused by perinatal hyperoxia, comparable to oxygen therapy in premature infants, might represent a previously unrecognized complication of such a medical intervention capable of aggravating medical conditions such as regional pneumonias, atelectases or general anaesthesia in adult life.

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

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

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.

Gap junctions support the sustained phase of hypoxic pulmonary vasoconstriction by facilitating calcium sensitization

AIMS

To determine the role of gap junctions (GJs) in hypoxic pulmonary vasoconstriction (HPV).

METHODS AND RESULTS

Studies were performed in rat isolated intrapulmonary arteries (IPAs) mounted on a myograph and in anaesthetized rats. Hypoxia induced a biphasic HPV response in IPAs preconstricted with prostaglandin F2α (PGF2α, 3 µM) or 20 mM K⁺. The GJ inhibitors 18β-glycyrrhetinic acid (18β-GA, 30 µM), heptanol (3.5 mM), or 2-aminoethoxydiphenyl borate (2-APB) (75 µM) had little effect on the transient Phase 1 of HPV, but abolished the sustained Phase 2 which is associated with Ca²⁺ sensitization. The voltage-dependent Ca²⁺ channel blocker diltiazem (10 µM) had no effect on HPV, and did not alter the inhibitory action of 18β-GA. Sustained HPV is enhanced by high glucose (15 mM) via potentiation of Ca²⁺ sensitization, in the presence of high glucose 18β-GA still abolished sustained HPV. Simultaneous measurement of tension and intracellular Ca²⁺ using Fura PE-3 demonstrated that whilst 18β-GA abolished tension development during sustained HPV, it did not affect the elevation of intracellular Ca²⁺. Consistent with this, 18β-GA abolished hypoxia-induced phosphorylation of the Rho kinase target MYPT-1. In anaesthetized rats hypoxia caused a biphasic increase in systolic right ventricular pressure. Treatment with oral 18β-GA (25 mg/kg) abolished the sustained component of the hypoxic pressor response.

CONCLUSION

These results imply that GJs are critically involved in the signalling pathways leading to Rho kinase-dependent Ca²⁺ sensitization during sustained HPV, but not elevation of intracellular Ca²⁺, and may explain the dependence of the former on an intact endothelium.

Hypoxic pulmonary vasoconstriction in the absence of pretone: essential role for intracellular Ca2+ release

  Hypoxic pulmonary vasoconstriction (HPV) maintains blood oxygenation during acute hypoxia but contributes to pulmonary hypertension during chronic hypoxia. The mechanisms of HPV remain controversial, in part because HPV is usually studied in the presence of agonist-induced preconstriction ('pretone'). This potentiates HPV but may obscure and distort its underlying mechanisms. We therefore carried out an extensive assessment of proposed mechanisms contributing to HPV in isolated intrapulmonary arteries (IPAs) in the absence of pretone by using a conventional small vessel myograph. Hypoxia elicited a biphasic constriction consisting of a small transient (phase 1) superimposed upon a sustained (phase 2) component. Neither phase was affected by the L-type Ca2+ channel antagonists diltiazem (10 and 30 μm) or nifedipine (3 μm). Application of the store-operated Ca2+ entry (SOCE) blockers BTP2 (10 μm) or SKF96365 (50 μm) attenuated phase 2 but not phase 1, whereas a lengthy (30 min) incubation in Ca2+-free physiological saline solution similarly reduced phase 2 but abolished phase 1. No further effect of inhibition of HPV was observed if the sarco/endoplasmic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (30 μm) was also applied during the 30 min incubation in Ca2+-free physiological saline solution. Pretreatment with 10 μm ryanodine and 15 mm caffeine abolished both phases, whereas treatment with 100 μm ryanodine attenuated both phases. The two-pore channel blocker NED-19 (1 μm) and the nicotinic acid adenine dinucleotide phosphate (NAADP) antagonist BZ194 (200 μm) had no effect on either phase of HPV. The lysosomal Ca2+-depleting agent concanamycin (1 μm) enhanced HPV if applied during hypoxia, but had no effect on HPV during a subsequent hypoxic challenge. The cyclic ADP ribose antagonist 8-bromo-cyclic ADP ribose (30 μm) had no effect on either phase of HPV. Neither the Ca2+-sensing receptor (CaSR) blocker NPS2390 (0.1 and 10 μm) nor FK506 (10 μm), a drug which displaces FKBP12.6 from ryanodine receptor 2 (RyR2), had any effect on HPV. HPV was virtually abolished by the rho kinase blocker Y-27632 (1 μm) and attenuated by the protein kinase C inhibitor Gö6983 (3 μm). Hypoxia for 45 min caused a significant increase in the ratio of oxidised to reduced glutathione (GSSG/GSH). HPV was unaffected by the NADPH oxidase inhibitor VAS2870 (10 μm), whereas phase 2 was inhibited but phase 1 was unaffected by the antioxidants ebselen (100 μm) and TEMPOL (3 mm). We conclude that both phases of HPV in this model are mainly dependent on [Ca2+]i release from the sarcoplasmic reticulum. Neither phase of HPV requires voltage-gated Ca2+ entry, but SOCE contributes to phase 2. We can detect no requirement for cyclic ADP ribose, NAADP-dependent lysosomal Ca2+ release, activation of the CaSR, or displacement of FKBP12.6 from RyR2 for either phase of HPV. Sustained HPV is associated with an oxidising shift in the GSSG/GSH redox potential and is inhibited by the antioxidants ebselen and TEMPOL, consistent with the concept that it requires an oxidising shift in the cell redox state or the generation of reactive oxygen species.

Cyclic ADP-Ribose and NAADP in Vascular Regulation and Diseases

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP), two intracellular Ca²⁺ mobilizing second messengers, have been recognized as a fundamental signaling mechanism regulating a variety of cell or organ functions in different biological systems. Here we reviewed the literature regarding these ADP-ribosylcyclase products in vascular cells with a major focus on their production, physiological roles, and related underlying mechanisms mediating their actions. In particular, several hot topics in this area of research are comprehensively discussed, which may help understand some of the controversial evidence provided by different studies. For example, some new models are emerging for the agonist receptor coupling of CD38 or ADP-ribosylcyclase and for the formation of an acidic microenvironment to facilitate the production of NAADP in vascular cells. We also summarized the evidence regarding the NAADP-mediated two-phase Ca²⁺ release with a slow Ca²⁺-induced Ca²⁺ release (CICR) and corresponding physiological relevance. The possibility of a permanent structural space between lysosomes and sarcoplasmic reticulum (SR), as well as the critical role of lysosome trafficking in phase 2 Ca²⁺ release in response to some agonists are also explored. With respect to the molecular targets of NAADP within cells, several possible candidates including SR ryanodine receptors (RyRs), lysosomal transient receptor potential-mucolipin 1 (TRP-ML1) and two pore channels (TPCs) are presented with supporting and opposing evidence. Finally, the possible role of NAADP-mediated regulation of lysosome function in autophagy and atherogenesis is discussed, which may indicate a new direction for further studies on the pathological roles of cADPR and NAADP in the vascular system.

Pan-junctional sarcoplasmic reticulum in vascular smooth muscle: nanospace Ca2+ transport for site- and function-specific Ca2+ signalling

This review focuses on how smooth muscle sarcoplasmic reticulum (SR), the major releasable Ca(2+) store in these cells, performs its many functions by communicating with the plasma membrane (PM) and other organelles across cytoplasmic nanospaces, defined by membrane-membrane junctions less than 50 nm across. In spite of accumulating evidence in favour of the view that cytoplasmic nanospaces are a prerequisite for effective control of diverse cellular functions, our current understanding of how smooth muscle cells accomplish site- and function-specific Ca(2+) signalling remains in its infancy. We first present evidence in support of the view that effective Ca(2+) signalling depends on the restricted diffusion of Ca(2+) within cytoplasmic nanospaces. We then develop an evidence-based model of the smooth muscle SR - the 'pan-junctional SR' model - that incorporates a network of tubules and quilts that are capable of auto-regulating their Ca(2+) content and determining junctional [Ca(2+)]i through loading and unloading at membrane-membrane nanojunctions. Thereby, we provide a novel working hypothesis in order to inform future investigation into the control of a variety of cellular functions by local Ca(2+) signals at junctional nanospaces, from contraction and energy metabolism to nuclear transcription. Based on the current literature, we discuss the molecular mechanisms whereby the SR mediates these multiple functions through the interaction of ion channels and pumps embedded in apposing membranes within inter-organellar junctions. We finally highlight the fact that although most current hypotheses are qualitatively supported by experimental data, solid quantitative simulations are seriously lacking. Considering that at physiological concentrations the number of calcium ions in a typical junctional nanospace between the PM and SR is of the order of 1, ion concentration variability plays a major role as the currency of information transfer and stochastic quantitative modelling will be required to both test and develop working hypotheses.

TRPC1 and Orai1 interact with STIM1 and mediate capacitative Ca2+ entry caused by acute hypoxia in mouse pulmonary arterial smooth muscle cells

Previous studies in pulmonary artery smooth muscle cells (PASMCs) showed that acute hypoxia activates capacitative Ca2+ entry (CCE) but the molecular candidate(s) mediating CCE caused by acute hypoxia remain unclear. The present study aimed to determine if transient receptor potential canonical 1 (TRPC1) and Orai1 interact with stromal interacting molecule 1 (STIM1) and mediate CCE caused by acute hypoxia in mouse PASMCs. In primary cultured PASMCs loaded with fura-2, acute hypoxia caused a transient followed by a sustained rise in intracellular Ca2+ concentration ([Ca2+]i). The transient but not sustained rise in [Ca2+]i was partially inhibited by nifedipine. Acute hypoxia also increased the rate of Mn2+ quench of fura-2 fluorescence that was inhibited by SKF 96365, Ni2+, La3+, and Gd3+, exhibiting pharmacological properties characteristic of CCE. The nifedipine-insensitive rise in [Ca2+]i and the increase in Mn2+ quench rate were both inhibited in cells treated with TRPC1 antibody or TRPC1 small interfering (si)RNA, in STIM1 siRNA-transfected cells and in Orai1 siRNA-transfected cells. Moreover, overexpression of STIM1 resulted in a marked increase in [Ca2+]i and Mn2+ quench rate caused by acute hypoxia, and they were reduced in cells treated with TRPC1 antibody and in cells transfected with Orai1 siRNA. Furthermore, TRPC1 and Orai1 coimmunoprecipitated with STIM1 and the precipitation levels of TRPC1 and Orai1 were increased in cells exposed to acute hypoxia. Immunostaining showed colocalizations of TRPC1-STIM1 and Orai1-STIM1, and the colocalizations of these proteins were more apparent in acute hypoxia. These data provide direct evidence that TRPC1 and Orai1 channels mediate CCE through activation of STIM1 in acute hypoxic mouse PASMCs.

Adaptive and maladaptive cardiorespiratory responses to continuous and intermittent hypoxia mediated by hypoxia-inducible factors 1 and 2

Hypoxia is a fundamental stimulus that impacts cells, tissues, organs, and physiological systems. The discovery of hypoxia-inducible factor-1 (HIF-1) and subsequent identification of other members of the HIF family of transcriptional activators has provided insight into the molecular underpinnings of oxygen homeostasis. This review focuses on the mechanisms of HIF activation and their roles in physiological and pathophysiological responses to hypoxia, with an emphasis on the cardiorespiratory systems. HIFs are heterodimers comprised of an O(2)-regulated HIF-1α or HIF-2α subunit and a constitutively expressed HIF-1β subunit. Induction of HIF activity under conditions of reduced O(2) availability requires stabilization of HIF-1α and HIF-2α due to reduced prolyl hydroxylation, dimerization with HIF-1β, and interaction with coactivators due to decreased asparaginyl hydroxylation. Stimuli other than hypoxia, such as nitric oxide and reactive oxygen species, can also activate HIFs. HIF-1 and HIF-2 are essential for acute O(2) sensing by the carotid body, and their coordinated transcriptional activation is critical for physiological adaptations to chronic hypoxia including erythropoiesis, vascularization, metabolic reprogramming, and ventilatory acclimatization. In contrast, intermittent hypoxia, which occurs in association with sleep-disordered breathing, results in an imbalance between HIF-1α and HIF-2α that causes oxidative stress, leading to cardiorespiratory pathology.

Effect of Yifei Huoxue Granule on the proliferation of rat pulmonary artery smooth muscle cells upon exposure to chronic hypoxic conditions in vitro

OBJECTIVE

To investigate the inhibitory effect of Yifei Huoxue Granule (, YFHXG) on the hypoxia-induced proliferation of rat pulmonary artery smooth muscle cells (PASMCs) and its mechanism of decreasing pulmonary arterial pressure.

METHODS

Twenty male Sprague-Dawley (SD) rats were randomly divided into four groups: saline, and 0.66, 3.30 and 16.50 g/kg of YFHXG groups, the saline and different concentrations of YFHXG were given twice daily for 7 days, respectively. Serum-pharmacology method was used in the preparation of YFHXG serum. Tissue block anchorage was employed in the primary culture of rat PASMCs. The PASMCs were randomly divided into normoxia group, hypoxia group, and hypoxia+YFHXG group (0.66, 3.30 and 16.50 g/kg doses of YFHXG-treated serum groups, exposed to hypoxic condition). PASMCs in normoxia and hypoxia group were cultured with saline serum, hypoxia+YFHXG groups were cultured with different concentrations of YFHXG serum. Cell viability was assessed with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Cell cycle was analyzed using flow cytometry. In addition, hypoxia inducible factor-1-alpha (HIF-1α) protein expression was evaluated by immunocytochemistry analysis, the concentration of intracellular reactive oxygen species (ROS) and Ca(2+) were determined by laser scanning confocal microscopy (LSCM).

RESULTS

MTT assay and flow cytometry showed that hypoxia could directly activate the proliferation of PASMCs, while YFHXG dose-dependently inhibited hypoxia-induced proliferation of rat PASMCs. Immunocytochemistry showed that hypoxia enhanced HIF-1α protein expression, and LSCM showed that hypoxia significantly increased intracellular ROS and Ca(2+), while YFHXG decreased the expression of HIF- 1α and attenuated the hypoxia-induced increase in intracellular concentration of ROS and Ca(2+).

CONCLUSIONS

YFHXG could inhibit hypoxia-induced proliferation of rat PASMCs, which may decrease pulmonary arterial pressure and vascular remodeling. The anti-hypoxia effect of YFHXG may be explained by its regulation of HIF-1α expression and of the levels of intracellular ROS and Ca(2+).

Ca2+ responses of pulmonary arterial myocytes to acute hypoxia require release from ryanodine and inositol trisphosphate receptors in sarcoplasmic reticulum

In pulmonary arterial smooth muscle cells (PASMC), acute hypoxia increases intracellular Ca(2+) concentration ([Ca(2+)](i)) by inducing Ca(2+) release from the sarcoplasmic reticulum (SR) and Ca(2+) influx through store- and voltage-operated Ca(2+) channels in sarcolemma. To evaluate the mechanisms of hypoxic Ca(2+) release, we measured [Ca(2+)](i) with fluorescent microscopy in primary cultures of rat distal PASMC. In cells perfused with Ca(2+)-free Krebs Ringer bicarbonate solution (KRBS), brief exposures to caffeine (30 mM) and norepinephrine (300 μM), which activate SR ryanodine and inositol trisphosphate receptors (RyR, IP(3)R), respectively, or 4% O(2) caused rapid transient increases in [Ca(2+)](i), indicating intracellular Ca(2+) release. Preexposure of these cells to caffeine, norepinephrine, or the SR Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA; 10 μM) blocked subsequent Ca(2+) release to caffeine, norepinephrine, and hypoxia. The RyR antagonist ryanodine (10 μM) blocked Ca(2+) release to caffeine and hypoxia but not norepinephrine. The IP(3)R antagonist xestospongin C (XeC, 0.1 μM) blocked Ca(2+) release to norepinephrine and hypoxia but not caffeine. In PASMC perfused with normal KRBS, acute hypoxia caused a sustained increase in [Ca(2+)](i) that was abolished by ryanodine or XeC. These results suggest that in rat distal PASMC 1) the initial increase in [Ca(2+)](i) induced by hypoxia, as well as the subsequent Ca(2+) influx that sustained this increase, required release of Ca(2+) from both RyR and IP(3)R, and 2) the SR Ca(2+) stores accessed by RyR, IP(3)R, and hypoxia functioned as a common store, which was replenished by a CPA-inhibitable Ca(2+)-ATPase.

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.

Role of thromboxane A2-activated nonselective cation channels in hypoxic pulmonary vasoconstriction of rat

Hypoxia-induced pulmonary vasoconstriction (HPV) is critical for matching of ventilation/perfusion in lungs. Although hypoxic inhibition of K+ channels has been a leading hypothesis for depolarization of pulmonary arterial smooth muscle cells (PASMCs) under hypoxia, pharmacological inhibition of K+ channels does not induce significant contraction in rat pulmonary arteries. Because a partial contraction by thromboxane A2 (TXA2) is required for induction of HPV, we hypothesize that TXA2 receptor (TP) stimulation might activate depolarizing nonselective cation channels (NSCs). Consistently, we found that 5–10 nM U46619, a stable agonist for TP, was indispensible for contraction of rat pulmonary arteries by 4-aminopyridine, a blocker of voltage-gated K+ channel (Kv). Whole cell voltage clamp with rat PASMC revealed that U46619 induced a NSC current ( INSC,TXA2) with weakly outward rectifying current-voltage relation. INSC,TXA2 was blocked by ruthenium red (RR), an antagonist of the transient receptor potential vanilloid-related channel (TRPV) subfamily. 2-Aminoethoxydiphenyl borate, an agonist for TRPV1–3, consistently activated NSC channels in PASMCs. In contrast, agonists for TRPV1 (capsaicin), TRPV3 (camphor), or TRPV4 (α-PDD) rarely induced an increase in the membrane conductance of PASMCs. RT-PCR analysis showed the expression of transcripts for TRPV2 and -4 in rat PASMCs. Finally, it was confirmed that pretreatment with RR largely inhibited HPV in the presence of U46619. The pretreatment with agonists for TRPV1 (capsaicin) and TRPV4 (α-PDD) was ineffective as pretone agents for HPV. Taken together, it is suggested that the concerted effects of INSC,TXA2 activation and Kv inhibition under hypoxia induce membrane depolarization sufficient for HPV. TRPV2 is carefully suggested as the TXA2-activated NSC in rat PASMC.

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.

Hypoxic pulmonary vasoconstriction: mechanisms of oxygen-sensing

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Hypoxia induces intracellular Ca2+ release by causing reactive oxygen species-mediated dissociation of FK506-binding protein 12.6 from ryanodine receptor 2 in pulmonary artery myocytes

Here we attempted to test a novel hypothesis that hypoxia may induce Ca(2+) release through reactive oxygen species (ROS)-mediated dissociation of FK506-binding protein 12.6 (FKBP12.6) from ryanodine receptors (RyRs) on the sarcoplasmic reticulum (SR) in pulmonary artery smooth muscle cells (PASMCs). The results reveal that hypoxic exposure significantly decreased the amount of FKBP12.6 on the SR of PAs and increased FKBP12.6 in the cytosol. The colocalization of FKBP12.6 with RyRs was decreased in intact PASMCs. Pharmacological and genetic inhibition of intracellular ROS generation prevented hypoxia from decreasing FKBP12.6 on the SR and increasing FKBP12.6 in the cytosol. Exogenous ROS (H(2)O(2)) reduced FKBP12.6 on the SR and augmented FKBP12.6 in the cytosol. Oxidized FKBP12.6 was absent on the SR from PAs pretreated with and without hypoxia, but it was present with a higher amount in the cytosol from PAs pretreated with than without hypoxia. Hypoxia and H(2)O(2) diminished the association of FKBP12.6 from type 2 RyRs (RyR2). The activity of RyRs was increased in PAs pretreated with hypoxia or H(2)O(2). FKBP12.6 removal enhanced, whereas RyR2 gene deletion blocked the hypoxic increase in [Ca(2+)](i) in PASMCs. Collectively, we conclude that hypoxia may induce Ca(2+) release by causing ROS-mediated dissociation of FKBP12.6 from RyR2 in PASMCs.

Cell redox state and hypoxic pulmonary vasoconstriction: recent evidence and possible mechanisms

During alveolar hypoxia, hypoxic pulmonary vasoconstriction (HPV) maintains blood oxygenation near optimum via incompletely defined mechanisms. It is proposed that a hypoxia-induced rise in the intracellular concentration of reactive oxygen species (ROS) or an oxidising shift in the cytoplasmic redox state provides the signal which initiates the constriction of pulmonary arteries (PA), although this is controversial. Here, we review recent investigations demonstrating that hypoxia causes a rise in [ROS] in PA smooth muscle, and that ROS and antioxidants have effects on PA which would be predicted if cell oxidation causes contraction. We argue that intracellular Ca2+ release and Ca2+-sensitisation are the key effector mechanisms causing HPV, and discuss evidence that both processes are promoted by ROS or oxidative protein modifications. We conclude that while it is plausible that an increase in cytoplasmic [ROS] activates HPV effector mechanisms, proving this link will require the determination of whether hypoxia causes oxidative modifications of proteins involved in Ca2+ homeostasis and sensitisation.

Redox signaling and reactive oxygen species in hypoxic pulmonary vasoconstriction

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

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.

Role of ROS signaling in differential hypoxic Ca2+ and contractile responses in pulmonary and systemic vascular smooth muscle cells

Hypoxia causes a large increase in [Ca2+]i and attendant contraction in pulmonary artery smooth muscle cells (PASMCs), but not in systemic artery SMCs. The different responses meet the respective functional needs in these two distinct vascular myocytes; however, the underlying molecular mechanisms are not well known. We and other investigators have provided extensive evidence to reveal that voltage-dependent K+ (KV) channels, canonical transient receptor potential (TRPC) channels, ryanodine receptor Ca2+ release channels (RyRs), cyclic adenosine diphosphate-ribose, FK506 binding protein 12.6, protein kinase C, NADPH oxidase and reactive oxygen species (ROS) are the essential effectors and signaling intermediates in the hypoxic increase in [Ca2+]i in PASMCs and HPV, but they may not primarily underlie the diverse cellular responses in pulmonary and systemic vascular myocytes. Hypoxia significantly increases mitochondrial ROS generation in PASMCs, which can induce intracellular Ca2+ release by opening RyRs, and may also cause extracellular Ca2+ influx by inhibiting KV channels and activating TRPC channels, leading to a large increase in [Ca2+]i in PASMCs and HPV. In contrast, hypoxia has no or a minor effect on mitochondrial ROS generation in systemic SMCs, thereby causing no change or a negligible increase in [Ca2+]i and contraction. Further preliminary work indicates that Rieske iron-sulfur protein in the mitochondrial complex III may perhaps serve as a key initial molecular determinant for the hypoxic increase in [Ca2+]i in PASMCs and HPV, suggesting its potential important role in different cellular changes to respond to hypoxic stimulation in pulmonary and systemic artery myocytes. All these findings have greatly improved our understanding of the molecular processes for the differential hypoxic Ca2+ and contractile responses in vascular SMCs from distinct pulmonary and systemic circulation systems.

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.

ROS-dependent signaling mechanisms for hypoxic Ca(2+) responses in pulmonary artery myocytes

Hypoxic exposure causes pulmonary vasoconstriction, which serves as a critical physiologic process that ensures regional alveolar ventilation and pulmonary perfusion in the lungs, but may become an essential pathologic factor leading to pulmonary hypertension. Although the molecular mechanisms underlying hypoxic pulmonary vasoconstriction and associated pulmonary hypertension are uncertain, increasing evidence indicates that hypoxia can result in a significant increase in intracellular reactive oxygen species concentration ([ROS](i)) through the mitochondrial electron-transport chain in pulmonary artery smooth muscle cells (PASMCs). The increased mitochondrial ROS subsequently activate protein kinase C-epsilon (PKCepsilon) and NADPH oxidase (Nox), providing positive mechanisms that further increase [ROS](i). ROS may directly cause extracellular Ca(2+) influx by inhibiting voltage-dependent K(+) (K(V)) channels and opening of store-operated Ca(2+) (SOC) channels, as well as intracellular Ca(2+) release by activating ryanodine receptors (RyRs), leading to an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and associated contraction. In concert with ROS, PKCepsilon may also affect K(V) channels, SOC channels, and RyRs, contributing to hypoxic Ca(2+) and contractile responses in PASMCs.

Identification of functionally segregated sarcoplasmic reticulum calcium stores in pulmonary arterial smooth muscle

In pulmonary arterial smooth muscle, Ca(2+) release from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) may induce constriction and dilation in a manner that is not mutually exclusive. We show here that the targeting of different sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA) and RyR subtypes to discrete SR regions explains this paradox. Western blots identified protein bands for SERCA2a and SERCA2b, whereas immunofluorescence labeling of isolated pulmonary arterial smooth muscle cells revealed striking differences in the spatial distribution of SERCA2a and SERCA2b and RyR1, RyR2, and RyR3, respectively. Almost all SERCA2a and RyR3 labeling was restricted to a region within 1.5 microm of the nucleus. In marked contrast, SERCA2b labeling was primarily found within 1.5 microm of the plasma membrane, where labeling for RyR1 was maximal. The majority of labeling for RyR2 lay in between these two regions of the cell. Application of the vasoconstrictor endothelin-1 induced global Ca(2+) waves in pulmonary arterial smooth muscle cells, which were markedly attenuated upon depletion of SR Ca(2+) stores by preincubation of cells with the SERCA inhibitor thapsigargin but remained unaffected after preincubation of cells with a second SERCA antagonist, cyclopiazonic acid. We conclude that functionally segregated SR Ca(2+) stores exist within pulmonary arterial smooth muscle cells. One sits proximal to the plasma membrane, receives Ca(2+) via SERCA2b, and likely releases Ca(2+) via RyR1 to mediate vasodilation. The other is located centrally, receives Ca(2+) via SERCA2a, and likely releases Ca(2+) via RyR3 and RyR2 to initiate vasoconstriction.

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

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

Hypoxic pulmonary vasodilation: a paradigm shift with a hydrogen sulfide mechanism

Hypoxic pulmonary vasoconstriction (HVC), an intrinsic and assumed ubiquitous response of mammalian pulmonary blood vessels, matches regional ventilation to perfusion via an unknown O(2)-sensing mechanism. Global pulmonary hypoxia experienced by individuals suffering from chronic obstructive pulmonary disease or numerous hypoventilation syndromes, including sleep apnea, often produces maladaptive pulmonary hypertension, but pulmonary hypertension is not observed in diving mammals, where profound hypoxia is routine. Here we examined the response of cow and sea lion pulmonary arteries (PA) to hypoxia and observed the expected HVC in the former and a unique hypoxic vasodilation in resistance vessels in the latter. We then used this disparate response to examine the O(2)-sensing mechanism. In both animals, exogenous H(2)S mimicked the vasoactive effects of hypoxia in isolated PA. H(2)S-synthesizing enzymes, cystathionine beta-synthase, cystathionine gamma-lyase, and 3-mercaptopyruvate sulfur transferase, were identified in lung tissue from both animals by one-dimensional Western blot analysis and immunohistochemistry. The relationship between H(2)S production/consumption and O(2) was examined in real time by use of amperometric H(2)S and O(2) sensors. H(2)S was produced by sea lion and cow lung homogenate in the absence of O(2), but it was rapidly consumed when O(2) was present. Furthermore, consumption of exogenous H(2)S by cow lung homogenate, PA smooth muscle cells, and heart mitochondria was O(2) dependent and exhibited maximal sensitivity at physiologically relevant Po(2) levels. These studies show that HVC is not an intrinsic property of PA and provide further evidence for O(2)-dependent H(2)S metabolism in O(2) sensing.

Hypoxia triggers subcellular compartmental redox signaling in vascular smooth muscle cells

RATIONALE

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

OBJECTIVE

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

METHODS AND RESULTS

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

CONCLUSIONS

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

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

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

Mechanisms of oxygen sensing: a key to therapy of pulmonary hypertension and patent ductus arteriosus

Specialized tissues that sense acute changes in the local oxygen tension include type 1 cells of the carotid body, neuroepithelial bodies in the lungs, and smooth muscle cells of the resistance pulmonary arteries and the ductus arteriosus (DA). Hypoxia inhibits outward potassium current in carotid body type 1 cells, leading to depolarization and calcium entry through L-type calcium channels. Increased intracellular calcium concentration ([Ca+ +]i) leads to exocytosis of neurotransmitters, thus stimulating the carotid sinus nerve and respiration. The same K+ channel inhibition occurs with hypoxia in pulmonary artery smooth muscle cells (PASMCs), causing contraction and providing part of the mechanism of hypoxic pulmonary vasoconstriction (HPV). In the SMCs of the DA, the mechanism works in reverse. It is the shift from hypoxia to normoxia that inhibits K+ channels and causes normoxic ductal contraction. In both PA and DA, the contraction is augmented by release of Ca+ + from the sarcoplasmic reticulum, entry of Ca+ + through store-operated channels (SOC) and by Ca+ + sensitization. The same three 'executive' mechanisms are partly responsible for idiopathic pulmonary arterial hypertension (IPAH). While vasoconstrictor mediators constrict both PA and DA and vasodilators dilate both vessels, only redox changes mimic oxygen by having directly opposite effects on the K+ channels, membrane potential, [Ca(++)]i and tone in the PA and DA. There are several different hypotheses as to how redox might alter tone, which remain to be resolved. However, understanding the mechanism will facilitate drug development for pulmonary hypertension and patent DA.

AMP-activated protein kinase and hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction is a vital homeostatic mechanism that aids ventilation-perfusion matching in the lung, for which the underlying mechanism(s) remains controversial. However, our most recent investigations strongly suggest that hypoxic pulmonary vasoconstriction is precipitated, at least in part, by the inhibition of mitochondrial oxidative phosphorylation by hypoxia, an increase in the AMP/ATP ratio and consequent activation of AMP-activated protein kinase (AMPK). Unfortunately, these studies lacked the definitive proof that can only be provided by selectively blocking AMPK-dependent signalling cascades. The aim of the present study was, therefore, to determine the effects of the AMPK inhibitor compound C upon: (1) phosphorylation in response to hypoxia of a classical AMPK substrate, acetyl CoA carboxylase, in rat pulmonary arterial smooth muscle and (2) hypoxic pulmonary vasoconstriction in rat isolated intrapulmonary arteries. Acetyl CoA carboxylase phosphorylation was increased approximately 3 fold in the presence of hypoxia (pO(2) = 16-21 mm Hg, 1 h) and 5-aminoimidazole-4-carboxamide riboside (AICAR; 1 mM; 4 h) and in a manner that was significantly attenuated by the AMPK antagonist compound C (40 microM). Most importantly, pre-incubation of intrapulmonary arteries with compound C (40 microM) inhibited phase II, but not phase I, of hypoxic pulmonary vasoconstriction. Likewise, compound C (40 microM) inhibited constriction by AICAR (1 mM). The results of the present study are consistent with the activation of AMPK being a key event in the initiation of the contractile response of pulmonary arteries to acute hypoxia.

Genetic evidence for functional role of ryanodine receptor 1 in pulmonary artery smooth muscle cells

Ryanodine receptor 1 (RyR1) is well-known to be expressed in systemic and pulmonary vascular smooth muscle cells (SMCs); however, its functional roles remain largely unknown. In the present study, we attempted to determine the potential importance of RyR1 in membrane depolarization-, neurotransmitter-, and hypoxia-induced Ca2+ release and contraction in pulmonary artery SMCs (PASMCs) using RyR1 homozygous and heterozygous gene deletion (RyR1-/- and RyR1+/-) mice. Our results indicate that spontaneous local Ca2+ release and caffeine-induced global Ca2+ release are significantly reduced in embryonic RyR1-/- and adult RyR+/- cells. An increase in [Ca2+]i following membrane depolarization with high K+ is markedly attenuated in RyR1-/- and RyR1+/- PASMCs in normal Ca2+ or Ca2+-free extracellular solution. Similarly, muscle contraction evoked by membrane depolarization is reduced in RyR1+/- pulmonary arteries in the presence or absence of extracellular Ca2+. Neurotransmitter receptor agonists and inositol 1,4,5-triphosphate elicit a much smaller increase in [Ca2+]i in both RyR1-/- and RyR1+/- cells. We have also found that neurotransmitter-evoked muscle contraction is significantly inhibited in RyR1+/- pulmonary arteries. Hypoxia-induced increase in [Ca2+]i and contraction are largely blocked in RyR1-/- and/or RyR1+/- PASMCs. Collectively, our findings provide genetic evidence for the functional importance of RyR1 in spontaneous local Ca2+ release, and membrane depolarization-, neurotransmitter-, as well as hypoxia-induced global Ca2+ release and attendant contraction in PASMCs.

Relationship of intracellular free Ca2+ concentration and calcium-activated chloride channels of pulmonary artery smooth muscle cells in rats under hypoxic conditions

To investigate the relationship between intracellular free Ca2+ concentration ([Ca2+]i) and calcium-activated chloride (Clca) channels of pulmonary artery smooth muscle cells (PASMCs) in rats under acute and chronic hypoxic conditions, acute hypoxia-induced contraction was observed in rat pulmonary artery by using routine blood vascular perfusion in vitro. The fluorescence Ca2+ indicator Fura-2/AM was used to observe [Ca2+]i of rat PASMCs under normal and chronic hypoxic condition. The effect of Clca channels on PASMCs proliferation was assessed by MTT assay. The Clca channel blockers niflumic acid (NFA) and indanyloxyacetic acid (IAA-94) exerted inhibitory effects on acute hypoxia-evoked contractions in the pulmonary artery. Under chronic hypoxic condition, [Ca2+]i was increased. Under normoxic condition, [Ca2+]i was (123.63 +/- 18.98) nmol/L, and in hypoxic condition, [Ca2+]i was (281.75 +/- 16.48) nmol/L (P<0.01). Under normoxic condition, [Ca2+]i showed no significant change and no effect on Clca channels was observed (P> 0.05). Chronic hypoxia increased [Ca2+]i which opened Clca channels. The NFA and IAA-94 blocked the channels and decreased [Ca2+]i from (281.75 +/- 16.48) nmol/L to (117.66 +/- 15.36) nmol/L (P<0.01). MTT assay showed that under chronic hypoxic condition NFA and IAA-94 decreased the value of absorbency (A value) from 0.459 +/- 0.058 to 0.224 +/- 0.025 (P<0.01). Hypoxia increased [Ca2+]i which opened Clca channels and had a positive-feedback in [Ca2+]i. This may play an important role in hypoxic pulmonary hypertension. Under chronic hypoxic condition, Clca channel may play a part in the regulation of proliferation of PASMCs.

Heterogeneous gene expression and functional activity of ryanodine receptors in resistance and conduit pulmonary as well as mesenteric artery smooth muscle cells

BACKGROUND

Hypoxia causes heterogeneous contractile responses in resistance and conduit pulmonary as well as systemic (mesenteric) artery smooth muscle cells (RPASMCs, CPASMCs and MASMCs), but the underlying mechanisms are largely unknown. In this study, we aimed to investigate whether the gene expression and functional activity of ryanodine receptors (RyRs) would be different in these 3 cell types.

METHODS

RyR mRNA expression, Ca(2+) sparks and [Ca(2+)](i) were measured by real-time quantitative RT-PCR, laser scanning confocal microscopy and wide-field fluorescence microscopy, respectively.

RESULTS

All 3 RyR subtype (RyR1, RyR2 and RyR3) mRNAs are expressed in RPASMCs, CPASMCs and MASMCs, but their expression levels are different. Spontaneous Ca(2+) sparks (functional events of RyRs) show distinct frequency, amplitude, duration, size and kinetics in these 3 cell types. Similarly, activation of RyRs by caffeine, 4-chloro-m-cresol or high K(+) induces differential Ca(2+) release. Moreover, hypoxia-induced increase in [Ca(2+)](i) is largest in MASMCs relative to CPSAMCs and smallest in RPASMCs.

CONCLUSION

This study provides comprehensive evidence that RyRs are heterogeneous in gene expression and functional activity in RPASMCs, CPASMCs and MASMCs, which may contribute to the diversity of excitation-contraction coupling and hypoxic Ca(2+) responses in different vascular smooth muscle cells.

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

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