Diminished NO release in chronic hypoxic human endothelial cells

Effects of different nitric oxide synthases on pulmonary and systemic hemodynamics in hypoxic stress rat model

BACKGROUND

Under hypoxia, exaggerated compensatory responses may lead to acute mountain sickness. The excessive vasodilatory effect of nitric oxide (NO) can lower the hypoxic pulmonary vasoconstriction (HPV) and peripheral blood pressure. While NO is catalyzed by various nitric oxide synthase (NOS) isoforms, the regulatory roles of these types in the hemodynamics of pulmonary and systemic circulation in living hypoxic animals remain unclear. Therefore, this study aims to investigate the regulatory effects of different NOS isoforms on pulmonary and systemic circulation in hypoxic rats by employing selective NOS inhibitors and continuously monitoring hemodynamic parameters of both pulmonary and systemic circulation.

METHODS

Forty healthy male Sprague-Dawley (SD) rats were randomly divided into four groups: Control group (NG-nitro-D-arginine methyl ester, D-NAME), L-NAME group (non-selective NOS inhibitor, NG-nitro-L-arginine methyl ester), AG group (inducible NOS inhibitor group, aminoguanidine), and 7-NI group (neurological NOS inhibitor, 7-nitroindazole). Hemodynamic parameters of rats were monitored for 10 min after inhibitor administration and 5 min after induction of hypoxia [15% O2, 2200 m a. sl., 582 mmHg (76.5 kPa), Xining, China] using the real-time dynamic monitoring model for pulmonary and systemic circulation hemodynamics in vivo. Serum NO concentrations and blood gas analysis were measured.

RESULTS

Under normoxia, mean arterial pressure and total peripheral vascular resistance were increased, and ascending aortic blood flow and serum NO concentration were decreased in the L-NAME and AG groups. During hypoxia, pulmonary arterial pressure and pulmonary vascular resistance were significantly increased in the L-NAME and AG groups.

CONCLUSIONS

This compensatory mechanism activated by inducible NOS and endothelial NOS effectively counteracts the pulmonary hemodynamic changes induced by hypoxic stress. It plays a crucial role in alleviating hypoxia-induced pulmonary arterial hypertension.

Endothelial cells signaling and patterning under hypoxia: a mechanistic integrative computational model including the Notch-Dll4 pathway

Introduction: Several signaling pathways are activated during hypoxia to promote angiogenesis, leading to endothelial cell patterning, interaction, and downstream signaling. Understanding the mechanistic signaling differences between endothelial cells under normoxia and hypoxia and their response to different stimuli can guide therapies to modulate angiogenesis. We present a novel mechanistic model of interacting endothelial cells, including the main pathways involved in angiogenesis. Methods: We calibrate and fit the model parameters based on well-established modeling techniques that include structural and practical parameter identifiability, uncertainty quantification, and global sensitivity. Results: Our results indicate that the main pathways involved in patterning tip and stalk endothelial cells under hypoxia differ, and the time under hypoxia interferes with how different stimuli affect patterning. Additionally, our simulations indicate that Notch signaling might regulate vascular permeability and establish different Nitric Oxide release patterns for tip/stalk cells. Following simulations with various stimuli, our model suggests that factors such as time under hypoxia and oxygen availability must be considered for EC pattern control. Discussion: This project provides insights into the signaling and patterning of endothelial cells under various oxygen levels and stimulation by VEGFA and is our first integrative approach toward achieving EC control as a method for improving angiogenesis. Overall, our model provides a computational framework that can be built on to test angiogenesis-related therapies by modulation of different pathways, such as the Notch pathway.

Hypoxia-induced signaling in the cardiovascular system: pathogenesis and therapeutic targets

Hypoxia, characterized by reduced oxygen concentration, is a significant stressor that affects the survival of aerobic species and plays a prominent role in cardiovascular diseases. From the research history and milestone events related to hypoxia in cardiovascular development and diseases, The "hypoxia-inducible factors (HIFs) switch" can be observed from both temporal and spatial perspectives, encompassing the occurrence and progression of hypoxia (gradual decline in oxygen concentration), the acute and chronic manifestations of hypoxia, and the geographical characteristics of hypoxia (natural selection at high altitudes). Furthermore, hypoxia signaling pathways are associated with natural rhythms, such as diurnal and hibernation processes. In addition to innate factors and natural selection, it has been found that epigenetics, as a postnatal factor, profoundly influences the hypoxic response and progression within the cardiovascular system. Within this intricate process, interactions between different tissues and organs within the cardiovascular system and other systems in the context of hypoxia signaling pathways have been established. Thus, it is the time to summarize and to construct a multi-level regulatory framework of hypoxia signaling and mechanisms in cardiovascular diseases for developing more therapeutic targets and make reasonable advancements in clinical research, including FDA-approved drugs and ongoing clinical trials, to guide future clinical practice in the field of hypoxia signaling in cardiovascular diseases.

Molecular Mechanisms of High-Altitude Acclimatization

High-altitude illnesses (HAIs) result from acute exposure to high altitude/hypoxia. Numerous molecular mechanisms affect appropriate acclimatization to hypobaric and/or normobaric hypoxia and curtail the development of HAIs. The understanding of these mechanisms is essential to optimize hypoxic acclimatization for efficient prophylaxis and treatment of HAIs. This review aims to link outcomes of molecular mechanisms to either adverse effects of acute high-altitude/hypoxia exposure or the developing tolerance with acclimatization. After summarizing systemic physiological responses to acute high-altitude exposure, the associated acclimatization, and the epidemiology and pathophysiology of various HAIs, the article focuses on molecular adjustments and maladjustments during acute exposure and acclimatization to high altitude/hypoxia. Pivotal modifying mechanisms include molecular responses orchestrated by transcription factors, most notably hypoxia inducible factors, and reciprocal effects on mitochondrial functions and REDOX homeostasis. In addition, discussed are genetic factors and the resultant proteomic profiles determining these hypoxia-modifying mechanisms culminating in successful high-altitude acclimatization. Lastly, the article discusses practical considerations related to the molecular aspects of acclimatization and altitude training strategies.

Mitochondrial dysfunction in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterized by the increased pulmonary vascular resistance due to pulmonary vasoconstriction and vascular remodeling. PAH has high disability, high mortality and poor prognosis, which is becoming a more common global health issue. There is currently no drug that can permanently cure PAH patients. The pathogenesis of PAH is still not fully elucidated. However, the role of metabolic theory in the pathogenesis of PAH is becoming clearer, especially mitochondrial metabolism. With the deepening of mitochondrial researches in recent years, more and more studies have shown that the occurrence and development of PAH are closely related to mitochondrial dysfunction, including the tricarboxylic acid cycle, redox homeostasis, enhanced glycolysis, and increased reactive oxygen species production, calcium dysregulation, mitophagy, etc. This review will further elucidate the relationship between mitochondrial metabolism and pulmonary vasoconstriction and pulmonary vascular remodeling. It might be possible to explore more comprehensive and specific treatment strategies for PAH by understanding these mitochondrial metabolic mechanisms.

Adaptive Responses to Hypoxia and/or Hyperoxia in Humans

Significance: Oxygen is indispensable for aerobic life, but its utilization exposes cells and tissues to oxidative stress; thus, tight regulation of cellular, tissue, and systemic oxygen concentrations is crucial. Here, we review the current understanding of how the human organism (mal-)adapts to low (hypoxia) and high (hyperoxia) oxygen levels and how these adaptations may be harnessed as therapeutic or performance enhancing strategies at the systemic level. Recent Advances: Hyperbaric oxygen therapy is already a cornerstone of modern medicine, and the application of mild hypoxia, that is, hypoxia conditioning (HC), to strengthen the resilience of organs or the whole body to severe hypoxic insults is an important preparation for high-altitude sojourns or to protect the cardiovascular system from hypoxic/ischemic damage. Many other applications of adaptations to hypo- and/or hyperoxia are only just emerging. HC-sometimes in combination with hyperoxic interventions-is gaining traction for the treatment of chronic diseases, including numerous neurological disorders, and for performance enhancement. Critical Issues: The dose- and intensity-dependent effects of varying oxygen concentrations render hypoxia- and/or hyperoxia-based interventions potentially highly beneficial, yet hazardous, although the risks versus benefits are as yet ill-defined. Future Directions: The field of low and high oxygen conditioning is expanding rapidly, and novel applications are increasingly recognized, for example, the modulation of aging processes, mood disorders, or metabolic diseases. To advance hypoxia/hyperoxia conditioning to clinical applications, more research on the effects of the intensity, duration, and frequency of altered oxygen concentrations, as well as on individual vulnerabilities to such interventions, is paramount. Antioxid. Redox Signal. 37, 887-912.

Endothelial Nitric Oxide Production and Antioxidant Response in Breath-Hold Diving: Genetic Predisposition or Environment Related?

Introduction

Nitric oxide (NO) is an essential signaling molecule modulating the endothelial adaptation during breath-hold diving (BH-diving). This study aimed to investigate changes in NO derivatives (NOx) and total antioxidant capacity (TAC), searching for correlations with different environmental and hyperbaric exposure.

Materials and methods

Blood samples were obtained from 50 breath-hold divers (BH-divers) before, and 30 and 60 min after the end of training sessions performed both in a swimming pool or the sea. Samples were tested for NOx and TAC differences in different groups related to their hyperbaric exposure, experience, and additional genetic polymorphism.

Results

We found statistically significant differences in NOx plasma concentration during the follow-up (decrease at T30 and increase at T60) compared with the pre-dive values. At T30, we found a significantly lower decrease of NOx in subjects with a higher diving experience, but no difference was detected between the swimming pool and Sea. No significant difference was found in TAC levels, as well as between NOx and TAC levels and the genetic variants.

Conclusion

These data showed how NO consumption in BH-diving is significantly lower in the expert group, indicating a possible training-related adaptation process. Data confirm a significant NO use during BH-diving, compatible with the well-known BH-diving related circulatory adaptation suggesting that the reduction in NOx 30 min after diving can be ascribed to the lower NO availability in the first few minutes after the dives. Expert BH-divers suffered higher oxidative stress. A preliminary genetic investigation seems to indicate a less significant influence of genetic predisposition.

Endothelial Dysfunction Driven by Hypoxia-The Influence of Oxygen Deficiency on NO Bioavailability

Cardiovascular diseases (CVDs) are the leading cause of death worldwide. The initial stage of CVDs is characterized by endothelial dysfunction, defined as the limited bioavailability of nitric oxide (NO). Thus, any factors that interfere with the synthesis or metabolism of NO in endothelial cells are involved in CVD pathogenesis. It is well established that hypoxia is both the triggering factor as well as the accompanying factor in cardiovascular disease, and diminished tissue oxygen levels have been reported to influence endothelial NO bioavailability. In endothelial cells, NO is produced by endothelial nitric oxide synthase (eNOS) from L-Arg, with tetrahydrobiopterin (BH₄) as an essential cofactor. Here, we discuss the mechanisms by which hypoxia affects NO bioavailability, including regulation of eNOS expression and activity. What is particularly important is the fact that hypoxia contributes to the depletion of cofactor BH₄ and deficiency of substrate L-Arg, and thus elicits eNOS uncoupling-a state in which the enzyme produces superoxide instead of NO. eNOS uncoupling and the resulting oxidative stress is the major driver of endothelial dysfunction and atherogenesis. Moreover, hypoxia induces impairment in mitochondrial respiration and endothelial cell activation; thus, oxidative stress and inflammation, along with the hypoxic response, contribute to the development of endothelial dysfunction.

The Nitric Oxide Pathway in Pulmonary Arterial Hypertension: Pathomechanism, Biomarkers and Drug Targets

The altered Nitric Oxide (NO) pathway in the pulmonary endothelium leads to increased vascular smooth muscle tone and vascular remodelling, and thus contributes to the development and progression of pulmonary arterial hypertension (PAH). The pulmonary NO signalling is abrogated by the decreased expression and dysfunction of the endothelial NO synthase (eNOS) and the accumulation of factors blocking eNOS functionality. The NO deficiency of the pulmonary vasculature can be assessed by detecting nitric oxide in the exhaled breath or measuring the degradation products of NO (nitrite, nitrate, S-nitrosothiol) in blood or urine. These non-invasive biomarkers might show the potential to correlate with changes in pulmonary haemodynamics and predict response to therapies. Current pharmacological therapies aim to stimulate pulmonary NO signalling by suppressing the degradation of NO (phosphodiesterase- 5 inhibitors) or increasing the formation of the endothelial cyclic guanosine monophosphate, which mediates the downstream effects of the pathway (soluble guanylate cyclase sensitizers). Recent data support that nitrite compounds and dietary supplements rich in nitrate might increase pulmonary NO availability and lessen vascular resistance. This review summarizes current knowledge on the involvement of the NO pathway in the pathomechanism of PAH, explores novel and easy-to-detect biomarkers of the pulmonary NO.

Nitric Oxide and Oxidative Stress Changes at Depth in Breath-Hold Diving

Background

Several mechanisms allow humans to resist the extreme conditions encountered during breath-hold diving. Available nitric oxide (NO) is one of the major contributors to such complex adaptations at depth and oxidative stress is one of the major collateral effects of diving. Due to technical difficulties, these biomarkers have not so far been studied in vivo while at depth. The aim of this study is to investigate nitrate and nitrite (NOx) concentration, total antioxidant capacity (TAC) and lipid peroxidation (TBARS) before, during, and after repetitive breath-hold dives in healthy volunteers.

Materials and Methods

Blood plasma, obtained from 14 expert breath-hold divers, was tested for differences in NOx, TAC, and TBARS between pre-dive, bottom, surface, 30 and 60 min post-dive samples.

Results

We observed a statistically significant increase of NOx plasma concentration in the “bottom blood draw” as compared to the pre-dive condition while we did not find any difference in the following samples We found a statistically significant decrease in TAC at the bottom but the value returned to normality immediately after reaching the surface. We did not find any statistically significant difference in TBARS.

Discussion

The increased plasma NOx values found at the bottom were not observed at surface and post dive sampling (T0, T30, T60), showing a very rapid return to the pre-dive values. Also TAC values returned to pre- diving levels immediately after the end of hyperbaric exposure, probably as a consequence of the activation of endogenous antioxidant defenses. TBARS did not show any difference during the protocol.

Influence of Active Exposure to Tobacco Smoke on Nitric Oxide Status of Pregnant Women

Smoking tobacco can impair proper vascular endothelial functioning. This is exhibited through reduced nitric oxide synthesis as well as activity due to accompanying oxidative stress. We examined the relationship between nitric oxide and markers of oxidative stress/antioxidant defense in serum of smoking and non-smoking pregnant women. Subjects included 99 healthy pregnant women, who were tested for nitric oxide (NO), endothelial (eNOS) and inducible (iNOS) nitric oxide synthase, total oxidant capacity (TOC), and total antioxidant capacity (TAC). NO, eNOS, and TAC serum concentrations were significantly lower (p < 0.005), but iNOS (p < 0.05) and TOC (p < 0.001) were higher in smokers than in non-smokers. Multivariate regression analysis showed associations between NO concentration and eNOS, TAC, and smoking status in the whole group of patients. In the model estimated separately for smokers, the highest impact of eNOS (β = 0.375; p = 0.021) and cotinine (β = −0.323; p = 0.037) was indicated for NO concentration. In the model of non-smokers, eNOS (β = 0.291, p = 0.030) and TAC (β = 0.350; p = 0.015) were important for NO level. Smoking during pregnancy could exacerbate oxidative stress, impair the action of nitric oxide synthases, and adversely affect the balance of oxygen and nitrogen metabolism. Relationships between NO concentrations and TAC in the studied women’s blood can confirm the antioxidant nature of nitric oxide.

eNOS expression and NO release during hypoxia is inhibited by miR-200b in human endothelial cells

The nitric oxide (NO) secreted by vascular endothelium is required for the maintenance of cardiovascular homeostasis. Diminished release of NO generated by endothelial NO synthase contributes to endothelial dysfunction. Hypoxia and ischemia reduce endothelial eNOS expression via posttranscriptional mechanisms that result in NOS3 transcript destabilization. Here, we examine whether microRNAs contribute to this mechanism. We followed the kinetics of hypoxia-induced changes in NOS3 mRNA and eNOS protein levels in primary human umbilical vein endothelial cells (HUVECs). Utilizing in silico predictive protocols to identify potential miRNAs that regulate eNOS expression, we identified miR-200b as a candidate. We established the functional miR-200b target sequence within the NOS3 3′UTR, and demonstrated that manipulation of the miRNA levels during hypoxia using miR-200b mimics and antagomirs regulates eNOS levels, and established that miR-200b physiologically limits eNOS expression during hypoxia. Furthermore, we demonstrated that the specific ablation of the hypoxic induction of miR-200b in HUVECs restored eNOS-driven hypoxic NO release to the normoxic levels. To determine whether miR-200b might be the only miRNA that had this effect, we utilized Next Generation Sequencing (NGS) to follow hypoxia-induced changes in the miRNA levels in HUVECS and found 83 novel hypoxamiRs, with two candidate miRNAs besides miR-200b that could potentially influence eNOS levels. Taken together, the data establish miR-200b-eNOS regulation as a first hypoxamiR-based mechanism that limits NO bioavailability during hypoxia in endothelial cells, and show that hypoxamiRs could become useful therapeutic targets for cardiovascular diseases and other hypoxic-related diseases including various types of cancer.

Blunted nitric oxide regulation in Tibetans under high-altitude hypoxia

Nitric oxide (NO) is an important molecule for vasomotor tone, and elevated NO signaling was previously hypothesized as a unique and adaptive physiological change in highland Tibetans. However, there has been lack of NO data from Tibetans living at low altitude and lowlander immigrants living at high altitude, which is crucial to test this hypothesis. Here, through cross-altitude (1990–5018 m) and cross-population (Tibetans and Han Chinese) analyses of serum NO metabolites (NOx) of 2086 individuals, we demonstrate that although Tibetans have a higher serum NOx level compared to lowlanders, Han Chinese immigrants living at high altitude show an even higher level than Tibetans. Consequently, our data contradict the previous proposal of increased NO signaling as the unique adaptive strategy in Tibetans. Instead, Tibetans have a relatively lower circulating NOx level at high altitude. This observation is further supported by data from the hypoxic experiments using human umbilical vein endothelial cells and gene knockout mice. No difference is detected between Tibetans and Han Chinese for endothelial nitric oxide synthase (eNOS), the key enzyme for circulating NO synthesis, suggesting that eNOS itself is unlikely to be the cause. We show that other NO synthesis-related genes (e.g. GCH1) carry Tibetan-enriched mutations significantly associated with the level of circulating NOx in Tibetans. Furthermore, gene network analysis revealed that the downregulation and upregulation of NOx is possibly achieved through distinct pathways. Collectively, our findings provide novel insights into the physiological and genetic mechanisms of the evolutionary adaptation of Tibetans to high-altitude hypoxia.

One-week cocoa flavanol intake increases prefrontal cortex oxygenation at rest and during moderate-intensity exercise in normoxia and hypoxia

During exercise in hypoxia, O₂ delivery to brain and muscle is compromised, and oxidative stress is elicited. Cocoa flavanols (CF) have antioxidant capacities and can increase blood flow by stimulating endothelial function. We aimed to examine the effects of 7-day CF intake on oxidative stress, nitric oxide production, and tissue oxygenation in response to exercise in normobaric hypoxia (14.3% O₂). In a randomized, double-blind, cross-over study, 14 well-trained male cyclists completed four trials: exercise in normoxia or hypoxia, after 7-day CF or placebo intake. Flow-mediated dilation (FMD) was measured before intake of the last dose CF or placebo. One hundred minutes later, 20-min steady-state (SS; 45% V̇o_2max) and 20-min time trial (TT) (cycling) were performed. Blood samples were taken. Prefrontal and muscular oxygenation was assessed by near-infrared spectroscopy. At baseline, FMD was increased by CF. Hypoxia increased exercise-induced elevations in lipid peroxidation and antioxidant capacity. CF suppressed exercise-induced lipid peroxidation but did not influence antioxidant capacity. At rest and during SS, prefrontal and muscular oxygenation was decreased by hypoxia. CF elevated prefrontal oxygenation but did not impact muscular oxygenation. During TT, hypoxia accelerated the exercise-induced decrease in prefrontal oxygenation, but not in muscular oxygenation. During TT, CF did not alter prefrontal and muscular oxygenation. CF did not change plasma nitrite, nitrate, and arginine:citrulline. During high-intensity exercise, CF improved neither tissue oxygenation nor performance in well-trained athletes. At rest and during moderate-intensity exercise, CF reduced exercise-induced lipid peroxidation and partially restored the hypoxia-induced decline in prefrontal oxygenation. NEW & NOTEWORTHY For the first time, we showed that CF had beneficial effects on endothelial function at rest, as well as on prefrontal oxygenation at rest and during moderate-intensity exercise, both in normoxia and hypoxia. Moreover, we showed that CF intake inhibited oxidative stress during exhaustive exercise in hypoxia.

Phosphorylated vasodilator-stimulated phosphoprotein (P-VASPSer239) in platelets is increased by nitrite and partially deoxygenated erythrocytes

Nitrite is recognized as a bioactive nitric oxide (NO) metabolite. We have shown that nitrite inhibits platelet activation and increases platelet cGMP levels in the presence of partially deoxygenated erythrocytes. In this study, we investigated the effect of nitrite on phosphorylation of vasodilator-stimulated phosphoprotein on residue serine 239 (P-VASPSer239), a marker of protein kinase G (PKG) activation, in human platelets. In platelet-rich plasma (PRP), nitrite itself had no effect on levels of P-VASPSer239 while DEANONOate increased P-VASPSer239. Deoxygenation of PRP + erythrocytes (20% hematocrit) raised baseline P-VASPSer239 in platelets. At 20% hematocrit, nitrite (10 μM) increased P-VASPSer239 in platelets about 31% at 10-20 minutes of incubation while the levels of P-VASPSer157, a marker of protein kinase A (PKA) activation, were not changed. Nitrite increased P-VASPSer239 in platelets in the presence of deoxygenated erythrocytes at 20-40% hematocrit, but the effects were slightly greater at 20% hematocrit. In conclusion, our data confirm that nitrite increases P-VASPSer239 in platelets in the presence of deoxygenated erythrocytes. They also further support the idea that partially deoxygenated erythrocytes may modulate platelet activity, at least in part, via the NO/sGC/PKG pathway from NO formed by reduction of circulating nitrite ions.

Hybrid Nitric Oxide Donor and its Carrier for the Treatment of Peripheral Arterial Diseases

Nitric oxide (NO) has been known to promote physiological angiogenesis to treat peripheral arterial diseases (PAD) by increasing the vascular endothelial growth factor (VEGF) level in endothelial cells (ECs) and preventing platelet adherence and leukocyte chemotaxis. However, the ongoing ischemic event during peripheral ischemia produces superoxide and diminishes the NO bioavailability by forming toxic peroxynitrite anion. Here we disclose an efficacious hybrid molecule 4-(5-Amino-1,2,3-oxadiazol-3-yl)-2,2,6,6-tetramethyl-1-piperidinol (SA-2) containing both antioxidant and NO donor functionalities that provide a therapeutic level of NO necessary to promote angiogenesis and to protect ECs against hydrogen peroxide-induced oxidative stress. Compound SA-2 scavenged reactive oxygen species, inhibited proliferation and migration of smooth muscle cells (SMCs) and promoted the tube formation from ECs. Copolymer poly(lactic-co-glycolic acid) (PLGA) nanoparticles loaded with SA-2 provided a sustained release of NO over days, improved aqueous stability in serum, protected ECs against oxidative stress, and enhanced angiogenesis under stress conditions as compared to that of the control in the in vitro matrigel tube formation assay. These results indicated the potential use of SA-2 nanoparticles as an alternative therapy to treat PAD.

A PP2A-mediated feedback mechanism controls Ca2+-dependent NO synthesis under physiological oxygen

Intracellular O₂ is a key regulator of NO signaling, yet most in vitro studies are conducted in atmospheric O₂ levels, hyperoxic with respect to the physiologic milieu. We investigated NO signaling in endothelial cells cultured in physiologic (5%) O₂ and stimulated with histamine or shear stress. Culture of cells in 5% O₂ (>5 d) decreased histamine- but not shear stress–stimulated endothelial (e)NOS activity. Unlike cells adapted to a hypoxic environment (1% O₂), those cultured in 5% O₂ still mobilized sufficient Ca²⁺ to activate AMPK. Enhanced expression and membrane targeting of PP2A-C was observed in 5% O₂, resulting in greater interaction with eNOS in response to histamine. Moreover, increased dephosphorylation of eNOS in 5% O₂ was Ca²⁺-sensitive and reversed by okadaic acid or PP2A-C siRNA. The present findings establish that Ca²⁺ mobilization stimulates both NO synthesis and PP2A-mediated eNOS dephosphorylation, thus constituting a novel negative feedback mechanism regulating eNOS activity not present in response to shear stress. This, coupled with enhanced NO bioavailability, underpins differences in NO signaling induced by inflammatory and physiologic stimuli that are apparent only in physiologic O₂ levels. Furthermore, an explicit delineation between physiologic normoxia and genuine hypoxia is defined here, with implications for our understanding of pathophysiological hypoxia.—Keeley, T. P., Siow, R. C. M., Jacob, R., Mann, G. E. A PP2A-mediated feedback mechanism controls Ca²⁺-dependent NO synthesis under physiological oxygen.

Effectiveness of isosorbide dinitrate in cyanide poisoning as a function of the administration timing

BACKGROUND

Better and safer antidotes against cyanide poisoning are needed. Prior study has shown a favorable effect of isosorbide dinitrate (ISDN) on the survival of cyanide-poisoned rabbits when administered as early as 1 min after poisoning. The aim of the current study was to further evaluate the efficacy of intravenous ISDN administered in clinically relevant timing for first responders.

METHODS

A comparative animal study using 24 rabbits in 4 randomized study groups was performed. Animals were poisoned with intravenous potassium cyanide (1 mg/kg). Animals in Group 1 served as controls and received no treatment. Groups 2-4 animals were treated intravenously with ISDN (50 μg/kg) after poisoning; one group after 3 min, another group after 5 min and the last after 7 min. Animals were observed for 30 min after poisoning. The study endpoints included survival rate, clinical status, blood pressure, pulse per minute, blood lactate and pH.

RESULTS

Five of 6 animals (83.3%) from every treatment group survived the whole observation period while all control untreated animals died. All the rabbits collapsed immediately after exposure, showing rapidly deteriorated vital signs with lactic metabolic acidosis (peak blood lactate levels of 18.1 to 19.0 mmol/L on average at 10 min post exposure). Vital signs, clinical scores, and blood gases of treated rabbits gradually improved.

CONCLUSION

Poisoned rabbits showed improved short-term survival following the administration of ISDN up to 7 min after lethal cyanide poisoning of. We see a potential for ISDN as an antidote against cyanide poisoning.

Folic Acid Promotes Recycling of Tetrahydrobiopterin and Protects Against Hypoxia-Induced Pulmonary Hypertension by Recoupling Endothelial Nitric Oxide Synthase

AIMS

Nitric oxide (NO) derived from endothelial NO synthase (eNOS) has been implicated in the adaptive response to hypoxia. An imbalance between 5,6,7,8-tetrahydrobiopterin (BH4) and 7,8-dihydrobiopterin (BH2) can result in eNOS uncoupling and the generation of superoxide instead of NO. Dihydrofolate reductase (DHFR) can recycle BH2 to BH4, leading to eNOS recoupling. However, the role of DHFR and eNOS recoupling in the response to hypoxia is not well understood. We hypothesized that increasing the capacity to recycle BH4 from BH2 would improve NO bioavailability as well as pulmonary vascular remodeling (PVR) and right ventricular hypertrophy (RVH) as indicators of pulmonary hypertension (PH) under hypoxic conditions.

RESULTS

In human pulmonary artery endothelial cells and murine pulmonary arteries exposed to hypoxia, eNOS was uncoupled as indicated by reduced superoxide production in the presence of the nitric oxide synthase inhibitor, L-(G)-nitro-L-arginine methyl ester (L-NAME). Concomitantly, NO levels, BH4 availability, and expression of DHFR were diminished under hypoxia. Application of folic acid (FA) restored DHFR levels, NO bioavailability, and BH4 levels under hypoxia. Importantly, FA prevented the development of hypoxia-induced PVR, right ventricular pressure increase, and RVH.

INNOVATION

FA-induced upregulation of DHFR recouples eNOS under hypoxia by improving BH4 recycling, thus preventing hypoxia-induced PH.

CONCLUSION

FA might serve as a novel therapeutic option combating PH.

Rho-kinase activation contributes to Lps-induced impairment of endothelial nitric oxide synthase activation by endothelin-1 in cultured hepatic sinusoidal endothelial cells

The purpose of this study is to understand the role of rho-kinase (ROCK-2) in the regulation of liver microcirculation after inflammatory stress. Endothelin-1 (ET-1)-induced nitric oxide (NO) is essential in the regulation of blood flow in hepatic sinusoids. Lipopolysaccharide (LPS) inhibits this ET-1-induced NO production and disrupts liver microcirculation; however, the exact molecular mechanism is unknown. Liver sinusoidal endothelial cells were isolated, pretreated with 10 ng/mL LPS for 6 h, and treated with 10 μM Y27632 (ROCK-2 inhibitor) for 30 min and 10 nM ET-1 for 30 min. Lipopolysaccharide induced RhoA membrane translocation that was attenuated by methyl-β-cyclodextrin (cholesterol sequester) or targeted mutation of caveolin-1. Lipopolysaccharide increased ROCK-2 expressions (+60%) and ROCK-2 activity (+36%). Endothelin-1 increased endothelial NO synthase (eNOS) activity (+70%), but LPS inhibited this ET-1-mediated eNOS response. Treatment with Y27632 restored ET-1-mediated eNOS activity (+61%) and stimulated NO production in the perinuclear region after LPS pretreatment. This treatment reduced cofilin-Ser3 phosphorylation (-73%), increased vasodilator-stimulated phosphoprotein-Ser239 phosphorylation (+88%), and stimulated globular actin/eNOS association. Lipopolysaccharide induces Rho/ROCKs signaling pathway to disrupt the ET-1-mediated eNOS activation in liver sinusoidal endothelial cells. Rho-kinase ROCK-2 inhibition restores ET-1-mediated NO production after the LPS pretreatment, in part, through an increase in actin depolymerization.

Early administration of isosorbide dinitrate improves survival of cyanide-poisoned rabbits

CONTEXT

More effective, rapidly delivered, safer antidotes are needed for cyanide poisoning. Previous study has demonstrated a beneficial effect of isosorbide dinitrate on the survival of cyanide-poisoned mice.

OBJECTIVE

To evaluate the effectiveness of isosorbide dinitrate compared with that of sodium nitrite in cyanide poisoning.

MATERIALS AND METHODS

A comparative animal study was performed using 18 rabbits, randomized into 3 study groups. Animals were poisoned intravenously with potassium cyanide (1 mg/kg). The first group was not given any further treatment. The second and third groups were treated intravenously 1 min after poisoning with sodium nitrite (6 mg/kg) and isosorbide dinitrate (50 μg/kg), respectively. The primary outcome was short-term survival of up to 30 min. Secondary outcomes included time to death, a clinical score, mean blood pressure, pulse, blood pH, and lactate and methemoglobin levels.

RESULTS

Rabbits treated with isosorbide dinitrate or sodium nitrite survived while only one untreated rabbit survived. Median time to death of the 5 poisoned and untreated animals was 10 min. All the animals collapsed soon after poisoning, exhibiting rapidly disturbed vital signs and developed lactic metabolic acidosis; average peak blood lactate levels were 15.5-19.1 mmol/L at 10 min after poisoning. The treated animals improved gradually with practically full recovery of the clinical scores, vital signs, and blood gas levels. Sodium nitrite administration raised methemoglobin to an average peak of 7.9%, while isosorbide dinitrate did not change methemoglobin levels.

CONCLUSION

Early administration of isosorbide dinitrate improved the short-term survival of cyanide-poisoned rabbits. Isosorbide dinitrate shows potential as an antidote for cyanide poisoning and may exert its effect using a nitric-oxide-dependent mechanism.

Dietary nitrate supplementation: effects on plasma nitrite and pulmonary O2 uptake dynamics during exercise in hypoxia and normoxia

We investigated the effects of dietary nitrate (NO3 (-)) supplementation on the concentration of plasma nitrite ([NO2 (-)]), oxygen uptake (V̇o2) kinetics, and exercise tolerance in normoxia (N) and hypoxia (H). In a double-blind, crossover study, 12 healthy subjects completed cycle exercise tests, twice in N (20.9% O2) and twice in H (13.1% O2). Subjects ingested either 140 ml/day of NO3 (-)-rich beetroot juice (8.4 mmol NO3; BR) or NO3 (-)-depleted beetroot juice (PL) for 3 days prior to moderate-intensity and severe-intensity exercise tests in H and N. Preexercise plasma [NO2 (-)] was significantly elevated in H-BR and N-BR compared with H-PL (P < 0.01) and N-PL (P < 0.01). The rate of decline in plasma [NO2 (-)] was greater during severe-intensity exercise in H-BR [-30 ± 22 nM/min, 95% confidence interval (CI); -44, -16] compared with H-PL (-7 ± 10 nM/min, 95% CI; -13, -1; P < 0.01) and in N-BR (-26 ± 19 nM/min, 95% CI; -38, -14) compared with N-PL (-1 ± 6 nM/min, 95% CI; -5, 2; P < 0.01). During moderate-intensity exercise, steady-state pulmonary V̇o2 was lower in H-BR (1.91 ± 0.28 l/min, 95% CI; 1.77, 2.13) compared with H-PL (2.05 ± 0.25 l/min, 95% CI; 1.93, 2.26; P = 0.02), and V̇o2 kinetics was faster in H-BR (τ: 24 ± 13 s, 95% CI; 15, 32) compared with H-PL (31 ± 11 s, 95% CI; 23, 38; P = 0.04). NO3 (-) supplementation had no significant effect on V̇o2 kinetics during severe-intensity exercise in hypoxia, or during moderate-intensity or severe-intensity exercise in normoxia. Tolerance to severe-intensity exercise was improved by NO3 (-) in hypoxia (H-PL: 197 ± 28; 95% CI; 173, 220 vs. H-BR: 214 ± 43 s, 95% CI; 177, 249; P = 0.04) but not normoxia. The metabolism of NO2 (-) during exercise is altered by NO3 (-) supplementation, exercise, and to a lesser extent, hypoxia. In hypoxia, NO3 (-) supplementation enhances V̇o2 kinetics during moderate-intensity exercise and improves severe-intensity exercise tolerance. These findings may have important implications for individuals exercising at altitude.

Nitrite induces the extravasation of iron oxide nanoparticles in hypoxic tumor tissue

Nitrite undergoes reconversion to nitric oxide under conditions characteristic of the tumor microenvironment, such as hypoxia and low pH. This selective conversion of nitrite into nitric oxide in tumor tissue has led to the possibility of using nitrite to enhance drug delivery and the radiation response. In this work, we propose to serially characterize the vascular response of brain tumor-bearing rats to nitrite using contrast-enhanced R2 * mapping. Imaging is performed using a multi-echo gradient echo sequence at baseline, post iron oxide nanoparticle injection and post-nitrite injection, whilst the animal is breathing air. The results indicate that nitrite sufficiently increases the vascular permeability in C6 gliomas, such that the iron oxide nanoparticles accumulate within the tumor tissue. When animals breathed 100% oxygen, the contrast agent remained within the vasculature, indicating that the conversion of nitrite to nitric oxide occurs in the presence of hypoxia within the tumor. The hypoxia-dependent, nitrite-induced extravasation of iron oxide nanoparticles observed herein has implications for the enhancement of conventional and nanotherapeutic drug delivery.

Competition and collaboration between RNA-binding proteins and microRNAs

Posttranscriptional regulation of mRNA species represents a major regulatory checkpoint in the control of gene expression. Historically, RNA-binding proteins (RBPs) have been regarded as the primary regulators of mRNA stability and translation. More recently, however, microRNAs have emerged as a class of potent and pervasive posttranscriptional rheostats that similarly affect mRNA stability and translation. The observation that both microRNAs and RBPs regulate mRNA stability and translation has initiated a newer area of research that involves the examination of dynamic interactions between these two important classes of posttranscriptional regulators, the myriad of factors that influence these biological interactions, and ultimately, their effects on target mRNAs. Specifically, microRNAs and RBPs can act synergistically to effect mRNA destabilization and translational inhibition. They can also engage in competition with each other and exert opposing effects on target mRNAs. To date, several key studies have provided critical details regarding the mechanisms and principles of interaction between these molecules. Additionally, these findings raise important questions regarding the regulation of these interactions, including the roles of posttranslational modification, subcellular localization, target inhibition versus activation, and changes in expression levels of these regulatory factors, especially under stimulus- and cell-specific conditions. Indeed, further experimentation is warranted to address these key issues that pertain to the collaboration and competition between microRNAs and RBPs. Significantly, the elucidation of these important details bears critical implications for disease management, especially for those diseases in which these cellular factors are dysregulated.

Intermittent maternal hypoxia has an influence on regional expression of endothelial nitric oxide synthase in fetal arteries of rabbits

BACKGROUND

Maternal hypoxia induces sustained fetal adaptations associated with changes in gene expression. We hypothesized that intermittent maternal hypoxia has an influence on regional expression of endothelial nitric oxide synthase (eNOS) in fetal arteries of New Zealand White rabbits.

METHODS

Timed-pregnant New Zealand White rabbits (term = 30 ± 1 d) were randomly assigned to a normoxic control group (n = 5) or a hypoxia group (12% O2, n = 5) during days 10-29 of pregnancy. At the end of pregnancy (29 d gestation), blood samples were collected from mothers and fetuses. Carotid and femoral arteries of fetuses were extracted for eNOS mRNA and protein concentration and analysis of total NOS activities.

RESULTS

Our data demonstrate that chronic intermittent maternal hypoxia significantly increased eNOS mRNA and protein concentrations and total NOS activities in carotid artery segments but decreased eNOS mRNA and protein concentrations and total NOS activities in femoral artery segments in the same fetuses. Vascular endothelial cells, but not smooth muscle cells, of fetal rabbits exhibited positive immunostaining for the eNOS protein.

CONCLUSION

These observations suggest that chronic hypoxia can regulate regional expression of eNOS as an adaptive response to hypoxic stress in fetal arteries.

Κ-opioid receptor stimulation improves endothelial function in hypoxic pulmonary hypertension

The present study was designed to investigate the effect of κ-opioid receptor stimulation with U50,488H on endothelial function and underlying mechanism in rats with hypoxic pulmonary hypertension (HPH). Chronic hypoxia-induced HPH was simulated by exposing the rats to 10% oxygen for 2 wk. After hypoxia, mean pulmonary arterial pressure (mPAP), right ventricular pressure (RVP) and right ventricular hypertrophy index (RVHI) were measured. Relaxation of pulmonary artery in response to acetylcholine (ACh) was determined. Expression and activity of endothelial nitric oxide (NO) synthase (eNOS) and inducible NO synthase (iNOS) with NO production, total antioxidant capacity (T-AOC), gp91(phox) expression and nitrotyrosine content were measured. The effect of U50,488H administration during chronic hypoxia was investigated. Administration of U50,488H significantly decreased mPAP and right ventricular hypertrophy as evidenced by reduction in RVP and RVHI. These effects were mediated by κ-opioid receptor. In the meantime, treatment with U50,488H significantly improved endothelial function as evidenced by enhanced relaxation in response to ACh. Moreover, U50,488H resulted in a significant increase in eNOS phosphorylation, NO content in serum, and T-AOC in pulmonary artery of HPH rats. In addition, the activity of eNOS was enhanced, but the activity of iNOS was attenuated in the pulmonary artery of chronic hypoxic rats treated with U50,488H. On the other hand, U50,488H markedly blunted HPH-induced elevation of gp91(phox) expression and nitrotyrosine content in pulmonary artery, and these effects were blocked by nor-BNI, a selective κ-opioid receptor antagonist. These data suggest that κ-opioid receptor stimulation with U50,488H improves endothelial function in rats with HPH. The mechanism of action might be attributed to the preservation of eNOS activity, enhancement of eNOS phosphorylation, downregulation of iNOS activity and its antioxidative/nitrative effect.

Ghrelin protects human pulmonary artery endothelial cells against hypoxia-induced injury via PI3-kinase/Akt

Endothelial injury and diminished NO release induced by hypoxia is thought to be a critical factor in the development of pulmonary artery hypertension (PAH). Ghrelin (Ghr) is a well-characterized hormone and has protective effects on the cardiovascular system, specifically by promoting the vascular endothelial cell function. The aim of this study was to investigate the effect of the Ghr on the hypoxia-induced injury in human pulmonary artery endothelial cells (HPAECs) and on the involved transduction pathway. Effects were investigated by treating cells with varying concentrations of Ghr in the absence or presence of inhibitors that target phosphoinositide 3-kinase (PI3K), in normoxic or hypoxic conditions for 24h. Our results indicated that the treatment with 10(-7) mol/l Ghr significantly enhanced cell viability (P<0.05, n=5) and upregulated the ratio of Bcl-2/Bax under hypoxic condition (P<0.05, n=4), as compared with the hypoxic condition alone. However, an addition of the PI3K/Akt inhibitor LY294002 inhibited these Ghr-mediated effects. Moreover, the Ghr (10(-7)mol/l) significantly increased NO secretion and eNOS phosphorylation in comparison with the hypoxia or normoxia alone group (P<0.05, n=4). Nevertheless, the treatment with LY294002 (20 μmol/l) decreased the Ghr-induced NO release as well as the eNOS activity. In conclusion, the Ghr could inhibit hypoxia-mediated HPAECs dysfunction via the PI3K/Akt pathway, and the bcl-2/bax ratio was also involved in the protective action of the Ghr in HPAECs. As such, the Ghr demonstrates a significant potential to prevent and treat PAH affected by the endothelial dysfunction.

Crosstalk between oxygen- and nitric oxide-dependent signaling pathways in angiogenesis

With every heart beat blood rushes through a complex network of tubes to deliver essential ingredients of life, oxygen and nutrients. Consequently, this network of blood vessels is an indispensable part of vertebrate physiology. Its organization and architecture is highly dynamic in its form and function. Understanding how blood vessels develop, a process referred to as angiogenesis, is equally important as to know how they function considering that failure or misalignment of this process results in disorder and disease, in many cases of which death is inevitable. Much has been learned about the angiogenic process and the critical contributors of blood vessel function. A central determinant is oxygen, an evident contributor given the fact that oxygen delivery is a primary feature of blood vessel function. Not only is oxygen however essential for mitochondrial energy production, it also serves as a key molecule in various biochemical reactions, such as the formation of nitric oxide (NO), on its part a critical regulator of vascular tone and vessel homeostasis. Hence, oxygen abundance relates to the production of NO, and NO in turn regulates oxygen delivery and consumption. Given the importance of the intrinsic link these two molecules exert on angiogenesis and vessel function; this review shall highlight our current understanding on how these two molecules cooperate to form blood vessels.

The oxidative denitrosylation mechanism and nitric oxide release from human fetal and adult hemoglobin, an experimentally based model simulation study

Generation of unbound nitric oxide (NO) via the oxidative denitrosylation (ODN) mechanism is proposed to involve the simultaneous reaction of nitrite with oxy and deoxy hemoglobin (Hb(O2) (k1) and Hb (k13)) to yield respectively, *NO2 and Hb(+2)(NO). These two reaction pathways are coupled when *NO2 reacts with Hb(+2)(NO) to yield Hb(+3)(NO) (k22), a species that releases NO rapidly. Here, I have constructed an experimentally based molecular model of the ODN mechanism (k1-k31), focusing on the high nitrite reductase activity of R-state hemoglobin. This model was used to test the hypothesis that human fetal hemoglobin (HbF) can generate unbound NO faster and to a greater extent than HbA, consequent to a 25-fold larger value of k1, which was determined in an earlier study. The results show that despite the use of identical values for k22, there was a 44-fold larger apparent rate of reaction of *NO2 with HbF(NO) compared to HbA(NO), for reactions simulated at 410 μM nitrite and 100 μM hemoglobin (heme basis), 50% oxygen saturation at pH 7.4 and 37°C. This faster reaction was associated with the generation of about 11 μM peak unbound NO. In contrast, HbA failed to generate unbound NO rapidly under the same conditions. However, raising the concentration of nitrite into the millimolar range did generate unbound NO in the HbA simulation, in agreement with the experimental literature, and that result was associated with acceleration in the rate of reaction of NO2 with HbA(NO). Unbound NO could be generated at 410 μM nitrite in the HbA simulation by lowering the pH. This too was associated with an acceleration in the rate of reaction of NO2 with HbA(NO). Furthermore, generation of unbound NO could be assigned to the pH-dependent increase in k1, independent of the associated increase in k(13). Finally, selective exchange of the HbA value of k1 for the HbF value, keeping all other constants and conditions unchanged, generated kinetic patterns for the various species of the "k1-modified" HbA simulation, which were virtually indistinguishable from those seen in the HbF simulation. Taken together, these findings show that rapid and extensive generation of unbound NO within the ODN mechanism is controlled by the value of k1. The faster and more extensive generation of unbound NO by HbF at micromolar nitrite concentration suggests a possible second function for HbF in sickle cell disease, namely enhanced vasodilation. The failure of 410 μM nitrite to generate unbound NO in the HbA simulation at pH 7.4, contrasts with evidence in the literature showing that exposure of intact red cells to 100 to 200 μM nitrite in PBS, promoted NO release into the gas phase. I point out that this difference in outcome may be due to the higher activity of HbA when bound to the cytoplasmic domain of the red cell membrane anion transport protein SLC4A1 (band 3) and to the demonstrated capacity of band 3 to transport nitrite.

Nitric oxide signaling in the microcirculation

Several apparent paradoxes are evident when one compares mathematical predictions from models of nitric oxide (NO) diffusion and convection in vasculature structures with experimental measurements of NO (or related metabolites) in animal and human studies. Values for NO predicted from mathematical models are generally much lower than in vivo NO values reported in the literature for experiments, specifically with NO microelectrodes positioned at perivascular locations next to different sizes of blood vessels in the microcirculation and NO electrodes inserted into a wide range of tissues supplied by the microcirculation of each specific organ system under investigation. There continues to be uncertainty about the roles of NO scavenging by hemoglobin versus a storage function that may conserve NO, and other signaling targets for NO need to be considered. This review describes model predictions and relevant experimental data with respect to several signaling pathways in the microcirculation that involve NO.

Iptakalim rescues human pulmonary artery endothelial cells from hypoxia-induced nitric oxide system dysfunction

The aim of this study was to assess whether hypoxia inhibits endothelial nitric oxide synthase (eNOS) activity and nitric oxide (NO) production, and whether iptakalim may rescue human pulmonary artery endothelial cells (HPAECs) from hypoxia-induced NO system dysfunction. HPAECs were cultured under hypoxic conditions in the absence or presence of 0.1, 10 and 1,000 μM iptakalim or the combination of 10 μM iptakalim and 1, 10 and 100 μM glibenclamide for 24 h, and the eNOS activity and NO levels were measured in the conditioned medium from the HPAEC cultures. The eNOS activity and NO levels were reduced significantly in the conditioned medium from HPAEC cultures under hypoxic conditions. Pre-treatment with 10 μM iptakalim normalized the reduction of the eNOS activity and NO levels caused by hypoxia in the conditioned medium from HPAEC cultures. Iptakalim raised the eNOS activity and NO levels under hypoxic conditions, but was blocked by the K(ATP) channel blocker, glibenclamide. Our results indicate that hypoxia impairs NO system function, whereas the ATP-sensitive K(+) channel opener, iptakalim, may rescue HPAECs from hypoxia-induced NO system dysfunction.

Oxygen-dependent regulation of nitric oxide production by inducible nitric oxide synthase

Inducible nitric oxide synthase (iNOS) catalyzes the reaction that converts the substrates O(2) and l-arginine to the products nitric oxide (NO) and l-citrulline. Macrophages, and many other cell types, upregulate and express iNOS primarily in response to inflammatory stimuli. Physiological and pathophysiological oxygen tension can regulate NO production by iNOS at multiple levels, including transcriptional, translational, posttranslational, enzyme dimerization, cofactor availability, and substrate dependence. Cell culture techniques that emphasize control of cellular PO(2), and measurement of NO or its stable products, have been used by several investigators for in vitro study of the O(2) dependence of NO production at one or more of these levels. In most cell types, prior or concurrent exposure to cytokines or other inflammatory stimuli is required for the upregulation of iNOS mRNA and protein by hypoxia. Important transcription factors that target the iNOS promoter in hypoxia include hypoxia-inducible factor 1 and/or nuclear factor κB. In contrast to the upregulation of iNOS by hypoxia, in most cell types NO production is reduced by hypoxia. Recent work suggests a prominent role for O(2) substrate dependence in the short-term regulation of iNOS-mediated NO production.

Opening of small and intermediate calcium-activated potassium channels induces relaxation mainly mediated by nitric-oxide release in large arteries and endothelium-derived hyperpolarizing factor in small arteries from rat

This study was designed to investigate whether calcium-activated potassium channels of small (SK(Ca) or K(Ca)2) and intermediate (IK(Ca) or K(Ca)3.1) conductance activated by 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309) are involved in both nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in large and small rat mesenteric arteries. Segments of rat superior and small mesenteric arteries were mounted in myographs for functional studies. NO was recorded using NO microsensors. SK(Ca) and IK(Ca) channel currents and mRNA expression were investigated in human umbilical vein endothelial cells (HUVECs), and calcium concentrations were investigated in both HUVECs and mesenteric arterial endothelial cells. In both superior (∼1093 μm) and small mesenteric (∼300 μm) arteries, NS309 evoked endothelium- and concentration-dependent relaxations. In superior mesenteric arteries, NS309 relaxations and NO release were inhibited by both N(G),N(G)-asymmetric dimethyl-l-arginine (ADMA) (300 μM), an inhibitor of NO synthase, and apamin (0.5 μM) plus 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34) (1 μM), blockers of SK(Ca) and IK(Ca) channels, respectively. In small mesenteric arteries, NS309 relaxations were reduced slightly by ADMA, whereas apamin plus an IK(Ca) channel blocker almost abolished relaxation. Iberiotoxin did not change NS309 relaxation. HUVECs expressed mRNA for SK(Ca) and IK(Ca) channels, and NS309 induced increases in calcium, outward current, and NO release that were blocked by apamin and TRAM-34 or charybdotoxin. These findings suggest that opening of SK(Ca) and IK(Ca) channels leads to endothelium-dependent relaxation that is mediated mainly by NO in large mesenteric arteries and by EDHF-type relaxation in small mesenteric arteries. NS309-induced calcium influx appears to contribute to the formation of NO.

The quaternary hemoglobin conformation regulates the formation of the nitrite-induced bioactive intermediate and the dissociation of nitric oxide from this intermediate

Deoxyhemoglobin reduces nitrite to nitric oxide (NO). In order to study the effect of the hemoglobin quaternary conformation on the nitrite reaction, we compared T-state deoxyhemoglobin with R-state deoxyhemoglobin produced by reacting hemoglobin with carboxypeptidase-A prior to deoxygenation. The nitrite reaction with deoxyhemoglobin was followed by chemiluminescence, electron paramagnetic resonance and visible spectroscopy. The initial steps in this reaction involve the binding of nitrite to deoxyhemoglobin followed by the formation of an electron delocalized metastable intermediate that retains potential NO bioactivity. This reaction is shown by visible spectroscopy to occur 5.6 times faster in the R-state than in the T-state. However, the dissociation of NO from the delocalized intermediate is shown to be facilitated by the T-quaternary conformation with a 9.6 fold increase in the rate constant. The preferred NO-release in the T-state, which has a higher affinity for the membrane, can result in the NO diffusing out of the RBC and being released to the vasculature at low partial pressures of oxygen.

BAY 41-2272 inhibits the development of chronic hypoxic pulmonary hypertension in rats

The present study investigated whether BAY 41-2272(5-cyclopropyl-2-[1-(2-fluoro-benzyl)-1H-pyrazolo[3,4-b]pyridin-3-yl]-pyrimidin-4-ylamine), a novel pyrazolopyridine that activates guanylyl cyclase and sensitizes the enzyme towards nitric oxide (NO), inhibits the development of pulmonary hypertension. BAY 41-2272 (1 or 10 mg/kg/day) was administered intraperitoneally, and sildenafil (25 mg/kg/day), an inhibitor phosphodiesterase type 5, was given in the drinking water to rats kept under chronic hypobaric hypoxia for two weeks. Right ventricular systolic pressure and hypertrophy, degree of muscularization and relaxation of pulmonary arteries were measured, and immunoblotting was performed. Chronic hypoxia increased right ventricular systolic pressure and expression of soluble guanylyl cyclase and phosphorylated vasodilator-stimulated phosphoprotein (VASP-P(ser239)). BAY 41-2272 prevented hypoxia-induced increase in right ventricular systolic pressure and right ventricular hypertrophy to the same extent as sildenafil. Only sildenafil significantly decreased hypoxia-induced muscularization of pulmonary arteries. Expressed relative to soluble guanylyl cyclase expression, VASP-P(ser239) was increased in lungs from rats treated with BAY 41-2272. Acutely BAY 41-2272 caused pulmonary as well as systemic vasodilatation. In the chronic setting systemic blood pressure was not different to baseline at trough after intraperitoneally administered BAY 41-2272. BAY 41-2272 vasorelaxation in isolated pulmonary resistance arteries was inhibited by an inhibitor of guanylyl cyclase, ODQ (1H-[1,2,4] oxadiazolo[4,3-a]quinoxaline-1-one), and of Na(+)-K(+)-ATPase, ouabain. In conclusion, in an adult rat model of chronic hypoxic pulmonary hypertension, BAY 41-2272 to a similar degree as sildenafil prevents pulmonary hypertension. Thus, BAY 41-2272 may provide a novel therapeutic compound for treating chronic hypoxic pulmonary hypertension.

Beta-actin association with endothelial nitric-oxide synthase modulates nitric oxide and superoxide generation from the enzyme

Protein-protein interactions represent an important post-translational mechanism for endothelial nitric-oxide synthase (eNOS) regulation. We have previously reported that beta-actin is associated with eNOS oxygenase domain and that association of eNOS with beta-actin increases eNOS activity and nitric oxide (NO) production. In the present study, we found that beta-actin-induced increase in NO production was accompanied by decrease in superoxide formation. A synthetic actin-binding sequence (ABS) peptide 326 with amino acid sequence corresponding to residues 326-333 of human eNOS, one of the putative ABSs, specifically bound to beta-actin and prevented eNOS association with beta-actin in vitro. Peptide 326 also prevented beta-actin-induced decrease in superoxide formation and increase in NO and L-citrulline production. A modified peptide 326 replacing hydrophobic amino acids leucine and tryptophan with neutral alanine was unable to interfere with eNOS-beta-actin binding and to prevent beta-actin-induced changes in NO and superoxide formation. Site-directed mutagenesis of the actin-binding domain of eNOS replacing leucine and tryptophan with alanine yielded an eNOS mutant that exhibited reduced eNOS-beta-actin association, decreased NO production, and increased superoxide formation in COS-7 cells. Disruption of eNOS-beta-actin interaction in endothelial cells using ABS peptide 326 resulted in decreased NO production, increased superoxide formation, and decreased endothelial monolayer wound repair, which was prevented by PEG-SOD and NO donor NOC-18. Taken together, this novel finding indicates that beta-actin binding to eNOS through residues 326-333 in the eNOS protein results in shifting the enzymatic activity from superoxide formation toward NO production. Modulation of NO and superoxide formation from eNOS by beta-actin plays an important role in endothelial function.

Proteomics reveals lowering oxygen alters cytoskeletal and endoplasmatic stress proteins in human endothelial cells

A proteomic approach was applied to explore the signalling pathways elicited by lowering O(2) in endothelial cells. Endothelial cells isolated from native umbilical cords were subjected to 21, 5, or 1% O(2) for 24 h. 2-D PAGE was performed and candidate proteins were identified using LC-MS/MS. Lowering of O(2) from 21 to 5% induced upregulation of cofilin-1, cyclophilin A, tubulin and tubulin fragments, a fragment of glucose-regulated protein 78 (Grp78) and calmodulin. The upregulation of Grp78 suggested that ER stress proteins were altered and indeed Grp94 and caspase 12 expression were increased in cells exposed to 5% O(2). The presence of ER stress is also supported by findings of blunted caffeine-evoked ER calcium release in cells exposed to 5 and 1% O(2). Exposure to 1% O(2) caused increases in cofilin-1, cyclophilin A, and caspase 12 as well as a decrease of beta-actin, but it did not alter the expression of calmodulin, tubulin, Grp78, and Grp94. Incubation with CoCl(2), a stabilizer of the hypoxia-inducible factor, increased the expression of several of the proteins. The present investigations reveal that lowering O(2), probably in part through hypoxia-inducible factor, alter the expression of a series of proteins mainly involved in cytoskeletal changes (e.g. cofilin-1, tubulin, and beta-actin) and in ER stress/apoptosis (e.g. Grp78/94, caspase 12, and cyclophilin A).