Arterial hypertension in a murine model of sleep apnea: role of NADPH oxidase 2

Pde5 Inhibition Reduced Blood Pressure and Alleviated Target Organ Damage in Chronic Intermittent Hypoxia

ABSTRACT

The role of phosphodiesterase 5 (Pde5) in obstructive sleep apnea-induced damage remains unclear. Our study aimed to investigate the role of Pde5 in the chronic intermittent hypoxia (CIH) model. C57BL/6J wild-type (WT) mice (n = 48) and Pde5 knockout (Pde5 -/- ) mice (n = 24) were randomly assigned to CIH group and room air group. After 6 weeks, some WT mice (n = 24) in CIH group were given sildenafil or saline gavage for another 4 weeks. Blood pressure was regularly measured during the experiment. Echocardiography was used to estimate cardiac function. We collected organs from each group of mice and measured their physical indicators. Histochemical staining was used to explore the size of cardiomyocyte and fibrosis area of various organs. Cyclic guanosine monophosphate and malondialdehyde concentrations in serum were measured by ELISA assay. Compared with the RA-treated group, the 6-week CIH resulted in a significant increase in blood pressure, altered heart structure, and reduced serum cyclic guanosine monophosphate in WT mice. Pde5 -/- mice and sildenafil intragastric administration significantly reduced systolic blood pressure in CIH condition and attenuated the damage of target organs. In CIH model, we found that the cardiomyocyte size and fibrosis area of heart and kidney significantly reduced in Pde5 -/- groups. Besides, endogenous and exogenous inhibition of Pde5 reduced malondialdehyde level and inflammatory and oxidative stress markers expression in CIH condition. In this study, we found that Pde5 inhibition could reduce blood pressure and alleviate target organ damage in the CIH model, which may be mediated through the oxidative stress pathway.

Murine Pro-Inflammatory Responses to Acute and Sustained Intermittent Hypoxia: Implications for Obstructive Sleep Apnea Research

OBJECTIVES

Obstructive sleep apnea (OSA) is characterized by chronic systemic inflammation; however, the mechanisms underlying these pathologic consequences are incompletely understood. Our objective was to determine the effects of short- versus long-term exposure to intermittent hypoxia (IH) on pro-inflammatory mediators within vulnerable organs impacted by OSA.

STUDY DESIGN

Experimental animal study.

METHODS

A total of 8-10 week old C57BL/6J mice were exposed to normoxic or IH conditions for 7 days (short-term) or 6 weeks (long-term) under 12 h light, 12 h dark cycles. After exposure, multiple tissues were collected over a 24 h period. These tissues were processed and evaluated for gene expression and protein levels of pro-inflammatory mediators from peripheral tissues.

RESULTS

We observed a global decrease in immune response pathways in the heart, lung, and liver compared with other peripheral organs after short-term exposure to IH. Although there were tissue-specific alterations in the gene expression of pro-inflammatory mediators, with down-regulation in the lung and up-regulation in the heart, we also observed reduced protein levels of pro-inflammatory mediators in the serum, lung, and heart following short-term exposure to IH. Long-term exposure to IH resulted in an overall increase in the levels of inflammatory mediators in the serum, lung, and heart.

CONCLUSIONS

We demonstrated novel, longitudinal changes in the inflammatory cascade in a mouse model of OSA. The duration of exposure to IH led to significant variability of inflammatory responses within blood and cardiopulmonary tissues. Our findings further elucidate how inflammatory responses change over the course of the disease in vulnerable organs.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:S1-S11, 2024.

Exploring the potential relationships among obstructive sleep apnea, erectile dysfunction, and gut microbiota: a narrative review

INTRODUCTION

Poor sleep quality is closely associated with comorbidities affecting a multitude of organ systems. Among the sleep disorders in the population, there has recently been an increase in the prevalence of obstructive sleep apnea (OSA), which has particularly affected men. The intermittent hypoxia and sleep fragmentation associated with OSA can result in the manifestation or aggravation of a number of pathophysiologic conditions, including the impairment of reproductive function in men and women. In this context, erectile dysfunction (ED) is of particular concern. Other consequences of OSA are changes in the gastrointestinal microbiota, with the resultant dysbiosis having potentially harmful consequences that promote downstream exacerbation of various comorbidities.

OBJECTIVES

This narrative review aims to explore the potential relationships among ED, gut microbiota, and OSA.

METHODS

A search of the relevant literature was performed in the PubMed, Embase, Medline, and Web of Science databases.

RESULTS

Sleep is important for regulating the body's functions, and sleep deprivation can negatively affect health. OSA can damage organic functions, including reproductive function, and can lead to ED. Restoring the microbiota and improving sleep can help to improve sexual function or reverse ED and enhance other associated conditions mediated through the gut-brain axis relationship. Probiotics and prebiotics can be used as supportive strategies in the prevention and treatment of OSA, as they help to reduce systemic inflammation and improve intestinal barrier function.

CONCLUSION

A good diet, a healthy lifestyle, and proper bowel function are essential in controlling depression and several other pathologies. Modulating the gut microbiota through probiotics and prebiotics can provide a viable strategy for developing new therapeutic options in treating many conditions. A better understanding of these a priori unrelated phenomena would foster our understanding of the effects of OSA on human fertility and how changes in gut microbiota may play a role.

Long non-coding RNA FKSG29 regulates oxidative stress and endothelial dysfunction in obstructive sleep apnea

Altered expressions of pro-/anti-oxidant genes are known to regulate the pathophysiology of obstructive sleep apnea (OSA).We aim to explore the role of a novel long non-coding (lnc) RNA FKSG29 in the development of intermittent hypoxia with re-oxygenation (IHR)-induced endothelial dysfunction in OSA. Gene expression levels of key pro-/anti-oxidant genes, vasoactive genes, and the FKSG29 were measured in peripheral blood mononuclear cells from 12 subjects with primary snoring (PS) and 36 OSA patients. Human monocytic THP-1 cells and human umbilical vein endothelial cells (HUVEC) were used for gene knockout and double luciferase under IHR exposure. Gene expression levels of the FKSG29 lncRNA, NOX2, NOX5, and VEGFA genes were increased in OSA patients versus PS subjects, while SOD2 and VEGFB gene expressions were decreased. Subgroup analysis showed that gene expression of the miR-23a-3p, an endogenous competitive microRNA of the FKSG29, was decreased in sleep-disordered breathing patients with hypertension versus those without hypertension. In vitro IHR experiments showed that knock-down of the FKSG29 reversed IHR-induced ROS overt production, early apoptosis, up-regulations of the HIF1A/HIF2A/NOX2/NOX4/NOX5/VEGFA/VEGFB genes, and down-regulations of the VEGFB/SOD2 genes, while the protective effects of FKSG29 knock-down were abolished by miR-23a-3p knock-down. Dual-luciferase reporter assays confirmed that FKSG29 was a sponge of miR-23a-3p, which regulated IL6R directly. Immunofluorescence stain further demonstrated that FKSGH29 knock-down decreased IHR-induced uptake of oxidized low density lipoprotein and reversed IHR-induced IL6R/STAT3/GATA6/ICAM1/VCAM1 up-regulations. The findings indicate that the combined RNA interference may be a novel therapy for OSA-related endothelial dysfunction via regulating pro-/anti-oxidant imbalance or targeting miR-23a-IL6R-ICAM1/VCAM1 signaling.

Obstructive sleep apnea in a mouse model is associated with tissue-specific transcriptomic changes in circadian rhythmicity and mean 24-hour gene expression

Intermittent hypoxia (IH) is a major clinical feature of obstructive sleep apnea (OSA). The mechanisms that become dysregulated after periods of exposure to IH are unclear, particularly in the early stages of disease. The circadian clock governs a wide array of biological functions and is intimately associated with stabilization of hypoxia-inducible factors (HIFs) under hypoxic conditions. In patients, IH occurs during the sleep phase of the 24-hour sleep-wake cycle, potentially affecting their circadian rhythms. Alterations in the circadian clock have the potential to accelerate pathological processes, including other comorbid conditions that can be associated with chronic, untreated OSA. We hypothesized that changes in the circadian clock would manifest differently in those organs and systems known to be impacted by OSA. Using an IH model to represent OSA, we evaluated circadian rhythmicity and mean 24-hour expression of the transcriptome in 6 different mouse tissues, including the liver, lung, kidney, muscle, heart, and cerebellum, after a 7-day exposure to IH. We found that transcriptomic changes within cardiopulmonary tissues were more affected by IH than other tissues. Also, IH exposure resulted in an overall increase in core body temperature. Our findings demonstrate a relationship between early exposure to IH and changes in specific physiological outcomes. This study provides insight into the early pathophysiological mechanisms associated with IH.

The Cardiovascular and Metabolic Effects of Chronic Hypoxia in Animal Models: A Mini-Review

Animal models are useful to understand the myriad physiological effects of hypoxia. Such models attempt to recapitulate the hypoxemia of human disease in various ways. In this mini-review, we consider the various animal models which have been deployed to understand the effects of chronic hypoxia on pulmonary and systemic blood pressure, glucose and lipid metabolism, atherosclerosis, and stroke. Chronic sustained hypoxia (CSH)-a model of chronic lung or heart diseases in which hypoxemia may be longstanding and persistent, or of high altitude, in which effective atmospheric oxygen concentration is low-reliably induces pulmonary hypertension in rodents, and appears to have protective effects on glucose metabolism. Chronic intermittent hypoxia (CIH) has long been used as a model of obstructive sleep apnea (OSA), in which recurrent airway occlusion results in intermittent reductions in oxyhemoglobin saturations throughout the night. CIH was first shown to increase systemic blood pressure, but has also been associated with other maladaptive physiological changes, including glucose dysregulation, atherosclerosis, progression of nonalcoholic fatty liver disease, and endothelial dysfunction. However, models of CIH have generally been implemented so as to mimic severe human OSA, with comparatively less focus on milder hypoxic regimens. Here we discuss CSH and CIH conceptually, the effects of these stimuli, and limitations of the available data.

Effect of chronic intermittent hypoxia (CIH) on neuromuscular junctions and mitochondria in slow- and fast-twitch skeletal muscles of mice—the role of iNOS

Background

Obstructive sleep apnea (OSA) imposes vascular and metabolic risks through chronic intermittent hypoxia (CIH) and impairs skeletal muscle performance. As studies addressing limb muscles are rare, the reasons for the lower exercise capacity are unknown. We hypothesize that CIH-related morphological alterations in neuromuscular junctions (NMJ) and mitochondrial integrity might be the cause of functional disorders in skeletal muscles.

Methods

Mice were kept under 6 weeks of CIH (alternating 7% and 21% O₂ fractions every 30 s, 8 h/day, 5 days/week) compared to normoxia (NOX). Analyses included neuromuscular junctions (NMJ) postsynaptic morphology and integrity, fiber cross-sectional area (CSA) and composition (ATPase), mitochondrial ultrastructure (transmission-electron-microscopy), and relevant transcripts (RT-qPCR). Besides wildtype (WT), we included inducible nitric oxide synthase knockout mice (iNOS^−/−) to evaluate whether iNOS is protective or risk-mediating.

Results

In WT soleus muscle, CIH vs. NOX reduced NMJ size (− 37.0%, p < 0.001) and length (− 25.0%, p < 0.05) together with fiber CSA of type IIa fibers (− 14%, p < 0.05) and increased centronucleated fiber fraction (p < 0.001). Moreover, CIH vs. NOX increased the fraction of damaged mitochondria (1.8-fold, p < 0.001). Compared to WT, iNOS^−/− similarly decreased NMJ area and length with NOX (− 55%, p < 0.001 and − 33%, p < 0.05, respectively) or with CIH (− 37%, p < 0.05 and − 29%, p < 0.05), however, prompted no fiber atrophy. Moreover, increased fractions of damaged (2.1-fold, p < 0.001) or swollen (> 6-fold, p < 0.001) mitochondria were observed with iNOS^−/− vs. WT under NOX and similarly under CIH. Both, CIH- and iNOS^−/− massively upregulated suppressor-of-cytokine-signaling-3 (SOCS3) > 10-fold without changes in IL6 mRNA expression. Furthermore, inflammatory markers like CD68 (macrophages) and IL1β were significantly lower in CIH vs. NOX. None of these morphological alterations with CIH- or iNOS^−/− were detected in the gastrocnemius muscle. Notably, iNOS expression was undetectable in WT muscle, unlike the liver, where it was massively decreased with CIH.

Conclusion

CIH leads to NMJ and mitochondrial damage associated with fiber atrophy/centronucleation selectively in slow-twitch muscle of WT. This effect is largely mimicked by iNOS^−/− at NOX (except for atrophy). Both conditions involve massive SOCS3 upregulation likely through denervation without Il6 upregulation but accompanied by a decrease of macrophage density especially next to denervated endplates. In the absence of muscular iNOS expression in WT, this damage may arise from extramuscular, e.g., motoneuronal iNOS deficiency (through CIH or knockout) awaiting functional evaluation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-022-00288-7.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

Role of olfactory receptor78 in carotid body-dependent sympathetic activation and hypertension in murine models of chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) is a hallmark manifestation of obstructive sleep apnea (OSA), a widespread breathing disorder. CIH-treated rodents exhibit activation of the sympathetic nervous system and hypertension. Heightened carotid body (CB) activity has been implicated in CIH-induced hypertension. CB expresses high abundance of olfactory receptor (Olfr) 78, a G-protein coupled receptor. Olfr 78 null mice exhibit impaired CB sensory nerve response to acute hypoxia. Present study examined whether Olfr78 participates in CB-dependent activation of the sympathetic nervous system and hypertension in CIH-treated mice and in hemeoxygenase (HO)-2 null mice experiencing CIH as a consequence of naturally occurring OSA. CIH-treated wild-type (WT) mice showed hypertension, biomarkers of sympathetic nerve activation, and enhanced CB sensory nerve response to hypoxia and sensory long-term facilitation (sLTF), and these responses were absent in CIH-treated Olfr78 null mice. HO-2 null mice showed higher apnea index (AI) (58 ± 1.2 apneas/h) than WT mice (AI = 8 ± 0.8 apneas/h) and exhibited elevated blood pressure (BP), elevated plasma norepinephrine (NE) levels, and heightened CB sensory nerve response to hypoxia and sLTF. The magnitude of hypertension correlated with AI in HO-2 null mice. In contrast, HO-2/Olfr78 double null mice showed absence of elevated BP and plasma NE levels and augmented CB response to hypoxia and sLTF. These results demonstrate that Olfr78 participates in sympathetic nerve activation and hypertension and heightened CB activity in two murine models of CIH.NEW & NOTEWORTHY Carotid body (CB) sensory nerve activation is essential for sympathetic nerve excitation and hypertension in rodents treated with chronic intermittent hypoxia (CIH) simulating blood O₂ profiles during obstructive sleep apnea (OSA). Here, we report that CIH-treated mice and hemeoxygenase (HO)-2-deficient mice, which show OSA phenotype, exhibit sympathetic excitation, hypertension, and CB activation. These effects are absent in Olfr78 null and Olfr78/HO-2 double null mice.

High Plasma Cystine Levels Are Associated with Blood Pressure and Reversed by CPAP in Patients with Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) independent of obesity (OBS) imposes severe cardiovascular risk. To what extent plasma cystine concentration (CySS), a novel pro-oxidative vascular risk factor, is increased in OSA with or without OBS is presently unknown. We therefore studied CySS together with the redox state and precursor amino acids of glutathione (GSH) in peripheral blood mononuclear cells (PBMC) in untreated male patients with OSA (apnea-hypopnea-index (AHI) > 15 h^-1, n = 28) compared to healthy male controls (n = 25) stratifying for BMI ≥ or < 30 kg m^-2. Fifteen OSA patients were reassessed after 3-5-months CPAP. CySS correlated with cumulative time at an O₂-saturation <90% (Tu90%) (r = 0.34, p < 0.05) beside BMI (r = 0.58, p < 0.001) and was higher in subjects with "hypoxic stress" (59.4 ± 2.0 vs. 50.1 ± 2.7 µM, p < 0.01) defined as Tu90% ≥ 15.2 min (corresponding to AHI ≥ 15 h^-1). Moreover, CySS significantly correlated with systolic (r = 0.32, p < 0.05) and diastolic (r = 0.31, p < 0.05) blood pressure. CPAP significantly lowered CySS along with blood pressure at unchanged BMI. Unexpectedly, GSH antioxidant capacity in PBMC was increased with OSA and reversed with CPAP. Plasma CySS levels are increased with OSA-related hypoxic stress and associated with higher blood pressure. CPAP decreases both CySS and blood pressure. The role of CySS in OSA-related vascular endpoints and their prevention by CPAP warrants further studies.

Renal Dopamine Receptors and Oxidative Stress: Role in Hypertension

Significance: The kidney plays an important role in the long-term control of blood pressure. Oxidative stress is one of the fundamental mechanisms responsible for the development of hypertension. Dopamine, via five subtypes of receptors, plays an important role in the control of blood pressure by various mechanisms, including the inhibition of oxidative stress. Recent Advances: Dopamine receptors exert their regulatory function to decrease the oxidative stress in the kidney and ultimately maintain normal sodium balance and blood pressure homeostasis. An aberration of this regulation may be involved in the pathogenesis of hypertension. Critical Issues: Our present article reviews the important role of oxidative stress and intrarenal dopaminergic system in the regulation of blood pressure, summarizes the current knowledge on renal dopamine receptor-mediated antioxidation, including decreasing reactive oxygen species production, inhibiting pro-oxidant enzyme nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase, and stimulating antioxidative enzymes, and also discusses its underlying mechanisms, including the increased activity of G protein-coupled receptor kinase 4 (GRK4) and abnormal trafficking of renal dopamine receptors in hypertensive status. Future Directions: Identifying the mechanisms of renal dopamine receptors in the regulation of oxidative stress and their contribution to the pathogenesis of hypertension remains an important research focus. Increased understanding of the role of reciprocal regulation between renal dopamine receptors and oxidative stress in the regulation of blood pressure may give us novel insights into the pathogenesis of hypertension and provide a new treatment strategy for hypertension.

Lung oxidative stress and transcriptional regulations induced by estradiol and intermittent hypoxia

We determined the effects of chronic intermittent hypoxia (CIH) and estradiol (E₂) on oxidative stress and gene expression in the lungs. Female Sprague-Dawley rats were left intact (sham) or ovariectomized (OVX) and implanted with pumps delivering vehicle or E₂ (0.5 mg/kg/day). Two weeks following surgery, the rats were exposed to room air (RA) or CIH for 7 days (10% O₂, 10 cycles/hour, 8 h/day). Lung samples were used to measure the activities of pro- (NADPH and xanthine oxidases) and antioxidant (superoxide dismutase, catalase and glutathione peroxidase) enzymes, and concentrations of advanced oxidation of protein products (AOPP). We determined gene expression with an RNA microarray and enrichment analysis of differentially expressed genes. In rats exposed to RA, OVX and E₂ supplementation increased pro- and antioxidant activities and AOPP concentration. In rats exposed to CIH, AOPP concentration, pro- and antioxidant enzymes activities increased in sham, did not changed in OVX-Veh rats, and were reduced in OVX-E₂ rats. In rats exposed to RA, genes involved in extracellular matrix were up-regulated by OVX and down-regulated by E₂, while E₂ up-regulated genes involved in cell mobility/adherence and leukocytes migration. OVX downregulated expression of roughly 200 olfactory receptor genes without effect of E₂. CIH altered gene expression in sham and OVX-E₂, but not in OVX-Veh rats. Enrichment analysis confirmed the antioxidant effects of E₂ under CIH. There are important interactions between ovarian hormones and CIH that can be relevant to better understand the consequences of sleep apnea (i.e. CIH) on the occurrence of lung pathologies in women.

Short-term exposure to intermittent hypoxia leads to changes in gene expression seen in chronic pulmonary disease

Obstructive sleep apnea (OSA) results from episodes of airway collapse and intermittent hypoxia (IH) and is associated with a host of health complications. Although the lung is the first organ to sense changes in oxygen levels, little is known about the consequences of IH to the lung hypoxia-inducible factor-responsive pathways. We hypothesized that exposure to IH would lead to cell-specific up- and downregulation of diverse expression pathways. We identified changes in circadian and immune pathways in lungs from mice exposed to IH. Among all cell types, endothelial cells showed the most prominent transcriptional changes. Upregulated genes in myofibroblast cells were enriched for genes associated with pulmonary hypertension and included targets of several drugs currently used to treat chronic pulmonary diseases. A better understanding of the pathophysiologic mechanisms underlying diseases associated with OSA could improve our therapeutic approaches, directing therapies to the most relevant cells and molecular pathways.

The Role of the Renal Dopaminergic System and Oxidative Stress in the Pathogenesis of Hypertension

The kidney is critical in the long-term regulation of blood pressure. Oxidative stress is one of the many factors that is accountable for the development of hypertension. The five dopamine receptor subtypes (D₁R–D₅R) have important roles in the regulation of blood pressure through several mechanisms, such as inhibition of oxidative stress. Dopamine receptors, including those expressed in the kidney, reduce oxidative stress by inhibiting the expression or action of receptors that increase oxidative stress. In addition, dopamine receptors stimulate the expression or action of receptors that decrease oxidative stress. This article examines the importance and relationship between the renal dopaminergic system and oxidative stress in the regulation of renal sodium handling and blood pressure. It discusses the current information on renal dopamine receptor-mediated antioxidative network, which includes the production of reactive oxygen species and abnormalities of renal dopamine receptors. Recognizing the mechanisms by which renal dopamine receptors regulate oxidative stress and their degree of influence on the pathogenesis of hypertension would further advance the understanding of the pathophysiology of hypertension.

Oxidative Stress and Inflammation Biomarker Expression in Obstructive Sleep Apnea Patients

Obstructive Sleep Apnea Syndrome (OSAS) is a respiratory sleep disorder characterised by repeated episodes of partial or complete obstruction of the upper airway during the night. This obstruction usually occurs with a reduction (hypopnea) or complete cessation (apnea) of the airflow in the upper airways with the persistence of thoracic-diaphragmatic respiratory movements. During the hypopnea/apnea events, poor alveolar ventilation reduces the oxygen saturation in the arterial blood (SaO₂) and a gradual increase in the partial arterial pressure of carbon dioxide (PaCO₂). The direct consequence of the intermittent hypoxia is an oxidative imbalance, with reactive oxygen species production and the inflammatory cascade’s activation with pro and anti-inflammatory cytokines growth. Tumour necrosis factors, inflammatory cytokines (IL2, IL4, IL6), lipid peroxidation, and cell-free DNA have been found to increase in OSAS patients. However, even though different risk-related markers have been described and analysed in the literature, it has not yet been clarified whether specified inflammatory bio-markers better correlates with OSAS diagnosis and its clinical evolution/comorbidities. We perform a scientific literature review to discuss inflammatory and oxidative stress biomarkers currently tested in OSAS patients and their correlation with the disease’s severity and treatment.

Obstructive sleep apnoea and cardiovascular disease: a literature review

As obesity becomes more common worldwide, the prevalence of obstructive sleep apnoea (OSA) continues to rise. Obstructive sleep apnoea is a well-known disorder that causes chronic intermittent hypoxia (CIH), which is considered a risk factor for atherosclerosis directly and indirectly. Ischaemic heart disease remains the leading cause of death. Most risk factors for atherosclerosis are well understood. However, other factors such as CIH are less well understood. Several studies have investigated the pathophysiology of CIH, attempting to uncover its link to atherosclerosis and to determine whether OSA treatment can be a therapeutic modality to modify the risk for atherosclerosis. In this article, we will review the pathophysiology of OSA as an independent risk factor for cardiovascular disease and discuss the most common markers that have been studied. We will also examine the potential impact of OSA management as a risk factor modifier on the reversibility of atherosclerosis.

Effects of Normoxic Recovery on Intima-Media Thickness of Aorta and Pulmonary Artery Following Intermittent Hypoxia in Mice

Obstructive sleep apnea (OSA) patients are at risk for increased blood pressure and carotid intima-media thickness (IMT), with pulmonary hypertension and right-sided heart failure potentially developing as well. Chronic intermittent hypoxia (IH) has been used as an OSA model in animals, but its effects on vascular beds have not been evaluated using objective unbiased tools. Previously published and current experimental data in mice exposed to IH were evaluated for IMT in aorta and pulmonary artery (PA) after IH with or without normoxic recovery using software for meta-analysis, Review Manager 5. Because IMT data reports on PA were extremely scarce, atherosclerotic area percentage from lumen data was also evaluated. IH significantly increased IMT parameters in both aorta and PA as illustrated by Forest plots (P < 0.01), which also confirmed that IMT values after normoxic recovery were within the normal range in both vascular beds. One-sided scarce lower areas in Funnel Plots were seen for both aorta and PA indicating the likelihood of significant publication bias. Forest and Funnel plots, which provide unbiased assessments of published and current data, suggest that IH exposures may induce IMT thickening that may be reversed by normoxic recovery in both aorta and PA. In light of the potential likelihood of publication bias, future studies are needed to confirm or refute the findings. In conclusion, OSA may induce IMT thickening (e.g., aorta and/or PA), but the treatment (e.g., nasal continuous positive airway pressure) will likely lead to improvements in such findings.

The role of reactive oxygen species in cognitive impairment associated with sleep apnea

Obstructive sleep apnea (OSA), a common breathing and sleeping disorder, is associated with a broad range of neurocognitive difficulties. Intermittent hypoxia (IH), one major characteristic of OSA, has been shown to impair learning and memory due to increased levels of reactive oxygen species (ROS). Under normal conditions, ROS are produced in low concentrations and act as signaling molecules in different processes. However, IH treatment leads to elevated ROS production via multiple pathways, including mitochondrial electron transport chain dysfunction and in particular complex I dysfunction, and induces oxidative tissue damage. Moreover, elevated ROS results in the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) and increased activity of peroxisomes, such as NADPH oxidase, xanthine oxidase and phospholipase A2. Furthermore, oxidative tissue damage has been found in regions of the brains of patients with OSA, including the cortex and hippocampus, which are associated with memory and executive function. Furthermore, increased ROS levels in these regions of the brain induce damage via inflammation, apoptosis, ER stress and neuronal activity disturbance. The present review focuses on the mechanism of excessive ROS production in an OSA model and the relationship between ROS and cognitive impairment.

The dorsal and the ventral side of hypoglossal motor nucleus showed different response to chronic intermittent hypoxia in rats

PURPOSE

To study neurochemical reactions to chronic intermittent hypoxia (CIH) in the hypoglossal nucleus (HN) of rats.

METHODS

Adult male Sprague-Dawley rats (n = 12) were randomly divided into two groups (the CIH and the control group). The CIH rats were housed in a hypoxic chamber with the fraction of oxygen volume alternating between 21% and 5% by providing air for 60 s and then providing nitrogen for 60 s from 8:30 am to 16:30 pm each day for 35 days. The control group was housed in a cabin with normal oxygen levels. We studied the expression of c-fos protein, 5-hydroxytryptamine (5-HT) positive terminals, and its 2A receptors in hypoglossal nuclei by immunohistochemistry.

RESULTS

The expression of c-fos, 5-HT positive terminals, and accordingly 5-HT 2A receptors in the CIH group were significantly higher than that in the controls (p < 0.05). The ventral side of the HN showed a clearly higher expression of 5-HT and its 2A receptors than the dorsal side (p < 0.05).

CONCLUSION

There were 2 responses of the HN to CIH. First, CIH induced a higher expression of 5-HT positive terminals and its 2A receptors, and second, this reaction was much more evident in ventral side than in the dorsal side. We postulate that these responses may serve to be a protective and compensatory mechanism for CIH.

Connexins and Atrial Fibrillation in Obstructive Sleep Apnea

PURPOSE OF THE REVIEW

To summarize the potential interactions between obstructive sleep apnea (OSA), atrial fibrillation (AF), and connexins.

RECENT FINDINGS

OSA is highly prevalent in patients with cardiovascular disease, and is associated with increased risk for end-organ substantial morbidities linked to autonomic nervous system imbalance, increased oxidative stress and inflammation, ultimately leading to reduced life expectancy. Epidemiological studies indicate that OSA is associated with increased incidence and progression of coronary heart disease, heart failure, stroke, as well as arrhythmias, particularly AF. Conversely, AF is very common among subjects referred for suspected OSA, and the prevalence of AF increases with OSA severity. The interrelationships between AF and OSA along with the well-known epidemiological links between these two conditions and obesity may reflect shared pathophysiological pathways, which may depend on the intercellular diffusion of signaling molecules into either the extracellular space or require cell-to-cell contact. Connexin signaling is accomplished via direct exchanges of cytosolic molecules between adjacent cells at gap membrane junctions for cell-to-cell coupling. The role of connexins in AF is now quite well established, but the impact of OSA on cardiac connexins has only recently begun to be investigated. Understanding the biology and regulatory mechanisms of connexins in OSA at the transcriptional, translational, and post-translational levels will undoubtedly require major efforts to decipher the breadth and complexity of connexin functions in OSA-induced AF.

SUMMARY

The risk of end-organ morbidities has initiated the search for circulating mechanistic biomarker signatures and the implementation of biomarker-based algorithms for precision-based diagnosis and risk assessment. Here we summarize recent findings in OSA as they relate to AF risk, and also review potential mechanisms linking OSA, AF and connexins.

Low but not high frequency of intermittent hypoxia suppresses endothelium-dependent, oxidative stress-mediated contractions in carotid arteries of obese mice

Obstructive sleep apnea is characterized by intermittent hypoxia (IH) during sleep and predisposes to endothelial dysfunction. Obesity is a major risk factor for the occurrence of sleep apnea. The present study compared the functional impact of low- (IH10; 10 hypoxic events/h) and high-frequency (IH60; 60 hypoxic events/h) IH for 4 wk on endothelial function in male C57BL/6 mice with or without high-fat (HF) diet-induced obesity. Mean arterial blood pressure (tail cuff method) was increased in obese mice after IH60 exposure, i.e., HF + IH60 group. The serum levels of the oxidative stress marker malondialdehyde were augmented in lean IH60 and HF groups, with a further increase in HF + IH60 but a reduction in HF + IH10 mice compared with the HF group. Vascular responsiveness was assessed as changes in isometric tension in isolated arteries. Relaxations to the endothelium-dependent vasodilator acetylcholine were impaired in HF + IH60 aortae. Endothelium-dependent contractions (EDC; response to acetylcholine in the presence of the nitric oxide synthase inhibitor l-NAME) in carotid arteries were augmented in the HF group, but this HF-induced augmentation was suppressed by low-frequency IH exposure. The addition of apocynin (antioxidant) reduced EDC in HF and HF + IH60 groups but not in HF + IH10 group. In conclusion, these findings suggest that exposure of obese mice to mild IH exerts preconditioning-like suppression of endothelium-dependent and oxidative stress-mediated contractions. When IH severity increases, this suppression diminishes and endothelial dysfunction accelerates. NEW & NOTEWORTHY The present study demonstrates, for the first time, that low-frequency intermittent hypoxia may exert a preconditioning-like suppression of oxidative stress-induced endothelium-dependent contractions in mice with diet-induced obesity. This relative suppression was diminished as intermittent hypoxia became more severe, and a deleterious effect on endothelial function emerged.

Effect of Gender on Chronic Intermittent Hypoxic Fosb Expression in Cardiorespiratory-Related Brain Structures in Mice

We aimed to delineate sex-based differences in neuroplasticity that may be associated with previously reported sex-based differences in physiological alterations caused by repetitive succession of hypoxemia-reoxygenation encountered during obstructive sleep apnea (OSA). We examined long-term changes in the activity of brainstem and diencephalic cardiorespiratory neuronal populations induced by chronic intermittent hypoxia (CIH) in male and female mice by analyzing Fosb expression. Whereas the overall baseline and CIH-induced Fosb expression in females was higher than in males, possibly reflecting different neuroplastic dynamics, in contrast, structures responded to CIH by Fosb upregulation in males only. There was a sex-based difference at the level of the rostral ventrolateral reticular nucleus of the medulla, with an increase in the number of FOSB/ΔFOSB-positive cells induced by CIH in males but not females. This structure contains neurons that generate the sympathetic tone and which are involved in CIH-induced sustained hypertension during waking hours. We suggest that the sex-based difference in neuroplasticity of this structure contributes to the reported sex-based difference in CIH-induced hypertension. Moreover, we highlighted a sex-based dimorphic phenomenon in serotoninergic systems induced by CIH, with increased serotoninergic immunoreactivity in the hypoglossal nucleus and a decreased number of serotoninergic cells in the dorsal raphe nucleus in male but not female mice. We suggest that this dimorphism in the neuroplasticity of serotoninergic systems predisposes males to a greater alteration of neuronal control of the upper respiratory tract associated with the greater collapsibility of upper airways described in male OSA subjects.

Which place of pharmacological approaches beyond continuous positive airway pressure to treat vascular disease related to obstructive sleep apnea?

Obstructive sleep apnea (OSA) is characterized by recurrent episodes of partial or complete upper airway obstruction, occurring during sleep, leading to chronic intermittent hypoxia (IH), which harms the cardiovascular system. OSA is associated with both functional and structural vascular alterations that contribute to an increased prevalence of fatal and non-fatal cardiovascular events. OSA is a heterogeneous disease with respect to the severity of hypoxia, the presence of daytime symptoms, obesity, and cardiovascular comorbidities. Various clusters of OSA phenotypes have been described leading to more highly personalized treatment. The aim of this review is to describe the various therapeutic strategies including continuous positive airway pressure (CPAP), oral appliances, surgery, weight loss, and especially pharmacological interventions that have been evaluated to reduce vascular alterations in both OSA patients and preclinical animal models. Conventional therapies, predominantly CPAP, have a limited impact on vascular alterations in the presence of co-morbidities. A better knowledge of pharmacological therapies targeting IH-induced vascular alterations will facilitate the use of combined therapies and is crucial for designing clinical trials in well-defined OSA phenotypes.

Chronological Change of Right Ventricle by Chronic Intermittent Hypoxia in Mice

Study Objective

No studies have investigated sequential changes in the heart on magnetic resonance imaging (MRI), along with observation of functional lung phenotypes and genetics, over the duration of chronic intermittent hypoxia (CIH). We investigated chronological changes in heart and lung phenotypes after CIH using a mouse model to provide new insights into the pathophysiology of sleep apnea-induced cardiovascular disease.

Methods

C57BL/6J adult male mice were randomized to 4 or 8 weeks of CIH. Cardiac cine-MRI images were analyzed to assess functional parameters of right ventricle (RV). Histopathological features of myocytes and pulmonary vessels, as well as genes involved in the endothelin (ET) system, were investigated.

Results

Function of the RV reduced significantly at 4 weeks and continuously decreased following another 4 weeks of CIH, although the rate of decrease was attenuated. Notably, persistence of reduced ejection fraction and end-systole RV wall thickness (WT) and increases in the ET system of the lungs and blood strongly implied the development of pulmonary hypertension after 8 weeks of CIH.

Conclusions

RV dysfunction with reduced end-systole RV WT could be a late phenotype in long-standing CIH and possibly also in obstructive sleep apnea.

Sleep apnea: An overlooked cause of lipotoxicity?

Obstructive sleep apnea (OSA) is a common sleep disorder associated with diabetes and cardiovascular disease. However, the mechanisms by which OSA causes cardiometabolic dysfunction are not fully elucidated. OSA increases plasma free fatty acids (FFA) during sleep, reflecting excessive adipose tissue lipolysis. In animal studies, intermittent hypoxia simulating OSA also increases FFA, and the increase is attenuated by beta-adrenergic blockade. In other contexts, excessive plasma FFA can lead to ectopic fat accumulation, insulin resistance, vascular dysfunction, and dyslipidemia. Herein, we propose that OSA is a cause of excessive adipose tissue lipolysis contributing towards systemic "lipotoxicity". Since visceral and upper-body obesity contributes to OSA pathogenesis, OSA-induced lipolysis may further aggravate the consequences of this metabolically harmful state. If this hypothesis is correct, then OSA may represent a reversible risk factor for cardio-metabolic dysfunction, and this risk might be mitigated by preventing OSA-induced lipolysis during sleep.

Aorta macrophage inflammatory and epigenetic changes in a murine model of obstructive sleep apnea: Potential role of CD36

Obstructive sleep apnea (OSA) affects 8-10% of the population, is characterized by chronic intermittent hypoxia (CIH), and causally associates with cardiovascular morbidities. In CIH-exposed mice, closely mimicking the chronicity of human OSA, increased accumulation and proliferation of pro-inflammatory metabolic M1-like macrophages highly expressing CD36, emerged in aorta. Transcriptomic and MeDIP-seq approaches identified activation of pro-atherogenic pathways involving a complex interplay of histone modifications in functionally-relevant biological pathways, such as inflammation and oxidative stress in aorta macrophages. Discontinuation of CIH did not elicit significant improvements in aorta wall macrophage phenotype. However, CIH-induced aorta changes were absent in CD36 knockout mice, Our results provide mechanistic insights showing that CIH exposures during sleep in absence of concurrent pro-atherogenic settings (i.e., genetic propensity or dietary manipulation) lead to the recruitment of CD36(+)^high macrophages to the aortic wall and trigger atherogenesis. Furthermore, long-term CIH-induced changes may not be reversible with usual OSA treatment.

Perivascular Mast Cells Govern Shear Stress-Induced Arteriogenesis by Orchestrating Leukocyte Function

The body has the capacity to compensate for an occluded artery by creating a natural bypass upon increased fluid shear stress. How this mechanical force is translated into collateral artery growth (arteriogenesis) is unresolved. We show that extravasation of neutrophils mediated by the platelet receptor GPIbα and uPA results in Nox2-derived reactive oxygen radicals, which activate perivascular mast cells. These c-kit(+)/CXCR-4(+) cells stimulate arteriogenesis by recruiting additional neutrophils as well as growth-promoting monocytes and T cells. Additionally, mast cells may directly contribute to vascular remodeling and vascular cell proliferation through increased MMP activity and by supplying growth-promoting factors. Boosting mast cell recruitment and activation effectively promotes arteriogenesis, thereby protecting tissue from severe ischemic damage. We thus find that perivascular mast cells are central regulators of shear stress-induced arteriogenesis by orchestrating leukocyte function and growth factor/cytokine release, thus providing a therapeutic target for treatment of vascular occlusive diseases.

Targeting the ROS-HIF-1-endothelin axis as a therapeutic approach for the treatment of obstructive sleep apnea-related cardiovascular complications

Obstructive sleep apnea (OSA) is now recognized as an independent and important risk factor for cardiovascular diseases such as hypertension, coronary heart disease, heart failure and stroke. Clinical and experimental data have confirmed that intermittent hypoxia is a major contributor to these deleterious consequences. The repetitive occurrence of hypoxia-reoxygenation sequences generates significant amounts of free radicals, particularly in moderate to severe OSA patients. Moreover, in addition to hypoxia, reactive oxygen species (ROS) are potential inducers of the hypoxia inducible transcription factor-1 (HIF-1) that promotes the transcription of numerous adaptive genes some of which being deleterious for the cardiovascular system, such as the endothelin-1 gene. This review will focus on the involvement of the ROS-HIF-1-endothelin signaling pathway in OSA and intermittent hypoxia and discuss current and potential therapeutic approaches targeting this pathway to treat or prevent cardiovascular disease in moderate to severe OSA patients.

Chronic intermittent hypoxia induces cardiac hypertrophy by impairing autophagy through the adenosine 5'-monophosphate-activated protein kinase pathway

Autophagy is tightly regulated to maintain cardiac homeostasis. Impaired autophagy is closely associated with pathological cardiac hypertrophy. However, the relationship between autophagy and cardiac hypertrophy induced by chronic intermittent hypoxia (CIH) is not known. In the present study, we measured autophagy-related genes and autophagosomes during 10 weeks of CIH in rats, and 6 days in H9C2 cardiomyocytes, and showed that autophagy was impaired. This conclusion was confirmed by the autophagy flux assay. We detected significant hypertrophic changes in myocardium with impaired autophagy. Rapamycin, an autophagy enhancer, attenuated the cardiac hypertrophy induced by CIH. Moreover, silencing autophagy-related gene 5 (ATG5) exerted the opposite effect. The role of adenosine monophosphate-activated protein kinase (AMPK) in regulating autophagy under CIH was confirmed using AICAR to upregulate this enzyme and restore autophagy flux. Restoring autophagy by AICAR or rapamycin significantly reversed the hypertrophic changes in cardiomyocytes. To investigate the mechanism of autophagy impairment, we compared phospho (p)-AMPK, p-Akt, cathepsin D, and NFAT3 levels, along with calcineurin activity, between sham and CIH groups. CIH activated calcineurin, and inhibited AMPK and AMPK-mediated autophagy in an Akt- and NFAT3-independent manner. Collectively, these data demonstrated that impaired autophagy induced by CIH through the AMPK pathway contributed to cardiac hypertrophy.

Inflammatory Role of ROS-Sensitive AMP-Activated Protein Kinase in the Hypersensitivity of Lung Vagal C Fibers Induced by Intermittent Hypoxia in Rats

Obstructive sleep apnea (OSA), manifested by airway exposure to intermittent hypoxia (IH), is associated with excess reactive oxygen species (ROS) production in airways, airway inflammation, and hyperreactive airway diseases. The cause-effect relationship for these events remains unclear. We investigated the inflammatory role of ROS-sensitive AMP-activated protein kinase (AMPK) in IH-induced airway hypersensitivity mediated by lung vagal C fibers (LVCFs) in rats. Conscious rats were exposed to room air (RA) or IH with or without treatment with N-acetyl-L-cysteine (NAC, an antioxidant), Compound C (an AMPK inhibitor), ibuprofen (a cyclooxygenase inhibitor), or their vehicles. Immediately after exposure (24 h), we found that intravenous capsaicin, phenylbiguanide, or α,β-methylene-ATP evoked augmented LVCF-mediated apneic responses and LVCF afferent responses in rats subjected to IH exposure in comparison with those in RA rats. The potentiating effect of IH on LVCF responses decreased at 6 h after and vanished at 12 h after the termination of IH exposure. The potentiating effect of IH on LVCF-mediated apneic and LVCF afferent responses was significantly attenuated by treatment with NAC, compound C, or ibuprofen, but not by their vehicles. Further biochemical analysis revealed that rats exposed to IH displayed increased lung levels of lipid peroxidation (an index of oxidative stress), AMPK phosphorylation (an index of AMPK activation), and prostaglandin E₂ (a cyclooxygenase metabolite), compared with those exposed to RA. IH-induced increase in lipid peroxidation was considerably suppressed by treatment with NAC but not by compound C or ibuprofen. IH-induced increase in AMPK phosphorylation was totally abolished by NAC or compound C but not by ibuprofen. IH-induced increase in prostaglandin E₂ was considerably prevented by any of these three inhibitor treatments. The vehicles of these inhibitors exerted no significant effect on the three IH-induced responses. These results suggest that 24-h IH exposure sensitizes LVCFs, leading to an exaggerated reflex and afferent responses to chemical stimulants in rats. Moreover, this IH-induced LVCF sensitization is mediated through a cascade of inflammatory responses in the airways involving increases in ROS, AMPK activation, and cyclooxygenase metabolite release.

NADPH Oxidase-Derived ROS Induced by Chronic Intermittent Hypoxia Mediates Hypersensitivity of Lung Vagal C Fibers in Rats

Obstructive sleep apnea (OSA), manifested by exposure to chronic intermittent hypoxia (CIH) and excess production of reactive oxygen species (ROS) in the airways, is associated with hyperreactive airway diseases. ROS, particularly when created by NADPH oxidase, are known to sensitize lung vagal C fibers (LVCFs), which may contribute to airway hypersensitivity pathogenesis. We investigated whether CIH augments the reflex and afferent responses of LVCFs to chemical stimulants and the roles of ROS and NADPH oxidase in such airway hypersensitivity. Rats were exposed to room air (RA) or CIH with/without daily treatment with MnTMPyP (a superoxide anion scavenger), apocynin (an NADPH oxidase inhibitor), or vehicle. At 16 h after their last exposure, intravenous capsaicin, adenosine, or α,β-methylene-ATP evoked an augmented apneic response in anesthetized rats with 14-days CIH exposure, compared to anesthetized rats with 14-days RA exposure. The augmented apneic responses to these LVCF stimulants were abolished by bilateral vagotomy or perivagal capsaicin treatment, which block LVCFs neural conduction and were significantly suppressed by treatment with MnTMPyP or apocynin, but not vehicle. Electrophysiological studies revealed that 14-days CIH exposure potentiated the responses of LVCFs to these stimulants. This effect was inhibited by treatment with MnTMPyP or apocynin treatment and was not seen in rats who received 7-days of CIH exposure. Biochemical analysis indicated that 14-days CIH exposure increased both lung lipid peroxidation, which is indicative of oxidative stress, and expression of the p47^phox subunit in the membrane fraction of lung tissue, which is an index of NADPH oxidase activation. The former was prevented by treatment with either MnTMPyP or apocynin, while the later was prevented by treatment with apocynin only. These results suggest that 14-days CIH exposure sensitizes LVCFs in rats, leading to an exaggerated reflex and afferent responses to stimulants and that this sensitization is mediated via ROS generated by NADPH oxidase.

Biological plausibility linking sleep apnoea and metabolic dysfunction

Obstructive sleep apnoea (OSA) is a very common disorder that affects 10-25% of the general population. In the past two decades, OSA has emerged as a cardiometabolic risk factor in both paediatric and adult populations. OSA-induced metabolic perturbations include dyslipidaemia, atherogenesis, liver dysfunction and abnormal glucose metabolism. The mainstay of treatment for OSA is adenotonsillectomy in children and continuous positive airway pressure therapy in adults. Although these therapies are effective at resolving the sleep-disordered breathing component of OSA, they do not always produce beneficial effects on metabolic function. Thus, a deeper understanding of the underlying mechanisms by which OSA influences metabolic dysfunction might yield improved therapeutic approaches and outcomes. In this Review, we summarize the evidence obtained from animal models and studies of patients with OSA of potential mechanistic pathways linking the hallmarks of OSA (intermittent hypoxia and sleep fragmentation) with metabolic dysfunction. Special emphasis is given to adipose tissue dysfunction induced by sleep apnoea, which bears a striking resemblance to adipose dysfunction resulting from obesity. In addition, important gaps in current knowledge and promising lines of future investigation are identified.

Apolipoprotein E ɛ2/ɛ3/ɛ4 variant in association with obstructive sleep apnoea and lipid profile: A meta-analysis

Objective

A meta-analysis of the association between haplotypical variants of the apolipoprotein E (APOE) gene (ɛ2/ɛ3/ɛ4) and obstructive sleep apnoea (OSA) risk and changes in lipid profile.

Methods

Electronic databases were searched to retrieve articles that provided data on APOE gene ɛ2/ɛ3/ɛ4 variants in patients with OSA and healthy controls. Data were extracted from eligible articles and statistical analyses were performed.

Results

The meta-analysis included 14 articles involving 19 study populations (3198 patients and 6031 controls). There was no significant association between the presence of the ɛ4 allele and OSA risk. The presence of ɛ4 was associated with significantly increased total cholesterol and decreased high-density lipoprotein cholesterol, compared with ɛ4 allele negative individuals. There was a low probability of publication bias but significant heterogeneity.

Conclusions

There was no association between APOE ɛ2/ɛ3/ɛ4 and OSA susceptibility. The presence of APOE ɛ4 was associated with changes in lipid profile.

HFE p.C282Y gene variant is associated with varicose veins in Russian population

Recently, the association of polymorphism rs1800562 (p.C282Y) in the hemochromatosis (HFE) gene with the increased risk of venous ulceration was shown. We hypothesized that HFE gene polymorphism might be involved not only in ulceration process, but also in susceptibility to primary varicose veins. We genotyped HFE p.C282Y (rs1800562) and p.H63D (rs1799945) variants in patients with primary varicose veins (n = 463) and in the control group (n = 754). In our study, p.282Y variant (rs1800562 A allele) was significantly associated with the risk of varicose veins (OR 1.79, 95 % CI = 1.11-2.89, P = 0.02). A borderline significant reverse association of p.63D variant (rs1799945 G allele) with venous leg ulcer development was revealed in Russians (OR 0.25, 95 % CI = 0.06-1.00, P = 0.05), but not in the meta-analysis (P = 0.56). We conclude that the HFE gene polymorphism can affect the risk of developing primary varicose veins.

Obstructive sleep apnoea syndrome

Obstructive sleep apnoea syndrome (OSAS) is a common clinical condition in which the throat narrows or collapses repeatedly during sleep, causing obstructive sleep apnoea events. The syndrome is particularly prevalent in middle-aged and older adults. The mechanism by which the upper airway collapses is not fully understood but is multifactorial and includes obesity, craniofacial changes, alteration in upper airway muscle function, pharyngeal neuropathy and fluid shift towards the neck. The direct consequences of the collapse are intermittent hypoxia and hypercapnia, recurrent arousals and increase in respiratory efforts, leading to secondary sympathetic activation, oxidative stress and systemic inflammation. Excessive daytime sleepiness is a burden for the majority of patients. OSAS is also associated with cardiovascular co-morbidities, including hypertension, arrhythmias, stroke, coronary heart disease, atherosclerosis and overall increased cardiovascular mortality, as well as metabolic dysfunction. Whether treating sleep apnoea can fully reverse its chronic consequences remains to be established in adequately designed studies. Continuous positive airway pressure (CPAP) is the primary treatment modality in patients with severe OSAS, whereas oral appliances are also widely used in mild to moderate forms. Finally, combining different treatment modalities such as CPAP and weight control is beneficial, but need to be evaluated in randomized controlled trials. For an illustrated summary of this Primer, visit: http://go.nature.com/Lwc6te.

Obstructive Sleep Apnea, Oxidative Stress, and Cardiovascular Disease: Evidence from Human Studies

Obstructive sleep apnea (OSA) is a frequent disease mainly affecting obese people and caused by repetitive collapse of the upper airways during sleep. The increased morbidity and mortality of OSA are mainly thought to be the consequence of its adverse effects on cardiovascular (CV) health. In this context, oxidative stress induced by nocturnal intermittent hypoxia has been identified to play a major role. This is suggested by biomarker studies in OSA patients showing excessively generated reactive oxygen species from leukocytes, reduced plasma levels of nitrite and nitrate, increased lipid peroxidation, and reduced antioxidant capacity. Biopsy studies complement these findings by demonstrating reduced endothelial nitric oxide synthase expression and increased nitrotyrosine immunofluorescence in the vasculature of these patients. Furthermore, oxidative stress in OSA correlates with surrogate markers of CV disease such as endothelial function, intima-media thickness, and high blood pressure. Continuous positive airway pressure therapy reverses oxidative stress in OSA. The same may be true for antioxidants; however, more studies are needed to clarify this issue.

Resveratrol attenuates intermittent hypoxia-induced macrophage migration to visceral white adipose tissue and insulin resistance in male mice

Chronic intermittent hypoxia during sleep (IH), as occurs in sleep apnea, promotes systemic insulin resistance. Resveratrol (Resv) has been reported to ameliorate high-fat diet-induced obesity, inflammation, and insulin resistance. To examine the effect of Resv on IH-induced metabolic dysfunction, male mice were subjected to IH or room air conditions for 8 weeks and treated with either Resv or vehicle (Veh). Fasting plasma levels of glucose, insulin, and leptin were obtained, homeostatic model assessment of insulin resistance index levels were calculated, and insulin sensitivity tests (phosphorylated AKT [also known as protein kinase B]/total AKT) were performed in 2 visceral white adipose tissue (VWAT) depots (epididymal [Epi] and mesenteric [Mes]) along with flow cytometry assessments for VWAT macrophages and phenotypes (M1 and M2). IH-Veh and IH-Resv mice showed initial reductions in food intake with later recovery, with resultant lower body weights after 8 weeks but with IH-Resv showing better increases in body weight vs IH-Veh. IH-Veh and IH-Resv mice exhibited lower fasting glucose levels, but only IH-Veh had increased homeostatic model assessment of insulin resistance index vs all 3 other groups. Leptin levels were preserved in IH-Veh but were significantly lower in IH-Resv. Reduced VWAT phosphorylated-AKT/AKT responses to insulin emerged in both Mes and Epi in IH-Veh but normalized in IH-Resv. Increases total macrophage counts and in M1 to M2 ratios occurred in IH-Veh Mes and Epi compared all other 3 groups. Thus, Resv ameliorates food intake and weight gain during IH exposures and markedly attenuates VWAT inflammation and insulin resistance, thereby providing a potentially useful adjunctive therapy for metabolic morbidity in the context of sleep apnea.

Enhanced carotid body chemosensory activity and the cardiovascular alterations induced by intermittent hypoxia

The carotid body (CB) plays a main role in the maintenance of the oxygen homeostasis. The hypoxic stimulation of the CB increases the chemosensory discharge, which in turn elicits reflex sympathetic, cardiovascular, and ventilatory adjustments. An exacerbate carotid chemosensory activity has been associated with human sympathetic-mediated diseases such as hypertension, insulin resistance, heart failure, and obstructive sleep apnea (OSA). Indeed, the CB chemosensory discharge becomes tonically hypereactive in experimental models of OSA and heart failure. Chronic intermittent hypoxia (CIH), a main feature of OSA, enhances CB chemosensory baseline discharges in normoxia and in response to hypoxia, inducing sympathetic overactivity and hypertension. Oxidative stress, increased levels of ET-1, Angiotensin II and pro-inflammatory cytokines, along with a reduced production of NO in the CB, have been associated with the enhanced carotid chemosensory activity. In this review, we will discuss new evidence supporting a main role for the CB chemoreceptor in the autonomic and cardiorespiratory alterations induced by intermittent hypoxia, as well as the molecular mechanisms involved in the CB chemosensory potentiation.

Nox family NADPH oxidases: Molecular mechanisms of activation

NADPH oxidases of the Nox family are important enzymatic sources of reactive oxygen species (ROS). Numerous homologue-specific mechanisms control the activity of this enzyme family involving calcium, free fatty acids, protein-protein interactions, intracellular trafficking, and posttranslational modifications such as phosphorylation, acetylation, or sumoylation. After a brief review on the classic pathways of Nox activation, this article will focus on novel mechanisms of homologue-specific activity control and on cell-specific aspects which govern Nox activity. From these findings of the recent years it must be concluded that the activity control of Nox enzymes is much more complex than anticipated. Moreover, depending on the cellular activity state, Nox enzymes are selectively activated or inactivated. The complex upstream signaling aspects of these events make the development of "intelligent" Nox inhibitors plausible, which selectively attenuate disease-related Nox-mediated ROS formation without altering physiological signaling ROS. This approach might be of relevance for Nox-mediated tissue injury in ischemia-reperfusion and inflammation and also for chronic Nox overactivation as present in cancer initiation and cardiovascular disease.

The efficacy of antihypertensive drugs in chronic intermittent hypoxia conditions

Sleep apnea/hypopnea disorders include centrally originated diseases and obstructive sleep apnea (OSA). This last condition is renowned as a frequent secondary cause of hypertension (HT). The mechanisms involved in the pathogenesis of HT can be summarized in relation to two main pathways: sympathetic nervous system stimulation mediated mainly by activation of carotid body (CB) chemoreflexes and/or asphyxia, and, by no means the least important, the systemic effects of chronic intermittent hypoxia (CIH). The use of animal models has revealed that CIH is the critical stimulus underlying sympathetic activity and hypertension, and that this effect requires the presence of functional arterial chemoreceptors, which are hyperactive in CIH. These models of CIH mimic the HT observed in humans and allow the study of CIH independently without the mechanical obstruction component. The effect of continuous positive airway pressure (CPAP), the gold standard treatment for OSA patients, to reduce blood pressure seems to be modest and concomitant antihypertensive therapy is still required. We focus this review on the efficacy of pharmacological interventions to revert HT associated with CIH conditions in both animal models and humans. First, we explore the experimental animal models, developed to mimic HT related to CIH, which have been used to investigate the effect of antihypertensive drugs (AHDs). Second, we review what is known about drug efficacy to reverse HT induced by CIH in animals. Moreover, findings in humans with OSA are cited to demonstrate the lack of strong evidence for the establishment of a first-line antihypertensive regimen for these patients. Indeed, specific therapeutic guidelines for the pharmacological treatment of HT in these patients are still lacking. Finally, we discuss the future perspectives concerning the non-pharmacological and pharmacological management of this particular type of HT.