Vascular effects of intermittent hypoxia

Chronic Intermittent Hypoxia Enhances Pathological Tau Seeding, Propagation, and Accumulation and Exacerbates Alzheimer-like Memory and Synaptic Plasticity Deficits and Molecular Signatures

BACKGROUND

Obstructive sleep apnea, characterized by sleep fragmentation and chronic intermittent hypoxia (CIH), is a risk factor for Alzheimer's disease (AD) progression. Recent epidemiological studies point to CIH as the best predictor of developing cognitive decline and AD in older adults with obstructive sleep apnea. However, the precise underlying mechanisms remain unknown. This study was undertaken to evaluate the effect of CIH on pathological human tau seeding, propagation, and accumulation; cognition; synaptic plasticity; neuronal network excitability; and gene expression profiles in a P301S human mutant tau mouse model of AD and related tauopathies.

METHODS

We exposed 4- to 4.5-month-old male P301S and wild-type mice to an 8-week CIH protocol (6-min cycle: 21% O₂ to 8% O₂ to 21% O₂, 80 cycles per 8 hours during daytime) and assessed its effect on tau pathology and various AD-related phenotypic and molecular signatures. Age- and sex-matched P301S and wild-type mice were reared in normoxia (21% O₂) as experimental controls.

RESULTS

CIH significantly enhanced pathological human tau seeding and spread across connected brain circuitry in P301S mice; it also increased phosphorylated tau load. CIH also exacerbated memory and synaptic plasticity deficits in P301S mice. However, CIH had no effect on seizure susceptibility and network hyperexcitability in these mice. Finally, CIH exacerbated AD-related pathogenic molecular signaling in P301S mice.

CONCLUSIONS

CIH-induced increase in pathologic human tau seeding and spread and exacerbation of other AD-related impairments provide new insights into the role of CIH and obstructive sleep apnea in AD pathogenesis.

Airway hyperresponsiveness induced by intermittent hypoxia in rats

This study investigated whether intermittent hypoxia (IH) induces airway hyperresponsiveness (AHR) and associated with lung inflammation. Male Brown Norway rats were exposed to 14-day IH or room air (RA) for 6 h/day. One day after the last exposure, total lung resistance to various doses of methacholine was measured as an index of bronchoconstrictive responses. Compared with RA controls, methacholine significantly induced an augmented bronchoconstriction in IH-exposed rats. Moreover, IH exposure evoked lung inflammation which was reflected by increased inflammatory cell infiltration, concentrations of interleukin-6 and prostaglandin E₂ in bronchoalveolar lavage fluid, and lung lipid peroxidation. IH-induced AHR and lung inflammation were completely abolished by daily intraperitoneal injection of N-acetylcysteine (an antioxidant) or ibuprofen (a cyclooxygenase inhibitor), but not by apocynin (an inhibitor of NADPH oxidase) or vehicle. In conclusion, AHR and lung inflammation occur after 14-day IH exposure, with endogenous reactive oxygen species and cyclooxygenase metabolites being responsible for these responses.

Intermittent hypoxia modulates redox homeostasis, lipid metabolism associated inflammatory processes and redox post-translational modifications: Benefits at high altitude

Intermittent hypoxia, initially associated with adverse effects of sleep apnea, has now metamorphosed into a module for improved sports performance. The regimen followed for improved sports performance is milder intermittent hypoxic training (IHT) as compared to chronic and severe intermittent hypoxia observed in sleep apnea. Although several studies have indicated the mechanism and enough data on physiological parameters altered by IH is available, proteome perturbations remain largely unknown. Altitude induced hypobaric hypoxia is known to require acclimatization as it causes systemic redox stress and inflammation in humans. In the present study, a short IHT regimen consisting of previously reported physiologically beneficial FIO2 levels of 13.5% and 12% was administered to human subjects. These subjects were then airlifted to altitude of 3500 m and their plasma proteome along with associated redox parameters were analyzed on days 4 and 7 of high altitude stay. We observed that redox stress and associated post-translational modifications, perturbed lipid metabolism and inflammatory signaling were induced by IHT exposure at Baseline. However, this caused activation of antioxidants, energy homeostasis mechanisms and anti-inflammatory responses during subsequent high-altitude exposure. Thus, we propose IHT as a beneficial non-pharmacological intervention that benefits individuals venturing to high altitude areas.

Sex and age differentially affect GABAergic neurons in the mouse prefrontal cortex and hippocampus following chronic intermittent hypoxia

Obstructive sleep apnea (OSA), a chronic sleep disorder characterized by repetitive reduction or cessation of airflow during sleep, is widely prevalent and is associated with adverse neurocognitive sequelae including increased risk of Alzheimer's disease (AD). In humans, OSA is more common in elderly males. OSA is characterized by sleep fragmentation and chronic intermittent hypoxia (CIH), and recent epidemiological studies point to CIH as the best predictor of neurocognitive sequelae associated with OSA. The sex- and age- specific effects of OSA-associated CIH on specific cell populations such as γ-aminobutyric acid (GABA)-ergic neurons in the hippocampus and the medial prefrontal cortex (mPFC), regions important for cognitive function, remain largely unknown. The present study examined the effect of 35 days of either moderate (10% oxygen) or severe (5% oxygen) CIH on GABAergic neurons in the mPFC and hippocampus of young and aged male and female mice as well as post-accelerated ovarian failure (AOF) female mice. In the mPFC and hippocampus, the number of GABA-labeled neurons increased in aged and young severe CIH males compared to controls but not in young moderate CIH males. This change was not representative of the individual GABAergic cell subpopulations, as the number of parvalbumin-labeled neurons decreased while the number of somatostatin-labeled neurons increased in the hippocampus of severe CIH young males only. In all female groups, the number of GABA-labeled cells was not different between CIH and controls. However, in the mPFC, CIH increased the number of parvalbumin-labeled neurons in young females and the number of somatostatin-labeled cells in AOF females but decreased the number of somatostatin-labeled cells in aged females. In the hippocampus, CIH decreased the number of somatostatin-labeled neurons in young females. CIH decreased the density of vesicular GABA transporter in the mPFC of AOF females only. These findings suggest sex-specific changes in GABAergic neurons in the hippocampus and mPFC with males showing an increase of this cell population as compared to their female counterparts following CIH. Age at exposure and severity of CIH also differentially affect the GABAergic cell population in mice.

Proliferative Pathways of Vascular Smooth Muscle Cells in Response to Intermittent Hypoxia

Obstructive sleep apnea (OSA) is characterized by intermittent hypoxia (IH) and is a risk factor for cardiovascular diseases (e.g., atherosclerosis) and chronic inflammatory diseases (CID). The excessive proliferation of vascular smooth muscle cells (VSMCs) plays a pivotal role in the progression of atherosclerosis. Hypoxia-inducible factor-1 and nuclear factor-κB are thought to be the main factors involved in responses to IH and in regulating adaptations or inflammation pathways, however, further evidence is needed to demonstrate the underlying mechanisms of this process in VSMCs. Furthermore, few studies of IH have examined smooth muscle cell responses. Our previous studies demonstrated that increased interleukin (IL)-6, epidermal growth factor family ligands, and erbB2 receptor, some of which amplify inflammation and, consequently, induce CID, were induced by IH and were involved in the proliferation of VSMCs. Since IH increased IL-6 and epiregulin expression in VSMCs, the same phenomenon may also occur in other smooth muscle cells, and, consequently, may be related to the incidence or progression of several diseases. In the present review, we describe how IH can induce the excessive proliferation of VSMCs and we develop the suggestion that other CID may be related to the effects of IH on other smooth muscle cells.

Is Aberrant Reno-Renal Reflex Control of Blood Pressure a Contributor to Chronic Intermittent Hypoxia-Induced Hypertension?

Renal sensory nerves are important in the regulation of body fluid and electrolyte homeostasis, and blood pressure. Activation of renal mechanoreceptor afferents triggers a negative feedback reno-renal reflex that leads to the inhibition of sympathetic nervous outflow. Conversely, activation of renal chemoreceptor afferents elicits reflex sympathoexcitation. Dysregulation of reno-renal reflexes by suppression of the inhibitory reflex and/or activation of the excitatory reflex impairs blood pressure control, predisposing to hypertension. Obstructive sleep apnoea syndrome (OSAS) is causally related to hypertension. Renal denervation in patients with OSAS or in experimental models of chronic intermittent hypoxia (CIH), a cardinal feature of OSAS due to recurrent apnoeas (pauses in breathing), results in a decrease in circulating norepinephrine levels and attenuation of hypertension. The mechanism of the beneficial effect of renal denervation on blood pressure control in models of CIH and OSAS is not fully understood, since renal denervation interrupts renal afferent signaling to the brain and sympathetic efferent signals to the kidneys. Herein, we consider the currently proposed mechanisms involved in the development of hypertension in CIH disease models with a focus on oxidative and inflammatory mediators in the kidneys and their potential influence on renal afferent control of blood pressure, with wider consideration of the evidence available from a variety of hypertension models. We draw focus to the potential contribution of aberrant renal afferent signaling in the development, maintenance and progression of high blood pressure, which may have relevance to CIH-induced hypertension.

Murine models of sleep apnea: functional implications of altered macrophage polarity and epigenetic modifications in adipose and vascular tissues

Obstructive sleep apnea (OSA) is a highly prevalent disease across the lifespan, is characterized by chronic intermittent hypoxia and sleep fragmentation, and has been independently associated with substantial cardiometabolic morbidity. However, the reversibility of end-organ morbidity with treatment is not always apparent, suggesting that both tissue remodeling and epigenetic mechanisms may be operationally involved. Here, we review the cumulative evidence focused around murine models of OSA to illustrate the temporal dependencies of cardiometabolic dysfunction and its reversibility, and more particularly to discuss the critical contributions of tissue macrophages to adipose tissue insulin resistance and vascular atherogenesis. In addition, we describe initial findings potentially implicating epigenetic alterations in both the emergence of the cardiometabolic morbidity of OSA, and in its reversibility with treatment. We anticipate that improved understanding of macrophage biology and epigenetics in the context of intermittent hypoxia and sleep fragmentation will lead to discovery of novel therapeutic targets and improved cardiovascular and metabolic outcomes in OSA.

Prediction Model for Hemorrhagic Complications after Laparoscopic Sleeve Gastrectomy: Development of SLEEVE BLEED Calculator

Introduction

Laparoscopic sleeve gastrectomy (LSG) is one of the most frequently performed bariatric procedures. Hemorrhagic complications (HC) after surgery are common and require surgical revision. Accurate estimation of the risk of postoperative HC can improve surgical decision-making process and minimize the risk of reoperation. The aim of the present study was to develop a predictive model for HC after LSG.

Material and Methods

The retrospective analysis of 522 patients after primary LSG was performed. Patients underwent surgery from January 2013 to February 2015. The primary outcome was defined as a surgical revision due to hemorrhagic complications. Multiple regression analysis was performed.

Results

The rate of hemorrhagic complications was 4 %. The mean age of patients was 41.0 (±11.6) years and mean BMI was 47.3 (±7.3) kg/m². Of the 12 examined variables, four were associated with risk of HC. Protective factors for HC were no history of obstructive sleep apnea (odds ratio [OR] 0.22; 95 % CI 0.05–0.94) and no history of hypertension (OR 0.38; 95 % CI 0.14–1.05). The low level of expertise in bariatric surgery (OR 2.85; 95 % CI 1.08–7.53) and no staple line reinforcement (OR 3.34; 95 % CI 1.21–9.21) were associated with higher risk of HC.

Conclusions

The result revealed the association between hemorrhagic complications and the following factors: obstructive sleep apnea, hypertension, level of expertise in bariatric surgery, and reinforcement of the staple line. The risk assessment model for hemorrhagic complications after LSG can contribute to surgical decision-making process.

Heme oxygenase-1-dependent central cardiorespiratory adaptations to chronic intermittent hypoxia in mice

Chronic intermittent hypoxia (CIH) increases sympathetic tone and respiratory instability. Our previous work showed that chronic hypoxia induces the oxygen-sensing enzyme heme oxygenase-1 (HO-1) within the C1 sympathoexcitatory region and the pre-Bötzinger complex (pre-BötC). We therefore examined the effect of CIH on time course of induced expression of HO-1 within these regions and determined whether the induction of HO-1 correlated with changes in respiratory, sigh frequency, and sympathetic responses (spectral analysis of heart rate) to acute hypoxia (10% O₂) during 10 days of exposure to CIH in chronically instrumented awake wild-type (WT) and HO-1 null mice (HO-1^-/-). HO-1 was induced within the C1 and pre-BötC regions after 1 day of CIH. There were no significant differences in the baseline respiratory parameters between WT and HO-1^-/- Prior to CIH, acute hypoxia increased respiratory frequency in both WT and HO-1^-/-; however, minute diaphragm electromyogram activity increased in WT but not HO-1^-/- The hypoxic respiratory response after 1 and 10 days of CIH was restored in HO-1^-/- CIH resulted in an initial significant decline in 1) the hypoxic sigh frequency response, which was restored in WT but not HO-1^-/-, and 2) the baseline sympathetic activity in WT and HO-1^-/-, which remained stable subsequently in WT but not in HO-1^-/- We conclude that 1) CIH induces expression of HO-1 in the C1 and pre-BötC regions within 1 day and 2) HO-1 is necessary for hypoxia respiratory response and contributes to the maintenance of the hypoxic sigh responses and baseline sympathetic activity during CIH.

Endothelin-1 mediates intermittent hypoxia-induced inflammatory vascular remodeling through HIF-1 activation

Obstructive sleep apnea (OSA) is a major risk factor for cardiovascular mortality, and apnea-induced intermittent hypoxia (IH) is known to promote various cardiovascular alterations such as vascular remodeling. However, the mechanisms that underlie IH remain incompletely investigated. We previously demonstrated that the hypoxia-inducible factor-1 (HIF-1) and endothelin-1 (ET-1) are involved in arterial hypertension and myocardial susceptibility to infarction induced by IH. Thus the objective of the present study was to investigate whether both ET-1 and HIF-1 were also involved in the vascular inflammatory remodeling induced by IH. Mice partially deficient for the Hif1α gene (HIF-1α(+/-)) and their wild-type equivalents, as well as C57BL/6J mice, treated or not with bosentan, a dual endothelin receptor antagonist, were exposed to IH or normoxia for 2 wk, 8 h/day. Splenocyte proliferative and secretory capacities, aortic nuclear factor-κB (NF-κB) and HIF-1 activities, and expression of cytokines and intima-media thickness (IMT) were measured. IH induced a systemic and aortic inflammation characterized by an increase in splenocyte proliferative and secretory capacities, aortic NF-κB activity, and cytokine expression in the aortic wall. This was accompanied by an increase in IMT. These modifications were prevented in HIF-1α(+/-) and bosentan-treated mice. The results of this study suggest that ET-1 is a major contributor to the vascular inflammatory remodeling induced by OSA-related IH, probably through HIF-1-dependent activation of NF-κB.

Intermittent hypoxia in rats reduces activation of Ca2+ sparks in mesenteric arteries

Ca(+) sparks are vascular smooth muscle cell (VSMC) Ca(2+)-release events that are mediated by ryanodine receptors (RyR) and promote vasodilation by activating large-conductance Ca(2+)-activated potassium channels and inhibiting myogenic tone. We have previously reported that exposing rats to intermittent hypoxia (IH) to simulate sleep apnea augments myogenic tone in mesenteric arteries through loss of hydrogen sulfide (H2S)-induced dilation. Because we also observed that H2S can increase Ca(2+) spark activity, we hypothesized that loss of H2S after IH exposure reduces Ca(2+) spark activity and that blocking Ca(2+) spark generation reduces H2S-induced dilation. Ca(2+) spark activity was lower in VSMC of arteries from IH compared with sham-exposed rats. Furthermore, depolarizing VSMC by increasing luminal pressure (from 20 to 100 mmHg) or by elevating extracellular [K(+)] increased spark activity in VSMC of arteries from sham rats but had no effect in arteries from IH rats. Inhibiting endogenous H2S production in sham arteries prevented these increases. NaHS or phosphodiesterase inhibition increased spark activity to the same extent in sham and IH arteries. Depolarization-induced increases in Ca(2+) spark activity were due to increased sparks per site, whereas H2S increases in spark activity were due to increased spark sites per cell. Finally, inhibiting Ca(2+) spark activity with ryanodine (10 μM) enhanced myogenic tone in arteries from sham but not IH rats and blocked dilation to exogenous H2S in arteries from both sham and IH rats. Our results suggest that H2S regulates RyR activation and that H2S-induced dilation requires Ca(2+) spark activation. IH exposure decreases endogenous H2S-dependent Ca(2+) spark activation to cause membrane depolarization and enhance myogenic tone in mesenteric arteries.

Toll-like receptor-4 mediated inflammation is involved in the cardiometabolic alterations induced by intermittent hypoxia

Objective. Intermittent hypoxia (IH) is a major component of sleep apnea syndrome as its cardiometabolic complications have been mainly attributed to IH. The pathophysiology is still poorly understood but there are some similarities with the obesity-associated cardiometabolic complications. As the latter results from inflammation involving toll-like receptor-4 (TLR4) signaling, we assessed this pathway in the cardiometabolic consequences of IH. Methods. Lean adult male TLR4-deficient (TLR4^−/−) mice and their controls (C57BL/6 mice) were exposed to either IH (FiO₂ 21-5%, 1 min cycle, 8 h/day) or air (normoxic mice) for 4 weeks. Animals were assessed at 1-week exposure for insulin tolerance test and after 4-week exposure for morphological and inflammatory changes of the epididymal fat and thoracic aorta. Results. IH induced insulin resistance, morphological and inflammatory changes of the epididymal fat (smaller pads and adipocytes, higher release of TNF-α and IL-6) and aorta (larger intima-media thickness and higher NFκB-p50 activity). All these alterations were prevented by TLR4 deletion. Conclusion. IH induces metabolic and vascular alterations that involve TLR4 mediated inflammation. These results confirm the important role of inflammation in the cardiometabolic consequences of IH and suggest that targeting TLR4/NFκB pathway could represent a further therapeutic option for sleep apnea patients.

Inflammation in sleep apnea: an update

Obstructive sleep apnea (OSA) is a common disorder associated with cardiovascular disease (CVD). One theory to explain this relationship proposes that OSA can induce systemic inflammation, thereby inducing CVD. This theory is based on the premise that obesity is a pro-inflammatory state, and that physiological derangements during sleep in subjects with OSA further aggravate inflammation. In support of this theory, some clinical studies have shown elevated inflammatory biomarkers in OSA subjects, or improvement in these markers following treatment of OSA. However, the data are inconsistent and often confounded by the effects of comorbid obesity. Animal models of OSA have been developed, which involve exposure of rodents or cells to intermittent hypoxia, a hallmark feature of OSA. Several of these experiments demonstrate that intermittent hypoxia can stimulate inflammatory pathways and lead to cardiovascular or metabolic pathology. In this review, we review relationships between OSA and inflammation, with particular attention to studies published within the last year.

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.

New frontiers in obstructive sleep apnoea

OSA (obstructive sleep apnoea), the most common respiratory disorder of sleep, is caused by the loss of upper airway dilating muscle activity during sleep superimposed on a narrow upper airway. This results in recurrent nocturnal asphyxia. Termination of these events usually requires arousal from sleep and results in sleep fragmentation and hypoxaemia, which leads to poor quality sleep, excessive daytime sleepiness, reduced quality of life and numerous other serious health consequences. Furthermore, patients with untreated sleep apnoea are at an increased risk of hypertension, stroke, heart failure and atrial fibrillation. Although there are many predisposing risk factors for OSA, including male gender, endocrine disorders, use of muscle relaxants, smoking, fluid retention and increased age, the strongest risk factor is obesity. The aim of the present review is to focus on three cutting-edge topics with respect to OSA. The section on animal models covers various strategies used to simulate the physiology or the effects of OSA in animals, and how these have helped to understand some of the underlying mechanisms of OSA. The section on diabetes discusses current evidence in both humans and animal models demonstrating that intermittent hypoxia and sleep fragmentation has a negative impact on glucose tolerance. Finally, the section on cardiovascular biomarkers reviews the evidence supporting the use of these biomarkers to both measure some of the negative consequences of OSA, as well as the potential benefits of OSA therapies.

Chronic intermittent hypoxia increases sympathetic control of blood pressure: role of neuronal activity in the hypothalamic paraventricular nucleus

Like humans with sleep apnea, rats exposed to chronic intermittent hypoxia (CIH) experience arterial hypoxemias and develop hypertension characterized by exaggerated sympathetic nerve activity (SNA). To gain insights into the poorly understood mechanisms that initiate sleep apnea/CIH-associated hypertension, experiments were performed in rats exposed to CIH for only 7 days. Compared with sham-treated normoxic control rats, CIH-exposed rats (n = 8 rats/group) had significantly increased hematocrit (P < 0.001) and mean arterial pressure (MAP; P < 0.05). Blockade of ganglionic transmission caused a significantly (P < 0.05) greater reduction of MAP in rats exposed to CIH than control rats (n = 8 rats/group), indicating a greater contribution of SNA in the support of MAP even at this early stage of CIH hypertension. Chemical inhibition of neuronal discharge in the hypothalamic paraventricular nucleus (PVN) (100 pmol muscimol) had no effect on renal SNA but reduced lumbar SNA (P < 0.005) and MAP (P < 0.05) more in CIH-exposed rats (n = 8) than control rats (n = 7), indicating that CIH increased the contribution of PVN neuronal activity in the support of lumbar SNA and MAP. Because CIH activates brain regions controlling body fluid homeostasis, the effects of internal carotid artery injection of hypertonic saline were tested and determined to increase lumbar SNA more (P < 0.05) in CIH-exposed rats than in control rats (n = 9 rats/group). We conclude that neurogenic mechanisms are activated early in the development of CIH hypertension such that elevated MAP relies on increased sympathetic tonus and ongoing PVN neuronal activity. The increased sensitivity of Na(+)/osmosensitive circuitry in CIH-exposed rats suggests that early neuroadaptive responses among body fluid regulatory neurons could contribute to the initiation of CIH hypertension.

Animal models of sleep disorders

Problems with sleep affect a large part of the general population, with more than half of all people in the United States reporting difficulties with sleep or insufficient sleep at various times and about 40 million affected chronically. Sleep is a complex physiologic process that is influenced by many internal and environmental factors, and problems with sleep are often related to specific personal circumstances or are based on subjective reports from the affected person. Although human subjects are used widely in the study of sleep and sleep disorders, the study of animals has been invaluable in developing our understanding about the physiology of sleep and the underlying mechanisms of sleep disorders. Historically, the use of animals for the study of sleep disorders has arguably been most fruitful for the condition of narcolepsy, in which studies of dogs and mice revealed previously unsuspected mechanisms for this condition. The current overview considers animal models that have been used to study 4 of the most common human sleep disorders-insomnia, narcolepsy, restless legs syndrome, and sleep apnea-and summarizes considerations relevant to the use of animals for the study of sleep and sleep disorders. Animal-based research has been vital to the elucidation of mechanisms that underlie sleep, its regulation, and its disorders and undoubtedly will remain crucial for discovering and validating sleep mechanisms and testing interventions for sleep disorders.

Endothelin 1-dependent neurovascular dysfunction in chronic intermittent hypoxia

Obstructive sleep apnea, a condition resulting in chronic intermittent hypoxia (CIH), is an independent risk factor for stroke and dementia, but the mechanisms of the effect are unknown. We tested the hypothesis that CIH increases cerebrovascular risk by altering critical mechanisms regulating cerebral blood flow thereby lowering cerebrovascular reserves. Male C57Bl6/J mice were subjected to CIH (10% O(2) for 90 seconds/room air for 90 seconds; during sleep hours) or sham treatment for 35 days. Somatosensory cortex blood flow was assessed by laser Doppler flowmetry in anesthetized mice equipped with a cranial window. CIH increased mean arterial pressure (from 74±2 to 83±3 mm Hg; P<0.05) and attenuated the blood flow increase produced by neural activity (whisker stimulation; -39±2%; P<0.05) or neocortical application of endothelium-dependent vasodilators (acetylcholine response: -41±3%; P<0.05). The cerebrovascular dysfunction was associated with oxidative stress in cerebral resistance arterioles and was abrogated by free radical scavenging or NADPH oxidase inhibition. Furthermore, cerebrovascular dysfunction and free radical increase were not observed in mice lacking the NOX2 subunit of NADPH oxidase. CIH markedly increased endothelin 1 in cerebral blood vessels, whereas cerebrovascular dysfunction and oxidative stress were abrogated by neocortical application of the endothelin type A receptor antagonist BQ123. These data demonstrate for the first time that CIH alters key regulatory mechanisms of the cerebral circulation through endothelin 1 and NADPH oxidase-derived radicals. The ensuing cerebrovascular dysfunction may increase stroke risk in patients with sleep apnea by reducing cerebrovascular reserves and increasing the brain's susceptibility to cerebral ischemia.

Sleep is associated with the metabolic syndrome in a multi-ethnic cohort of midlife women: the SWAN Sleep Study

STUDY OBJECTIVES

We evaluated associations among subjective and objective measures of sleep and the metabolic syndrome in a multi-ethnic sample of midlife women.

DESIGN

Cross-sectional study.

SETTING

Participants' homes.

PARTICIPANTS

Caucasian (n = 158), African American (n = 125), and Chinese women (n = 57); mean age = 51 years. Age range = 46-57 years.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Metabolic syndrome was measured in the clinic and sleep quality was assessed by self-report. Indices of sleep duration, continuity/fragmentation, depth, and sleep disordered breathing were assessed by in-home polysomnography (PSG). Covariates included sociodemographics, menopausal status, use of medications that affect sleep, and self-reported health complaints and health behaviors known to influence metabolic syndrome risk. Logistic regression was used to test the hypothesis that the metabolic syndrome would be associated with increased subjective sleep complaints and PSG-assessed sleep disturbances. In univariate analyses, the metabolic syndrome was associated with decreased sleep duration and efficiency and increased NREM beta power and apnea-hypopnea index (AHI). After covariate adjustment, sleep efficiency (odds ratio [OR] = 2.06, 95% confidence interval [CI]: 1.08-3.93), NREM beta power (OR = 2.09, 95% CI: 1.09-3.98), and AHI (OR = 1.86, 95% CI: 1.40-2.48) remained significantly associated with the metabolic syndrome (odds ratio values are expressed in standard deviation units). These relationships did not differ by race.

CONCLUSIONS

Objective indices of sleep continuity, depth, and sleep disordered breathing are significant correlates of the metabolic syndrome in midlife women, independent of race, menopausal status and other factors that might otherwise account for these relationships.

Qidantongmai protects endothelial cells against hypoxia-induced damage through regulating the serum VEGF-a level

Qidantongmai (QDTM) is a Traditional Chinese Medicine (TCM) preparation that has long been used in folk medicine for the treatment of cardiovascular diseases. However, the underlying mechanisms are poorly understood. The present study was designed to determine the effects of QDTM on endothelial cells under hypoxic conditions both in vitro and in vivo. Primary human umbilical vein endothelial cells (HUVECs) were isolated, pretreated with QDTM medicated serum or saline control, and then cultured under hypoxia (2% oxygen) for 24 h. Sprague-Dawley rats were administered 1 ml/100 g of QDTM or saline twice a day for 4 days and treated with hypoxia (6 hours/day, discontinuous hypoxia, 360 mm Hg). QDTM not only protected HUVECs from hypoxia-induced damage by significantly retaining cell viability (P < 0.05) and decreasing apoptosis (P < 0.05) in vitro, but also protected liver endothelial cells from hypoxia-induced damage in vivo. Moreover, QDTM increased the serum VEGF-A level (P < 0.05) in rats treated with hypoxia for 7 days but suppressed the upregulation of serum VEGF-A in rats treated with hypoxia for 14 days. QDTM is a potent preparation that can protect endothelial cells against hypoxia-induced damage. The ability of QDTM to modulate the serum VEGF-A level may play an important role in its effects on endothelial cells.

Cardiovascular consequences of sleep apnea

Sleep apnea is a common health concern that is characterized by repetitive episodes of asphyxia. This condition has been linked to serious long-term adverse effects such as hypertension, metabolic dysregulation, and cardiovascular disease. Although the mechanism for the initiation and aggravation of cardiovascular disease has not been fully elucidated, oxidative stress and subsequent endothelial dysfunction play major roles. Animal models, which have the advantage of being free of comorbidities and/or behavioral variables (that commonly occur in humans), allow invasive measurements under well-controlled experimental conditions, and as such are useful tools in the study of the pathophysiological mechanisms of sleep apnea. This review summarizes currently available information on the cardiovascular consequences of sleep apnea and briefly describes common experimental approaches useful to sleep apnea in different animal models.

Effects of sleep apnea on nocturnal free fatty acids in subjects with heart failure

STUDY OBJECTIVES

Sleep apnea is common in patients with congestive heart failure, and may contribute to the progression of underlying heart disease. Cardiovascular and metabolic complications of sleep apnea have been attributed to intermittent hypoxia. Elevated free fatty acids (FFA) are also associated with the progression of metabolic, vascular, and cardiac dysfunction. The objective of this study was to determine the effect of intermittent hypoxia on FFA levels during sleep in patients with heart failure.

DESIGN AND INTERVENTIONS

During sleep, frequent blood samples were examined for FFA in patients with stable heart failure (ejection fraction < 40%). In patients with severe sleep apnea (apnea-hypopnea index = 65.5 ± 9.1 events/h; average low SpO₂ = 88.9%), FFA levels were compared to controls with milder sleep apnea (apnea-hypopnea index = 15.4 ± 3.7 events/h; average low SpO₂ = 93.6%). In patients with severe sleep apnea, supplemental oxygen at 2-4 liters/min was administered on a subsequent night to eliminate hypoxemia.

MEASUREMENTS AND RESULTS

Prior to sleep onset, controls and patients with severe apnea exhibited a similar FFA level. After sleep onset, patients with severe sleep apnea exhibited a marked and rapid increase in FFA relative to control subjects. This increase persisted throughout NREM and REM sleep exceeding serum FFA levels in control subjects by 0.134 mmol/L (P = 0.0038). Supplemental oxygen normalized the FFA profile without affecting sleep architecture or respiratory arousal frequency.

CONCLUSION

In patients with heart failure, severe sleep apnea causes surges in nocturnal FFA that may contribute to the accelerated progression of underlying heart disease. Supplemental oxygen prevents the FFA elevation.

Intermittent hypoxia conditioning prevents endothelial dysfunction and improves nitric oxide storage in spontaneously hypertensive rats

Although intermittent hypoxia is often associated with hypertension, experimental and clinical studies have demonstrated definite antihypertensive effects of some intermittent hypoxia conditioning (IHC) regimens. Mechanisms of this antihypertensive response are unknown. Endothelial dysfunction related to disturbed synthesis and/or reduced availability of nitric oxide (NO) has been linked to hypertension. Thus, experiments were conducted to determine if IHC can improve endothelium-dependent relaxation and formation of releasable vascular NO stores of young (4-8-week-old) spontaneously hypertensive rats (SHR). Rats were subjected to either IHC (9.5-10% O(2), 5-10 min, 5-8 times per day, 20 d) or to sham conditioning. Endothelium-dependent relaxation to acetylcholine was measured in norepinephrine-precontracted, isolated aortic rings, and the size of NO stores was evaluated by percent relaxation to N-acetylcysteine (NAC), which releases stored NO. The capacity of aortic rings for NO storage was evaluated by the relaxation to NAC after prior incubation with an NO donor. IHC significantly suppressed the development of hypertension in young SHR. Endothelial function decreased from 54.7 ± 4.6% to 28.1 ± 6.4% relaxation to acetylcholine after 20 d of sham IHC, whereas endothelial function was sustained (60.3 ± 6.0% relaxation) in IHC rats. IHC also induced formation of available NO stores and enhanced the capacity of aortic rings to store NO. Therefore, the antihypertensive effect of IHC in young SHR is associated with prevention of endothelial dysfunction and with increased accumulation of NO stores in vascular walls.

Chronic intermittent hypoxia induces NMDA receptor-dependent plasticity and suppresses nitric oxide signaling in the mouse hypothalamic paraventricular nucleus

Chronic intermittent hypoxia (CIH) is a concomitant of sleep apnea that produces a slowly developing chemosensory-dependent blood pressure elevation ascribed in part to NMDA receptor-dependent plasticity and reduced nitric oxide (NO) signaling in the carotid body. The hypothalamic paraventricular nucleus (PVN) is responsive to hypoxic stress and also contains neurons that express NMDA receptors and neuronal nitric oxide synthase (nNOS). We tested the hypothesis that extended (35 d) CIH results in a decrease in the surface/synaptic availability of the essential NMDA NR1 subunit in nNOS-containing neurons and NMDA-induced NO production in the PVN of mice. As compared with controls, the 35 d CIH-exposed mice showed a significant increase in blood pressure and an increased density of NR1 immunogold particles located in the cytoplasm of nNOS-containing dendrites. Neither of these between-group differences was seen after 14 d, even though there was already a reduction in the NR1 plasmalemmal density at this time point. Patch-clamp recording of PVN neurons in slices showed a significant reduction in NMDA currents after either 14 or 35 d exposure to CIH compared with sham controls. In contrast, NO production, as measured by the NO-sensitive fluorescent dye 4-amino-5-methylamino-2',7'-difluorofluorescein, was suppressed only in the 35 d CIH group. We conclude that CIH produces a reduction in the surface/synaptic targeting of NR1 in nNOS neurons and decreases NMDA receptor-mediated currents in the PVN before the emergence of hypertension, the development of which may be enabled by suppression of NO signaling in this brain region.

Mechanisms of intermittent hypoxia induced hypertension

Exposing rodents to brief episodes of hypoxia mimics the hypoxemia and the cardiovascular and metabolic effects observed in patients with obstructive sleep apnoea (OSA), a condition that affects between 5% and 20% of the population. Apart from daytime sleepiness, OSA is associated with a high incidence of systemic and pulmonary hypertension, peripheral vascular disease, stroke and sudden cardiac death. The development of animal models to study sleep apnoea has provided convincing evidence that recurrent exposure to intermittent hypoxia (IH) has significant vascular and haemodynamic impact that explain much of the cardiovascular morbidity and mortality observed in patients with sleep apnoea. However, the molecular and cellular mechanisms of how IH causes these changes is unclear and under investigation. This review focuses on the most recent findings addressing these mechanisms. It includes a discussion of the contribution of the nervous system, circulating and vascular factors, inflammatory mediators and transcription factors to IH-induced cardiovascular disease. It also highlights the importance of reactive oxygen species as a primary mediator of the systemic and pulmonary hypertension that develops in response to exposure to IH.