Circulating endothelin-1 in obstructive sleep apnea

Endothelin-1 receptor blockade does not alter the sympathetic and hemodynamic response to acute intermittent hypoxia in men

Repeat exposures to low oxygen (intermittent hypoxia, IH), like that observed in sleep apnea, elicit increases in muscle sympathetic nerve activity (MSNA) and blood pressure (BP) in men. Endothelin (ET) receptor antagonists can attenuate the sympathetic and BP response to IH in rodents; whether these data translate to humans are unclear. We hypothesized that ET-receptor antagonism would ameliorate any rise in MSNA and BP following acute IH in humans. Twelve healthy men (31 ± 1 yr) completed two visits (control, bosentan) separated by at least 1 wk. MSNA, BP, and baroreflex sensitivity (modified Oxford) were assessed during normoxic rest before and following 30 min of IH. The midpoint (T50) for each individual's baroreflex curve was calculated. Acute IH increased plasma ET-1 (P < 0.01), MSNA burst frequency (P = 0.03), and mean BP (P < 0.01). There was no effect of IH on baroreflex sensitivity (P = 0.46), although an increase in T50 was observed (P < 0.01). MSNA burst frequency was higher (P = 0.04) and mean BP (P < 0.01) was lower following bosentan treatment compared with control. There was no effect of bosentan on baroreflex sensitivity (P = 0.53), although a lower T50 was observed on the bosentan visit (P < 0.01). There was no effect of bosentan on increases in MSNA (P = 0.81) or mean BP (P = 0.12) following acute IH. Acute IH results in an increase in ET-1, MSNA, and BP in healthy young men. The effect of IH on MSNA and BP is not attenuated following ET-receptor inhibition. Present data suggest that acute IH does not increase MSNA or BP through activation of ET-receptors in healthy young men.NEW & NOTEWORTHY Repeat exposures to low oxygen (intermittent hypoxia, IH) elicit increases in muscle sympathetic nerve activity (MSNA) and blood pressure (BP) in men. Endothelin (ET) receptor antagonists can attenuate the sympathetic and BP response to IH in rodents; whether these data translate to humans were unclear. We show acute IH results in an increase in ET-1, MSNA, and BP in healthy young men; however, the effect of IH on MSNA and BP does not occur through activation of ET-receptors in healthy young men.

Endothelin-1 as a novel target for the prevention of metabolic dysfunction with intermittent hypoxia in male participants

We examined the effect of intermittent hypoxia (IH, a hallmark feature of sleep apnea) on adipose tissue lipolysis and the role of endothelin-1 (ET-1) in this response. We hypothesized that IH can increase ET-1 secretion and plasma free fatty acid (FFA) concentrations. We further hypothesized that inhibition of ET-1 receptor activation with bosentan could prevent any IH-mediated increase in FFA. To test this hypothesis, 16 healthy male participants (32 ± 5 yr, 26 ± 2 kg/m2) were exposed to 30 min of IH in the absence (control) and presence of bosentan (62.5 mg oral twice daily for 3 days prior). Arterial blood samples for ET-1, epinephrine, and FFA concentrations, as well as abdominal subcutaneous adipose tissue biopsies (to assess transcription of cellular receptors/proteins involved in lipolysis), were collected. Additional proof-of-concept studies were conducted in vitro using primary differentiated human white preadipocytes (HWPs). We show that IH increased circulating ET-1, epinephrine, and FFA (P < 0.05). Bosentan treatment reduced plasma epinephrine concentrations and blunted IH-mediated increases in FFA (P < 0.01). In adipose tissue, bosentan had no effect on cellular receptors and proteins involved in lipolysis (P > 0.05). ET-1 treatment did not directly induce lipolysis in differentiated HWP. In conclusion, IH increases plasma ET-1 and FFA concentrations. Inhibition of ET-1 receptors with bosentan attenuates the FFA increase in response to IH. Based on a lack of a direct effect of ET-1 in HWP, we speculate the effect of bosentan on circulating FFA in vivo may be secondary to its ability to reduce sympathoadrenal tone.

Sleep Apnea and Hypertension

Obstructive sleep apnea is a frequent finding in clinical practice especially with the obesity epidemic and the growing awareness of sleep-disordered breathing as a potential and treatable risk factor for cardiovascular diseases. It frequently coexists undiagnosed activating pathophysiological mechanisms known to participate in development and progression of cardiovascular diseases and resistance to therapeutical strategies. The sympathetic activation and the baroreflex and chemoreflex impairment appear to be the main pathophysiological factors that activating several mechanisms elicit cardiac and vascular damage. Data from cross-sectional population-based studies, prospective studies and meta-analysis have clearly shown the implication of OSA in the development of the hypertensive state and the benefits obtained by continuous positive airway pressure on daytime blood pressure and cardiovascular risk.

Effect of continuous positive airway pressure on endothelin-1 in patients with obstructive sleep apnea: a meta-analysis

PURPOSE

Obstructive sleep apnea (OSA) is related to endothelin-1 (ET-1). Continuous positive airway pressure (CPAP) is an effective therapy for OSA. However, the effectiveness of CPAP on ET-1 levels in patients with OSA yielded contradictory results. We conducted a meta-analysis to assess the effect of CPAP on ET-1 levels in OSA.

METHODS

The Embase, and Cochrane Library and PubMed were searched before March, 2018. The overall effects were measured by the standardized mean difference (SMD) with a 95% confidence interval (CI). Ten studies were included and the meta-analysis was conducted using Stata 14.0.

RESULTS

10 studies involving 375 patients were included in the meta-analysis. The result showed that there was a significant reduction in ET-1 levels in OSA patients before and after CPAP therapy (SMD = - 0.74, 95% CI = - 1.30 to - 0.17, z = 2.56, p = 0.01). Further, subgroup analysis demonstrated that Apnea-Hypopnea Index (AHI), CPAP therapy duration, and sample size also affected CPAP therapy.

CONCLUSIONS

Our meta-analysis indicated that CPAP treatment among OSA patients was significantly was related to a decrease in ET-1 levels. Further prospective long-term studies with a larger number of patients are needed to evaluate and clarify this issue.

Blockade of Endothelin-1 Receptor Type B Ameliorates Glucose Intolerance and Insulin Resistance in a Mouse Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is associated with insulin resistance (IR) and glucose intolerance. Elevated endothelin-1 (ET-1) levels have been observed in OSA patients and in mice exposed to intermittent hypoxia (IH). We examined whether pharmacological blockade of type A and type B ET-1 receptors (ETA and ETB) would ameliorate glucose intolerance and IR in mice exposed to IH. Subcutaneously implanted pumps delivered BQ-123 (ETA antagonist; 200 nmol/kg/day), BQ-788 (ETB antagonist; 200 nmol/kg/day) or vehicle (saline or propyleneglycol [PG]) for 14 days in C57BL6/J mice (10/group). During treatment, mice were exposed to IH (decreasing the FiO2 from 20.9% to 6%, 60/h) or intermittent air (IA). After IH or IA exposure, insulin (0.5 IU/kg) or glucose (1 mg/kg) was injected intraperitoneally and plasma glucose determined after injection and area under glucose curve (AUC) was calculated. Fourteen-day IH increased fasting glucose levels (122 ± 7 vs. 157 ± 8 mg/dL, PG: 118 ± 6 vs. 139 ± 8; both p < 0.05) and impaired glucose tolerance (AUCglucose: 19,249 ± 1105 vs. 29,124 ± 1444, PG AUCglucose: 18,066 ± 947 vs. 25,135 ± 797; both p < 0.05) in vehicle-treated animals. IH-induced impairments in glucose tolerance were partially ameliorated with BQ-788 treatment (AUCglucose: 21,969 ± 662; p < 0.05). Fourteen-day IH also induced IR (AUCglucose: 7185 ± 401 vs. 8699 ± 401; p < 0.05). Treatment with BQ-788 decreased IR under IA (AUCglucose: 5281 ± 401, p < 0.05) and reduced worsening of IR with IH (AUCglucose: 7302 ± 401, p < 0.05). There was no effect of BQ-123 on IH-induced impairments in glucose tolerance or IR. Our results suggest that ET-1 plays a role in IH-induced impairments in glucose homeostasis.

Interactions between and Shared Molecular Mechanisms of Diabetic Peripheral Neuropathy and Obstructive Sleep Apnea in Type 2 Diabetes Patients

Type 2 diabetes (T2D) accounts for about 90% of all diabetes patients and incurs a heavy global public health burden. Up to 50% of T2D patients will eventually develop neuropathy as T2D progresses. Diabetic peripheral neuropathy (DPN) is a common diabetic complication and one of the main causes of increased morbidity and mortality of T2D patients. Obstructive sleep apnea (OSA) affects over 15% of the general population and is associated with a higher prevalence of T2D. Growing evidence also indicates that OSA is highly prevalent in T2D patients probably due to diabetic peripheral neuropathy. However, the interrelations among diabetic peripheral neuropathy, OSA, and T2D hitherto have not been clearly elucidated. Numerous molecular mechanisms have been documented that underlie diabetic peripheral neuropathy and OSA, including oxidative stress, inflammation, endothelin-1, vascular endothelial growth factor (VEGF), accumulation of advanced glycation end products, protein kinase C (PKC) signaling, poly ADP ribose polymerase (PARP), nitrosative stress, plasminogen activator inhibitor-1, and vitamin D deficiency. In this review, we seek to illuminate the relationships among T2D, diabetic peripheral neuropathy, and OSA and how they interact with one another. In addition, we summarize and explain the shared molecular mechanisms involved in diabetic peripheral neuropathy and OSA for further mechanistic investigations and novel therapeutic strategies for attenuating and preventing the development and progression of diabetic peripheral neuropathy and OSA in T2D.

Endothelin contributes to the blood pressure rise triggered by hypoxia in severe obstructive sleep apnea

BACKGROUND

Obstructive sleep apnea (OSA) is strongly correlated with an increased risk of systemic hypertension. However, the link between systemic hypertension and nocturnal apneas remains incompletely understood. Animal studies suggest an implication of the endothelin system. The aim of the present study is to determine if endogenous endothelin plays a role in the increase in blood pressure observed during hypoxic episodes in OSA patients, in addition to peripheral chemoreflex and neural sympathetic activation.

METHODS

We assessed the effects of the nonspecific endothelin antagonist bosentan (500 mg; Tracleer; Actelion; Basel, Switzerland) on ventilation, hemodynamics, and muscle sympathetic nerve activity (MSNA) during normoxia and isocapnic hypoxia using a randomized, crossover, double-blinded, placebo-controlled study design, and in 13 severely untreated sleep apneic patients (age 50 ± 9 years, apnea-hypopnea index 35 ± 21/h).

RESULTS

Hypoxia increased blood pressure, MSNA, and minute ventilation as oxygen saturation decreased. Bosentan suppressed completely the increase in SBP during a 5-min hypoxic challenge (143 ± 5 mmHg during hypoxia vs. 133 ± 5 mmHg during normoxia with placebo and 127 ± 3 mmHg during hypoxia vs. 125 ± 3 mmHg during normoxia under bosentan, P = 0.023). DBP as well as the rise in MSNA and ventilation during isocapnic hypoxia did not differ between bosentan and placebo.

CONCLUSION

Endothelin contributes to the rise in SBP in response to acute hypoxia in patients with severely untreated OSA. This was not due to lower chemoreflex activation with bosentan.

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.

Peripheral chemoreception and arterial pressure responses to intermittent hypoxia

Carotid bodies are the principal peripheral chemoreceptors for detecting changes in arterial blood oxygen levels, and the resulting chemoreflex is a potent regulator of blood pressure. Recurrent apnea with intermittent hypoxia (IH) is a major clinical problem in adult humans and infants born preterm. Adult patients with recurrent apnea exhibit heightened sympathetic nerve activity and hypertension. Adults born preterm are predisposed to early onset of hypertension. Available evidence suggests that carotid body chemoreflex contributes to hypertension caused by IH in both adults and neonates. Experimental models of IH provided important insights into cellular and molecular mechanisms underlying carotid body chemoreflex-mediated hypertension. This article provides a comprehensive appraisal of how IH affects carotid body function, underlying cellular, molecular, and epigenetic mechanisms, and the contribution of chemoreflex to the hypertension.

Polymorphisms in nitric oxide synthase and endothelin genes among children with obstructive sleep apnea

Background

Obstructive sleep apnea (OSA) is associated with adverse and interdependent cognitive and cardiovascular consequences. Increasing evidence suggests that nitric oxide synthase (NOS) and endothelin family (EDN) genes underlie mechanistic aspects of OSA-associated morbidities. We aimed to identify single nucleotide polymorphisms (SNPs) in the NOS family (3 isoforms), and EDN family (3 isoforms) to identify potential associations of these SNPs in children with OSA.

Methods

A pediatric community cohort (ages 5–10 years) enriched for snoring underwent overnight polysomnographic (NPSG) and a fasting morning blood draw. The diagnostic criteria for OSA were an obstructive apnea-hypopnea Index (AHI) >2/h total sleep time (TST), snoring during the night, and a nadir oxyhemoglobin saturation <92%. Control children were defined as non-snoring children with AHI <2/h TST (NOSA). Endothelial function was assessed using a modified post-occlusive hyperemic test. The time to peak reperfusion (Tmax) was considered as the indicator for normal endothelial function (NEF; Tmax<45 sec), or ED (Tmax≥45 sec). Genomic DNA from peripheral blood was extracted and allelic frequencies were assessed for, NOS1 (209 SNPs), NOS2 (122 SNPs), NOS3 (50 SNPs), EDN1 (43 SNPs), EDN2 (48 SNPs), EDN3 (14 SNPs), endothelin receptor A, EDNRA, (27 SNPs), and endothelin receptor B, EDNRB (23 SNPs) using a custom SNPs array. The relative frequencies of NOS-1,-2, and −3, and EDN-1,-2,-3,-EDNRA, and-EDNRB genotypes were evaluated in 608 subjects [128 with OSA, and 480 without OSA (NOSA)]. Furthermore, subjects with OSA were divided into 2 subgroups: OSA with normal endothelial function (OSA-NEF), and OSA with endothelial dysfunction (OSA-ED). Linkage disequilibrium was analyzed using Haploview version 4.2 software.

Results

For NOSA vs. OSA groups, 15 differentially distributed SNPs for NOS1 gene, and 1 SNP for NOS3 emerged, while 4 SNPs for EDN1 and 1 SNP for both EDN2 and EDN3 were identified. However, in the smaller sub-group for whom endothelial function was available, none of the significant SNPs was retained due to lack of statistical power.

Conclusions

Differences in the distribution of polymorphisms among NOS and EDN gene families suggest that these SNPs could play a contributory role in the pathophysiology and risk of OSA-induced cardiovascular morbidity. Thus, analysis of genotype-phenotype interactions in children with OSA may assist in the formulation of categorical risk estimates.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Role for PKCβ in enhanced endothelin-1-induced pulmonary vasoconstrictor reactivity following intermittent hypoxia

Intermittent hypoxia (IH) resulting from sleep apnea causes both systemic and pulmonary hypertension. Enhanced endothelin-1 (ET-1)-induced vasoconstrictor reactivity is thought to play a central role in the systemic hypertensive response to IH. However, whether IH similarly increases pulmonary vasoreactivity and the signaling mechanisms involved are unknown. The objective of the present study was to test the hypothesis that IH augments ET-1-induced pulmonary vasoconstrictor reactivity through a PKCβ-dependent signaling pathway. Responses to ET-1 were assessed in endothelium-disrupted, pressurized pulmonary arteries (∼150 μm inner diameter) from eucapnic-IH [(E-IH) 3 min cycles, 5% O(2)-5% CO(2)/air flush, 7 h/day; 4 wk] and sham (air-cycled) rats. Arteries were loaded with fura-2 AM to monitor vascular smooth muscle (VSM) intracellular Ca(2+) concentration ([Ca(2+)](i)). E-IH increased vasoconstrictor reactivity without altering Ca(2+) responses, suggestive of myofilament Ca(2+) sensitization. Consistent with our hypothesis, inhibitors of both PKCα/β (myr-PKC) and PKCβ (LY-333-531) selectively decreased vasoconstriction to ET-1 in arteries from E-IH rats and normalized responses between groups, whereas Rho kinase (fasudil) and PKCδ (rottlerin) inhibition were without effect. Although E-IH did not alter arterial PKCα/β mRNA or protein expression, E-IH increased basal PKCβI/II membrane localization and caused ET-1-induced translocation of these isoforms away from the membrane fraction. We conclude that E-IH augments pulmonary vasoconstrictor reactivity to ET-1 through a novel PKCβ-dependent mechanism that is independent of altered PKC expression. These findings provide new insights into signaling mechanisms that contribute to vasoconstriction in the hypertensive pulmonary circulation.

Vascular changes, cardiovascular disease and obstructive sleep apnea

Vascular changes related to obstructive sleep apnea (OSA) can lead to chronic cardiovascular consequences such as hypertension. The cardiovascular consequences are owing to nocturnal perturbations related to intrathoracic pressure changes, intermittent hypoxia, sympathetic neural activation, endothelial dysfunction, oxidative stress and systemic inflammation. Intermittent hypoxia due to sleep-related events in OSA activates the renin-angiotensin system and increases the levels of endothelin-1. Intermittent hypoxia also results in oxidative stress, as evidenced by elevated levels of xanthine oxidoreductase, lipid peroxidation and the presence of reactive oxygen species. There is also evidence for a decrease in antioxidant capacity. Patients with OSA may have endothelial dysfunction that resolves with continuous positive airway pressure. OSA is a state of inflammation as evidenced by elevated levels of C-reactive protein, IL-6, NF-κB, TNF-α, ICAM-1, VCAM-1 and E-selectin. This may suggest that OSA is a predisposing factor for atherogenesis. This article will discuss the role of nocturnal perturbations consequent to OSA resulting in endothelial dysfunction, oxidative stress, and inflammation and how they may subsequently play a causative role in cardiovascular disorders.

Mechanisms of sympathetic activation and blood pressure elevation by intermittent hypoxia

Sleep disordered breathing with recurrent apneas is one of the most frequently encountered breathing disorder in adult humans and preterm infants. Recurrent apnea patients exhibit several co-morbidities including hypertension and persistent sympathetic activation. Intermittent hypoxia (IH) resulting from apneas appears to be the primary stimulus for evoking autonomic changes. The purpose of this article is to briefly review the effects of IH on chemo- and baro-reflexes and circulating vasoactive hormones and their contribution to sympathetic activation and blood pressures. Sleep apnea patients and IH-treated rodents exhibit exaggerated arterial chemo-reflex. Studies on rodent models demonstrated that IH leads to hyperactive carotid body response to hypoxia. On the other hand, baro-reflex function is attenuated in patients with sleep apnea and in IH-treated rodents. Circulating vasoactive hormone levels are elevated in sleep apnea patients and in rodent models of IH. Thus, persistent sympathetic activation and hypertension associated with sleep apneas seems to be due to a combination of altered chemo- and baro-reflexes resulting in sympathetic activation and action of elevated circulating levels of vasoactive hormones on vasculature.

Inflammation accelerates atherosclerotic processes in obstructive sleep apnea syndrome (OSAS)

Obstructive sleep apnea syndrome (OSAS) is an often underestimated sleep disorder that has been associated with cardiovascular disease. OSAS is characterized by cycles of apnea and/or hypopnea during sleep caused by the collapse of the upper airways. Intermittent hypoxia deriving from the cycles of apnea/arousals (to retrieve the ventilation) plays a pivotal role in the pathogenesis of the disease. Obesity is the most frequent predisposing condition of OSAS. Recent evidence suggests that OSAS could be considered as a pro-atherosclerotic disease, independently of visceral fat amount. Oxidative stress, cardiovascular inflammation, endothelial dysfunction, and metabolic abnormalities in OSAS could accelerate atherogenesis. The present review is focused on the possible pathophysiological mediators which could favor atherosclerosis in OSAS.

Retinal vein occlusions: The potential impact of a dysregulation of the retinal veins

A retinal vein occlusion (RVO) is a sight threatening disease. It can be divided into central vein occlusion and branch retinal vein occlusion. The pathogenesis of the condition remains to be solved. Mechanical compression of the vessel wall or thrombotic occlusion of the vessel lumen, sometimes combined with rheological disorders, are often assumed pathomechanisms. Accordingly, the therapy relies either on mechanical decompression, lyses of thrombi or improvement of rheology. A number of observations however, such as the relationship of RVO to atherosclerotic risk factors, spontaneous reversibility particularly in young patients, rest flow observed in angiography, occlusion despite anticoagulation or thrombocytopenia and finally the positive effect of anti-VEGF therapy are not explained by the present pathogenetic concept. As a new concept we propose a local venous constriction induced by vasoconstrictive molecules diffusing from neighbouring diseased arteries and/or from other neighbouring (hypoxic) tissues. Recognizing these postulated conditions might lead to an earlier identification of impending vein occlusions as well as to a treatment more tailored to the risk factor constellation of the particular patient.

NFATc3 contributes to intermittent hypoxia-induced arterial remodeling in mice

Sleep apnea (SA) is defined as intermittent respiratory arrest during sleep and affects up to 20% of the adult population. SA is also associated with an increased incidence of hypertension and peripheral vascular disease. Exposing rodents to intermittent hypoxia during sleep mimics the cyclical hypoxia/normoxia of SA. We have previously shown that in mice and rats intermittent hypoxia induces ET-1 upregulation and systemic hypertension. Furthermore, intermittent hypoxia (IH) in mice increases nuclear factor of activated T cells isoform 3 (NFATc3) transcriptional activity in aorta and mesenteric arteries, whereas the calcineurin/NFAT inhibitor cyclosporin A prevents IH-induced hypertension. More importantly, NFATc3 knockout (KO) mice do not develop IH-induced hypertension. The goals of this study were to determine the role of NFATc3 in IH-induced arterial remodeling and whether IH-induced NFATc3 activation is mediated by ET-1. Oral administration of both a dual (bosentan) and a selective endothelin receptor type A antagonist (PD155080) during 2 days of IH exposure attenuated NFAT activation in aorta and mesenteric arteries. Rho kinase inhibition with fasudil also prevented IH-induced NFAT activation. Mesenteric artery cross-sectional wall thickness was increased by IH in wild-type (WT) and vehicle-treated mice but not in bosentan-treated and NFATc3 KO mice. The arterial remodeling in mesenteric arteries after IH was characterized by increased expression of the hypertrophic NFATc3 target smooth muscle-alpha-actin in WT but not in KO mice. These results indicate that ET-1 is an upstream activator of NFATc3 during intermittent hypoxia, contributing to the resultant hypertension and increased wall thickness.

Pathophysiology of sleep apnea

Sleep-induced apnea and disordered breathing refers to intermittent, cyclical cessations or reductions of airflow, with or without obstructions of the upper airway (OSA). In the presence of an anatomically compromised, collapsible airway, the sleep-induced loss of compensatory tonic input to the upper airway dilator muscle motor neurons leads to collapse of the pharyngeal airway. In turn, the ability of the sleeping subject to compensate for this airway obstruction will determine the degree of cycling of these events. Several of the classic neurotransmitters and a growing list of neuromodulators have now been identified that contribute to neurochemical regulation of pharyngeal motor neuron activity and airway patency. Limited progress has been made in developing pharmacotherapies with acceptable specificity for the treatment of sleep-induced airway obstruction. We review three types of major long-term sequelae to severe OSA that have been assessed in humans through use of continuous positive airway pressure (CPAP) treatment and in animal models via long-term intermittent hypoxemia (IH): 1) cardiovascular. The evidence is strongest to support daytime systemic hypertension as a consequence of severe OSA, with less conclusive effects on pulmonary hypertension, stroke, coronary artery disease, and cardiac arrhythmias. The underlying mechanisms mediating hypertension include enhanced chemoreceptor sensitivity causing excessive daytime sympathetic vasoconstrictor activity, combined with overproduction of superoxide ion and inflammatory effects on resistance vessels. 2) Insulin sensitivity and homeostasis of glucose regulation are negatively impacted by both intermittent hypoxemia and sleep disruption, but whether these influences of OSA are sufficient, independent of obesity, to contribute significantly to the "metabolic syndrome" remains unsettled. 3) Neurocognitive effects include daytime sleepiness and impaired memory and concentration. These effects reflect hypoxic-induced "neural injury." We discuss future research into understanding the pathophysiology of sleep apnea as a basis for uncovering newer forms of treatment of both the ventilatory disorder and its multiple sequelae.

Endothelial function in obstructive sleep apnea

Untreated obstructive sleep apnea (OSA) is an independent risk factor for hypertension, myocardial infarction, and stroke. The repetitive hypoxia/reoxygenation and sleep fragmentation associated with OSA impair endothelial function. Endothelial dysfunction, in turn, may mediate increased risk for cardiovascular diseases. Specifically, in OSA, endothelial nitric oxide availability and repair capacity are reduced, whereas oxidative stress and inflammation are enhanced. Treatment of OSA improves endothelial vasomotor tone and reduces inflammation. We review the evidence and possible mechanisms of endothelial dysfunction as well as the effect of treatment on endothelial function in OSA.

Cardiovascular Changes in Children with Snoring and Obstructive Sleep Apnoea

Introduction: Adults with obstructive sleep apnoea (OSA) are well documented to be at high risk for cardiovascular abnormalities. Growing evidence suggests that OSA is also associated with cardiovascular consequences in children. The purpose of this review is to examine the available data on this association in children. Methods: Primary studies were extracted from a MEDLINE search limited to those published between 1970 and 2008. The keywords used included child, sleep disordered breathing, sleep apnoea, snoring, blood pressure and hearts. The relevant articles were selected by consensus between 2 authors. Results: The results suggested that OSA was consistently associated with hypertension. Meta-analysis of risk of hypertension in those with high apnoea-hypopnoea index was undertaken. A combined odds ratio equal to 3.15 was found (95% confidence interval, 2.01 to 4.93). There was evidence for increased sympathetic activation, decreased arterial distensibility and ventricular hypertrophy in children with OSA. Conclusion: Childhood OSA is associated with blood pressure dysregulation. The association of OSA with other cardiovascular morbidities requires further study in view of the limited data available currently. Key words: Atherosclerosis, Child, Hypertension

Endothelin type A receptor antagonist normalizes blood pressure in rats exposed to eucapnic intermittent hypoxia

We have reported that eucapnic intermittent hypoxia (E-IH) causes systemic hypertension, elevates plasma endothelin 1 (ET-1) levels, and augments vascular reactivity to ET-1 and that a nonspecific ET-1 receptor antagonist acutely lowers blood pressure in E-IH-exposed rats. However, the effect of chronic ET-1 receptor inhibition has not been evaluated, and the ET receptor subtype mediating the vascular effects has not been established. We hypothesized that E-IH causes systemic hypertension through the increased ET-1 activation of vascular ET type A (ET(A)) receptors. We found that mean arterial pressure (MAP) increased after 14 days of 7 h/day E-IH exposure (109 +/- 2 to 137 +/- 4 mmHg; P < 0.005) but did not change in sham-exposed rats. The ET(A) receptor antagonist BQ-123 (10 to 1,000 nmol/kg iv) acutely decreased MAP dose dependently in conscious E-IH but not sham rats, and continuous infusion of BQ-123 (100 nmol.kg(-1).day(-1) sc for 14 days) prevented E-IH-induced increases in MAP. ET-1-induced constriction was augmented in small mesenteric arteries from rats exposed 14 days to E-IH compared with those from sham rats. Constriction was blocked by the ET(A) receptor antagonist BQ-123 (10 microM) but not by the ET type B (ET(B)) receptor antagonist BQ-788 (100 microM). ET(A) receptor mRNA content was greater in renal medulla and coronary arteries from E-IH rats. ET(B) receptor mRNA was not different in any tissues examined, whereas ET-1 mRNA was increased in the heart and in the renal medulla. Thus augmented ET-1-dependent vasoconstriction via vascular ET(A) receptors appears to elevate blood pressure in E-IH-exposed rats.

Oxidative stress in obstructive sleep apnea: putative pathways to the cardiovascular complications

Obstructive sleep apnea (OSA) is a major public health problem because of its high prevalence in morbidity and mortality. A growing body of evidence suggests that OSA is an important risk factor for cardiovascular diseases. Although the mechanism for the initiation and aggravation of cardiovascular disease has not been fully elucidated, one theorized mechanism is intermittent hypoxia, which is produced by each sleep-disordered breathing event. This repeated hypoxia and reoxygenation cycle is similar to hypoxia-reperfusion injury, which initiates oxidative stress. Recent studies have suggested that OSA is associated with increased levels of oxidative stress or antioxidant deficiencies or both. Oxidative stress is involved in the activation of redox-sensitive transcription factors, which regulate downstream products such as inflammatory cytokines, chemokines, and adhesion molecules. This pathway may be able to explain the pathogenesis of atherosclerosis, a common pathologic factor underlying all types of cardiovascular disease. In addition, endothelial dysfunction derived from oxidative stress can contribute to cardiovascular diseases. This review summarizes current available evidence for and against the occurrence of oxidative stress in OSA and discusses the putative pathways initiating cardiovascular consequences associated with OSA.

NFATc3 is required for intermittent hypoxia-induced hypertension

Sleep apnea, defined as intermittent respiratory arrest during sleep, is associated with increased incidence of hypertension and peripheral vascular disease. Exposure of rodents to brief periods of intermittent hypercarbia/hypoxia (H-IH) during sleep mimics the cyclical hypoxia-normoxia of sleep apnea. Endothelin-1, an upstream activator of nuclear factor of activated T cells (NFAT), is increased during H-IH. Therefore, we hypothesized that NFATc3 is activated by H-IH and is required for H-IH-induced hypertension. Consistent with this hypothesis, we found that H-IH (20 brief exposures per hour to 5% O(2)-5% CO(2) for 7 h/day) induces systemic hypertension in mice [mean arterial pressure (MAP) = 97 +/- 2 vs. 124 +/- 2 mmHg, P < 0.05, n = 5] and increases NFATc3 transcriptional activity in aorta and mesenteric arteries. Cyclosporin A, an NFAT inhibitor, and genetic ablation of NFATc3 [NFATc3 knockout (KO)] prevented NFAT activation. More importantly, H-IH-induced hypertension was attenuated in cyclosporin A-treated mice and prevented in NFATc3 KO mice. MAP was significantly elevated in wild-type mice (Delta = 23.5 +/- 6.1 mmHg), but not in KO mice (Delta = -3.9 +/- 5.7). These results indicate that H-IH-induced increases in MAP require NFATc3 and that NFATc3 may contribute to the vascular changes associated with H-IH-induced hypertension.

Abnormally increased nitric oxide synthesis and increased endothelin-1 in plasma in patients with obstructive sleep apnoea

OBJECTIVE

Obstructive sleep apnoea (OSA) is a risk factor for cardiovascular morbidity and mortality. The mechanism is unknown, but endothelial dysfunction might contribute. We tested the hypothesis that patients with OSA have abnormal nitric oxide (NO) synthesis, which results in an abnormal response in blood pressure, renal hemodynamics, renal tubular function and vasoactive hormones in response to inhibition of the NO synthesis with L-NMMA.

MATERIAL AND METHODS

A randomized, placebo-controlled, single-blinded, crossover study was done in 13 OSA patients and 14 controls. We measured changes in systolic and diastolic blood pressure (SBP and DBP), pulse rate, glomerular filtration rate, renal plasma flow, fractional excretion of sodium and lithium and plasma concentrations of vasoactive hormones after injection of L-NMMA and placebo.

RESULTS

L-NMMA induced a significant increase in SBP and DBP in both groups. The placebo-corrected increase in DBP was more pronounced in patients with OSA than in controls (9 (5-11) versus 3 (1-6) mmHg; p=0.01). Endothelin-1 (ET-1) was significantly higher in patients compared with controls (1.1 (1.0-1.3) versus 0.8 (0.7-0.9) pg/mL; p<0.01). In controls, L-NMMA induced a small increase in ET-1, while no changes were seen in patients. The placebo-corrected change in ET-1 was lower in patients compared to controls (-01 (-0.3-0.0) versus 0.1 (0.0-0.1) pg/mL; p=0.04).

CONCLUSIONS

Patients with OSA have abnormally increased NO synthesis and an increased unresponsive level of ET-1 in plasma. This might be due to an imbalance between NO synthesis and ET-1 production.

Eucapnic intermittent hypoxia augments endothelin-1 vasoconstriction in rats: role of PKCdelta

We reported previously that simulating sleep apnea by exposing rats to eucapnic intermittent hypoxia (E-IH) causes endothelin-dependent hypertension and increases constrictor sensitivity to endothelin-1 (ET-1). In addition, augmented ET-1-induced constriction in small mesenteric arteries (sMA) is mediated by increased Ca(2+) sensitization independent of Rho-associated kinase. We hypothesized that exposing rats to E-IH augments ET-1-mediated vasoconstriction by increasing protein kinase C (PKC)-dependent Ca(2+) sensitization. In sMA, the nonselective PKC inhibitor GF-109203x (3 microM) significantly inhibited ET-1-stimulated constriction in E-IH arteries but did not affect ET-1-stimulated constriction in sham arteries. Phospholipase C inhibitor U-73122 (1 microM) also inhibited constriction by ET-1 in E-IH but not sham sMA. In contrast, the classical PKC (cPKC) inhibitor Gö-6976 (1 microM) had no effect on ET-1-mediated vasoconstriction in either group, but a PKCdelta-selective inhibitor (rottlerin, 3 microM) significantly decreased ET-1-mediated constriction in E-IH but not in sham sMA. ET-1 increased PKCdelta phosphorylation in E-IH but not sham sMA. In contrast, ET-1 constriction in thoracic aorta from both sham and E-IH rats was inhibited by Gö-6976 but not by rottlerin. These observations support our hypothesis that E-IH exposure significantly increases ET-1-mediated constriction of sMA through PKCdelta activation and modestly augments ET-1 contraction in thoracic aorta through activation of one or more cPKC isoforms. Therefore, upregulation of a PKC pathway may contribute to elevated ET-1-dependent vascular resistance in this model of hypertension.

Increased erythrocyte adhesiveness and aggregation in obstructive sleep apnea syndrome

BACKGROUND

Obstructive sleep apnea (OSA) is associated with an increased incidence of stroke and myocardial infarction as well as increased prothrombotic and inflammatory processes. Although erythrocyte adhesiveness/aggregation is known to be elevated in cardiovascular diseases, it has never been evaluated in OSA. The aim of this study was to examine the possible association of OSA and erythrocyte adhesiveness/aggregation.

METHODS

The study was conducted in the Sleep Laboratory of a tertiary university-affiliated medical center in 79 patients (age 57.1+/-12.9 years) with a diagnosis of OSA (apnea hypopnea index 41.2+/-25.9). Findings were compared with data on 1079 controls without clinical symptoms of OSA, matched for sex, age, and body mass index. Overnight polysomnography and blood sampling for erythrocyte sedimentation rate, levels of fibrinogen, high-sensitivity C-reactive protein, and erythrocyte adhesion/aggregation test consisting of measures of erythrocyte percentage and vacuum range on image analysis.

RESULTS

The study group had significantly higher values than controls of all three markers of inflammation (p<0.001 for erythrocyte sedimentation rate and fibrinogen; p=0.037 for C-reactive protein). Erythrocyte percentage was significantly lower in the sleep apnea group (84.05+/-15.97 vs. 90.79+/-11.23%, p<0.001), and vacuum range was significantly higher (8.22+/-7.98 vs. 4.63+/-4.05 microm, p<0.001), indicating stronger erythrocyte adhesion/aggregation. Both these factors were significantly correlated with erythrocyte sedimentation rate and to hs-CRP (percentage: r=-0.630; 0.258, p=0.005; 0.031; vacuum range: r=0.494; -0.328, p=0.001; 0.005 respectively).

CONCLUSION

OSA is associated with increased erythrocyte aggregation/adhesion, which is correlated with an increase in inflammation markers. These findings might help explain the increased cardiovascular morbidity in OSA.

Reactive oxygen species contribute to sleep apnea-induced hypertension in rats

In clinical studies, sleep apnea is associated with hypertension, oxidative stress, and increased circulating endothelin-1 (ET-1). We previously developed a model of sleep apnea by exposing rats to eucapnic intermittent hypoxia (IH-C) during sleep, which increases both blood pressure and plasma levels of ET-1. Because similar protocols in mice increase tissue and plasma markers of oxidative stress, we hypothesized that IH-C generation of reactive oxygen species (ROS) contributes to the development of ET-1-dependent hypertension in IH-C rats. To test this, male Sprague-Dawley rats were instrumented with indwelling blood pressure telemeters and drank either plain water or water containing the superoxide dismutase mimetic, Tempol (4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 mM). Mean arterial pressure (MAP) and heart rate (HR) were recorded for 3 control days and 14 treatment days with rats exposed 7 h/day to IH-C or air/air cycling (Sham). On day 14, MAP in IH-C rats treated with Tempol (107 +/- 2.29 mmHg) was significantly lower than in untreated IH-C rats (118 +/- 9 mmHg, P < 0.05). Tempol did not affect blood pressure in sham-operated rats (Tempol = 101 +/- 3, water = 101 +/- 2 mmHg). Immunoreactive ET-1 was greater in plasma from IH-C rats compared with plasma from sham-operated rats but was not different from Sham in Tempol-treated IH-C rats. Small mesenteric arteries from IH-C rats but not Tempol-treated IH-C rats had increased superoxide levels as measured by ferric cytochrome c reduction, lucigenin signaling, and dihydroethidium fluorescence. The data show that IH-C increases ET-1 production and vascular ROS levels and that scavenging superoxide prevents both. Thus oxidative stress appears to contribute to increases in ET-1 production and elevated arterial pressure in this rat model of sleep apnea-induced hypertension.

[Primary open-angle glaucoma and systemic diseases]

The exact pathomechanism of primary open-angle glaucoma (POAG) is still not completely understood. Besides elevated intraocular pressure, which has been identified as a major risk factor, there is mounting evidence for the involvement of systemic factors in the development of glaucomatous damage. Systemic peculiarities described in POAG include cardiovascular, endocrine, neurodegenerative, and sleep alterations. However, some of the studies available on systemic findings in glaucoma patients are contradictory, making further research necessary to identify the exact role of such disturbances in the pathogenesis of the damage. Another difficulty is that many studies are limited by their small sample size, their retrospective nature, and potential selection bias, thus making data interpretation more difficult. Moreover, it is not always clear whether we are dealing with coincidence or a true association between glaucoma and a particular systemic disease. Nevertheless, there is ample evidence for the involvement of vascular factors such as vascular dysregulation and blood pressure in the pathogenesis of POAG.

Altered carotid body function by intermittent hypoxia in neonates and adults: relevance to recurrent apneas

Chronic intermittent hypoxia (CIH) is associated with recurrent apneas in adults and premature infants. It has been proposed that reflexes arising from the carotid bodies contribute to the autonomic abnormalities associated with CIH. The purpose of this review is to summarize recent studies on the effects of CIH on adult and neonatal carotid bodies. CIH exerts two major effects on the adult carotid body that includes sensitization of the hypoxic sensory response and induction of sensory long-term facilitation (LTF). In neonates CIH leads to sensitization of the hypoxic response but does not induce sensory LTF. The effects of CIH on carotid bodies develop over time and could be reversed in adults but not in neonates. CIH-evoked changes in the carotid body involve reactive oxygen species (ROS)-mediated signaling and transcriptional activation by hypoxia-inducible factor-1. Augmented chemoreceptor sensitivity to hypoxia increases the likelihood of unstable breathing perpetuating the effects of CIH, whereas sensory LTF may contribute to increased sympathetic tone and systemic hypertension associated with recurrent apneas.

Sleep and the metabolic syndrome

The metabolic syndrome represents a clustering of several interrelated risk factors of metabolic origin that are thought to increase cardiovascular risk. It is still uncertain whether this clustering results from multiple underlying risk factors or whether it has a single cause. One metabolic abnormality that may underlie several clinical characteristics of the metabolic syndrome is insulin resistance. This review discusses the evidence that sleep disturbances (obstructive sleep apnoea, sleep deprivation and shift work) may independently lead to the development of both insulin resistance and individual clinical components of the metabolic syndrome. The converse may also be true, in that metabolic abnormalities associated with the metabolic syndrome and insulin resistance may potentially exacerbate sleep disorders. The notion that sleep disturbances exert detrimental metabolic effects may help explain the increasing prevalence of the metabolic syndrome and insulin resistance in the general population and may have important implications for population-based approaches to combat the increasing epidemic of metabolic and cardiovascular disease.

Plasma levels of vascular endothelial markers in obstructive sleep apnea

Although the link between obstructive sleep apnea syndrome and risk for cardiovascular disorders has yet to be fully described, the hypothetical involvement of endothelial dysfunction is pathophysiologically plausible. In order to test this hypothesis, we measured plasma levels of endothelial markers in 82 male subjects (41 subjects with obstructive sleep apnea syndrome and a 41-subject control group). Obstructive sleep apnea syndrome patients presented higher circulating levels of intercellular cell adhesion molecule-1, E-selectin, and endothelin-1 than the control group. On the other hand, no differences were found in the von Willebrand factor. Levels of E-selectin and intercellular cell adhesion molecule-1 were significantly correlated to total oxygen desaturation. However, no significant correlation was found in either endothelin-1 or von Willebrand factor. We conclude that obstructive sleep apnea syndrome is associated with changes in levels of adhesion molecules, and that this could be the result of obstructive sleep apnea syndrome-induced hypoxia.

Obstructive sleep apnea and insulin resistance: a role for microcirculation?

Obstructive sleep apnea is an increasingly recognized medical problem. The recent attention to its frequency in the general population and its important role in metabolic, vascular, and behavioral aspects have sharply increased the number and nature of investigations, thereby revealing new aspects that open new approaches in research. Whereas obstructive sleep apnea is a well-known phenomenon accompanying obesity and diabetes, new findings strongly suggest that this close relationship may also operate in the opposite direction. Indeed obstructive sleep apnea may be a primary feature inducing or aggravating a series of vascular and metabolic disturbances closely resembling the metabolic syndrome. This review will discuss established and potential mechanisms responsible for these changes. Obstructive sleep apnea indeed appears to gather all the elements necessary to induce insulin resistance, hypertension, and possibly heart failure. After careful analysis of these modifications and considering how they are intertwined, we propose that microcirculation could represent the common denominator mediating the progression of this pathology, as it is eventually the case in the metabolic syndrome and diabetes domain. This plausible hypothesis is discussed in detail and should be verified by appropriate preclinical and clinical protocols, which are now achievable by using noninvasive techniques in humans.

Obesity-related cardiovascular disease: implications of obstructive sleep apnea

Obesity and obstructive sleep apnea (OSA) often coexist. OSA has been linked to cardiovascular disease. Thus, OSA may contribute to the cardiovascular consequences of obesity. In this review, we explore clinical and pathophysiological interactions between obesity, cardiovascular disease and OSA. We discuss the mechanisms whereby OSA may contribute to hypertension, atherosclerosis, insulin resistance and atrial fibrillation associated with obesity, and emphasize the potential implications for understanding why only a subgroup of obese patients develop cardiovascular disease. Identification of the OSA-dependent and OSA-independent pathways in the cardiovascular pathophysiology of obesity may hold clinical and therapeutic promise.

A Sick Eye in a Sick Body? Systemic Findings in Patients with Primary Open-angle Glaucoma

Despite intense research, the pathogenesis of primary open-angle glaucoma (POAG) is still not completely understood. There is ample evidence for a pathophysiological role of elevated intraocular pressure; however, several systemic factors may influence onset and progression of the disease. Systemic peculiarities found in POAG include alterations of the cardiovascular system, autonomic nervous system, immune system, as well as endocrinological, psychological, and sleep disturbances. An association between POAG and other neurodegenerative diseases, such as Alzheimer disease and Parkinson disease, has also been described. Furthermore, the diagnosis of glaucoma can affect the patient's quality of life. By highlighting the systemic alterations found in POAG, this review attempts to bring glaucoma into a broader medical context.

The future of endothelin-receptor antagonism as treatment for systemic hypertension

Endothelin (ET) is an endogenous peptide secreted predominantly by endothelial cells that mediates its effects via vasoconstriction and hypertrophy of vascular smooth muscle. Because the role of ET has been described in multiple pathologic processes in cardiovascular disease, including hypertension, there has been a strong interest in the development of therapeutic agents that inhibit ET receptors. ET receptor antagonists have shown much promise in disease states such as pulmonary arterial hypertension, essential hypertension, and various forms of secondary hypertension. This review serves to summarize the current role of ET and ET receptor antagonists in both the pathophysiology and the treatment of hypertension.

Obstructive sleep apnea: Plasma endothelin-1 precursor but not endothelin-1 levels are elevated and decline with nasal continuous positive airway pressure

Assessment of plasma endothelin-1 (ET-1) reveals conflicting results in cerebral and noncerebral conditions. Obstructive sleep apnea (OSA) syndrome has been used as a definite challenge for the investigation of endothelin measurements. Despite marked sleep-related breathing disturbances in untreated patients peripherally measurable ET-1 concentrations remained within the normal range and did not change after an appropriate therapy with continuous positive airway pressure (CPAP). In contrast, its precursor, big ET-1, was considerably elevated in untreated patients and dropped to normal values after long-term CPAP depending on compliance. Relatively stable big ET-1 elevations in untreated patients, during sleep and wakefulness, suggest that a general endothelial alteration beyond that explained by a direct impact of nocturnal breathing disturbances on the vascular system occurs. CPAP-therapy effectively lowered plasma big ET-1 in compliant patients and thus possibly their related risk for vascular diseases. Big ET-1 has been demonstrated to be a more appropriate marker of endothelial alteration than ET-1 because of its longer half-life. Simultaneous measurements are to be recommended.

Obstructive Sleep Apnoea, Congestive Heart Failure and Cardiovascular Disease

Sleep disordered breathing is a common condition within the general community. Mostly this is represented by obstructive sleep apnoea (OSA), a condition characterized by repetitive occlusions of the upper airway due to retro-positioning of the tongue and pharyngeal collapse during sleep. This article covers the key evidence relating OSA to both causation and progression of congestive heart failure and cardiovascular disease including hypertension. The results of recent studies are summarized, and the authors conclude that whilst progress has been made, there remain many gaps in our knowledge in relation to the contribution to the burden of cardiac disease produced by associated conditions such as OSA. Larger studies with important primary endpoints will be required to demonstrate the merit of screening and treating this disorder.

Severe upper airway obstruction during sleep

Few disorders may manifest with predominantly sleep-related obstructive breathing. Obstructive sleep apnea (OSA) is a common disorder, varies in severity and is associated with significant cardiovascular and neurocognitive morbidity. It is estimated that between 8 and 18 million people in the United States have at least mild OSA. Although the exact mechanism of OSA is not well-delineated, multiple factors contribute to the development of upper airway obstruction and include anatomic, mechanical, neurologic, and inflammatory changes in the pharynx. OSA may occur concomitantly with asthma. Approximately 74% of asthmatics experience nocturnal symptoms of airflow obstruction secondary to reactive airways disease. Similar cytokine, chemokine, and histologic changes are seen in both disorders. Sleep deprivation, chronic upper airway edema, and inflammation associated with OSA may further exacerbate nocturnal asthma symptoms. Allergic rhinitis may contribute to both OSA and asthma. Continuous positive airway pressure (CPAP) is the gold standard treatment for OSA. Treatment with CPAP therapy has also been shown to improve both daytime and nighttime peak expiratory flow rates in patients with concomitant OSA and asthma. It is important for allergists to be aware of how OSA may complicate diagnosis and treatment of asthma and allergic rhinitis. A thorough sleep history and high clinical suspicion for OSA is indicated, particularly in asthma patients who are refractory to standard medication treatments.

Regulation of catecholamines by sustained and intermittent hypoxia in neuroendocrine cells and sympathetic neurons

Chronic intermittent hypoxia, a characteristic feature of sleep-disordered breathing, induces hypertension through augmented sympathetic nerve activity and requires the presence of functional carotid body arterial chemoreceptors. In contrast, chronic sustained hypoxia does not alter blood pressure. We therefore analyzed the biosynthetic pathways of catecholamines in peripheral nervous system structures involved in the pathogenesis of intermittent hypoxia-induced hypertension, namely, carotid bodies, superior cervical ganglia, and adrenal glands. Rats were exposed to either intermittent hypoxia (90 seconds of room air alternating with 90 seconds of 10% O2) or to sustained hypoxia (10% O2) for 1 to 30 days. Dopamine, norepinephrine, epinephrine, dihydroxyphenylacetic acid, and 5-hydroxytyptamine contents were measured by high-performance liquid chromatography. Expression of tyrosine hydroxylase and its phosphorylated forms, dopamine beta-hydroxylase, phenylethanolamine N-methyltransferase, and GTP cyclohydrolase-1 were determined by Western blot analyses. Both sustained and intermittent hypoxia significantly increased dopamine and norepinephrine content in carotid bodies but not in sympathetic ganglia or adrenal glands. In carotid bodies, both types of hypoxia augmented total levels of tyrosine hydroxylase protein and its phosphorylation on serines 19, 31, 40, as well as levels of GTP cyclohydrolase-1. However, the effects of intermittent hypoxia on catecholaminergic pathways were significantly smaller and delayed than those induced by sustained hypoxia. Thus, attenuated induction of catecholaminergic phenotype by intermittent hypoxia in carotid body may play a role in development of hypertension associated with sleep-disordered breathing. The effects of both types of hypoxia on expression of catecholaminergic enzymes in superior cervical neurons and adrenal glands were transient and small.

Hypertension and obstructive sleep apnea

Obstructive sleep apnea is a common disorder that is often unrecognized and underappreciated. Emerging evidence suggests that there is a causal link between obstructive sleep apnea and hypertension. This relationship appears to be independent of other comorbidities that have been previously linked to hypertension, such as obesity. The majority of studies support the contention that alleviation of sleep disordered breathing has a clinically significant beneficial impact on decreasing both nighttime and daytime blood pressure. A pathophysiologic basis for patients with sleep apnea having an increased risk for hypertension is not fully elucidated. However, there is consistent evidence that autonomic mechanisms are implicated. Sympathetic activation along with humoral responses to repetitive episodes of hypoxemia and apnea over the longer term may cause vasoconstriction, endothelial dysfunction, and possibly hypertension. Patients with sleep apnea are often obese and may be predisposed to weight gain. Hence, obesity may further contribute to hypertension in this patient population.

Cardiovascular consequences of obstructive sleep apnea

Sleep apnea is associated with several cardiovascular disease conditions. A causal relationship between sleep apnea and each of these diseases is likely, but remains to be proven. The clearest evidence implicating OSA in the development of new cardiovascular disease involves data that show an increased prevalence of new hypertension in patients with OSA followed over 4 years [3]. Circumstantial evidence and data from small study samples suggest that OSA, in the setting of existing cardiovascular disease, may exacerbate symptoms and accelerate disease progression. The diagnosis of OSA always should be considered in patients with refractory heart failure, resistant hypertension, nocturnal cardiac ischemia, and nocturnal arrhythmias, especially in individuals with risk factors for sleep apnea (e.g., central obesity, age, and male gender). Treating sleep apnea may help to achieve better clinical control in these diseases and may improve long-term cardiovascular prognosis.

Sleep-disordered breathing and stroke

Sleep-related breathing disorders are strongly associated with increased risk of stroke independent of known risk factors. The direction of causation favors sleep-disordered breathing leading to stroke rather than the other way around, although definitive proof of this awaits the results of prospective cohort studies. If causal, even a moderately elevated risk of stroke coupled with the high prevalence of sleep-disordered breathing could have significant public health implications. The relationship between sleep-disordered breathing and stroke risk factors is complex, and likely part of the risk for cerebrovascular events is because of higher cardiovascular risk factors in patients with increased RDI. The mechanisms underlying this increased risk of stroke are multi-factorial and include reduction in cerebral blood flow, altered cerebral autoregulation, impaired endothelial function, accelerated atherogenesis, thrombosis, and paradoxic embolism. Because of the effects of sleep-disordered breathing on vascular tone, hypertension is believed to be a major mechanism by which sleep-disordered breathing might influence risk of stroke. Because sleep-related breathing disorders are treatable patients with stroke/TIA should undergo investigation, with a thorough sleep history interview, physical examination, and polysomnography. Treatment of sleep apnea has been shown to improve quality of life, lower blood pressure, improve sleep quality, improve neurocognitive functioning, and decrease symptoms of excessive daytime sleepiness [98]. Further treatment trials are needed to determine whether treatment improves outcome after stroke and whether treatment may serve as secondary prophylaxis and modify the risk of recurrent stroke or death.

Genomic approaches to understanding obstructive sleep apnea

Obstructive sleep apnea (OSA) is an increasingly recognized, common chronic disease in the developed nations and is a complex disease that has high social and economic costs. OSA and its associated 'intermediate' phenotypes-craniofacial structure, body fat distribution and metabolism, and neurological control of the upper airway muscles and of sleep and circadian rhythm-are under a substantial degree of genetic control. Investigating the genetic aetiology of OSA offers a means of better understanding its pathogenesis, with the goal of improving preventive strategies, diagnostic tools and therapies. Molecular studies of OSA itself are in their infancy, but considerable effort and expense has already been expended in attempts to detect genetic loci contributing to OSA-associated intermediate phenotypes, such as obesity. However, many of the fundamental questions relating to the genetic epidemiology of OSA and associated factors remain unanswered. This chapter reviews the current state of knowledge of the genetics of OSA, with a focus on genomic approaches to understanding sleep disorders.

Obstructive sleep apnoea syndrome--an oxidative stress disorder

Obstructive sleep apnoea syndrome (OSA) is associated with increased cardiovascular morbidity and mortality. However, the underlying mechanisms are not entirely understood. This review will summarize the evidence that substantiates the notion that the repeated apnoea-related hypoxic events in OSA, similarly to hypoxia/reperfusion injury, initiate oxidative stress. Thus, affecting energy metabolism, redox-sensitive gene expression, and expression of adhesion molecules. A limited number of studies substantiate this hypothesis directly by demonstrating increased free radical production in OSA leukocytes and increased plasma-lipid peroxidation. A great number of studies, however, support this hypothesis indirectly. Increase in circulating levels of adenosine and urinary uric acid in OSA are implicated with increased production of reactive oxygen species (ROS). Activation of redox-sensitive gene expression is suggested by the increase in some protein products of these genes, including VEGF, erythropoietin, endothelin-1, inflammatory cytokines and adhesion molecules. These implicate the participation of redox-sensitive transcription factors as HIF-1 AP-1 and NFkappaB. Finally, adhesion molecule-dependent increased avidity of OSA monocytes to endothelial cells, combined with diminished NO bioavailability, lead to exaggerated endothelial cell damage and dysfunction. Cumulatively, these processes may exacerbate atherogenic sequelae in OSA.