Pulmonary Vascular Responses to Chronic Intermittent Hypoxia in a Guinea Pig Model of Obstructive Sleep Apnea

Experimental evidence suggests that chronic intermittent hypoxia (CIH), a major hallmark of obstructive sleep apnea (OSA), boosts carotid body (CB) responsiveness, thereby causing increased sympathetic activity, arterial and pulmonary hypertension, and cardiovascular disease. An enhanced circulatory chemoreflex, oxidative stress, and NO signaling appear to play important roles in these responses to CIH in rodents. Since the guinea pig has a hypofunctional CB (i.e., it is a natural CB knockout), in this study we used it as a model to investigate the CB dependence of the effects of CIH on pulmonary vascular responses, including those mediated by NO, by comparing them with those previously described in the rat. We have analyzed pulmonary artery pressure (PAP), the hypoxic pulmonary vasoconstriction (HPV) response, endothelial function both in vivo and in vitro, and vascular remodeling (intima-media thickness, collagen fiber content, and vessel lumen area). We demonstrate that 30 days of the exposure of guinea pigs to CIH (FiO2, 5% for 40 s, 30 cycles/h) induces pulmonary artery remodeling but does not alter endothelial function or the contractile response to phenylephrine (PE) in these arteries. In contrast, CIH exposure increased the systemic arterial pressure and enhanced the contractile response to PE while decreasing endothelium-dependent vasorelaxation to carbachol in the aorta without causing its remodeling. We conclude that since all of these effects are independent of CB sensitization, there must be other oxygen sensors, beyond the CB, with the capacity to alter the autonomic control of the heart and vascular function and structure in CIH.

The pathogenesis of OSA-related hypertension: what are the determining factors?

Sleep-disordered breathing has a relatively high prevalence, which varies from 3-7% in males and from 2-5% in females in the adult population. Studies published in the literature have shown that sleep apnea is closely related to an increased risk of developing various pathologies, among which arterial hypertension stands out. The prevalence of hypertension in patients suffering from obstructive sleep apnea (OSA) ranges from 35-80% and appears to be related to OSA severity. Approximately 40-50% of patients affected by hypertension are also affected by OSA and this association seems to be stronger in young and middle-aged adults (<50 years of age). The primary objective of this narrative review is to provide an update on what are the main contributing comorbidities to the development of a hypertensive state in patients suffering from OSA, an independent risk factor for diurnal hypertension, implicated as a risk factor for the first stroke, recurrent stroke, and post-stroke mortality. There are a lot of factors that contribute to developing a hypertensive state in OSA patients, some more decisive, others less. More evidence from longitudinal studies is needed on the impact of OSA on cardiovascular risk in females, on the causal link between OSA and arterial hypertension or metabolic diseases, like diabetes and glucose intolerance, and the effect of different kinds of OSA treatment.

Chronic intermittent hypoxia remodels catecholaminergic nerve innervation in mouse atria

Chronic intermittent hypoxia (CIH, a model for sleep apnoea) is a major risk factor for several cardiovascular diseases. Autonomic imbalance (sympathetic overactivity and parasympathetic withdrawal) has emerged as a causal contributor of CIH-induced cardiovascular disease. Previously, we showed that CIH remodels the parasympathetic pathway. However, whether CIH induces remodelling of the cardiac sympathetic innervation remains unknown. Mice (male, C57BL/6J, 2-3 months) were exposed to either room air (RA, 21% O2 ) or CIH (alternating 21% and 5.7% O2 , every 6 min, 10 h day-1 ) for 8-10 weeks. Flat-mounts of their left and right atria were immunohistochemically labelled for tyrosine hydroxylase (TH, a sympathetic marker). Using a confocal microscope (or fluorescence microscope) and Neurlocudia 360 digitization and tracing system, we scanned both the left and right atria and quantitatively analysed the sympathetic axon density in both groups. The segmentation data was mapped onto a 3D mouse heart scaffold. Our findings indicated that CIH significantly remodelled the TH immunoreactive (-IR) innervation of the atria by increasing its density at the sinoatrial node, the auricles and the major veins attached to the atria (P < 0.05, n = 7). Additionally, CIH increased the branching points of TH-IR axons and decreased the distance between varicosities. Abnormal patterns of TH-IR axons around intrinsic cardiac ganglia were also found following CIH. We postulate that the increased sympathetic innervation may further amplify the effects of enhanced CIH-induced central sympathetic drive to the heart. Our work provides an anatomical foundation for the understanding of CIH-induced autonomic imbalance. KEY POINTS: Chronic intermittent hypoxia (CIH, a model for sleep apnoea) causes sympathetic overactivity, cardiovascular remodelling and hypertension. We determined the effect of CIH on sympathetic innervation of the mouse atria. In vivo CIH for 8-10 weeks resulted in an aberrant axonal pattern around the principal neurons within intrinsic cardiac ganglia and an increase in the density, branching point, tortuosity of catecholaminergic axons and atrial wall thickness. Utilizing mapping tool available from NIH (SPARC) Program, the topographical distribution of the catecholaminergic innervation of the atria were integrated into a novel 3D heart scaffold for precise anatomical distribution and holistic quantitative comparison between normal and CIH mice. This work provides a unique neuroanatomical understanding of the pathophysiology of CIH-induced autonomic remodelling.

Chronic intermittent hypoxia-induced cardiovascular and renal dysfunction: from adaptation to maladaptation

Chronic intermittent hypoxia (CIH) is the dominant pathological feature of human obstructive sleep apnoea (OSA), which is highly prevalent and associated with cardiovascular and renal diseases. CIH causes hypertension, centred on sympathetic nervous overactivity, which persists following removal of the CIH stimulus. Molecular mechanisms contributing to CIH-induced hypertension have been carefully delineated. However, there is a dearth of knowledge on the efficacy of interventions to ameliorate high blood pressure in established disease. CIH causes endothelial dysfunction, aberrant structural remodelling of vessels and accelerates atherosclerotic processes. Pro-inflammatory and pro-oxidant pathways converge on disrupted nitric oxide signalling driving vascular dysfunction. In addition, CIH has adverse effects on the myocardium, manifesting atrial fibrillation, and cardiac remodelling progressing to contractile dysfunction. Sympatho-vagal imbalance, oxidative stress, inflammation, dysregulated HIF-1α transcriptional responses and resultant pro-apoptotic ER stress, calcium dysregulation, and mitochondrial dysfunction conspire to drive myocardial injury and failure. CIH elaborates direct and indirect effects in the kidney that initially contribute to the development of hypertension and later to chronic kidney disease. CIH-induced morphological damage of the kidney is dependent on TLR4/NF-κB/NLRP3/caspase-1 inflammasome activation and associated pyroptosis. Emerging potential therapies related to the gut-kidney axis and blockade of aryl hydrocarbon receptors (AhR) are promising. Cardiorenal outcomes in response to intermittent hypoxia present along a continuum from adaptation to maladaptation and are dependent on the intensity and duration of exposure to intermittent hypoxia. This heterogeneity of OSA is relevant to therapeutic treatment options and we argue the need for better stratification of OSA phenotypes.

Chronic Intermittent Hypoxia-Induced Diaphragm Muscle Weakness Is NADPH Oxidase-2 Dependent

Chronic intermittent hypoxia (CIH)-induced redox alterations underlie diaphragm muscle dysfunction. We sought to establish if NADPH oxidase 2 (NOX2)-derived reactive oxygen species (ROS) underpin CIH-induced changes in diaphragm muscle, which manifest as impaired muscle performance. Adult male mice (C57BL/6J) were assigned to one of three groups: normoxic controls (sham); chronic intermittent hypoxia-exposed (CIH, 12 cycles/hour, 8 h/day for 14 days); and CIH + apocynin (NOX2 inhibitor, 2 mM) administered in the drinking water throughout exposure to CIH. In separate studies, we examined sham and CIH-exposed NOX2-null mice (B6.129S-Cybb^TM1Din/J). Apocynin co-treatment or NOX2 deletion proved efficacious in entirely preventing diaphragm muscle dysfunction following exposure to CIH. Exposure to CIH had no effect on NOX2 expression. However, NOX4 mRNA expression was increased following exposure to CIH in wild-type and NOX2 null mice. There was no evidence of overt CIH-induced oxidative stress. A NOX2-dependent increase in genes related to muscle regeneration, antioxidant capacity, and autophagy and atrophy was evident following exposure to CIH. We suggest that NOX-dependent CIH-induced diaphragm muscle weakness has the potential to affect ventilatory and non-ventilatory performance of the respiratory system. Therapeutic strategies employing NOX2 blockade may function as an adjunct therapy to improve diaphragm muscle performance and reduce disease burden in diseases characterised by exposure to CIH, such as obstructive sleep apnoea.

Three Days of Chronic Intermittent Hypoxia Induce β1-Adrenoceptor Dependent Increases in Left Ventricular Contractility

Sleep apnea is characterized by bouts of chronic intermittent hypoxia (CIH) that elicit sympathetic hyperactivity resulting in residual hypertension. We previously demonstrated that exposure to CIH increases cardiac output and sought to determine if enhanced cardiac contractility manifests prior to hypertension.Male Wistar rats were exposed to cyclical bouts of hypoxia (FiO₂ = 0.05 nadir; 90 s) and normoxia (FiO₂ = 0.21; 210 s) 8 h/day for 3 days (CIH; n = 6). Control animals (n = 7) were exposed to room air. Data are presented as mean ± SD and were analyzed using unpaired Student t-tests.Three-day exposure to CIH did not elicit changes in heart rate and blood pressure (p > 0.05). However, baseline left ventricular contractility (dP/dt_MAX) was significantly increased in CIH-exposed animals compared with control (15300 ± 2002 vs. 12320 ± 2725 mmHg/s; p = 0.025), despite no difference in catecholamine concentrations. Acute β₁-adrenoceptor inhibition reduced contractility in CIH-exposed animals (-7604 ± 1298 vs. -4747 ± 2080 mmHg/s; p = 0.014), to levels equivalent to control, while preserving cardiovascular parameters. Sympathetic ganglion blockade (hexamethonium 25 mg/kg; i.v.) produced equivalent cardiovascular responses suggesting similar global sympathetic activity between groups. Interestingly, gene expression of the β₁-adrenoceptor pathway in cardiac tissue was unchanged.Our results suggest that CIH increases cardiac contractility via β₁-adrenoceptor dependent mechanisms prior to development of global sympathetic hyperactivity suggesting that positive cardiac inotropy contributes to the development of hypertension in CIH-exposed rats.

Cardiorespiratory alterations following intermittent photostimulation of RVLM C1 neurons: Implications for long-term blood pressure, breathing and sleep regulation in freely moving rats

AIM

Sympathoexcitation and sleep-disordered breathing are common contributors for disease progression. Catecholaminergic neurons from the rostral ventrolateral medulla (RVLM-C1) modulate sympathetic outflow and have anatomical projections to respiratory neurons; however, the contribution of highly selective activation of RVLM-C1 neurons on long-term autonomic and breathing (dys)regulation remains to be understood.

METHODS

To explore this relationship, a lentiviral vector carrying the light-sensitive cation channel channelrhodopsin-2 (LVV-PRSX8-ChR2-YFP) was unilaterally injected into the RVLM of healthy rats. On the contralateral side, LVV-PRSX8-ChR2-YFP was co-injected with a specific immunotoxin (DβH-SAP) targeted to eliminate C1 neurons.

RESULTS

Intermittent photostimulation of RVLM-C1 in vivo, in unrestrained freely moving rats, elicited long-term facilitation of the sympathetic drive, a rise in blood pressure and sympatho-respiratory coupling. In addition, photoactivation of RVLM-C1 induced long-lasting ventilatory instability, characterized by oscillations in tidal volume and increased breathing variability, but only during non-rapid eye movement sleep. These effects were not observed when photostimulation of the RVLM was performed in the presence of DβH-SAP toxin.

CONCLUSIONS

The finding that intermittent activation of RVLM-C1 neurons induces autonomic and breathing dysfunction suggest that episodic stimulation of RVLM-C1 may serve as a pathological substrate for the long-term development of cardiorespiratory disorders.

Caloric restriction prevents obesity- and intermittent hypoxia-induced cardiac remodeling in leptin-deficient ob/ob mice

Background: Intermittent hypoxia (IH), a key characteristic of obstructive sleep apnea, is independently associated with cardiometabolic impairment. While endogenous leptin levels may provide cardioprotective effects against hypoxia, leptin resistance is common among obese individuals presenting with obstructive sleep apnea.

Methods: Here, we assessed left ventricle (LV) function using M-mode echocardiography in lean wild-type, calorically-restricted ob/ob, and obese ob/ob mice before and after 6 days of IH to determine how obesity and intermittent hypoxia interact to affect cardiac function independent of leptin signaling.

Results: Calorically-restricting ob/ob mice for 4 weeks prior to IH exposure prevented weight gain (−2.1 ± 1.4 g) compared to free-fed ob/ob mice (8.7 ± 1.1 g). Free-fed ob/ob mice exhibited increased LV mass (0.713 ± 0.008 g) relative to wild-type mice (0.685 ± 0.004 g) and increased posterior wall thickness (0.089 ± 0.006 cm) relative to calorically-restricted ob/ob mice (0.072 ± 0.004 cm). Following 6 days of IH, free-fed ob/ob mice exhibited increases in cardiac output (44.81 ± 2.97 pre-IH vs. 57.14 ± 3.09 ml/min post-IH), LV diameter (0.400 ± 0.007 pre-IH vs. 0.428 ± 0.009 cm post-IH) and end diastolic volume (0.160 ± 0.007 pre-IH vs. 0.195 ± 0.012 ml post-IH) that were not detected in wild-type or calorically-restricted ob/ob mice.

Conclusion: Caloric restriction can prevent obesity-induced LV hypertrophy and protect against acute IH-induced cardiac remodeling independent of leptin signaling. These findings may have clinical implications for obstructive sleep apnea.

Blunted hypoxic pulmonary vasoconstriction in apnoea divers

NEW FINDINGS

What is new and noteworthy? What is the central question of this study? Does the hyperbaric, hypercapnic, acidotic, hypoxic stress of apnoea diving lead to greater pulmonary vasoreactivity and increased right-heart work in apnoea divers? What is the main finding and its importance? Compared to sex- and age-matched controls, Divers had a significantly lower change in total pulmonary resistance in response to short duration isocapnic hypoxia. With oral sildenafil (50 mg), there were no differences in total pulmonary resistance between groups, suggesting Divers can maintain normal pulmonary artery tone in hypoxic conditions. Blunted hypoxic pulmonary vasoconstriction may be beneficial during apnoea diving.

ABSTRACT

Competitive apnoea divers repetitively dive to depths beyond 50 m. During the final portions of ascent, Divers experience significant hypoxaemia. Additionally, hyperbaria during diving increases thoracic blood volume while simultaneously reducing lung volume, increasing pulmonary artery pressure. We hypothesized that Divers would have exaggerated hypoxic pulmonary vasoconstriction leading to increased right-heart work due to their repetitive hypoxaemia and hyperbaria, and that the administration of sildenafil would have a greater effect in reducing pulmonary resistance in Divers. We recruited 16 Divers and 16 age and sex matched non-diving controls (Controls). Using a double-blinded, placebo-controlled, cross-over design, participants were evaluated for normal cardiac and lung function, then their cardiopulmonary responses to 20-30 minutes of isocapnic hypoxia (end-tidal PO₂  = 50 mm Hg) were measured one hour following ingestion of 50 mg sildenafil or placebo. Cardiac structure and cardiopulmonary function were similar at baseline. With placebo, Divers had a significantly smaller increase in total pulmonary resistance than controls after 20-30 minutes isocapnic hypoxia (Δ -3.85 ± 72.85 vs 73.74 ± 91.06 dynes/sec/cm^-5 , p = .0222). With sildenafil, Divers and Controls had similarly blunted increases in total pulmonary resistance after 20-30 minutes of hypoxia. Divers also had a significantly lower systemic vascular resistance following sildenafil in normoxia. These data indicate that repetitive apnoea diving leads to a blunted hypoxic pulmonary vasoconstriction. We suggest this is a beneficial adaption allowing for increased cardiac output with reduced right heart work and thus reducing cardiac oxygen utilization under hypoxemic conditions. This article is protected by copyright. All rights reserved.

NADPH oxidase-2 is necessary for chronic intermittent hypoxia-induced sternohyoid muscle weakness in adult male mice

New Findings

What is the central question of this study?

Exposure to chronic intermittent hypoxia (CIH) evokes redox changes, culminating in impaired upper airway muscle function: what is the specific source of CIH‐induced reactive oxygen species?

What is the main finding and its importance?

Profound sternohyoid muscle dysfunction following exposure to CIH was entirely prevented by apocynin co‐treatment or NADPH oxidase 2 (NOX2) deletion. The results have implications for human obstructive sleep apnoea syndrome and point to antioxidant intervention, potentially targeting NOX2 blockade, as a therapeutic strategy.

Abstract

Exposure to chronic intermittent hypoxia (CIH) evokes redox changes, culminating in impaired upper airway muscle function. We sought to determine if NADPH oxidase 2 (NOX2)‐derived reactive oxygen species underpin CIH‐induced maladaptive changes in upper airway (sternohyoid) muscle performance. Adult male mice (C57BL/6J) were assigned to one of three groups: normoxic controls (sham); CIH‐exposed (CIH, 12 cycles/hour, 8 h/day for 14 days); and CIH + apocynin (NOX2 inhibitor, 2 mM) given in the drinking water throughout exposure to CIH. In addition, we studied sham and CIH‐exposed NOX2‐null mice (B6.129S‐CybbTM^1Din/J). Profound sternohyoid muscle dysfunction following exposure to CIH was entirely prevented by apocynin co‐treatment or NOX2 deletion. Exposure to CIH increased sternohyoid muscle NOX enzyme activity, with no alteration to the gene or protein expression of NOX subunits. There was no evidence of overt oxidative stress, muscle regeneration, inflammation or atrophy following exposure to CIH. We suggest that NOX‐dependent CIH‐induced upper airway muscle weakness increases vulnerability to upper airway obstruction. Our results have implications for human obstructive sleep apnoea syndrome and point to antioxidant intervention, potentially targeting NOX2 blockade, as a therapeutic strategy.

Breathing in Duchenne muscular dystrophy: Translation to therapy

Duchenne muscular dystrophy (DMD) is an X-linked neuromuscular disease caused by a deficiency in dystrophin - a structural protein which stabilizes muscle during contraction. Dystrophin deficiency adversely affects the respiratory system leading to sleep-disordered breathing, hypoventilation, and weakness of the expiratory and inspiratory musculature, which culminate in severe respiratory dysfunction. Muscle degeneration associated respiratory impairment in neuromuscular disease is a result of disruptions at multiple sites of the respiratory control network, including sensory and motor pathways. As a result of this pathology, respiratory failure is a leading cause of premature death in DMD patients. Currently available treatments for DMD respiratory insufficiency attenuate respiratory symptoms without completely reversing the underlying pathophysiology. This underscores the need to develop curative therapies to improve quality of life and longevity of DMD patients. This review summarises research findings on the pathophysiology of respiratory insufficiencies in DMD disease in humans and animal models, the clinical interventions available to ameliorate symptoms, and gene-based therapeutic strategies uncovered by preclinical animal studies. Abstract figure legend: Summary of the therapeutic strategies for respiratory insufficiency in DMD (Duchenne muscular dystrophy). Treatment options currently in clinical use only attenuate respiratory symptoms without reversing the underlying pathology of DMD-associated respiratory insufficiencies. Ongoing preclinical and clinical research is aimed at developing curative therapies that both improve quality of life and longevity of DMD patients. AAV - adeno-associated virus, PPMO - Peptide-conjugated phosphorodiamidate morpholino oligomer This article is protected by copyright. All rights reserved.

Intrarenal pelvic bradykinin-induced sympathoexcitatory reno-renal reflex is attenuated in rats exposed to chronic intermittent hypoxia

OBJECTIVE

In this study, we hypothesized that excitatory reno-renal reflex control of sympathetic outflow is enhanced in rats exposed to chronic intermittent hypoxia (CIH) with established hypertension.

METHODS

Under anaesthesia, renal sensory nerve endings in the renal pelvic wall were chemically activated using bradykinin (150, 400 and 700 μmol/l) and capsaicin (1.3 μmol/l), and cardiovascular parameters and renal sympathetic nerve activity (RSNA) were measured.

RESULTS

CIH-exposed rats were hypertensive with elevated basal heart rate and increased basal urine flow compared with sham. The intrarenal pelvic infusion of bradykinin was associated with contralateral increase in the RSNA and heart rate, without concomitant changes in blood pressure. This was associated with a drop in the glomerular filtration rate, which was significant during a 5 min period after termination of the infusion but without significant changes in urine flow and absolute sodium excretion. In response to intrarenal pelvic infusion of 700 μmol/l bradykinin, the increases in RSNA and heart rate were blunted in CIH-exposed rats compared with sham rats. Conversely, the intrarenal pelvic infusion of capsaicin evoked an equivalent sympathoexcitatory effect in CIH-exposed and sham rats. The blockade of bradykinin type 1 receptors (BK1R) suppressed the bradykinin-induced increase in RSNA by ∼33%, with a greater suppression obtained when bradykinin type 2 receptors (BK2R) and BK1R were contemporaneously blocked (∼66%).

CONCLUSION

Our findings reveal that the bradykinin-dependent excitatory reno-renal reflex does not contribute to CIH-induced sympathetic hyperactivity and hypertension. Rather, there is evidence that the excitatory reno-renal reflex is suppressed in CIH-exposed rats, which might relate to a downregulation of BK2R.

Cell Energy Budget Platform for Multiparametric Assessment of Cell and Tissue Metabolism

Specific bioenergetic signature reports on the current metabolic state of the cell, which may be affected by metabolic rearrangement, dysfunction or dysregulation of relevant signaling pathways, altered physiological condition or energy stress. A combined analysis of respiration , glycolytic flux, Krebs cycle activity, ATP levels, and total biomass allows informative initial assessment. Such simple, high-throughput, multiparametric methodology, called cell energy budget (CEB ) platform, is presented here and demonstrated with particular cell and tissue models. The CEB uses a commercial fluorescent lanthanide probe pH-Xtra™ to measure extracellular acidification (ECA) associated with lactate (L-ECA) and combined lactate/CO₂ (T-ECA), a phosphorescent probe MitoXpress^®-Xtra to measure oxygen consumption rate (OCR), a bioluminescent ATP kit, and an absorbance-based total protein assay. All the assays are performed on a standard multi-label reader. Using the same readouts, the CEB approach can be extended to more detailed mechanistic studies, by targeting specific pathways in cell bioenergetics and measuring other cellular parameters, such as NAD(P)H, Ca²⁺, mitochondrial pH, membrane potential, redox state, with conventional fluorescent or luminescent probes.

The role of NADPH oxidase in chronic intermittent hypoxia-induced respiratory plasticity in adult male mice

Reactive oxygen species (ROS) are proposed as mediators of chronic intermittent hypoxia (CIH)-induced respiratory plasticity. We sought to determine if NADPH oxidase 2 (NOX2)-derived ROS underpin CIH-induced maladaptive changes in respiratory control. Adult male mice (C57BL/6 J) were assigned to one of three groups: normoxic controls (sham); chronic intermittent hypoxia-exposed (CIH, 12 cycles/hour, 8 h/day for 14 days); and CIH + apocynin (NOX2 inhibitor, 2 mM) given in the drinking water throughout exposure to CIH. In addition, we studied sham and CIH-exposed NOX2-null mice (B6.129S-Cybb^TM1Din/J). Whole-body plethysmography was used to measure breathing and metabolic parameters. Ventilation (V̇_I/V̇CO₂) during normoxia was unaffected by CIH, but apnoea index was increased, which was prevented by apocynin, but not by NOX2 deletion. The ventilatory response to hypercapnia following exposure to CIH was potentiated in NOX2-null mice. Our results reveal ROS-dependent influences on the control of breathing and point to antioxidant intervention as a potential adjunctive therapeutic strategy in respiratory control disorders.

β-Adrenoceptor blockade prevents carotid body hyperactivity and elevated vascular sympathetic nerve density induced by chronic intermittent hypoxia

Carotid body (CB) hyperactivity promotes hypertension in response to chronic intermittent hypoxia (CIH). The plasma concentration of adrenaline is reported to be elevated in CIH and our previous work suggests that adrenaline directly activates the CB. However, a role for chronic adrenergic stimulation in mediating CB hyperactivity is currently unknown. This study evaluated whether beta-blocker treatment with propranolol (Prop) prevented the development of CB hyperactivity, vascular sympathetic nerve growth and hypertension caused by CIH. Adult male Wistar rats were assigned into 1 of 4 groups: Control (N), N + Prop, CIH and CIH + Prop. The CIH paradigm consisted of 8 cycles h-1, 8 h day-1, for 3 weeks. Propranolol was administered via drinking water to achieve a dose of 40 mg kg-1 day-1. Immunohistochemistry revealed the presence of both β1 and β2-adrenoceptor subtypes on the CB type I cell. CIH caused a 2-3-fold elevation in basal CB single-fibre chemoafferent activity and this was prevented by chronic propranolol treatment. Chemoafferent responses to hypoxia and mitochondrial inhibitors were attenuated by propranolol, an effect that was greater in CIH animals. Propranolol decreased respiratory frequency in normoxia and hypoxia in N and CIH. Propranolol also abolished the CIH mediated increase in vascular sympathetic nerve density. Arterial blood pressure was reduced in propranolol groups during hypoxia. Propranolol exaggerated the fall in blood pressure in most (6/7) CIH animals during hypoxia, suggestive of reduced sympathetic tone. These findings therefore identify new roles for β-adrenergic stimulation in evoking CB hyperactivity, sympathetic vascular hyperinnervation and altered blood pressure control in response to CIH.

Transthoracic echocardiography: from guidelines for humans to cardiac ultrasound of the heart in rats

Ultrasound examination of the heart is a cornerstone of clinical evaluation of patients with established or suspected cardiovascular conditions. Advancements in ultrasound imaging technology have brought transthoracic echocardiography to preclinical murine models of cardiovascular diseases. The translational potential of cardiac ultrasound is critically important in rat models of myocardial infarction and ischemia-reperfusion injury, congestive heart failure, arterial hypertension, cardiac hypertrophy, pulmonary hypertension, right heart failure, Takotsubo cardiomyopathy, hypertrophic and dilated cardiomyopathies, developmental disorders, and metabolic syndrome. Modern echocardiographic machines capable of high-frame-rate image acquisition and fitted with high-frequency transducers allow for cardiac ultrasound in rats that yields most of the echocardiographic measurements and indices recommended by international guidelines for cardiac ultrasound in human patients. Among them are dimensions of cardiac chambers and walls, indices of systolic and diastolic cardiac function, and valvular function. In addition, measurements of cardiac dimensions and ejection fraction can be significantly improved by intravenous administration of ultrasound enhancing agents (UEAs). In this article we discuss echocardiography in rats, describe a technique for minimally invasive intravenous administration of UEAs via the saphenous vein and present a step-by-step approach to cardiac ultrasound in rats.

Sex differences in integrated neurocardiovascular control of blood pressure following acute intermittent hypercapnic hypoxia

Repetitive hypoxic apneas, similar to those observed in sleep apnea, result in resetting of the sympathetic baroreflex to higher blood pressures (BP). This baroreflex resetting is associated with hypertension in preclinical models of sleep apnea (intermittent hypoxia, IH); however, the majority of understanding comes from males. There are data to suggest that female rats exposed to IH do not develop high BP. Clinical data further support sex differences in the development of hypertension in sleep apnea, but mechanistic data are lacking. Here we examined sex-related differences in the effect of IH on sympathetic control of BP in humans. We hypothesized that after acute IH we would observe a rise in muscle sympathetic nerve activity (MSNA) and arterial BP in young men (n = 30) that would be absent in young women (n = 19). BP and MSNA were measured during normoxic rest before and after 30 min of IH. Baroreflex sensitivity (modified Oxford) was evaluated before and after IH. A rise in mean BP following IH was observed in men (+2.0 ± 0.7 mmHg, P = 0.03), whereas no change was observed in women (-2.7 ± 1.2 mmHg, P = 0.11). The elevation in MSNA following IH was not different between groups (4.7 ± 1.1 vs. 3.8 ± 1.2 bursts/min, P = 0.65). Sympathetic baroreflex sensitivity did not change after IH in either group (P > 0.05). Our results support sex-related differences in the effect of IH on neurovascular control of BP and show that any BP-raising effects of IH are absent in young women. These data enhance our understanding of sex-specific mechanisms that may contribute to BP changes in sleep apnea.

Peripheral Dopamine 2-Receptor Antagonist Reverses Hypertension in a Chronic Intermittent Hypoxia Rat Model

The sleep apnea-hypopnea syndrome (SAHS) involves periods of intermittent hypoxia, experimentally reproduced by exposing animal models to oscillatory PO₂ patterns. In both situations, chronic intermittent hypoxia (CIH) exposure produces carotid body (CB) hyperactivation generating an increased input to the brainstem which originates sympathetic hyperactivity, followed by hypertension that is abolished by CB denervation. CB has dopamine (DA) receptors in chemoreceptor cells acting as DA-2 autoreceptors. The aim was to check if blocking DA-2 receptors could decrease the CB hypersensitivity produced by CIH, minimizing CIH-related effects. Domperidone (DOM), a selective peripheral DA-2 receptor antagonist that does not cross the blood-brain barrier, was used to examine its effect on CIH (30 days) exposed rats. Arterial pressure, CB secretory activity and whole-body plethysmography were measured. DOM, acute or chronically administered during the last 15 days of CIH, reversed the hypertension produced by CIH, an analogous effect to that obtained with CB denervation. DOM marginally decreased blood pressure in control animals and did not affect hypoxic ventilatory response in control or CIH animals. No adverse effects were observed. DOM, used as gastrokinetic and antiemetic drug, could be a therapeutic opportunity for hypertension in SAHS patients’ resistant to standard treatments.

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.

Hemoglobin Concentration Affects Hypertensive Basal Ganglia Hemorrhage After Surgery: Correlation Analysis in a High-Altitude Region

OBJECTIVE

Hypertensive cerebral hemorrhage leads to greater mortality and worse functional outcomes at high altitudes. Experimental studies have suggested that hemoglobin can lead to increased perihemorrhagic edema after intracerebral hemorrhage.

METHODS

Patients were divided into a high-hemoglobin (H-H) group (>180 g/L) and a low-hemoglobin (L-H) group (≤180 g/L). The distance from the cortex to the midline was used to indicate the degree of edema. At 1, 7, 14, and 21 days, the patients' status was scored using the Glasgow coma scale, and survival was plotted using Kaplan-Meier survival curves. Pearson correlation analysis showed that the difference between the postoperative and preoperative Glasgow coma scale score correlated with the hemoglobin concentration. The Glasgow outcome scale was used to assess neurological recovery after 6 months.

RESULTS

On days 7, 14, and 21, the edema of the H-H group was significantly greater than that of the L-H group (P < 0.01 and P < 0.001, respectively). The edema of the H-H group peaked at 14 and 21 days, but that of the L-H group peaked at 7 days. The hemoglobin concentration and postoperative neurological recovery had a linear relationship in the H-H group. The L-H group had greater survival compared with the H-H group (P < 0.05). The L-H group had higher Glasgow outcome scale scores compared with the H-H group (P < 0.05).

CONCLUSION

The hemoglobin concentration affects the mortality and morbidity from hypertensive cerebral hemorrhage in high-altitude regions, and a linear relationship exists between hemoglobin concentration and neurological recovery in the H-H group.

Protective roles of estradiol against vascular oxidative stress in ovariectomized female rats exposed to normoxia or intermittent hypoxia

AIM

We tested the hypothesis that estradiol (E₂ ) reduces aortic oxidative stress and endothelial dysfunction in ovariectomized (OVX) female rats exposed to room air (RA) or chronic intermittent hypoxia (CIH).

METHODS

We used intact or OVX female rats treated with vehicle or E₂ (0.5 mg/kg/d) and exposed to RA or CIH (21%-10% O₂ , 10 cycles/h, 8 h/d) for 7 or 35 days, and measured the arterial pressure, heart rate and plasma endothelin-1 levels. We also measured in thoracic aortic samples, the activities of the pro-oxidant enzymes NADPH (NOX) and xanthine oxidase (XO), the antioxidant enzymes superoxide dismutase, catalase, glutathione peroxidase and the advanced oxidation protein products (AOPP-oxidative stress marker). Finally, we used aortic rings to assess the contractile response to phenylephrine and the vasodilatory response to acetylcholine.

RESULTS

After 7 or 35 days of CIH, E₂ supplementation reduced arterial pressure. E₂ reduced plasma endothelin-1 levels after 7 days of CIH, but not after 35 days. Ovariectomy, but not CIH for 7 days, increased aortic oxidative stress and E₂ treatment prevented this effect. Remarkably, in animals exposed to RA, this was achieved by a reduction in NOX and XO activities, but in animals exposed to CIH this was achieved by increased catalase activity. In OVX female rats exposed to CIH for 7 days, E₂ supplementation improved the NO-mediated vasodilation. After 35 days of CIH, enzymatic activities, AOPP and aortic reactivity were similar in all groups.

CONCLUSION

E₂ -based therapy could help prevent the vascular consequences of CIH in apneic women.

Renal cortical oxygen tension is decreased following exposure to long-term but not short-term intermittent hypoxia in the rat

Chronic kidney disease (CKD) occurs in more than 50% of patients with obstructive sleep apnea (OSA). However, the impact of intermittent hypoxia (IH) on renal function and oxygen homeostasis is unclear. Male Sprague-Dawley rats were exposed to IH (270 s at 21% O₂; 90 s hypoxia, 6.5% O₂ at nadir) for 4 h [acute IH (AIH)] or to chronic IH (CIH) for 8 h/day for 2 wk. Animals were anesthetized and surgically prepared for the measurement of mean arterial pressure (MAP), and left renal excretory function, renal blood flow (RBF), and renal oxygen tension (Po₂). AIH had no effect on MAP (123 ± 14 vs. 129 ± 14 mmHg, means ± SE, sham vs. IH). The CIH group was hypertensive (122 ± 9 vs. 144 ± 15 mmHg, P < 0.05). Glomerular filtration rate (GFR) (0.92 ± 0.27 vs. 1.33 ± 0.33 ml/min), RBF (3.8 ± 1.5 vs. 7.2 ± 2.4 ml/min), and transported sodium (TNa) (132 ± 39 vs. 201 ± 47 μmol/min) were increased in the AIH group (all P < 0.05). In the CIH group, GFR (1.25 ± 0.28 vs. 0.86 ± 0.28 ml/min, P < 0.05) and TNa (160 ± 39 vs. 120 ± 40 μmol/min, P < 0.05) were decreased, while RBF (4.13 ± 1.5 vs. 3.08 ± 1.5 ml/min) was not significantly different. Oxygen consumption (QO₂) was increased in the AIH group (6.76 ± 2.60 vs. 13.60 ± 7.77 μmol/min, P < 0.05), but it was not significantly altered in the CIH group (3.97 ± 2.63 vs. 6.82 ± 3.29 μmol/min). Cortical Po₂ was not significantly different in the AIH group (46 ± 4 vs. 46 ± 3 mmHg), but it was decreased in the CIH group (44 ± 5 mmHg vs. 38 ± 2 mmHg, P < 0.05). For AIH, renal oxygen homeostasis was preserved through a maintained balance between O₂ supply (RBF) and consumption (GFR). For CIH, mismatched TNa and QO₂ reflect inefficient O₂ utilization and, thereby, sustained decrease in cortical Po₂.

Chronic intermittent hypoxia disrupts cardiorespiratory homeostasis and gut microbiota composition in adult male guinea-pigs

BACKGROUND

Carotid body (peripheral oxygen sensor) sensitisation is pivotal in the development of chronic intermittent hypoxia (CIH)-induced hypertension. We sought to determine if exposure to CIH, modelling human sleep apnoea, adversely affects cardiorespiratory control in guinea-pigs, a species with hypoxia-insensitive carotid bodies. We reasoned that CIH-induced disruption of gut microbiota would evoke cardiorespiratory morbidity.

METHODS

Adult male guinea-pigs were exposed to CIH (6.5% O₂ at nadir, 6 cycles.hour^-1) for 8 h.day^-1 for 12 consecutive days.

FINDINGS

CIH-exposed animals established reduced faecal microbiota species richness, with increased relative abundance of Bacteroidetes and reduced relative abundance of Firmicutes bacteria. Urinary corticosterone and noradrenaline levels were unchanged in CIH-exposed animals, but brainstem noradrenaline concentrations were lower compared with sham. Baseline ventilation was equivalent in CIH-exposed and sham animals; however, respiratory timing variability, sigh frequency and ventilation during hypoxic breathing were all lower in CIH-exposed animals. Baseline arterial blood pressure was unaffected by exposure to CIH, but β-adrenoceptor-dependent tachycardia and blunted bradycardia during phenylephrine-induced pressor responses was evident compared with sham controls.

INTERPRETATION

Increased carotid body chemo-afferent signalling appears obligatory for the development of CIH-induced hypertension and elevated chemoreflex control of breathing commonly reported in mammals, with hypoxia-sensitive carotid bodies. However, we reveal that exposure to modest CIH alters gut microbiota richness and composition, brainstem neurochemistry, and autonomic control of heart rate, independent of carotid body sensitisation, suggesting modulation of breathing and autonomic homeostasis via the microbiota-gut-brainstem axis. The findings have relevance to human sleep-disordered breathing.

FUNDING

The Department of Physiology, and APC Microbiome Ireland, UCC.

No evidence in support of a prodromal respiratory control signature in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative condition disturbing major brain networks, including those pivotal to the motor control of breathing. The aim of this study was to examine respiratory control in the TgF344-AD transgenic rat model of AD. At 8-11 months of age, basal minute ventilation and ventilatory responsiveness to chemostimulation were equivalent in conscious wild-type (WT) and TgF344-AD rats. Under urethane anesthesia, basal diaphragm and genioglossus EMG activities were similar in WT and TgF344-AD rats. The duration of phenylbiguanide-induced apnoea was significantly shorter in TgF344-AD rats compared with WT. Following bilateral cervical vagotomy, diaphragm and genioglossus EMG responsiveness to chemostimulation were intact in TgF344-AD rats. Amyloid precursor protein C-terminal fragments were elevated in the TgF344-AD brainstem, in the absence of amyloid-β accumulation or alterations in tau phosphorylation. Brainstem pro-inflammatory cytokine concentrations were not increased in TgF344-AD rats. We conclude that neural control of breathing is preserved in TgF344-AD rats at this stage of the disease.

Evans Blue Dye: A Revisit of Its Applications in Biomedicine

Evans blue (EB) dye has owned a long history as a biological dye and diagnostic agent since its first staining application by Herbert McLean Evans in 1914. Due to its high water solubility and slow excretion, as well as its tight binding to serum albumin, EB has been widely used in biomedicine, including its use in estimating blood volume and vascular permeability, detecting lymph nodes, and localizing the tumor lesions. Recently, a series of EB derivatives have been labeled with PET isotopes and can be used as theranostics with a broad potential due to their improved half-life in the blood and reduced release. Some of EB derivatives have even been used in translational applications in clinics. In addition, a novel necrosis-avid feature of EB has recently been reported in some preclinical animal studies. Given all these interesting and important advances in EB study, a comprehensive revisiting of EB has been made in its biomedical applications in the review.

Neural Control of Blood Pressure in Chronic Intermittent Hypoxia

Sleep apnea (SA) is increasing in prevalence and is commonly comorbid with hypertension. Chronic intermittent hypoxia is used to model the arterial hypoxemia seen in SA, and through this paradigm, the mechanisms that underlie SA-induced hypertension are becoming clear. Cyclic hypoxic exposure during sleep chronically stimulates the carotid chemoreflexes, inducing sensory long-term facilitation, and drives sympathetic outflow from the hindbrain. The elevated sympathetic tone drives hypertension and renal sympathetic activity to the kidneys resulting in increased plasma renin activity and eventually angiotensin II (Ang II) peripherally. Upon waking, when respiration is normalized, the sympathetic activity does not diminish. This is partially because of adaptations leading to overactivation of the hindbrain regions controlling sympathetic outflow such as the nucleus tractus solitarius (NTS), and rostral ventrolateral medulla (RVLM). The sustained sympathetic activity is also due to enhanced synaptic signaling from the forebrain through the paraventricular nucleus (PVN). During the waking hours, when the chemoreceptors are not exposed to hypoxia, the forebrain circumventricular organs (CVOs) are stimulated by peripherally circulating Ang II from the elevated plasma renin activity. The CVOs and median preoptic nucleus chronically activate the PVN due to the Ang II signaling. All together, this leads to elevated nocturnal mean arterial pressure (MAP) as a response to hypoxemia, as well as inappropriately elevated diurnal MAP in response to maladaptations.

Intradialytic Hypoxemia and Clinical Outcomes in Patients on Hemodialysis

BACKGROUND AND OBJECTIVES

Intradialytic hypoxemia has been recognized for decades, but its associations with outcomes have not yet been assessed in a large patient cohort.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Our retrospective cohort study was conducted between January of 2012 and January of 2015. We recorded blood oxygen saturation every minute during hemodialysis in patients with arteriovenous access. A 6-month baseline period with at least 10 treatments with oxygen saturation measurements preceded a 12-month follow-up. Patients were stratified by the presence or absence of prolonged intradialytic hypoxemia defined as oxygen saturation <90% for at least one third of the treatment time. Demographic, laboratory, and treatment data and hospitalization and mortality rates were compared between the groups. Multivariate Cox regression analysis was used to assess baseline predictors of all-cause mortality during follow-up.

RESULTS

In total, 100 (10%) of 983 patients had prolonged intradialytic hypoxemia. These patients were older (+3.6 years; 95% confidence interval, 0.8 to 6.3), had longer dialysis vintage (+1.2 years; 95% confidence interval, 0.3 to 2.1), and had higher prevalence of congestive heart failure (+10.8%; 95% confidence interval, 1.6 to 20.7) and chronic obstructive pulmonary disease (+13%; 95% confidence interval, 5 to 21.2). They also resembled an inflammatory phenotype, with lower serum albumin levels (-0.1 g/dl; 95% confidence interval, -0.2 to 0) and higher neutrophil-to-lymphocyte ratios (+1; 95% confidence interval, 0.5 to 1.6). They had lower hemoglobin levels (-0.2 g/dl; 95% confidence interval, -0.4 to 0) and required more erythropoietin (+1374 U per hemodialysis treatment; 95% confidence interval, 343 to 2405). During follow-up, all-cause hospitalization (1113 hospitalizations; univariate hazard ratio, 1.46; 95% confidence interval, 1.22 to 1.73) and mortality (89 deaths; adjusted hazard ratio, 1.98; 95% confidence interval, 1.14 to 3.43) were higher in patients with prolonged intradialytic hypoxemia.

CONCLUSIONS

Prolonged intradialytic hypoxemia was associated with laboratory indicators of inflammation, higher erythropoietin requirements, and higher all-cause hospitalization and mortality.

Mild Chronic Intermittent Hypoxia in Wistar Rats Evokes Significant Cardiovascular Pathophysiology but No Overt Changes in Carotid Body-Mediated Respiratory Responses

Models of chronic intermittent hypoxia (CIH), the main feature of obstructive sleep apnoea (OSA), have demonstrated dysregulation of the cardiovascular and respiratory systems resulting in hypertension, cardiac hypertrophy and alterations in the hypoxic ventilatory response (HVR) due to changes in sympathetic and respiratory control by the carotid body. In the UK, treatment of OSA is only offered to patients with an apnoea-hypopnoea index (AHI) >15, we investigated whether mild CIH produced significant pathophysiological changes, which might inform treatment guidelines.Rats were exposed to CIH (6 h(-1), 8 h day(-1), 5 % O(2) nadir) for 2 weeks and then arterial blood pressure (ABP), heart rate (HR) and ventilation were recorded in these and normoxic control rats (N) under Alfaxan anaesthesia, at baseline and in response to Dejours test, graded hypoxia and hypercapnia. Hearts were analysed post-mortem.CIH induced significant increases in baseline ABP (142 ± 5 vs 122 ± 2 mmHg), HR (448 ± 9 vs 412 ± 5 bpm) and cardiac mass (3.5 ± 0.1 vs 2.7 ± 0.1 g kg body mass(-1)) as a result of a selective left ventricular hypertrophy (1.6 ± 0.1 vs 1.3 ± 0.08 g kg body mass(-1); FCSA 464 ± 32 μm(2) vs 314 ± 9 μm(2)). There was no significant difference between N and CIH in baseline respiration or the response to Dejours test, graded hypoxia and hypercapnia.These results demonstrate that mild CIH can induce the significant cardiovascular changes associated with OSA without overt changes in respiratory function. Given evidence that CIH changes carotid body sensory activity, a possible explanation for these results is that there is differential integration of chemoreceptor input with respiratory and cardiac sympathetic outputs.

Augmented cardiovascular responses to episodes of repetitive compared with isolated respiratory events in preschool children with sleep-disordered breathing

BACKGROUND

Childhood sleep disordered breathing (SDB) presents as isolated respiratory events or episodes of consecutive repetitive events. We hypothesized that the surge in blood pressure (BP) and heart rate (HR) would be greater at the termination of events during episodes of repetitive events than following isolated events.

METHODS

% change in HR and pulse transit time (PTT; inverse surrogate of BP) were calculated from the last half of an event to: (i) between successive repetitive events; (ii) termination of the last repetitive event; (iii) event termination for isolated events.

RESULTS

69% of the children exhibiting both isolated and repetitive events had more repetitive than isolated events. %HR change between repetitive events (27 ± 1%) was greater than at event termination for isolated events (17 ± 1%; P < 0.001). %PTT change at the termination of the last repetitive event (-8 ± 2%) was greater than at the termination of isolated events (-2 ± 2%; P < 0.05).

CONCLUSION

Episodes of repetitive respiratory events evoke a greater acute cardiovascular response, including surges in BP and HR between events, than do isolated events. Given that the majority of respiratory events in preschool children occur as repetitive episodes, this finding should be taken into account when assessing the impact of respiratory events for a given child.