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

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.

Pathophysiological mechanisms and therapeutic approaches in obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) is a common breathing disorder in sleep in which the airways narrow or collapse during sleep, causing obstructive sleep apnea. The prevalence of OSAS continues to rise worldwide, particularly in middle-aged and elderly individuals. The mechanism of upper airway collapse is incompletely understood but is associated with several factors, including obesity, craniofacial changes, altered muscle function in the upper airway, pharyngeal neuropathy, and fluid shifts to the neck. The main characteristics of OSAS are recurrent pauses in respiration, which lead to intermittent hypoxia (IH) and hypercapnia, accompanied by blood oxygen desaturation and arousal during sleep, which sharply increases the risk of several diseases. This paper first briefly describes the epidemiology, incidence, and pathophysiological mechanisms of OSAS. Next, the alterations in relevant signaling pathways induced by IH are systematically reviewed and discussed. For example, IH can induce gut microbiota (GM) dysbiosis, impair the intestinal barrier, and alter intestinal metabolites. These mechanisms ultimately lead to secondary oxidative stress, systemic inflammation, and sympathetic activation. We then summarize the effects of IH on disease pathogenesis, including cardiocerebrovascular disorders, neurological disorders, metabolic diseases, cancer, reproductive disorders, and COVID-19. Finally, different therapeutic strategies for OSAS caused by different causes are proposed. Multidisciplinary approaches and shared decision-making are necessary for the successful treatment of OSAS in the future, but more randomized controlled trials are needed for further evaluation to define what treatments are best for specific OSAS patients.

Vasopressin: a possible link between hypoxia and hypertension

Cardiovascular and respiratory diseases are frequently associated with transient and prolonged hypoxia, whereas hypoxia exerts pro-hypertensive effects, through stimulation of the sympathetic system and release of pressor endocrine factors. This review is focused on the role of arginine vasopressin (AVP) in dysregulation of the cardiovascular system during hypoxia associated with cardiovascular disorders. AVP is synthesized mainly in the neuroendocrine neurons of the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON), which send axons to the posterior pituitary and various regions of the central nervous system (CNS). Vasopressinergic neurons are innervated by multiple neuronal projections releasing several neurotransmitters and other regulatory molecules. AVP interacts with V1a, V1b and V2 receptors that are present in the brain and peripheral organs, including the heart, vessels, lungs, and kidneys. Release of vasopressin is intensified during hypernatremia, hypovolemia, inflammation, stress, pain, and hypoxia which frequently occur in cardiovascular patients, and blood AVP concentration is markedly elevated in cardiovascular diseases associated with hypoxemia. There is evidence that hypoxia stimulates AVP release through stimulation of chemoreceptors. It is suggested that acting in the carotid bodies, AVP may fine-tune respiratory and hemodynamic responses to hypoxia and that this effect is intensified in hypertension. There is also evidence that during hypoxia, augmentation of pro-hypertensive effects of vasopressin may result from inappropriate interaction of this hormone with other compounds regulating the cardiovascular system (catecholamines, angiotensins, natriuretic peptides, steroids, nitric oxide). In conclusion, current literature indicates that abnormal mutual interactions between hypoxia and vasopressin may significantly contribute to pathogenesis of hypertension.

Inhibition of peripheral chemoreceptors improves ventilatory efficiency during exercise in heart failure with preserved ejection fraction − a role of tonic activity and acute reflex response

Peripheral chemoreceptors (PChRs) play a significant role in maintaining adequate oxygenation in the bloodstream. PChRs functionality comprises two components: tonic activity (PChT) which regulates ventilation during normoxia and acute reflex response (peripheral chemosensitivity, PChS), which increases ventilation following a specific stimulus. There is a clear link between augmented PChS and exercise intolerance in patients with heart failure with reduced ejection fraction. It has been also shown that inhibition of PChRs leads to the improvement in exercise capacity. However, it has not been established yet: 1) whether similar mechanisms take part in heart failure with preserved ejection fraction (HFpEF) and 2) which component of PChRs functionality (PChT vs. PChS) is responsible for the benefit seen after the acute experimental blockade. To answer those questions we enrolled 12 stable patients with HFpEF. All participants underwent an assessment of PChT (attenuation of minute ventilation in response to low-dose dopamine infusion), PChS (enhancement of minute ventilation in response to hypoxia) and a symptom-limited cardiopulmonary exercise test on cycle ergometer. All tests were placebo-controlled, double-blinded and performed in a randomized order. Under resting conditions and at normoxia dopamine attenuated minute ventilation and systemic vascular resistance (p = 0.03 for both). These changes were not seen with placebo. Dopamine also decreased ventilatory and mean arterial pressure responses to hypoxia (p < 0.05 for both). Inhibition of PChRs led to a decrease in V˙E/V˙CO₂ comparing to placebo (36 ± 3.6 vs. 34.3 ± 3.7, p = 0.04), with no effect on peak oxygen consumption. We found a significant relationship between PChT and the relative decrement of V˙E/V˙CO₂ on dopamine comparing to placebo (R = 0.76, p = 0.005). There was a trend for correlation between PChS (on placebo) and V˙E/V˙CO₂ during placebo infusion (R = 0.56, p = 0.059), but the relative improvement in V˙E/V˙CO₂ was not related to the change in PChS (dopamine vs. placebo). We did not find a significant relationship between PChT and PChS. In conclusion, inhibition of PChRs in HFpEF population improves ventilatory efficiency during exercise. Increased PChS is associated with worse (higher) V˙E/V˙CO₂, whereas PChT predicts an improvement in V˙E/V˙CO₂ after PChRs inhibition. This results may be meaningful for patient selection in further clinical trials involving PChRs modulation.

Sleep dysregulation in sympathetic-mediated diseases: implications for disease progression

The autonomic nervous system (ANS) plays an important role in the coordination of several physiological functions including sleep/wake process. Significant changes in ANS activity occur during wake-to-sleep transition maintaining the adequate cardiorespiratory regulation and brain activity. Since sleep is a complex homeostatic function, partly regulated by the ANS, it is not surprising that sleep disruption trigger and/or evidence symptoms of ANS impairment. Indeed, several studies suggest a bidirectional relationship between impaired ANS function (i.e. enhanced sympathetic drive), and the emergence/development of sleep disorders. Furthermore, several epidemiological studies described a strong association between sympathetic-mediated diseases and the development and maintenance of sleep disorders resulting in a vicious cycle with adverse outcomes and increased mortality risk. However, which and how the sleep/wake control and ANS circuitry becomes affected during the progression of ANS-related diseases remains poorly understood. Thus, understanding the physiological mechanisms underpinning sleep/wake-dependent sympathetic modulation could provide insights into diseases involving autonomic dysfunction. The purpose of this review is to explore potential neural mechanisms involved in both the onset/maintenance of sympathetic-mediated diseases (Rett syndrome, congenital central hypoventilation syndrome, obstructive sleep apnoea, type 2 diabetes, obesity, heart failure, hypertension, and neurodegenerative diseases) and their plausible contribution to the generation of sleep disorders in order to review evidence that may serve to establish a causal link between sleep disorders and heightened sympathetic activity.

Crucial Role of Stromal Interaction Molecule-Activated TRPC-ORAI Channels in Vascular Remodeling and Pulmonary Hypertension Induced by Intermittent Hypoxia

Obstructive sleep apnea (OSA), a sleep breathing disorder featured by chronic intermittent hypoxia (CIH), is associate with pulmonary hypertension. Rats exposed to CIH develop lung vascular remodeling and pulmonary hypertension, which paralleled the upregulation of stromal interaction molecule (STIM)-activated TRPC-ORAI Ca2+ channels (STOC) in the lung, suggesting that STOC participate in the pulmonary vascular alterations. Accordingly, to evaluate the role played by STOC in pulmonary hypertension we studied whether the STOC blocker 2-aminoethoxydiphenyl borate (2-APB) may prevent the vascular remodeling and the pulmonary hypertension induced by CIH in a rat model of OSA. We assessed the effects of 2-APB on right ventricular systolic pressure (RVSP), pulmonary vascular remodeling, α-actin and proliferation marker Ki-67 levels in pulmonary arterial smooth muscle cells (PASMC), mRNA levels of STOC subunits, and systemic and pulmonary oxidative stress (TBARS) in male Sprague-Dawley (200 g) rats exposed to CIH (5% O2, 12 times/h for 8h) for 28 days. At 14 days of CIH, osmotic pumps containing 2-APB (10 mg/kg/day) or its vehicle were implanted and rats were kept for 2 more weeks in CIH. Exposure to CIH for 28 days raised RVSP &gt; 35 mm Hg, increased the medial layer thickness and the levels of α-actin and Ki-67 in PASMC, and increased the gene expression of TRPC1, TRPC4, TRPC6 and ORAI1 subunits. Treatment with 2-APB prevented the raise in RVSP and the increment of the medial layer thickness, as well as the increased levels of α-actin and Ki-67 in PASMC, and the increased gene expression of STOC subunits. In addition, 2-APB did not reduced the lung and systemic oxidative stress, suggesting that the effects of 2-APB on vascular remodeling and pulmonary hypertension are independent on the reduction of the oxidative stress. Thus, our results supported that STIM-activated TRPC-ORAI Ca2+ channels contributes to the lung vascular remodeling and pulmonary hypertension induced by CIH.

Blood Pressure Load: An Effective Indicator of Systemic Circulation Status in Individuals With Acute Altitude Sickness

Background: Acute high altitude (HA) exposure results in blood pressure (BP) variations in most subjects. Previous studies have demonstrated that higher BP is potentially correlated with acute mountain sickness (AMS). The BP load may be of clinical significance regarding systemic circulation status.

Objectives: This study aimed to examine HA-induced BP changes in patients with AMS compared to those in healthy subjects. Further, we provided clinical information about the relationship between variations in 24-h ambulatory parameters (BP level, BP variability, and BP load) and AMS.

Methods: Sixty-nine subjects were enrolled and all participants ascended Litang (4,100 m above sea level). They were monitored using a 24-h ambulatory blood pressure device and underwent echocardiography within 24 h of altitude exposure. The 2018 Lake Louise questionnaire was used to evaluate AMS.

Results: The AMS group comprised more women than men [15 (65.2%) vs. 13 (28.3%), P < 0.001] and fewer smokers [4 (17.4%) vs. 23 (50.0%), P = 0.009]. The AMS group exhibited significant increases in 24-h BP compared to the non-AMS group (24-h SBP variation: 10.52 ± 6.48 vs. 6.03 ± 9.27 mmHg, P = 0.041; 24-h DBP variation: 8.70 ± 4.57 vs. 5.03 ± 4.98 mmHg, P = 0.004). The variation of mean 24-h cBPL (cumulative BP load) (mean 24-h cSBPL: 10.58 ± 10.99 vs. 4.02 ± 10.58, P = 0.016; 24-h mean cDBPL: 6.03 ± 5.87 vs. 2.89 ± 4.99, P = 0.034) was also obviously higher in AMS subjects than in non-AMS subjects after HA exposure. 24-h mean cSBPL variation (OR = 1.07, P = 0.024) and 24-h mean cDBPL variation (OR = 1.14, P = 0.034) were independent risk factors of AMS. Moreover, variation of 24-h mean cSBPL showed a good correlation with AMS score (R = 0.504, P < 0.001).

Conclusions: Our study demonstrated that patients with AMS had higher BP and BP load changes after altitude exposure than healthy subjects. Excessive BP load variations were associated with AMS. Thus, BP load could be an effective indicator regarding systemic circulation status of AMS.

Maladaptive Pulmonary Vascular Responses to Chronic Sustained and Chronic Intermittent Hypoxia in Rat

Chronic sustained hypoxia (CSH), as found in individuals living at a high altitude or in patients suffering respiratory disorders, initiates physiological adaptations such as carotid body stimulation to maintain oxygen levels, but has deleterious effects such as pulmonary hypertension (PH). Obstructive sleep apnea (OSA), a respiratory disorder of increasing prevalence, is characterized by a situation of chronic intermittent hypoxia (CIH). OSA is associated with the development of systemic hypertension and cardiovascular pathologies, due to carotid body and sympathetic overactivation. There is growing evidence that CIH can also compromise the pulmonary circulation, causing pulmonary hypertension in OSA patients and animal models. The aim of this work was to compare hemodynamics, vascular contractility, and L-arginine-NO metabolism in two models of PH in rats, associated with CSH and CIH exposure. We demonstrate that whereas CSH and CIH cause several common effects such as an increased hematocrit, weight loss, and an increase in pulmonary artery pressure (PAP), compared to CIH, CSH seems to have more of an effect on the pulmonary circulation, whereas the effects of CIH are apparently more targeted on the systemic circulation. The results suggest that the endothelial dysfunction evident in pulmonary arteries with both hypoxia protocols are not due to an increase in methylated arginines in these arteries, although an increase in plasma SDMA could contribute to the apparent loss of basal NO-dependent vasodilation and, therefore, the increase in PAP that results from CIH.

Size of the Carotid Body in Patients with Cardiovascular and Respiratory Diseases Measured by Computed Tomography Angiography: A Case-Control Study

OBJECTIVES

Carotid bodies (CBs) play an important role in regulating sympathetic nervous system activity. Thus, they are likely to be enlarged in patients with certain cardiovascular and respiratory diseases. The aim of this case-control study was to verify this hypothesis using computed tomography angiography (CTA).

METHODS

We retrospectively analysed 141 CTAs including 16 controls, 96 patients with only hypertension (HT), 12 with HT and previous acute myocardial infarction (AMI), 9 with HT and heart failure (HF), and 8 with HT and chronic obstructive pulmonary disease (COPD). We assessed the data using analysis of variance, with p < 0.05 indicating significance.

RESULTS

CB average areas in the controls were 2.31 mm2 (right side (RS)) vs. 2.34 mm2 (left side (LS)). CB size was significantly enlarged in patients with HT: 3.07 mm2 (RS) (p=0.019) vs. 2.91 mm2 (LS) (p=0.002). If AMI (RS: 3.5 mm2; LS: 3.44 mm2) or HF (RS: 4.01 mm2; LS: 4.55 mm2) was associated with HT, the CB size was even more enlarged. COPD did not affect CB size (RS: 2.40 mm2; LS: 2.29 mm2).

CONCLUSIONS

Our data showed that certain diseases with increased activity of the sympathetic nervous system were associated with significantly enlarged CBs.

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

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

Carbamylated form of human erythropoietin normalizes cardiorespiratory disorders triggered by intermittent hypoxia mimicking sleep apnea syndrome

BACKGROUND AND OBJECTIVE

Chronic intermittent hypoxia (CIH), one of the main features of obstructive sleep apnea (OSA), enhances carotid body-mediated chemoreflex and induces hypertension and breathing disorders. The carbamylated form of erythropoietin (cEpo) may have beneficial effects as it retains its antioxidant/anti-inflammatory and neuroprotective profile without increasing red blood cells number. However, no studies have evaluated the potential therapeutic effect of cEpo on CIH-related cardiorespiratory disorders. We aimed to determine whether cEpo normalized the CIH-enhanced carotid body ventilatory chemoreflex, the hypertension and ventilatory disorders in rats.

METHODS

Male Sprague-Dawley rats (250 g) were exposed to CIH (5% O2, 12/h, 8 h/day) for 28 days. cEPO (20 μg/kg, i.p) was administrated from day 21 every other day for one more week. Cardiovascular and respiratory function were assessed in freely moving animals.

RESULTS

Twenty-one days of CIH increased carotid body-mediated chemoreflex responses as evidenced by a significant increase in the hypoxic ventilatory response (FiO2 10%) and triggered irregular eupneic breathing, active expiration, and produced hypertension. cEpo treatment significantly reduced the carotid body--chemoreflex responses, normalizes breathing patterns and the hypertension in CIH. In addition, cEpo treatment effectively normalized carotid body chemosensory responses evoked by acute hypoxic stimulation in CIH rats.

CONCLUSION

Present results strongly support beneficial cardiorespiratory therapeutic effects of cEpo during CIH exposure.

Alterations in sympathetic and parasympathetic brain networks in obstructive sleep apnea

OBJECTIVE/BACKGROUND

Obstructive sleep apnea (OSA) patients experience hypoxia and, potentially, autonomic impairments stemming from neural damage. In this study, the executive control networks (ECNs), salience networks (SNs), and default mode networks (DMNs) of adult OSA patients, as well as their relationships with autonomic impairment, were investigated through independent component analysis (ICA).

PATIENTS/METHODS

A total of 41 OSA patients and 19 healthy controls volunteers were recruited and subjected to polysomnography to ascertain their degree, if any, of sleep apnea. Each participant also underwent a cardiovascular autonomic survey, with the participant's baroreflex sensitivity (BRS) being determined based on heart rate and blood pressure alterations. The resting fMRI data of the participants was separated using probabilistic ICA, and six autonomic resting-state networks were established for group comparisons. The differences in autonomic parameters, autonomic functional connectivity (FC), and clinical severity were then correlated.

RESULTS

The OSA group had significantly worse BRS values than the controls, as well as lower FC in the posterior and anterior SNs, bilateral ECNs, and the ventral DMN, and higher FC in the left ECN. These intrinsic connectivity networks showed dissociable correlations with greater baroreflex impairment and clinical disease severity. The higher FC in the left ECN was associated with the lower FC in the ventral DMN.

CONCLUSIONS

Our findings suggest that autonomic dysfunction in OSA might be accompanied by central autonomic network alterations. The stronger sympathetic-associated regions in ECNs and the weaker parasympathetic-associated regions in DMNs may represent intrinsic neural architecture fluctuations underlining their consequent processes in OSA.

Effects of Perinatal Hyperoxia on Breathing

Air-breathing animals do not experience hyperoxia (inspired O₂ > 21%) in nature, but preterm and full-term infants often experience hyperoxia/hyperoxemia in clinical settings. This article focuses on the effects of normobaric hyperoxia during the perinatal period on breathing in humans and other mammals, with an emphasis on the neural control of breathing during hyperoxia, after return to normoxia, and in response to subsequent hypoxic and hypercapnic challenges. Acute hyperoxia typically evokes an immediate ventilatory depression that is often, but not always, followed by hyperpnea. The hypoxic ventilatory response (HVR) is enhanced by brief periods of hyperoxia in adult mammals, but the limited data available suggest that this may not be the case for newborns. Chronic exposure to mild-to-moderate levels of hyperoxia (e.g., 30-60% O₂ for several days to a few weeks) elicits several changes in breathing in nonhuman animals, some of which are unique to perinatal exposures (i.e., developmental plasticity). Examples of this developmental plasticity include hypoventilation after return to normoxia and long-lasting attenuation of the HVR. Although both peripheral and CNS mechanisms are implicated in hyperoxia-induced plasticity, it is particularly clear that perinatal hyperoxia affects carotid body development. Some of these effects may be transient (e.g., decreased O₂ sensitivity of carotid body glomus cells) while others may be permanent (e.g., carotid body hypoplasia, loss of chemoafferent neurons). Whether the hyperoxic exposures routinely experienced by human infants in clinical settings are sufficient to alter respiratory control development remains an open question and requires further research. © 2020 American Physiological Society. Compr Physiol 10:597-636, 2020.

Physiology of the Carotid Body: From Molecules to Disease

The carotid body (CB) is an arterial chemoreceptor organ located in the carotid bifurcation and has a well-recognized role in cardiorespiratory regulation. The CB contains neurosecretory sensory cells (glomus cells), which release transmitters in response to hypoxia, hypercapnia, and acidemia to activate afferent sensory fibers terminating in the respiratory and autonomic brainstem centers. Knowledge of the physiology of the CB has progressed enormously in recent years. Herein we review advances concerning the organization and function of the cellular elements of the CB, with emphasis on the molecular mechanisms of acute oxygen sensing by glomus cells. We introduce the modern view of the CB as a multimodal integrated metabolic sensor and describe the properties of the CB stem cell niche, which support CB growth during acclimatization to chronic hypoxia. Finally, we discuss the increasing medical relevance of CB dysfunction and its potential impact on the mechanisms of disease.

Reactive oxygen/nitrogen species (ROS/RNS) and oxidative stress in arthroplasty

The interplay between implant design, biomaterial characteristics, and the local microenvironment adjacent to the implant is of utmost importance for implant performance and success of the joint replacement surgery. Reactive oxygen and nitrogen species (ROS/RNS) are among the various factors affecting the host as well as the implant components. Excessive formation of ROS and RNS can lead to oxidative stress, a condition that is known to damage cells and tissues and also to affect signaling pathways. It may further compromise implant longevity by accelerating implant degradation, primarily through activation of inflammatory cells. In addition, wear products of metallic, ceramic, polyethylene, or bone cement origin may also generate oxidative stress themselves. This review outlines the generation of free radicals and oxidative stress in arthroplasty and provides a conceptual framework on its implications for soft tissue remodeling and bone resorption (osteolysis) as well as implant longevity. Key findings derived from cell culture studies, animal models, and patients' samples are presented. Strategies to control oxidative stress by implant design and antioxidants are explored and areas of controversy and challenges are highlighted. Finally, directions for future research are identified. A better understanding of the host-implant interplay and the role of free radicals and oxidative stress will help to evaluate therapeutic approaches and will ultimately improve implant performance in arthroplasty.

Vaso-Occlusion in Sickle Cell Disease: Is Autonomic Dysregulation of the Microvasculature the Trigger?

Sickle cell disease (SCD) is an inherited hemoglobinopathy characterized by polymerization of hemoglobin S upon deoxygenation that results in the formation of rigid sickled-shaped red blood cells that can occlude the microvasculature, which leads to sudden onsets of pain. The severity of vaso-occlusive crises (VOC) is quite variable among patients, which is not fully explained by their genetic and biological profiles. The mechanism that initiates the transition from steady state to VOC remains unknown, as is the role of clinically reported triggers such as stress, cold and pain. The rate of hemoglobin S polymerization after deoxygenation is an important determinant of vaso-occlusion. Similarly, the microvascular blood flow rate plays a critical role as fast-moving red blood cells are better able to escape the microvasculature before polymerization of deoxy-hemoglobin S causes the red cells to become rigid and lodge in small vessels. The role of the autonomic nervous system (ANS) activity in VOC initiation and propagation has been underestimated considering that the ANS is the major regulator of microvascular blood flow and that most triggers of VOC can alter the autonomic balance. Here, we will briefly review the evidence supporting the presence of ANS dysfunction in SCD, its implications in the onset of VOC, and how differences in autonomic vasoreactivity might potentially contribute to variability in VOC severity.

Endothelin-1 enhanced carotid body chemosensory activity in chronic intermittent hypoxia through PLC, PKC and p38MAPK signaling pathways

Endothelin-1 (ET-1), as it functions as a neuromodulator, has been associated with hypertension in chronic intermittent hypoxia (CIH) which attribute to enhanced carotid body sensibility to hypoxia. However, the molecular mechanism of ET-1 on carotid body sensibility in CIH is still not clear. Here, effect of ET-1 on carotid body chemosensory stimulation in rats exposed to either CIH or room air (Normoxia) was explored. Furthermore, Phospholipase C (PLC), Protein kinase C (PKC) or p38 MAPK antagonists were adopted to clarify the signalling pathways involved. Results showed that ET-1 induced a higher increase of carotid sinus nerve activity (CSNA) in animals exposed to CIH. Both ETA and ETB receptor expression were up-regulated by CIH exposure, but only ETA is responsible for ET-1 induced CSNA increase. Additional, the increase was inhibited by PLC, PKC, p38 MAPK antagonists and calcium channel blocker. Our findings support that ETA receptor mediates ET-1-induced CSNA increase through PLC, PKC and p38 MAPK signalling pathways in chronic intermittent hypoxia. Also, our study indicated that calcium influx was necessary for enhancing effect of ET-1 on CSNA.

TRPV1 channels contribute to spontaneous glutamate release in nucleus tractus solitarii following chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) reduces afferent-evoked excitatory postsynaptic currents (EPSCs) but enhances basal spontaneous (s) and asynchronous (a) EPSCs in second-order neurons of nucleus tractus solitarii (nTS), a major area for cardiorespiratory control. The net result is an increase in synaptic transmission. The mechanisms by which this occurs are unknown. The N-type calcium channel and transient receptor potential cation channel TRPV1 play prominent roles in nTS sEPSCs and aEPSCs. The functional role of these channels in CIH-mediated afferent-evoked EPSC, sEPSC, and aEPSC was tested in rat nTS slices following antagonist inhibition and in mouse nTS slices that lack TRPV1. Block of N-type channels decreased aEPSCs in normoxic and, to a lesser extent, CIH-exposed rats. sEPSCs examined in the presence of TTX (miniature EPSCs) were also decreased by N-type block in normoxic but not CIH-exposed rats. Antagonist inhibition of TRPV1 reduced the normoxic and the CIH-mediated increase in sEPSCs, aEPSCs, and mEPSCs. As in rats, in TRPV1+/+ control mice, aEPSCs, sEPSCs, and mEPSCs were enhanced following CIH. However, none were enhanced in TRPV1-/- null mice. Normoxic tractus solitarii (TS)-evoked EPSC amplitude, and the decrease after CIH, were comparable in control and null mice. In rats, TRPV1 was localized in the nodose-petrosal ganglia (NPG) and their central branches. CIH did not alter TRPV1 mRNA but increased its protein in NPG consistent with an increased contribution of TRPV1. Together, our studies indicate TRPV1 contributes to the CIH increase in aEPSCs and mEPSCs, but the CIH reduction in TS-EPSC amplitude occurs via an alternative mechanism. NEW & NOTEWORTHY This study provides information on the underlying mechanisms responsible for the chronic intermittent hypoxia (CIH) increase in synaptic transmission that leads to exaggerated sympathetic nervous and respiratory activity at baseline and in response to low oxygen. We demonstrate that the CIH increase in asynchronous and spontaneous excitatory postsynaptic currents (EPSCs) and miniature EPSCs, but not decrease in afferent-driven EPSCs, is dependent on transient receptor potential vanilloid type 1 (TRPV1). Thus TRPV1 is important in controlling nucleus tractus solitarii synaptic activity during CIH.

Caffeine and supplemental oxygen effectively suppress periodic breathing with only minor effects during long episodes of apnoea in preterm infants

AIM

We investigated the characteristics and effects of sleep stage, supplemental oxygen and caffeine on periodic breathing (PB) and apnoea of prematurity (AOP) in preterm infants.

METHODS

This 2013-2015 study recruited 21 preterm infants on neonatal wards in the Helsinki and Uusimaa Hospital District, Finland, at a median corrected gestational age of 35.7 weeks and performed polysomnography at baseline, during supplemental oxygen and during caffeine treatment.

RESULTS

All infants demonstrated PB, during a median of 11% of sleep time and 85% of PB occurred during non-rapid eye movement sleep (NREM). Apnoea episodes were brief during PB, but 66% were associated with oxygen desaturation. Supplemental oxygen substantially reduced PB time by 99% and caffeine by 91%. Oxygen desaturation decreased from 38 per hour at baseline to 8.5 with oxygen and 24 with caffeine (all p < 0.001). AOPs decreased from 1.4 per hour at baseline to 0.4 with oxygen (p = 0.03) and 0.3 with caffeine (p = 0.07). Most (84%) apnoea episodes over 15 seconds were mixed episodes during REM sleep.

CONCLUSION

PB occurred predominantly during NREM sleep, caused intermittent hypoxia, and was suppressed by supplemental oxygen and caffeine. In contrast, long apnoea episodes representing AOP were only modestly decreased.

Ventilatory and Autonomic Regulation in Sleep Apnea Syndrome: A Potential Protective Role for Erythropoietin?

Obstructive sleep apnea (OSA) is the most common form of sleep disordered breathing and is associated with wide array of cardiovascular morbidities. It has been proposed that during OSA, the respiratory control center (RCC) is affected by exaggerated afferent signals coming from peripheral/central chemoreceptors which leads to ventilatory instability and may perpetuate apnea generation. Treatments focused on decreasing hyperactivity of peripheral/central chemoreceptors may be useful to improving ventilatory instability in OSA patients. Previous studies indicate that oxidative stress and inflammation are key players in the increased peripheral/central chemoreflex drive associated with OSA. Recent data suggest that erythropoietin (Epo) could also be involved in modulating chemoreflex activity as functional Epo receptors are constitutively expressed in peripheral and central chemoreceptors cells. Additionally, there is some evidence that Epo has anti-oxidant/anti-inflammatory effects. Accordingly, we propose that Epo treatment during OSA may reduce enhanced peripheral/central chemoreflex drive and normalize the activity of the RCC which in turn may help to abrogate ventilatory instability. In this perspective article we discuss the potential beneficial effects of Epo administration on ventilatory regulation in the setting of OSA.

Carotid body removal normalizes arterial blood pressure and respiratory frequency in offspring of protein-restricted mothers

The aim of this study is to evaluate the short-term and long-term effects elicited by carotid body removal (CBR) on ventilatory function and the development of hypertension in the offspring of malnourished rats. Wistar rats were fed a normo-protein (NP, 17% casein) or low-protein (LP, 8% casein) diet during pregnancy and lactation. At 29 days of age, the animals were submitted to CBR or a sham surgery, according to the following groups: NP-cbr, LP-cbr, NP-sham, or LP-sham. In the short-term, at 30 days of age, the respiratory frequency (RF) and immunoreactivity for Fos on the retrotrapezoid nucleus (RTN; brainstem site containing CO₂ sensitive neurons) after exposure to CO₂ were evaluated. In the long term, at 90 days of age, arterial pressure (AP), heart rate (HR), and cardiovascular variability were evaluated. In the short term, an increase in the baseline RF (~6%), response to CO₂ (~8%), and Fos in the RTN (~27%) occurred in the LP-sham group compared with the NP-sham group. Interestingly, the CBR in the LP group normalized the RF in response to CO₂ as well as RTN cell activation. In the long term, CBR reduced the mean AP by ~20 mmHg in malnourished rats. The normalization of the arterial pressure was associated with a decrease in the low-frequency (LF) oscillatory component of AP (~58%) and in the sympathetic tonus to the cardiovascular system (~29%). In conclusion, carotid body inputs in malnourished offspring may be responsible for the following: (i) enhanced respiratory frequency and CO₂ chemosensitivity in early life and (ii) the production of autonomic imbalance and the development of hypertension.

Hemodynamic characteristics of postural hyperventilation: POTS with hyperventilation versus panic versus voluntary hyperventilation

Upright hyperventilation occurs in ~25% of our patients with postural tachycardia syndrome (POTS). Poikilocapnic hyperventilation alone causes tachycardia. Here, we examined changes in respiration and hemodynamics comprising cardiac output (CO), systemic vascular resistance (SVR), and blood pressure (BP) measured during head-up tilt (HUT) in three groups: patients with POTS and hyperventilation (POTS-HV), patients with panic disorder who hyperventilate (Panic), and healthy controls performing voluntary upright hyperpnea (Voluntary-HV). Though all were comparably tachycardic during hyperventilation, POTS-HV manifested hyperpnea, decreased CO, increased SVR, and increased BP during HUT; Panic patients showed both hyperpnea and tachypnea, increased CO, and increased SVR as BP increased during HUT; and Voluntary-HV were hyperpneic by design and had increased CO, decreased SVR, and decreased BP during upright hyperventilation. Mechanisms of hyperventilation and hemodynamic changes differed among POTS-HV, Panic, and Voluntary-HV subjects. We hypothesize that the hyperventilation in POTS is caused by a mechanism involving peripheral chemoreflex sensitization by intermittent ischemic hypoxia. NEW & NOTEWORTHY Hyperventilation is common in postural tachycardia syndrome (POTS) and has distinctive cardiovascular characteristics when compared with hyperventilation in panic disorder or with voluntary hyperventilation. Hyperventilation in POTS is hyperpnea only, distinct from panic in which tachypnea also occurs. Cardiac output is decreased in POTS, whereas peripheral resistance and blood pressure (BP) are increased. This is distinct from voluntary hyperventilation where cardiac output is increased and resistance and BP are decreased and from panic where they are all increased.

Chronic Intermittent Hypoxia-Induced Vascular Dysfunction in Rats is Reverted by N-Acetylcysteine Supplementation and Arginase Inhibition

Chronic intermittent hypoxia (CIH), the main attribute of obstructive sleep apnea (OSA), produces oxidative stress, endothelial dysfunction, and hypertension. Nitric oxide (NO) plays a critical role in controlling the vasomotor tone. The NO level depends on the L-arginine level, which can be reduced by arginase enzymatic activity, and its reaction with the superoxide radical to produce peroxynitrite. Accordingly, we hypothesized whether a combination of an arginase inhibitor and an antioxidant may restore the endothelial function and reduced arterial blood pressure (BP) in CIH-induced hypertensive rats. Male Sprague-Dawley rats 200 g were exposed either to CIH (5% O₂, 12 times/h 8 h/day) or sham condition for 35 days. BP was continuously measured by radio-telemetry in conscious animals. After 14 days, rats were treated with 2(S)-amino-6-boronohexanoic acid (ABH 400 μg/kg day, osmotic pump), N-acetylcysteine (NAC 100 mg/kg day, drinking water), or the combination of both drugs until day 35. At the end of the experiments, external carotid and femoral arteries were isolated to determine vasoactive contractile responses induced by KCL and acetylcholine (ACh) with wire-myography. CIH-induced hypertension (~8 mmHg) was reverted by ABH, NAC, and ABH/NAC administration. Carotid arteries from CIH-treated rats showed higher contraction induced by KCl (3.4 ± 0.4 vs. 2.4 ± 0.2 N/m²) and diminished vasorelaxation elicits by ACh compared to sham rats (12.8 ± 1.5 vs. 30.5 ± 4.6%). ABH reverted the increased contraction (2.5 ± 0.2 N/m²) and the reduced vasorelaxation induced by ACh in carotid arteries from CIH-rats (38.1 ± 4.9%). However, NAC failed to revert the enhanced vasocontraction (3.9 ± 0.6 N/m²) induced by KCl and the diminished ACh-induced vasorelaxation in carotid arteries (10.7 ± 0.8%). Femoral arteries from CIH rats showed an increased contractile response, an effect partially reverted by ABH, but completely reverted by NAC and ABH/NAC. The impaired endothelial-dependent relaxation in femoral arteries from CIH rats was reverted by ABH and ABH/NAC. In addition, ABH/NAC at high doses had no effect on liver and kidney gross morphology and biochemical parameters. Thus, although ABH, and NAC alone and the combination of ABH/NAC were able to normalize the elevated BP, only the combined treatment of ABH/NAC normalized the vascular reactivity and the systemic oxidative stress in CIH-treated rats.

Revisiting the Role of TRP, Orai, and ASIC Channels in the Pulmonary Arterial Response to Hypoxia

The pulmonary arteries are exquisitely responsive to oxygen changes. They rapidly and proportionally contract as arterial PO₂ decrease, and they relax as arterial PO₂ is re-established. The hypoxic pulmonary vasoconstriction (HPV) is intrinsic since it does not require neural or endocrine factors, as evidenced in isolated vessels. On the other hand, pulmonary arteries also respond to sustained hypoxia with structural and functional remodeling, involving growth of smooth muscle medial layer and later recruitment of adventitial fibroblasts, secreted mitogens from endothelium and changes in the response to vasoconstrictor and vasodilator stimuli. Hypoxic pulmonary arterial vasoconstriction and remodeling are relevant biological responses both under physiological and pathological conditions, to explain matching between ventilation and perfusion, fetal to neonatal transition of pulmonary circulation and pulmonary artery over-constriction and thickening in pulmonary hypertension. Store operated channels (SOC) and receptor operated channels (ROC) are plasma membrane cationic channels that mediate calcium influx in response to depletion of internal calcium stores or receptor activation, respectively. They are involved in both HPV and pathological remodeling since their pharmacological blockade or genetic suppression of several of the Stim, Orai, TRP, or ASIC proteins in SOC or ROC complexes attenuate the calcium increase, the tension development, the pulmonary artery smooth muscle proliferation, and pulmonary arterial hypertension. In this Mini Review, we discussed the evidence obtained in in vivo animal models, at the level of isolated organ or cells of pulmonary arteries, and we identified and discussed the questions for future research needed to validate these signaling complexes as targets against pulmonary hypertension.

Translating carotid body function into clinical medicine

The carotid body (CB) is considered the main O₂ chemoreceptor, which contributes to cardiorespiratory homeostasis and ventilatory acclimatization. In clinical medicine, the most common pathologies associated with the CB are tumours. However, a growing body of evidence supports the novel idea that an enhanced CB chemosensory discharge contributes to the autonomic dysfunction and pathological consequences in obstructive sleep apnoea (OSA), hypertension, systolic heart failure (HF) and cardiometabolic diseases. Heightened CB chemosensory reactivity elicited by oxidative stress has been involved in sympathetic hyperactivity, cardiorespiratory instability, hypertension and insulin resistance. CB ablation, which reduces sympathetic hyperactivity, decreases hypertension in animal models of OSA and hypertension, eliminates breathing instability and improves animal survival in HF, and restores insulin tolerance in cardiometabolic models. Thus, data obtained from preclinical studies highlight the importance of the CB in the progression of sympathetic-related diseases, supporting the idea that appeasing the enhanced CB chemosensory drive may be useful in improving cardiovascular, respiratory and endocrine alterations. Accordingly, CB ablation has been proposed and used as a treatment for moderating resistant hypertension and HF-induced sympathetic hyperactivity in humans. First-in-human studies have shown that CB ablation reduces sympathetic overactivity, transiently reduces severe hypertension and improves quality of life in HF patients. Thus, CB ablation would be a useful therapy to reverse sympathetic overactivation in HF and severe hypertension, but caution is required before it is widely used due to the crucial physiological function played by the CB. Further studies in preclinical models are required to assess side-effects of CB ablation.

The role of the Nox4-derived ROS-mediated RhoA/Rho kinase pathway in rat hypertension induced by chronic intermittent hypoxia

BACKGROUND

Obstructive sleep apnea syndrome, which is a risk factor for resistant hypertension, is characterized by chronic intermittent hypoxia (CIH) and is associated with many cardiovascular diseases. CIH elicits systemic oxidative stress and sympathetic hyperactivity, which lead to hypertension. Rho kinases (ROCKs) are considered to be major effectors of the small GTPase RhoA and have been extensively studied in the cardiovascular field. Upregulation of the RhoA/ROCK signaling cascade is observed in various cardiovascular disorders, such as atherosclerosis, pulmonary hypertension, and stroke. However, the exact molecular function of RhoA/ROCK in CIH remains unclear and requires further study.

OBJECTIVE

This study aimed to investigate the role of the NADPH oxidase 4 (Nox4)-induced ROS/RhoA/ROCK pathway in CIH-induced hypertension in rats.

METHODS

Male Sprague-Dawley rats were exposed to CIH for 21 days (intermittent hypoxia of 21% O₂ for 60 s and 5% O₂ for 30 s, cyclically repeated for 8 h/day). We randomly assigned 56 male rats to groups of normoxia (RA) or vertically implemented CIH together with vehicle (CIH-V), GKT137831 (CIH-G), N-acetyl cysteine (NAC) (CIH-N), or Y27632 (CIH-Y). The rats in the RA group were continuously exposed to room air, whereas the rats in the other groups were exposed to CIH. Systolic blood pressure (BP) was monitored at the beginning of each week. After the experiment, renal sympathetic nerve activity (RSNA) was recorded, and serum and renal tissues were subjected to molecular biological and biochemical analyses.

RESULTS

Compared with the BP of RA rats, the BP of CIH-V rats started to increase 2 weeks after the beginning of the experiment, subsequently stabilizing at a high level at the end of the third week. CIH increased both RSNA and oxidative stress. This response was attenuated by treatment of the rats with GKT137831 or NAC. Inhibiting Nox4 activity or ROS production reduced RhoA/ROCK expression. Treatment with Y27632 reduced both BP and RSNA in rats exposed to CIH.

CONCLUSION

Hypertension can be induced by CIH in SD rats. The CIH-induced elevation of BP is at least partially mediated via the Nox4-induced ROS/RhoA/ROCK pathway.

Oxidative stress augments chemoreflex sensitivity in rats exposed to chronic intermittent hypoxia

Chronic exposure to intermittent hypoxia (CIH) elicits plasticity of the carotid sinus and phrenic nerves via reactive oxygen species (ROS). To determine whether CIH-induced alterations in ventilation, metabolism, and heart rate are also dependent on ROS, we measured responses to acute hypoxia in conscious rats after 14 and 21 d of either CIH or normoxia (NORM), with or without concomitant administration of allopurinol (xanthine oxidase inhibitor), combined allopurinol plus losartan (angiotensin II type 1 receptor antagonist), or apocynin (NADPH oxidase inhibitor). Carotid body nitrotyrosine production was measured by immunohistochemistry. CIH produced an increase in the ventilatory response to acute hypoxia that was virtually eliminated by all three pharmacologic interventions. CIH caused a robust increase in carotid body nitrotyrosine production that was greatly attenuated by allopurinol plus losartan and by apocynin but unaffected by allopurinol. CIH caused a decrease in metabolic rate and a reduction in hypoxic bradycardia. Both of these effects were prevented by allopurinol, allopurinol plus losartan, and apocynin.

Carotid Body Ablation Abrogates Hypertension and Autonomic Alterations Induced by Intermittent Hypoxia in Rats

Chronic intermittent hypoxia (CIH), the main feature of obstructive sleep apnea, enhances carotid body (CB) chemosensory responses to hypoxia and produces autonomic dysfunction, cardiac arrhythmias, and hypertension. We tested whether autonomic alterations, arrhythmogenesis, and the progression of hypertension induced by CIH depend on the enhanced CB chemosensory drive, by ablation of the CB chemoreceptors. Male Sprague-Dawley rats were exposed to control (Sham) conditions for 7 days and then to CIH (5% O2, 12/h 8 h/d) for a total of 28 days. At 21 days of CIH exposure, rats underwent bilateral CB ablation and then exposed to CIH for 7 additional days. Arterial blood pressure and ventilatory chemoreflex response to hypoxia were measured in conscious rats. In addition, cardiac autonomic imbalance, cardiac baroreflex gain, and arrhythmia score were assessed during the length of the experiments. In separate experimental series, we measured extracellular matrix remodeling content in cardiac atrial tissue and systemic oxidative stress. CIH induced hypertension, enhanced ventilatory response to hypoxia, induced autonomic imbalance toward sympathetic preponderance, reduced baroreflex gain, and increased arrhythmias and atrial fibrosis. CB ablation normalized blood pressure, reduced ventilatory response to hypoxia, and restored cardiac autonomic and baroreflex function. In addition, CB ablation reduced the number of arrhythmias, but not extracellular matrix remodeling or systemic oxidative stress, suggesting that reductions in arrhythmia incidence during CIH were related to normalization of cardiac autonomic balance. Present results show that autonomic alterations induced by CIH are critically dependent on the CB and support a main role for the CB in the CIH-induced hypertension.

Obstructive Sleep Apnea and Vascular Diseases

Obstructive sleep apnea (OSA) affects a large proportion of adults, and is as an independent risk factor for cerebrovascular and cardiovascular disease. The repetitive airway obstruction that characterizes OSA results in intermittent hypoxia, intrathoracic pressure swings, and sleep fragmentation, which in turn lead to sympathetic activation, oxidative stress, inflammation, and endothelial dysfunction. This review outlines the associations between OSA and vascular diseases and describes basic mechanisms that may be responsible for this association, in both the micro- and macrocirculation. It also reports on interventional studies that aim to ameliorate OSA and thereby reduce vascular disease burden. © 2016 American Physiological Society. Compr Physiol 6:1519-1528, 2016.

Effect of Continuous Positive Airway Pressure on Cardiovascular Biomarkers: The Sleep Apnea Stress Randomized Controlled Trial

BACKGROUND

Although existing research highlights the relationship of OSA and cardiovascular disease, the effect of OSA treatment on cardiovascular biomarkers remains unclear. We evaluated the effect of OSA treatment on oxidative stress/inflammation measures.

METHODS

We conducted a parallel, randomized controlled trial in moderate to severe OSA (apnea-hypopnea index ≥ 15) patients to examine effects of 2-month CPAP vs sham-CPAP on the primary outcome of oxidative stress/inflammation (F2-isoprostanes: ng/mg) and myeloperoxidase: pmol/L) and secondary oxidative stress measures. Exploratory secondary analyses included vascular and systemic inflammation markers. Linear models adjusted for baseline values examined effect of CPAP on biomarker change (least squares means, 95% CI) including secondary stratified analyses examining CPAP adherence and degree of hypoxia.

RESULTS

Of 153 participants, 76 were randomized to CPAP and 77 to sham-CPAP. In an intent-to-treat analyses, no significant change was observed in the sham and CPAP groups respectively: F2-isoprostanes (-0.02 [-0.12 to 0.10] vs -0.08 [-0.18 to 0.03]) or myeloperoxidase (-3.33 [-17.02 to 10.37] vs -5.15 [-18.65 to 8.35]), nor other oxidative markers; findings that persisted in analyses stratified by adherence and hypoxia. Exploratory analyses revealed percentage reduction of soluble IL-6 receptor (ng/mL) levels (-0.04 [-0.08 to -0.01] vs 0.02 [-0.02 to 0.06], P = .019) and augmentation index (%) (-6.49 [-9.32 to -3.65] vs 0.44 [-2.22 to 3.10], P < .001) with CPAP compared with sham, respectively.

CONCLUSIONS

In moderate to severe OSA, 2-month CPAP vs sham did not reduce oxidative stress despite consideration of a broad range of measures, positive airway pressure adherence, and hypoxia burden. These findings suggest that nonoxidative stress pathways primarily modulate OSA-related cardiovascular consequences.

TRIAL REGISTRATION

ClinicalTrials.govNCT00607893.

Carotid body chemoreceptors, sympathetic neural activation, and cardiometabolic disease

The carotid body (CB) is the main peripheral chemoreceptor that senses the arterial PO₂, PCO₂ and pH. In response to hypoxemia, hypercapnia and acidosis, carotid chemosensory discharge elicits reflex respiratory, autonomic and cardiovascular adjustments. The classical construct considers the CB as the main peripheral oxygen sensor, triggering reflex physiological responses to acute hypoxemia and facilitating the ventilatory acclimation to chronic hypoxemia at high altitude. However, a growing body of experimental evidence supports the novel concept that an abnormally enhanced CB chemosensory input to the brainstem contributes to overactivation of the sympathetic nervous system, and consequent pathology. Indeed, the CB has been implicated in several diseases associated with increases in central sympathetic outflow. These include hypertension, heart failure, sleep apnea, chronic obstructive pulmonary disease and metabolic syndrome. Indeed, ablation of the CB has been proposed for the treatment of severe and resistant hypertension in humans. In this review, we will analyze and discuss new evidence supporting an important role for the CB chemoreceptor in the progression of autonomic and cardiorespiratory alterations induced by heart failure, obstructive sleep apnea, chronic obstructive pulmonary disease and metabolic syndrome.

Intermittent Hypoxia-Induced Carotid Body Chemosensory Potentiation and Hypertension Are Critically Dependent on Peroxynitrite Formation

Oxidative stress is involved in the development of carotid body (CB) chemosensory potentiation and systemic hypertension induced by chronic intermittent hypoxia (CIH), the main feature of obstructive sleep apnea. We tested whether peroxynitrite (ONOO⁻), a highly reactive nitrogen species, is involved in the enhanced CB oxygen chemosensitivity and the hypertension during CIH. Accordingly, we studied effects of Ebselen, an ONOO⁻ scavenger, on 3-nitrotyrosine immunoreactivity (3-NT-ir) in the CB, the CB chemosensory discharge, and arterial blood pressure (BP) in rats exposed to CIH. Male Sprague-Dawley rats were exposed to CIH (5% O₂, 12 times/h, 8 h/day) for 7 days. Ebselen (10 mg/kg/day) was administrated using osmotic minipumps and BP measured with radiotelemetry. Compared to the sham animals, CIH-treated rats showed increased 3-NT-ir within the CB, enhanced CB chemosensory responses to hypoxia, increased BP response to acute hypoxia, and hypertension. Rats treated with Ebselen and exposed to CIH displayed a significant reduction in 3-NT-ir levels (60.8 ± 14.9 versus 22.9 ± 4.2 a.u.), reduced CB chemosensory response to 5% O₂ (266.5 ± 13.4 versus 168.6 ± 16.8 Hz), and decreased mean BP (116.9 ± 13.2 versus 82.1 ± 5.1 mmHg). Our results suggest that CIH-induced CB chemosensory potentiation and hypertension are critically dependent on ONOO⁻ formation.

Relevance of the Carotid Body Chemoreflex in the Progression of Heart Failure

Chronic heart failure (CHF) is a global health problem affecting millions of people. Autonomic dysfunction and disordered breathing patterns are commonly observed in patients with CHF, and both are strongly related to poor prognosis and high mortality risk. Tonic activation of carotid body (CB) chemoreceptors contributes to sympathoexcitation and disordered breathing patterns in experimental models of CHF. Recent studies show that ablation of the CB chemoreceptors improves autonomic function and breathing control in CHF and improves survival. These exciting findings indicate that alterations in CB function are critical to the progression of CHF. Therefore, better understanding of the physiology of the CB chemoreflex in CHF could lead to improvements in current treatments and clinical management of patients with CHF characterized by high chemosensitivity. Accordingly, the main focus of this brief review is to summarize current knowledge of CB chemoreflex function in different experimental models of CHF and to comment on their potential translation to treatment of human CHF.

Hydrogen sulfide and hypoxia-induced changes in TASK (K2P3/9) activity and intracellular Ca(2+) concentration in rat carotid body glomus cells

Acute hypoxia depolarizes carotid body chemoreceptor (glomus) cells and elevates intracellular Ca(2+) concentration ([Ca(2+)]i). Recent studies suggest that hydrogen sulfide (H2S) may serve as an oxygen sensor/signal in the carotid body during acute hypoxia. To further test such a role for H2S, we studied the effects of H2S on the activity of TASK channel and [Ca(2+)]i, which are considered important for mediating the glomus cell response to hypoxia. Like hypoxia, NaHS (a H2S donor) inhibited TASK activity and elevated [Ca(2+)]i. To inhibit the production of H2S, glomus cells were incubated (3h) with inhibitors of cystathionine-β-synthase and cystathionine-γ-lyase (DL-propargylglycine, aminooxyacetic acid, β-cyano-L-alanine; 0.3 mM). SF7 fluorescence was used to assess the level of H2S production. The inhibitors blocked L-cysteine- and hypoxia-induced elevation of SF7 fluorescence intensity. In cells treated with the inhibitors, hypoxia produced an inhibition of TASK activity and a rise in [Ca(2+)]i, similar in magnitude to those observed in control cells. L-cysteine produced no effect on TASK activity or [Ca(2+)]i and did not affect hypoxia-induced inhibition of TASK and elevation of [Ca(2+)]i. These findings suggest that under normal conditions, H2S is not a major signal in hypoxia-induced modulation of TASK channels and [Ca(2+)]i in isolated glomus cells.

Design of a dual-function peptide probe as a binder of angiotensin II and an inducer of silver nanoparticle aggregation for use in label-free colorimetric assays

Label-free colorimetric assays using metallic nanoparticles have received much recent attention, for their application in simple and sensitive methods for detection of biomolecules. Short peptide probes that can bind to analyte biomolecules are attractive ligands in molecular nanotechnology; however, identification of biological recognition motifs is usually based on trial-and-error experiments. Herein, a peptide probe was screened for colorimetric detection of angiotensin II (Ang II) using a mechanism for non-crosslinking aggregation of silver nanoparticles (AgNPs). The dual-function peptides, which bind to the analyte and induce AgNP aggregation, were identified using a two-step strategy: (1) screening of an Ang II-binding peptide from an Ang II receptor sequence library, using SPOT technology, which enable peptides synthesis on cellulose membranes via an Fmoc method and (2) selection of peptide probes that effectively induce aggregation of AgNPs using a photolinker modified peptide array. Using the identified peptide probe, KGKNKRRR, aggregation of AgNPs was detected by observation of a pink color in the absence of Ang II, whereas AgNPs remained dispersed in the presence of Ang II (yellow). The color changes were not observed in the presence of other hormone molecules. Ang II could be detected within 15 min, with a detection limit of 10 µM, by measuring the ratio of absorbance at 400 nm and 568 nm; the signal could also be observed with the naked eye. These data suggest that the peptide identified here could be used as a probe for simple and rapid colorimetric detection of Ang II. This strategy for the identification of functional peptides shows promise for the development of colorimetric detection of various diagnostically important biomolecules.